The potential of color centers in hexagonal boron nitride (hBN) for quantum technology applications has driven research to create emitters across a broad spectral range by using diverse techniques. Electron beam irradiation is one such approach that creates yellow emitters at room temperature,. however, their behavior at low temperatures remains unexplored. Here, we present a comprehensive photophysical characterization of these yellow emitters in hBN under cryogenic conditions. We identify a bright and photostable defect with a zero-phonon line (ZPL) at 547.5 nm and a phonon sideband (PSB) approximately 90 meV from the ZPL. Excitation through this PSB enhances the emission intensity by nearly 5-fold at 4.5 K. Temperature-dependent photoluminescence (PL) from 4.5 to 220 K shows a decreasing Debye-Waller (DW) factor with elevated temperature, reflecting enhanced phonon-assisted emission. Further analysis reveals the presence of an additional low-energy phonon mode, leading to a T 3 dependence of the ZPL line width and a T 2 dependence of the ZPL peak shift. These observations deepen our understanding of the nature of the emitters, opening new avenues for the precise tuning of quantum light sources.

We explore the zero-phonon line of single photon emitters in helium-ion treated monolayer MoS2, which are currently understood in terms of single sulfur-site vacancies. By comparing the linewidths of the zero-phonon line as extracted directly from optical spectra with values inferred from the first-order autocorrelation function of the photoluminescence, we quantify bounds of the homogeneous broadening and of phonon-assisted contributions. The results are discussed in terms of both the independent boson model and ab-initio results as computed from GW and Bethe-Salpeter equation approximations.

We investigate the interaction between interlayer excitons and ferroelectric domains in hBN-encapsulated 3R-MoS2/MoSe2 heterostructures, combining photoluminescence experiments with density functional theory and many-body Green's function calculations. Low-temperature photoluminescence spectroscopy reveals a strong redshift of the interlayer exciton energy with increasing MoS2 layer thickness, attributed to band renormalization and dielectric effects. We observe local variations in exciton energy that correlate with local ferroelectric domain polarization of the 3R-MoS2 layer, showcasing distinct domain-dependent interlayer exciton transition energies. Gate voltage experiments demonstrate that the interlayer exciton energy can be tuned by electrically induced domain switching. These results highlight the potential for interlayer exciton control by local ferroelectric order and establish a foundation for future ferroelectric optoelectronic devices based on van der Waals heterostructures.

The development of deterministic single-photon sources emitting in the telecommunication bands is a key challenge for quantum communication and photonic quantum computing. Here, we investigate the optical properties and single-photon emission of molecular beam epitaxy (MBE) grown semiconductor quantum dots (QDs) emitting in the telecom O-and C-bands. The QDs are embedded in an InGaAs matrix with fixed indium content grown on top of a compositionally graded InGaAs buffer. This structure allows for the future implementation of electrically contacted nanocavities to enable high-quality and bright QD emission. In detailed optical characterizations we observe linewidths as low as 50 mu eV, close to the spectrometer resolution limit, low fine structure splittings close to 10 mu eV, and g(2)(0) values as low as 0.08. These results advance the current performance metrics for MBE-grown QDs on GaAs substrates emitting in the telecom bands and showcase the potential of the heterostructures presented for further integration into photonic devices.

We study the emission from a molecular photonic cavity formed by two proximal photonic crystal defect cavities containing a small number ( < 3 ) of In(Ga)As quantum dots. Under strong excitation, we observe photoluminescence from the bonding and antibonding modes in agreement with ab initio numerical simulations. Power dependent measurements, however, reveal an unexpected peak, emerging at an energy between the bonding and antibonding modes of the molecule. Temperature-dependent measurements indicate that this unexpected feature is photonic in origin. Time-resolved measurements show the emergent peak exhibits a lifetime tau M = 0.75(10) ns, similar to both bonding and antibonding coupled modes. Comparisons of experimental results with quantum optical modeling suggest that this new feature arises from a coexistence of weak and strong coupling, due to the molecule emitting in an environment whose configuration permits or, on the contrary, impedes its strong coupling. This scenario is reproduced theoretically with a master equation reduced to the key ingredients of its dynamics and that roots the mechanism to a dissipative coupling between bare modes of the system. Excellent qualitative agreement is obtained between experiment and theory, showing how solid-state cavity QED can reveal intriguing new regimes of light-matter interaction.

Spin defects in semiconductors are widely investigated for various applications in quantum sensing. Conventional host materials such as diamond and hexagonal boron nitride (hBN) provide bulk or low-dimensional platforms for optically addressable spin systems, but often lack the structural properties needed for chemical sensing. Here, we introduce a new class of quantum sensors based on naturally occurring spin defects in boron nitride nanotubes (BNNTs), which combine high surface area with omnidirectional spin control, key features for enhanced sensing performance. First, we present strong evidence that these defects consist of weakly-coupled spin pairs, akin to recently identified centers in hBN, and demonstrate coherent spin control over ensembles embedded within randomly oriented, dense, BNNTs networks. Using dynamical decoupling, we enhance spin coherence times by a factor exceeding 300 times and implement high-resolution detection of radiofrequency signals. By integrating the BNNT mesh sensor into a microfluidic platform we demonstrate chemical sensing of paramagnetic ions in solution, with detectable concentrations reaching levels nearly 1000 times lower than previously demonstrated using comparable hBN-based systems. This highly porous and flexible architecture positions BNNTs as a powerful new host material for quantum sensing.

In this work, we experimentally and theoretically study the dressed-state emission of the biexciton-exciton cascade in a semiconductor quantum dot under pulsed, resonant, two-photon excitation. Building on the wellcharacterized steady-state dressed emission of the four-level system, we examine its dynamic counterpart under pulsed, resonant excitation, addressing both experimental observations and theoretical modeling. Here we report several sidebands emerging from the biexciton-to-exciton transition, whose number and spectral width depend on the excitation pulse duration and the effective pulse area, while no sidebands emerge from the exciton-to-groundstate transition. Since the biexciton state population follows a nonlinear pulse area function, sidebands with a small spectral nonlinearity result. Detuning- and time-dependent measurements provide deeper insight into the emission properties of the dressed states. They show that side peak emission only occurs in the presence of the excitation pulse. Moreover, when the system is excited by a Gaussian-shaped laser pulse, side peak emission takes place sequentially.

Spins confined in optically active quantum dot molecules (QDMs) can be used for the deterministic generation of photonic graph states with tailored entanglement structures. Their usefulness for the generation of such nonclassical states of light is determined by orbital and spin decoherence mechanisms, particularly phonon-mediated processes dominant at energy scales up to a few millielectronvolts. Here, we directly measure the spectral function of orbital phonon relaxation between the energy states of the neutral exciton in a QDM and benchmark our findings against microscopic k <middle dot> p theory. Our results reveal pronounced resonances and antiresonances in the phonon-relaxation rates, ranging from tens of mu s-1up to tens of ns-1. Comparison with a kinetic model reveals the voltage (energy) dependent phonon coupling strength and fully explains the interplay between phonon-assisted relaxation and radiative recombination. The resonances and antiresonances enable further tunability of the exciton lifetime which can be leveraged to increase the lifetime of energetically unfavorable charge configurations needed for realizing efficient spin-photon interfaces and multidimensional cluster states.

The growth of two-dimensional epitaxial materials on industrially relevant substrates is critical for enabling their scalable synthesis and integration into next-generation technologies. Here we present a comprehensive study of the molecular beam epitaxial growth of gallium selenide on 2-inch c-plane sapphire substrates. Using in situ reflection high-energy electron diffraction (RHEED), in situ Raman spectroscopy, optical and scanning electron microscopies, we construct a diagram of the gallium selenide growth modes as a function of substrate temperature (530-650 degrees C) and Se/Ga flux ratio (5-110). The growth mode diagram reveals distinct regimes, including the growth of layered post-transition metal monochalcogenide GaSe with an unstrained in-plane lattice constant of 3.71 +/- 0.01 A degrees\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{{{{\rm{A}}}}}$$\end{document} and a partial epitaxial alignment on sapphire. This work demonstrates a RHEED-based pathway for synthesizing gallium selenide of specific phase and morphology, and the construction of a phase diagram for high vapor pressure III-VI compounds that can be applied to a wide range of other metal chalcogenide materials.

We present superconducting nanowire photodetectors based on hBN-encapsulated, few-layer NbSe2, showing signatures of single-photon sensitivity. The top-down fabrication process preserves the superconducting properties of NbSe2, as confirmed by low-temperature transport measurements showing comparable results to unpatterned sheets, and it maintains a contact and wiring resistance down to similar to 30 Omega at T = 4 K. Meandered NbSe2 nanowires exhibit high responsivity (up to 4.9 x 10(4) V/W) over a spectral range of 650-1550 nm in a closed-cycle cryostat at 4 K, outperforming samples with different geometries in this work. The meander achieves a recovery time of (135 +/- 36) ns, a system timing jitter of (1103 +/- 7) ps, and a detection efficiency of similar to 0.01% at 0.95I(c). A linear increase of the count rate with the number of photons between the noise level and the latching threshold offers a signature of single-photon sensitivity at 1100 nm.

Excitons dominate the optical response of two-dimensional (2D) semiconductors. Strong interactions produce peculiar excitonic complexes, which provide a testing ground for exciton and quantum many-body theories. Here, we report a hitherto unobserved many-body exciton that emerges upon filling both the K and Q valleys of WSe2. We optically probe the exciton landscape using charge-tunable devices with unusually thin dielectrics that facilitate doping up to several 1013 cm-2. We observe the emergence of the thermodynamically stable complex when 10 valleys are electrostatically filled. We gain insight into its physics using magneto-optical measurements. Our results are well-described by a model where the number of distinguishable Fermi seas interacting with the photoexcited electron-hole pair defines the complex's behavior. In addition to expanding the repertoire of excitons in 2D semiconductors, this complex could probe the limit of exciton models and answer open questions about screened Coulomb interactions in 2D semiconductors.

In the single-photon emission from a two-level system, multiphotons are generally regarded as accidental, undesired, and unrelated to the mechanism. In coherently driven systems, however, they form the cornerstone of single-photon emission, which arises from quantum interferences between virtual multiphoton fluctuations of the emitter and the Poissonian superposition of all number states induced by the driving. Here, we demonstrate how one can control the multiphoton dynamics by disrupting these quantum interferences through an external homodyne control of the emitter's mean field. Experimentally, we observed a transition from single-photon to multiphoton emission, up to three-photon correlations. We show that, counterintuitively, quantum fluctuations always play a major qualitative role, even and, in fact, especially when their quantitative contribution is vanishing. Our findings provide distinct insights into the paradoxical character of quantum mechanics and open pathways for mean-field engineering as a tool for precision multiphoton control.

We present evidence for the strong participation of hot phonons in the photo-physics of interlayer excitons (IXs) in 2H - and 3R - stacked MoSe2/WSe2 heterobilayers. Photoluminescence (PL) excitation spectroscopy reveals that excess energy associated with relaxation of intra-layer excitons towards IXs profoundly shapes the overall IX-PL lineshape, while the energy of the spectrally narrow discrete emission lines conventionally associated with trapped moir & eacute,. IXs remain unaffected. A strikingly uniform line-spacing of the discrete emission lines is observed, along with characteristic temperature and excitation level dependence. Results suggest an entirely new picture of the discrete IX emission in which non-thermal phonons play a crucial role in shaping the spectrum. Excitation power and time resolved data indicate that these features are most likely polaronic in nature. Our findings extend the understanding of the photophysics of IXs beyond current interpretations based primarily on moir & eacute,.-trapped IXs.

We investigate the coupling between an ensemble of individual emitters and multiple photons in a high-Q cavity at the mesoscopic excitation level. The master equation theory is used to calculate the emission spectrum of the cavity QED system. The increasing excitation level not only pumps the system to the high-energy multi-emitter-photon states, but also introduces the pump-induced dephasing that suppresses the coherent energy exchange (coupling) between photons and emitters. When the emitter lifetime exceeds a threshold, we observe the mesoscopic excitation level i.e., the system is pumped to high energy states whilst the coherent couplings between these states are not yet suppressed. The mesoscopic excitation enables the couplings between multi-emitter-photon states, and thereby, paves the way to building quantum photonic devices based on multiple photons and nonlinear effects.

We investigate the confinement of neutral excitons in a one-dimensional (1D) potential engineered by proximizing hexagonal boron nitride (hBN)-encapsulated monolayer MoSe2 to ferroelectric domain walls (DWs) in periodically poled LiNbO3. Our device exploits the nanometer scale in-plane electric field gradient at the DW to induce dipolar exciton confinement via the DC Stark effect. Spatially resolved photoluminescence spectroscopy reveals the emergence of narrow emission lines redshifted from the MoSe2 neutral exciton by up to similar to 100 meV, depending on the sample structure. The spatial distribution, excitation energy response, and polarization properties of the emission are consistent with the signatures of 1D-confined excitons. The large electric-field gradients accessible via proximal ferroelectric systems open up new avenues for the creation of robust quantum-confined excitons in atomically thin materials and their heterostructures.

Self-assembled optically active quantum dot molecules (QDMs) allow the creation of protected qubits via singlet-triplet spin states. The qubit energy splitting of these states is defined by the tunnel coupling strength and is, therefore, determined by the potential landscape and is fixed during growth. Applying an in-plane magnetic field increases the confinement of the hybridized wave functions within the quantum dots, leading to a decrease of the tunnel coupling strength. We achieve a tuning of the coupling strength by 53.4% +/- 1.7%. The ability to fine-tune this coupling is essential for quantum network and computing applications that require quantum systems with near identical performance.

Hexagonal boron nitride (hBN) is a layered material with a wide variety of excellent properties for emergent applications in quantum photonics using atomically thin materials. For example, it hosts single-photon emitters that operate up to room-temperature, it can be exploited for atomically flat tunnel barriers, and it can be used to form high finesse photonic nanocavities. Moreover, it is an ideal encapsulating dielectric for two-dimensional (2D) materials and heterostructures, with highly beneficial effects on their electronic and optical properties. Depending on the use case, the thickness of hBN is a critical parameter and needs to be carefully controlled from the monolayer to hundreds of layers. This calls for quick and non-invasive methods to unambiguously identify the thickness of exfoliated flakes. Here, we show that the apparent color of hBN flakes on different SiO2/Si substrates can be made to be highly indicative of the flake thickness, providing a simple method to infer the hBN thickness. Using experimental determination of the colour of hBN flakes and calculating the optical contrast, we derived the optimal substrates for the most reliable hBN thickness identification for flakes with thickness ranging from a few layers towards bulk-like hBN. Our results offer a practical guide for the determination of hBN flake thickness for widespread applications using 2D materials and heterostructures.

Superconducting nanowire single-photon detectors (SNSPDs) are indispensable in fields such as quantum science and technology, astronomy, and biomedical imaging, where high detection efficiency, low dark count rates, and high timing accuracy are required. Recently, helium (He) ion irradiation was shown to be a promising method to enhance SNSPD performance. Here, we study how changes in the underlying superconducting NbTiN film and the SiO2Si substrate affect device performance. While irradiated and unirradiated NbTiN films show similar crystallinity, we observe He bubble formation below the SiO2Si interface and an amorphization of the Si substrate. Both reduce the thermal conductance between the superconducting thin film and the substrate from 210 to 70 W m−2 K−4 after irradiation with 2000 ions nm−2. This effect, combined with the lateral straggle of He ions in the substrate, allows the modification of the superconductor-to-substrate thermal conductance of an SNSPD by selectively irradiating only the regions around the nanowire. With this approach, we achieved a broader bias current range (9.8 μA vs 3.7 µA) in which the detector operates at its maximum detection efficiency, which is beneficial for reducing dark counts while maintaining high sensitivity. Moreover, the photon-assisted critical current remained similar to that of the unirradiated reference device (59.0 µA vs 60.1 µA), while full irradiation reduced it to 22.4 µA. Our results suggest that the irradiation-induced reduction of the thermal conductance significantly enhances SNSPD sensitivity, offering a novel approach to locally engineer substrate properties for improved detector performance.

Detecting single photons is essential for applications such as dark matter detection, quantum science and technology, and biomedical imaging. Superconducting nanowire single-photon detectors (SNSPDs) excel in this task due to their near-unity detection efficiency, subhertz dark count rates, and picosecond timing jitter. However, a local increase of current density (current crowding) in the bends of meander-shaped SNSPDs limits these performance metrics. By locally irradiating the SNSPD's straight segments with helium ions while leaving the bends unirradiated, we realize current crowding-free SNSPDs with simultaneously enhanced sensitivity: After irradiation with 800 ions nm-2, locally irradiated SNSPDs showed a relative saturation plateau width of 37%, while fully irradiated SNSPDs reached only 10%. This larger relative plateau width allows operation at lower relative bias currents, thereby reducing the dark count rate while still detecting single photons efficiently. We achieve an internal detection efficiency of 94% with 7 mHz dark count rate near the onset of saturating detection efficiency for a wavelength of 780 nm.

Coulomb interactions in atomically thin materials are remarkably sensitive to variations in the dielectric screening of the environment, which can be used to control exotic quantum many-body phases and engineer exciton potential landscapes. For decades, static or frequency-independent approximations of the dielectric response, where increased dielectric screening is predicted to cause an energy redshift of the exciton resonance, have been sufficient. These approximations were first applied to quantum wells and were more recently extended with initial success to layered transition metal dichalcogenides (TMDs). Here, we use charge-tunable exciton resonances to investigate screening effects in TMD monolayers embedded in materials with low-frequency dielectric constants ranging from 4 to more than 1000, a range of 2 orders of magnitude larger than in previous studies. In contrast to the redshift predicted by static models, we observe a blueshift of the exciton resonance exceeding 30 meV in higher dielectric constant environments. We explain our observations by introducing a dynamical screening model based on a solution to the Bethe-Salpeter equation (BSE). When dynamical effects are strong, we find that the exciton binding energy remains mostly controlled by the low-frequency dielectric response, while the exciton self-energy is dominated by the high-frequency one. Our results supplant the understanding of screening in layered materials and their heterostructures, introduce a knob to tune selected many-body effects, and reshape the framework for detecting and controlling correlated quantum many-body states and designing optoelectronic and quantum devices.

We investigate the growth of amorphous MoSi thin films using magnetron co-sputtering and optimize the growth conditions with respect to crystal structure and superconducting properties (e.g., critical temperature \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\text {c}}$$\end{document}). The deposition pressure, Mo:Si stoichiometry and substrate temperature are systematically varied to achieve a transition temperature of 8.4(3) K for films with a thickness of 17.7(8) nm and 6.2(9) K for a 4.3(4) nm thick film. For Mo concentrations above 81% the crystalline phase Mo3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Mo}_\text {3}$$\end{document}Si is observed in grazing incidence X-ray diffraction measurements. The same phase appears when the working pressure during deposition is reduced below 3.1x10-3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1 \times 10<^>{\text {-3}}\hspace{1.66656pt}$$\end{document}mbar and when the substrate temperature during deposition is increased above \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$100\hspace{1.66656pt}<^>{\circ } $$\end{document}C. By choosing a sufficient Si concentration and optimum deposition pressure we identify deposition conditions that ensure a homogeneous amorphous growth of the superconducting thin film. We then fabricate superconducting nanowire single-photon detectors which exhibit an unitary internal efficiency to single photons at an operational temperature of 1.2 K while simultaneously having a dark count rate below 1 Hz. Our results establish the link between MoSi film deposition, morphology and the performance of SSPD.

We explore the optical dipole orientation of single photon emitters in monolayer MoS2 as produced by a focused helium ion beam. The single photon emitters can be understood as single-sulfur vacancies. The corresponding far-field luminescence spectra reveal several photoluminescence lines below the dominating luminescence of the exciton in MoS2. These sub-bandgap emission lines were predicted by ab initio theory, but they have never been resolved in luminescence experiments because of their small amplitude. We reveal the lines by their dependence as a function of the photon energy and momentum as measured in the back focal plane of the optical circuitry. The agreement between theory and experiment suggests that the defect states interact strongly within the Brillouin zone.

InAs semiconductor quantum dots (QDs) emitting in the near-infrared are promising platforms for on-demand single-photon sources and spin-photon interfaces. However, the realization of quantum-photonic nanodevices emitting in the telecom windows with similar performance remains an open challenge. In particular, nanophotonic devices incorporating quantum light emitting diodes in the telecom C-band based on GaAs substrates are still lacking due to the relaxation of the lattice constant along the InGaAs graded layer which makes the implementation of electrically contacted devices challenging. Here, we report an optimized heterostructure design for QDs emitting in the telecom O- and C-bands grown by means of molecular beam epitaxy. The InAs QDs are embedded in mostly relaxed InGaAs matrices with fixed indium content grown on top of compositionally graded InGaAs buffers. Reciprocal space maps of the indium profiles and optical absorption spectra are used to optimize In0.22Ga0.78As and In0.30Ga0.70As matrices, accounting for the chosen indium grading profile. This approach results in a tunable QD photoluminescence (PL) emission from 1200 up to 1600 nm. Power and polarization dependent micro-PL measurements performed at 4 K reveal exciton-biexciton complexes from quantum dots emitting in the telecom O- and C-bands. The presented study establishes a flexible platform that can be an essential component for advanced photonic devices based on InAs/GaAs that serve as building blocks for future quantum networks.

Predicting the optical properties of large-scale ensembles of luminescent nanowire arrays that host active quantum heterostructures is of paramount interest for on-chip integrated photonic and quantum photonic devices. However, this has remained challenging due to the vast geometrical parameter space and variations at the single object level. Here, we demonstrate high-throughput spectroscopy on 16800 individual InGaAs quantum heterostructures grown by site-selective epitaxy on silicon, with varying geometrical parameters to assess uniformity/yield in luminescence efficiency, and emission energy trends. The luminescence uniformity/yield enhances significantly at prepatterned array mask opening diameters (d 0) greater than 50 nm. Additionally, the emission energy exhibits anomalous behavior with respect to d 0, which is notably attributed to rotational twinning within the InGaAs region, inducing significant energy shifts due to quantum confinement effects. These findings provide useful insights for mapping and optimizing the interdependencies between geometrical parameters and electronic/optical properties of widely tunable sets of quantum nanowire heterostructures.

Developing coherent excitation methods for quantum emitters ensuring high brightness, optimal single-photon purity and indistinguishability of the emitted photons has been a key challenge in the past years. While various methods have been proposed and explored, they all have specific advantages and disadvantages. This study investigates the dynamics of the recent swing-up scheme as an excitation method for a two-level system and its performance in single-photon generation. By applying two far red-detuned laser pulses, the two-level system can be prepared in the excited state with near-unity fidelity. The successful operation and coherent character of this technique are demonstrated using a semiconductor quantum dot (QD). Moreover, the multi-dimensional parameter space of the two laser pulses is explored to analyze its impact on excitation fidelity. Finally, the performance of the scheme as an excitation method for generating high-quality single photons is analyzed. The swing-up scheme itself proves effective, exhibiting nearly perfect single-photon purity, while the observed indistinguishability in the studied sample is limited by the influence of the inevitable high excitation powers on the semiconductor environment of the quantum dot. This study explores the coherent dynamics of the swing-up excitation scheme of a two-level system. Utilizing two far red-detuned laser pulses allows near-unity fidelity in preparing the system in the excited state. Demonstrated with a semiconductor quantum dot, the study analyzes the impact of the two laser pulses' multi-dimensional parameter space on excitation fidelity. image

Radial Bragg gratings are commonly used to enhance light extraction from quantum emitters, but lack a well-suited, fast simulation method for optimization beyond periodic designs. To overcome this limitation, an algorithm based on the transfer matrix model (TMM) to calculate the free-space emission of such gratings is proposed and demonstrated. Using finite difference time domain (FDTD) simulations, free-space emission, and transfer matrices of single grating components are characterized. The TMM then combines any number of components to receive the total emission. Randomized benchmarks verify that results from this method agree within 98% with FDTD while reducing simulation time by one to two orders of magnitude. The speed advantage of this approach is shown by maximizing emission of a fifteen-trench circular grating into a Gaussian mode. It is expected that this novel algorithm will facilitate the optimization of radial gratings, enabling quantum light sources with unprecedented collection efficiencies. Using finite difference time domain (FDTD) simulations, free-space emission and transfer matrices of single radial grating components are characterized. The transfer matrix model introduced here then combines any number of components to receive the total emission of the radial grating. Benchmarks show 98% agreement with FDTD but 10- to 100-fold speed advantage, allowing efficient optimization of aperiodic radial grating designs.image

Electrically controllable quantum-dot molecules (QDMs) are a promising platform for deterministic entanglement generation and, as such, a resource for quantum-repeater networks. A microscopic open-quantum-systems approach based on a time-dependent Bloch-Redfield equation is developed to model the generation of entangled spin states with high fidelity. The state preparation is a crucial step in a protocol for deterministic entangled-photon-pair generation that is proposed for quantum repeater applications. The theory takes into account the quantum-dot molecules' electronic properties that are controlled by time-dependent electric fields as well as dissipation due to electron-phonon interaction. The transition between adiabatic and non-adiabatic regimes is quantified, which provides insights into the dynamics of adiabatic control of QDM charge states in the presence of dissipative processes. From this, the maximum speed of entangled-state preparation is inferred under different experimental conditions, which serves as a first step toward simulation of attainable entangled photon-pair generation rates. The developed formalism opens the possibility for device-realistic descriptions of repeater protocol implementations. Entanglement generation is crucial for many quantum technology applications, including quantum repeaters. In the context of quantum repeater protocols, an open-quantum systems formalism is developed to simulate gate operations performed on quantum-dot molecules in switchable electric fields. From this, the maximum speed of entangled-state preparation under different experimental conditions can be obtained.image

Group IV vacancy color centers in diamond are promising spin-photon interfaces with strong potential for applications in photonic quantum technologies. Reliable methods for controlling and stabilizing their charge state are urgently needed for scaling to multiqubit devices. Here, we manipulate the charge state of silicon vacancy (SiV) ensembles by combining luminescence and photocurrent spectroscopy. We controllably convert the charge state between the optically active SiV- and dark SiV2- with megahertz rates and >90% contrast by judiciously choosing the local potential applied to in-plane surface electrodes and the laser excitation wavelength. We observe intense SiV- photoluminescence under hole capture, measure the intrinsic conversion time from the dark SiV2- to the bright SiV- to be 36.4(67) ms, and demonstrate how it can be enhanced by a factor of 105 via optical pumping. Moreover, we obtain previously unknown information on the defects that contribute to photoconductivity, indicating the presence of substitutional nitrogen and divacancies.

We report lasing of moiré trapped interlayer excitons (IXs) by integrating a pristine hBN-encapsulated MoSe2/WSe2 heterobilayer into a high-Q (>10(4)) nanophotonic cavity. We control the cavity-IX detuning using a magnetic field and measure their dipolar coupling strength to be 78 +/- 4 micro-electron volts, fully consistent with the 82 micro-electron volts predicted by theory. The emission from the cavity mode shows clear threshold-like behavior as the transition is tuned into resonance with the cavity. We observe a superlinear power dependence accompanied by a narrowing of the linewidth as the distinct features of lasing. The onset and prominence of these threshold-like behaviors are pronounced at resonance while weak off-resonance. Our results show that a lasing transition can be induced in interacting moiré IXs with macroscopic coherence extending over the length scale of the cavity mode. Such systems raise interesting perspectives for low-power switching and synaptic nanophotonic devices using two-dimensional materials.

The interaction of a resonant light field with a quantum two-level system is of key interest both for fundamental quantum optics and quantum technological applications employing resonant excitation. While emission under resonant continuous-wave excitation has been well studied, the more complex emission spectrum of dynamically dressed states-a quantum two-level system driven by resonant pulsed excitation -has so far been investigated in detail only theoretically. Here, we present the first experimental observation of the complete resonance fluorescence emission spectrum of a single quantum two-level system, in the form of an excitonic transition in a semiconductor quantum dot, driven by finite Gaussian pulses. We observe multiple emerging sidebands as predicted by theory, with an increase of their number and spectral detuning with excitation pulse intensity and a dependence of their spectral shape and intensity on the pulse length. Detuning-dependent measurements provide additional insights into the emission features. The experimental results are in excellent agreement with theoretical calculations of the emission spectra, corroborating our findings.

Ternary GaAsSb nanowires (NW) are key materials for integrated high-speed photonic applications on silicon (Si), where homogeneous, high aspect-ratio dimensions and high-quality properties for controlled absorption, mode confinement and waveguiding are much desired. Here, we demonstrate a unique high-temperature (high-T >650 degrees C) molecular beam epitaxial (MBE) approach to realize self-catalyzed GaAsSb NWs site-selectively on Si with high aspect-ratio and non-tapered morphologies under antimony (Sb)-saturated conditions. While hitherto reported low-moderate temperature growth processes result in early growth termination and inhomogeneous morphologies, the non-tapered nature of NWs under high-T growth is independent of the supply rates of relevant growth species. Analysis of dedicated Ga-flux and growth time series, allows us to pinpoint the microscopic mechanisms responsible for the elimination of tapering, namely concurrent vapor-solid, step-flow growth along NW side-facets enabled by enhanced Ga diffusion under the high-T growth. Performing growth in an Sb-saturated regime, leads to high Sb-content in VLS-GaAsSb NW close to 30% that is independent of Ga-flux. This independence enables multi-step growth via sequentially increased Ga-flux to realize uniform and very long (>7 mu m) GaAsSb NWs. The excellent properties of these NWs are confirmed by a completely phase-pure, twin-free zincblende (ZB) crystal structure, a homogeneous Sb-content along the VLS-GaAsSb NW growth axis, along with remarkably narrow, single-peak low-temperature photoluminescence linewidth (<15 meV) at wavelengths of similar to 1100-1200 nm.

III-V semiconductor nanowire (NW) heterostructures with axial InGaAs active regions hold large potential for diverse on-chip device applications, including site-selectively integrated quantum light sources, NW lasers with high material gain, as well as resonant tunneling diodes and avalanche photodiodes. Despite various promising efforts toward high-quality single or multiple axial InGaAs heterostacks using noncatalytic growth mechanisms, the important roles of facet-dependent shape evolution, crystal defects, and the applicability to more universal growth schemes have remained elusive. Here, we report the growth of optically active InGaAs axial NW heterostructures via completely catalyst-free, selective-area molecular beam epitaxy directly on silicon (Si) using GaAs-(Sb) NW arrays as tunable, high-uniformity growth templates and highlight fundamental relationships between structural, morphological, and optical properties of the InGaAs region. Structural, compositional, and 3D-tomographic characterizations affirm the desired directional growth along the NW axis with no radial growth observed. Clearly distinct luminescence from the InGaAs active region is demonstrated, where tunable array-geometry parameters and In content up to 20% are further investigated. Based on the underlying twin-induced growth mode, we further describe the facet-dependent shape and interface evolution of the InGaAs segment and its direct correlation with emission energy.

We report on the optical absorption characteristics of selectively positioned sulfur vacancies in monolayer MoS2, as observed by photovoltage and photocurrent experiments in an atomistic vertical tunneling circuit at cryogenic and room temperature. Charge carriers are resonantly photoexcited within the defect states before they tunnel through an hBN tunneling barrier to a graphene-based drain contact. Both photovoltage and photocurrent characteristics confirm the optical absorption spectrum as derived from ab initio GW and Bethe-Salpeter equation approximations. Our results reveal the potential of single-vacancy tunneling devices as atomic-scale photodiodes.

Achieving homogeneous performance metrics between nominally identical pixels is challenging for the operation of arrays of superconducting nanowire single-photon detectors (SNSPDs). Here, local helium ion irradiation is utilized to post-process and tune single-photon detection efficiency, switching current, and critical temperature of individual devices on the same chip. For 12 nm thick highly absorptive SNSPDs, which are barely sensitive to single photons with a wavelength of 780 nm prior to He ion irradiation, an increase of the system detection efficiency from <0.05% to (55.3 +/- 1.1)% is observed following irradiation. Moreover, the internal detection efficiency saturates at a temperature of 4.5 K after irradiation with 1800 ions nm(-2). Compared to 8 nm SNSPDs of similar detection efficiency, a doubling of the switching current (to 20 <mu>A) is observed for irradiated 10 nm thick detectors, increasing the amplitude of detection voltage pulses. Investigations of the scaling of superconducting thin film properties with irradiation up to a fluence of 2600 ions nm(-2) revealed an increase of sheet resistance and a decrease of critical temperature towards high fluences. A physical model accounting for defect generation and sputtering during helium ion irradiation is presented and shows good qualitative agreement with experiments.

Janus transition metal dichalcogenides are an emerging class of atomically thin materials with engineered broken mirror symmetry that gives rise to long-lived dipolar excitons, Rashba splitting, and topologically protected solitons. They hold great promise as a versatile nonlinear optical platform due to their broadband harmonic generation tunability, ease of integration on photonic structures, and nonlinearities beyond the basal crystal plane. Here, second and third harmonic generation in MoSSe and WSSe Janus monolayers is studied. Polarization-resolved spectroscopy is used to map the full second-order susceptibility tensor of MoSSe, including its out-of-plane components. In addition, the effective third-order susceptibility and the second-order nonlinear dispersion close to exciton resonances for both MoSSe and WSSe are measured at room and cryogenic temperatures. This work sets a bedrock for understanding the nonlinear optical properties of Janus transition metal dichalcogenides and probing their use in the next-generation on-chip multifaceted photonic devices.

In recent years, there has been a significant increase in interest in tuning the emission wavelength of InAs quantum dots (QDs) to wavelengths compatible with the already existing silica fiber networks. In this work, we develop and explore compositionally graded In x Ga 1-x As metamorphic buffer layers (MBLs), with lattice constant carefully tailored to tune the emission wavelengths of InAs QDs towards the telecom O-band. The designed heterostructure is grown by molecular beam epitaxy (MBE), where a single layer of InAs QDs is grown on top of the MBL and is capped with a layer having a fixed indium (In) content. We investigate the structural properties of the grown MBLs by reciprocal space mapping, as well as transmission electron microscopy, and verify the dependence of the absorption edge of the MBL on the In-content by photothermal deflection spectroscopy measurements. This allows us to identify a growth temperature range for which the MBLs achieve a near-equilibrium strain relaxation for In-content up to similar to 30 % . Furthermore, we explore the emission wavelength tunability of QDs grown on top of a residual strained layer with a low density of dislocations. Specifically, we demonstrate a characteristic red-shift of the QD photoluminescence towards the telecom O-band (1300 nm) at low temperature. This study provides insights into the relaxation profiles and dislocation propagation in compositionally graded MBLs grown via MBE thus paving the way for realizing MBE-grown heterostructures containing InAs QDs for advanced nanophotonic devices emitting in the telecom bands.

We investigate the growth conditions for thin ( <= 200 nm) sputtered aluminum films. These coatings are needed for various applications, e.g. for advanced manufacturing processes in the aerospace industry or for nanostructures for quantum devices. Obtaining high-quality films, with low roughness, requires precise optimization of the deposition process. To this end, we tune various sputtering parameters such as the deposition rate, temperature and power, which enables 50 nm thin films with a root mean square roughness of less than 1 nm and high reflectivity. Finally, we confirm the high-quality of the deposited films by realizing superconducting single-photon detectors integrated into multi-layer heterostructures consisting of an aluminum mirror and a silicon dioxide dielectric spacer. We achieve an improvement in detection efficiency at 780 nm from 40% to 70% by this integration approach.

Nanowire lasers can be monolithically and site-selectively integrated onto silicon photonic circuits. To assess their full potential for ultrafast optoelectronic devices, a detailed understanding of their lasing dynamics is crucial. However, the roles played by their resonator geometry and the microscopic processes that mediate energy exchange between the photonic, electronic, and phononic subsystems are largely unexplored. Here, we study the dynamics of GaAs-AlGaAs core-shell nanowire lasers at cryogenic temperatures using a combined experimental and theoretical approach. Our results indicate that these NW lasers exhibit sustained intensity oscillations with frequencies ranging from 160 GHz to 260 GHz. As the underlying physical mechanism, we have identified self-induced electron-hole plasma temperature oscillations resulting from a dynamic competition between photoinduced carrier heating and cooling via phonon scattering. These dynamics are intimately linked to the strong interaction between the lasing mode and the gain material, which arises from the wavelength-scale dimensions of these lasers. We anticipate that our results could lead to optimised approaches for ultrafast intensity and phase modulation of chip-integrated semiconductor lasers at the nanoscale.

We report on scalable heterointegration of superconducting electrodes and epitaxial semiconductor quantum dots (QDs) on strong piezoelectric and optically nonlinear lithium niobate. The implemented processes combine the sputter-deposited thin film superconductor niobium nitride and III-V compound semiconductor membranes onto the host substrate. The superconducting thin film is employed as a zero-resistivity electrode material for a surface acoustic wave resonator with internal quality factors Q approximate to 17 000 representing a three-fold enhancement compared to identical devices with normal conducting electrodes. Superconducting operation of approximate to 400 MHz resonators is achieved to temperatures T > 7 K and electrical radio frequency powers P-rf > +9 dBm. Heterogeneously integrated single QDs couple to the resonant phononic field of the surface acoustic wave resonator operated in the superconducting regime. Position and frequency selective coupling mediated by deformation potential coupling is validated using time-integrated and time-resolved optical spectroscopy. Furthermore, acoustoelectric charge state control is achieved in a modified device geometry harnessing large piezoelectric fields inside the resonator. The hybrid QD-surface acoustic wave resonator can be scaled to higher operation frequencies and smaller mode volumes for quantum phase modulation and transduction between photons and phonons via the QD. Finally, the employed materials allow for the realization of other types of optoelectronic devices, including superconducting single photon detectors and integrated photonic and phononic circuits.

Two-dimensional (2D) heterostructures integrated into nanophotonic cavities have emerged as a promising approach towards novel photonic and opto-electronic devices. However, the thickness of the 2D heterostructure has a strong influence on the resonance frequency of the nanocavity. For a single cavity, the resonance frequency shifts approximately linearly with the thickness. Here, we propose to use the inherent non-linearity of the mode coupling to render the cavity mode insensitive to the thickness of the 2D heterostructure. Based on the coupled mode theory, we reveal that this goal can be achieved using either a homoatomic molecule with a filtered coupling or heteroatomic molecules. We perform numerical simulations to further demonstrate the robustness of the eigenfrequency in the proposed photonic molecules. Our results render nanophotonic structures insensitive to the thickness of 2D materials, thus owing appealing potential in energy- or detuning-sensitive applications such as cavity quantum electrodynamics.

Photonic bound states in the continuum (BICs) provide a standout platform for strong light-matter coupling with transition metal dichalcogenides (TMDCs) but have so far mostly been implemented as traditional all-dielectric metasurfaces with adjacent TMDC layers, incurring limitations related to strain, mode overlap and material integration. Here, we demonstrate intrinsic strong coupling in BIC-driven metasurfaces composed of nanostructured bulk tungsten disulfide (WS2) and exhibiting resonances with sharp, tailored linewidths and selective enhancement of light-matter interactions. Tuning of the BIC resonances across the exciton resonance in bulk WS2 is achieved by varying the metasurface unit cells, enabling strong coupling with an anticrossing pattern and a Rabi splitting of 116 meV. Crucially, the coupling strength itself can be controlled and is shown to be independent of material-intrinsic losses. Our self-hybridized metasurface platform can readily incorporate other TMDCs or excitonic materials to deliver fundamental insights and practical device concepts for polaritonic applications. The authors demonstrate strong coupling in bound state in the continuum metasurfaces on nanostructured bulk WS2 and exhibiting sharp resonances with tailored linewidths and controllable light-matter coupling strength.

The increasing roleof two-dimensional (2D) devices requires thedevelopment of new techniques for ultrafast control of physical propertiesin 2D van der Waals (vdW) nanolayers. A special feature of heterobilayersassembled from vdW monolayers is femtosecond separation of photoexcitedelectrons and holes between the neighboring layers, resulting in theformation of Coulomb force. Using laser pulses, we generate a 0.8THz coherent breathing mode in MoSe2/WSe2 heterobilayers,which modulates the thickness of the heterobilayer and should modulatethe photogenerated electric field in the vdW gap. While the phononfrequency and decay time are independent of the stacking angle betweenthe MoSe2 and WSe2 monolayers, the amplitudedecreases at intermediate angles, which is explained by a decreasein the photogenerated electric field between the layers. The modulationof the vdW gap by coherent phonons enables a new technology for thegeneration of THz radiation in 2D nanodevices with vdW heterobilayers.

Negatively-charged boron vacancy centers (V-B(-)) in hexagonal Boron Nitride (hBN) are attracting increasing interest since they represent optically-addressable qubits in a van der Waals material. In particular, these spin defects have shown promise as sensors for temperature, pressure, and static magnetic fields. However, their short spin coherence time limits their scope for quantum technology. Here, we apply dynamical decoupling techniques to suppress magnetic noise and extend the spin coherence time by two orders of magni-tude, approaching the fundamental T-1 relaxation limit. Based on this improvement, we demonstrate advanced spin control and a set of quantum sensing protocols to detect radiofrequency signals with sub-Hz resolution. The corresponding sensitivity is benchmarked against that of state-of-the-art NV-diamond quantum sensors. This work lays the foundation for nanoscale sensing using spin defects in an exfoliable material and opens a promising path to quantum sensors and quantum networks integrated into ultra-thin structures.

The electron-hole exchange interaction is a fundamental mechanism that drives valley depolarization via intervalley exciton hopping in semiconductor multivalley systems. Here, we report polarization-resolved photoluminescence spectroscopy of neutral excitons and negatively charged trions in monolayer MoSe2 and WSe2 under biaxial strain. We observe a marked enhancement (reduction) on the WSe2 triplet trion valley polarization with compressive (tensile) strain while the trion in MoSe2 is unaffected. The origin of this effect is shown to be a strain-dependent tuning of the electron-hole exchange interaction. A combined analysis of the strain-dependent polarization degree using ab initio calculations and rate equations shows that strain affects intervalley scattering beyond what is expected from strain-dependent band-gap modulations. The results evidence how strain can be used to tune valley physics in energetically degenerate multivalley systems.

Semiconductor quantum dot molecules are considered promising candidates for quantum technological applications due to their wide tunability of optical properties and coverage of different energy scales associated with charge and spin physics. While previous works have studied the tunnel-coupling of the different excitonic charge complexes shared by the two quantum dots by conventional optical spectroscopy, we here report on the first demonstration of a coherently controlled interdot tunnel-coupling focusing on the quantum coherence of the optically active trion transitions. We employ ultrafast four-wave mixing spectroscopy to resonantly generate a quantum coherence in one trion complex, transfer it to and probe it in another trion configuration. With the help of theoretical modeling on different levels of complexity, we give an instructive explanation of the underlying coupling mechanism and dynamical processes.

Quantum dot molecules (QDMs) are one of the few quantum light sources that promise deterministic gener-ation of one-and two-dimensional photonic graph states. The proposed protocols rely on coherent excitation of the tunnel-coupled and spatially indirect exciton states. Here, we demonstrate power-dependent Rabi oscillations of direct excitons, spatially indirect excitons, and excitons with a hybridized electron wave function. An off-resonant detection technique based on phonon-mediated state transfer allows for spectrally filtered detection under resonant excitation. Applying a gate voltage to the QDM device enables a continuous transition between direct and indirect excitons and, thereby, control of the overlap of the electron and hole wave function. This does not only vary the Rabi frequency of the investigated transition by a factor of approximate to 3, but also allows to optimize graph state generation in terms of optical pulse power and reduction of radiative lifetimes.

Large-scale two-dimensional (2D) moire superlattices are driving a revolution in designer quantum materials. The electronic interactions in these superlattices, strongly dependent on the periodicity and symmetry of the moire pattern, critically determine the emergent properties and phase diagrams. To date, the relative twist angle between two layers has been the primary tuning parameter for a given choice of constituent crystals. Here, we establish strain as a powerful mechanism to in situ modify the moire periodicity and symmetry. We develop an analytically exact mathematical description for the moire lattice under arbitrary in-plane heterostrain acting on any bilayer structure. We demonstrate the ability to fine-tune the moire lattice near critical points, such as the magic angle in bilayer graphene, or fully reconfigure the moire lattice symmetry beyond that imposed by the unstrained constituent crystals. Due to this unprecedented simultaneous control over the strength of electronic interactions and lattice symmetry, 2D heterostrain provides a powerful platform to engineer, tune, and probe strongly correlated moire materials.

Single spin-defects in 2D transition-metal dichalcogenides are natural spin-photon interfaces for quantum applications. Here we report high-field magneto-photoluminescence spectroscopy from three emission lines (Q1, Q2, and Q*) of He-ion induced sulfur vacancies in monolayer MoS2. Analysis of the asymmetric PL lineshapes in combination with the diamagnetic shift of Q1 and Q2 yields a consistent picture of localized emitters with a wave function extent of similar to 3.5 nm. The distinct valley-Zeeman splitting in out-of-plane B-fields and the brightening of dark states through in-plane B-fields necessitates spin-valley selectivity of the defect states and lifted spin-degeneracy at zero field. Comparing our results to ab initio calculations identifies the nature of Q1 and Q2 and suggests that Q* is the emission from a chemically functionalized defect. Analysis of the optical degree of circular polarization reveals that the Fermi level is a parameter that enables the tunability of the emitter. These results show that defects in 2D semiconductors may be utilized for quantum technologies.

Vapor-liquid-solid (VLS) growth is the mainstream method in realizing advanced semiconductor nanowires (NWs), as widely applied to many III-V compounds. It is exclusively explored also for antimony (Sb) compounds, such as the relevant GaAsSb-based NW materials, although morphological inhomogeneities, phase segregation, and limitations in the supersaturation due to Sb strongly inhibit their growth dynamics. Fundamental advances are now reported here via entirely catalyst-free GaAsSb NWs, where particularly the Sb-mediated effects on the NW growth dynamics and physical properties are investigated in this novel growth regime. Remarkably, depending on GaAsSb composition and nature of the growth surface, both surfactant and anti-surfactant action is found, as seen by transitions between growth acceleration and deceleration characteristics. For threshold Sb-contents up to 3-4%, adatom diffusion lengths are increased approximate to sevenfold compared to Sb-free GaAs NWs, evidencing the significant surfactant effect. Furthermore, microstructural analysis reveals unique Sb-mediated transitions in compositional structure, as well as substantial reduction in twin defect density, approximate to tenfold over only small compositional range (1.5-6% Sb), exhibiting much larger dynamics as found in VLS-type GaAsSb NWs. The effect of such extended twin-free domains is corroborated by approximate to threefold increases in exciton lifetime (approximate to 4.5 ns) due to enlarged electron-hole pair separation in these phase-pure NWs.

Triangular cross-section silicon carbide (SiC) photonic devices have been studied as an efficient and scalable route for integration of color centers into quantum hardware. In this work, we explore efficient collection and detection of color center emission in a triangular cross-section SiC waveguide by introducing a photonic crystal mirror on its one side and a superconducting nanowire single photon detector (SNSPD) on the other. Our modeled triangular cross-section devices with a randomly positioned emitter have a maximum coupling efficiency of 89% into the desired optical mode and a high coupling efficiency ( > 75%) in more than half of the configurations. For the first time, NbTiN thin films were sputtered on 4H-SiC and the electrical and optical properties of the thin films were measured. We found that the transport properties are similar to the case of NbTiN on SiO2 substrates, while the extinction coefficient is up to 50% higher for 1680 nm wavelength. Finally, we performed finite-difference time-domain simulations of triangular cross-section waveguide integrated with an SNSPD to identify optimal nanowire geometries for efficient detection of light from transverse electric and transverse magnetic polarized modes.

We report resonant Raman spectroscopy of neutral excitons X0 and intravalley trions X- in hBN-encapsulated MoS2 monolayer embedded in a nanobeam cavity. By temperature tuning the detuning between Raman modes of MoS2 lattice phonons and X0/X- emission peaks, we probe the mutual coupling of excitons, lattice phonons and cavity vibrational phonons. We observe an enhancement of X0-induced Raman scattering and a suppression for X--induced, and explain our findings as arising from the tripartite exciton-phonon-phonon coupling. The cavity vibrational phonons provide intermediate replica states of X0 for resonance conditions in the scattering of lattice phonons, thus enhancing the Raman intensity. In contrast, the tripartite coupling involving X- is found to be much weaker, an observation explained by the geometry-dependent polarity of the electron and hole deformation potentials. Our results indicate that phononic hybridization between lattice and nanomechanical modes plays a key role in the excitonic photophysics and light-matter interaction in 2D-material nanophotonic systems.

We demonstrate that semiconductor quantum dots can be excited efficiently in a resonant three-photon process, while resonant two-photon excitation is highly suppressed. Time-dependent Floquet theory is used to quantify the strength of the multiphoton processes and model the experimental results. The efficiency of these transitions can be drawn directly from parity considerations in the electron and hole wave functions in semiconductor quantum dots. Finally, we exploit this technique to probe intrinsic properties of InGaN quantum dots. In contrast to nonresonant excitation, slow relaxation of charge carriers is avoided, which allows us to measure directly the radiative lifetime of the lowest energy exciton states. Since the emission energy is detuned far from the resonant driving laser field, polarization filtering is not required and emission with a greater degree of linear polarization is observed compared to nonresonant excitation.

Optoelectronic properties of van der Waals homostructures can be selectively engineered by the relative twist angle between layers. Here, we study the twist-dependent moire ' coupling in MoSe2 homobilayers. For small angles, we find a pronounced redshift of the K -K and Gamma-K excitons accompanied by a transition from K -K to Gamma-K emission. Both effects can be traced back to the underlying moire ' pattern in the MoSe2 homobilayers, as confirmed by our low-energy continuum model for different moire ' excitons. We identify two distinct intralayer moire ' excitons for R stacking, while H stacking yields two degenerate intralayer excitons due to inversion symmetry. In both cases, bright interlayer excitons are found at higher energies. The performed calculations are in excellent agreement with experiment and allow us to characterize the observed exciton resonances, providing insight about the layer composition and relevant stacking configuration of different moire ' exciton species.

We investigated the timing jitter of superconducting nanowire single-photon detectors (SNSPDs) and found a strong dependence on the detector response. By varying the multi-layer structure, we observed changes in pulse shape which are attributed to capacitive behaviour affecting the pulse heights, rise times and consequently timing jitter. Moreover, we developed a technique to predict the timing jitter of a single device within certain limits by capturing only a single detector pulse, eliminating the need for detailed jitter measurement using a pulsed laser when a rough estimate of the timing jitter is sufficient.

Entanglement swapping and heralding are at the heart of many protocols for distributed quantum information. For photons, this typically involves Bell-state measurements based on two-photon interference effects. In this context, hybrid systems that combine high rate, ultrastable, and pure quantum sources with long-lived quantum memories are particularly interesting. Here, we develop a theoretical description of pulsed two-photon interference of photons from dissimilar sources to predict the outcomes of second-order cross-correlation measurements. These are directly related to, and hence used to quantify, photon indistinguishability. We study their dependence on critical system parameters such as quantum state lifetime and emission frequency, and quantify the impact of time jitter, pure dephasing, and spectral wandering. We show that for a fixed lifetime of one of the two emitters, for each frequency detuning there is an optimal lifetime of the second emitter that leads to the highest photon indistinguishability. Expectations for different hybrid combinations involving III-V semiconductor quantum dots, color centers in diamond, atom-scale defects in two-dimensional materials and neutral atoms are quantitatively compared for realworld system parameters. Our work provides a theoretical basis for the treatment of dissimilar emitters and enables assessment of which imperfections can be tolerated in hybrid photonic quantum networks.

Hybrid quantum technologies synergistically combine different types of systems with complementary strengths. Here, the authors show monolithic integration and control of quantum dots and the emitted single photons in a surface acoustic wave-driven GaAs integrated quantum photonic circuit. Integrated photonic circuits are key components for photonic quantum technologies and for the implementation of chip-based quantum devices. Future applications demand flexible architectures to overcome common limitations of many current devices, for instance the lack of tuneabilty or built-in quantum light sources. Here, we report on a dynamically reconfigurable integrated photonic circuit comprising integrated quantum dots (QDs), a Mach-Zehnder interferometer (MZI) and surface acoustic wave (SAW) transducers directly fabricated on a monolithic semiconductor platform. We demonstrate on-chip single photon generation by the QD and its sub-nanosecond dynamic on-chip control. Two independently applied SAWs piezo-optomechanically rotate the single photon in the MZI or spectrally modulate the QD emission wavelength. In the MZI, SAWs imprint a time-dependent optical phase and modulate the qubit rotation to the output superposition state. This enables dynamic single photon routing with frequencies exceeding one gigahertz. Finally, the combination of the dynamic single photon control and spectral tuning of the QD realizes wavelength multiplexing of the input photon state and demultiplexing it at the output. Our approach is scalable to multi-component integrated quantum photonic circuits and is compatible with hybrid photonic architectures and other key components for instance photonic resonators or on-chip detectors.

We utilize cavity-enhanced extinction spectroscopy to directly quantify the optical absorption of defects in MoS2 generated by helium ion bombardment. We achieve hyperspectral imaging of specific defect patterns with a detection limit below 0.01% extinction, corresponding to a detectable defect density below 1 x 10(11) cm(-2). The corresponding spectra reveal a broad subgap absorption, being consistent with theoretical predictions related to sulfur vacancy-bound excitons in MoS2. Our results highlight cavity-enhanced extinction spectroscopy as efficient means for the detection of optical transitions in nanoscale thin films with weak absorption, applicable to a broad range of materials.

Tunnel-coupled pairs of optically active quantum dots-quantum dot molecules (QDMs)-offer the possibility to combine excellent optical properties such as strong light-matter coupling with two-spin singlet-triplet (S-T0$S-T_0$) qubits having extended coherence times. The S-T0$S-T_0$ basis formed using two spins is inherently protected against electric and magnetic field noise. However, since a single gate voltage is typically used to stabilize the charge occupancy of the dots and control the inter-dot orbital couplings, operation of the S-T0$S-T_0$ qubits under optimal conditions remains challenging. Here, an electric field tunable QDM that can be optically charged with one (1h) or two holes (2h) on demand is presented. A four-phase optical and electric field control sequence facilitates the sequential preparation of the 2h charge state and subsequently allows flexible control of the inter-dot coupling. Charges are loaded via optical pumping and electron tunnel ionization. One- and two-hole charging efficiencies of (93.5 +/- 0.8)% and (80.5 +/- 1.3)% are achieved, respectively. Combining efficient charge state preparation and precise setting of inter-dot coupling allows for the control of few-spin qubits, as would be required for the on-demand generation of 2D photonic cluster states or quantum transduction between microwaves and photons.

We demonstrate that optically active emitters can be locally generated by focusing a He-ion beam onto monolayer WS2 encapsulated in hBN. The emitters show a low-temperature photoluminescence spectrum, which is well described by an independent Boson model for localized emitters. Consistently, the photoluminescence intensity of the emitters saturates at low excitation intensities, which is distinct to the photoluminescence of excitonic transitions in the investigated WS2 monolayers. The demonstrated method allows us to position defect emitters in WS2 monolayers on demand. A statistical analysis suggests the generation yield of individual emitters to be as high as 11% at the highest investigated He-ion doses.

We report the effects of antimony (Sb) surfactant on the growth and correlated structural and optical properties of non-catalytic GaAs nanowires (NW) grown by selective area epitaxy on silicon. Strong enhancements in the axial growth with very high aspect ratio up to 50 are observed by the addition of small traces of Sb (1%-2%), contrasting the commonly reported growth limiting behavior of Sb in GaAs(Sb) NWs. The Sb surfactant effect modifies the growth facet structure from a pyramidal-shaped growth front terminated by {1-1-0} planes to a flat (111)B growth plane, that is even further improved by the presence of Si co-dopants. Additional benefits are seen by the substantial change in microstructure, from a heavily defected layer stacking in Sb-free GaAs NWs to a twinned phase-pure zinc blende structure in Sb-mediated GaAs(Sb) NWs. We directly confirm the impact of the altered microstructure on the optical emission and carrier recombination dynamics via observation of long, few-ns carrier lifetimes in the GaAs(Sb) NWs using steady-state and time-resolved photoluminescence spectroscopy. (C) 2022 Author(s).

Atomically thin semiconductors can be readily integrated into a wide range of nanophotonic architectures for applications in quantum photonics and novel optoelectronic devices. We report the observation of nonlocal interactions of ???free??? trions in pristine hBN/MoS2/hBN heterostructures coupled to single mode (Q > 104) quasi 0D nanocavities. The high excitonic and photonic quality of the interaction system stems from our integrated nanofabrication approach simultaneously with the hBN encapsulation and the maximized local cavity field amplitude within the MoS2 monolayer. We observe a nonmonotonic temperature dependence of the cavity-trion interaction strength, consistent with the nonlocal light-matter interactions in which the extent of the center-of-mass (c.m.) wave function is comparable to the cavity mode volume in space. Our approach can be generalized to other optically active 2D materials, opening the way toward harnessing novel light-matter interaction regimes for applications in quantum photonics.

Nucleation control of self-assembled quantum dots is challenging. Here, the authors employ conventional molecular beam epitaxy to achieve wafer-scale density modulation of high-quality quantum dots with tunable periodicity on unpatterned substrates. Precise control of the properties of semiconductor quantum dots (QDs) is vital for creating novel devices for quantum photonics and advanced opto-electronics. Suitable low QD-densities for single QD devices and experiments are challenging to control during epitaxy and are typically found only in limited regions of the wafer. Here, we demonstrate how conventional molecular beam epitaxy (MBE) can be used to modulate the density of optically active QDs in one- and two- dimensional patterns, while still retaining excellent quality. We find that material thickness gradients during layer-by-layer growth result in surface roughness modulations across the whole wafer. Growth on such templates strongly influences the QD nucleation probability. We obtain density modulations between 1 and 10 QDs/mu m(2) and periods ranging from several millimeters down to at least a few hundred microns. This method is universal and expected to be applicable to a wide variety of different semiconductor material systems. We apply the method to enable growth of ultra-low noise QDs across an entire 3-inch semiconductor wafer.

We propose a scheme for the generation of highly indistinguishable single photons using semiconductor quantum dots and demonstrate its performance and potential. The scheme is based on the resonant twophoton excitation of the biexciton followed by stimulation of the biexciton to selectively prepare an exciton. Quantum-optical simulations and experiments are in good agreement and show that the scheme provides significant advantages over previously demonstrated excitation methods. The two-photon excitation of the biexciton suppresses re-excitation and enables ultralow multiphoton errors, while the precisely timed stimulation pulse results in very low timing jitter of the photons, and consequently, high indistinguishability. In addition, the polarization of the stimulation pulse allows us to deterministically program the polarization of the emitted photon (H or V). This ensures that all emission of interest occurs in the polarization of the detection channel, resulting in higher brightness than cross-polarized resonant excitation.

Manipulating electronic interlayer coupling in layered van der Waals (vdW) materials is essential for designing optoelectronic devices. Here, we control vibrational and electronic interlayer coupling in bi- and trilayer 2H-MoS2 using large external electric fields in a microcapacitor device. The electric field lifts Raman selection rules and activates phonon modes in excellent agreement with ab initio calculations. Through polarization-resolved photoluminescence spectroscopy in the same device, we observe a strongly tunable valley dichroism with maximum circular polarization degree of similar to 60% in bilayer and similar to 35% in trilayer MoS2 that is fully consistent with a rate equation model which includes input from electronic band structure calculations. We identify the highly delocalized electron wave function between the layers close to the high-symmetry Q points as the origin of the tunable circular dichroism. Our results demonstrate the possibility of electric-field-tunable interlayer coupling for controlling emergent spin-valley physics and hybridization-driven effects in vdW materials and their heterostructures.

Rapid development in integrated optoelectronic devices and quantum photonic architectures creates a need for optical fiber to chip coupling with low losses. Here we present a fast and generic approach that allows temperature stable self-aligning connections of nanophotonic devices to optical fibers. We show that the attainable precision of our approach is equal to that of DRIE-process based couplings. Specifically, the initial alignment precision is 1.2 +/- 0.4 mu m, the average shift caused by mating < 0.5 mu m, which is in the order of the precision of the concentricity of the employed fiber, and the thermal cycling stability is < 0.2 mu m. From these values the expected overall alignment offset is calculated as 1.4 +/- 0.4 mu m. These results show that our process offers an easy to implement, versatile, robust and DRIE-free method for coupling photonic devices to optical fibers. It can be fully automated and is therefore scalable for coupling to novel devices for quantum photonic systems.

We report magnetophotoluminescence spectroscopy of gated MoS2 monolayers in high magnetic fields to 28 T. At B = 0 T and electron density n(s) similar to 10(12) cm(-2), we observe three trion resonances that cannot be explained within a single-particle picture. Employing ab initio calculations that take into account three-particle correlation effects as well as local and nonlocal electron-hole exchange interaction, we identify those features as quantum superpositions of inter- and intravalley spin states. We experimentally investigate the mixed character of the trion wave function via the filling factor dependent valley Zeeman shift in positive and negative magnetic fields. Our results highlight the importance of exchange interactions for exciton physics in monolayer MoS2 and provide insights into the microscopic understanding of trion physics in two-dimensional multivalley semiconductors for low excess carrier densities.

Nanoplasmonic systems combined with optically active two-dimensional materials provide intriguing opportunities to explore and control light-matter interactions at extreme subwavelength length scales approaching the exciton Bohr radius. Here, we present room- and cryogenic-temperature investigations of a MoSe2 monolayer on individual gold dipole nanoantennas. By controlling nanoantenna size, the dipolar resonance is tuned relative to the exciton achieving a total tuning of similar to 130 meV. Differential reflectance measurements performed on >100 structures reveal an apparent avoided crossing between exciton and dipolar mode and an exciton-plasmon coupling constant of g = 55 meV, representing g/(h omega(X)) >= 3% of the transition energy. This places our hybrid system in the intermediate-coupling regime where spectra exhibit a characteristic Fano-like shape. We demonstrate active control by varying the polarization of the excitation light to programmably suppress coupling to the dipole mode. We further study the emerging optical signatures of the monolayer localized at dipole nanoantennas at 10 K.

We design a quantum dot (QD) embedded in a vertical-cavity photonic nanowire (NW), deterministically integrated on a silicon-on-insulator (SOI) waveguide (WG), as a novel quantum light source in a quantum photonic integrated circuit (QPIC). Using a broadband QD emitter, we perform finite-difference time domain simulations to systematically tune key geometrical parameters and to explore the coupling mechanisms of the emission to the NW and WG modes. We find distinct Fabry-Perot resonances in the Purcell enhanced emission that govern the outcoupled power into the fundamental TE mode of the SOI-WG. With an optimized geometry that places the QD emitter in a finite NW in close proximity to the WG, we obtain peak outcoupling efficiencies for polarized emission as high as eighty percent. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.

Visualizing eigenmodes is crucial in understanding the behavior of state-of-the-art micromechanical devices. We demonstrate a method to optically map multiple modes of mechanical structures simultaneously. The fast and robust method, based on a modified phase-lock loop, is demonstrated on a silicon nitride membrane and shown to outperform three alternative approaches. Line traces and two-dimensional maps of different modes are acquired. The high quality data enables us to determine the weights of individual contributions in superpositions of degenerate modes.

Quantum technologies that rely on photonic qubits require a precise controllability of their properties. For this purpose hybrid approaches are particularly attractive because they offer a large flexibility to address different aspects of the photonic degrees of freedom. When combining photonics with other quantum platforms like phonons, quantum transducers have to be realized that convert between the mechanical and optical domain. Here, we realize this interface between phonons in the form of surface acoustic waves (SAWs) and single photons, mediated by a single semiconductor quantum dot exciton. In this combined theoretical and experimental study, we show that the different sidebands exhibit characteristic blinking dynamics that can be controlled by detuning the laser from the exciton transition. By developing analytical approximations we gain a better understanding of the involved internal dynamics. Our specific SAW approach allows us to reach the ideal frequency range of around 1 GHz that enables simultaneous temporal and spectral phonon sideband resolution close to the combined fundamental time-bandwidth limit.

Materials combining semiconductor functionalities with spin control are desired for the advancement of quantum technologies. Here, we study the magneto-optical properties of novel paramagnetic Ruddlesden-Popper hybrid perovskites Mn:(PEA)2PbI4 (PEA = phenethylammonium) and report magnetically brightened excitonic luminescence with strong circular polarization from the interaction with isolated Mn2+ ions. Using a combination of superconducting quantum interference device (SQUID) magnetometry, magneto-absorption and transient optical spectroscopy, we find that a dark exciton population is brightened by state mixing with the bright excitons in the presence of a magnetic field. Unexpectedly, the circular polarization of the dark exciton luminescence follows the Brillouin-shaped magnetization with a saturation polarization of 13% at 4 K and 6 T. From high-field transient magneto-luminescence we attribute our observations to spin-dependent exciton dynamics at early times after excitation, with first indications for a Mn-mediated spin-flip process. Our findings demonstrate manganese doping as a powerful approach to control excitonic spin physics in Ruddlesden-Popper perovskites, which will stimulate research on this highly tuneable material platform with promise for tailored interactions between magnetic moments and excitonic states.

For two-dimensional (2D) layered semiconductors, control over atomic defects and understanding of their electronic and optical functionality represent major challenges towards developing a mature semiconductor technology using such materials. Here, we correlate generation, optical spectroscopy, atomic resolution imaging, and ab initio theory of chalcogen vacancies in monolayer MoS2. Chalcogen vacancies are selectively generated by in-vacuo annealing, but also focused ion beam exposure. The defect generation rate, atomic imaging and the optical signatures support this claim. We discriminate the narrow linewidth photoluminescence signatures of vacancies, resulting predominantly from localized defect orbitals, from broad luminescence features in the same spectral range, resulting from adsorbates. Vacancies can be patterned with a precision below 10nm by ion beams, show single photon emission, and open the possibility for advanced defect engineering of 2D semiconductors at the ultimate scale. The relation between the microscopic structure and the optical properties of atomic defects in 2D semiconductors is still debated. Here, the authors correlate different fabrication processes, optical spectroscopy and electron microscopy to identify the optical signatures of chalcogen vacancies in monolayer MoS2.

We report magneto-optical spectroscopy of gated monolayer MoS2 in high magnetic fields up to 28 T and obtain new insights on the many-body interaction of neutral and charged excitons with the resident charges of distinct spin and valley texture. For neutral excitons at low electron doping, we observe a nonlinear valley Zeeman shift due to dipolar spin-interactions that depends sensitively on the local carrier concentration. As the Fermi energy increases to dominate over the other relevant energy scales in the system, the magneto-optical response depends on the occupation of the fully spin-polarized Landau levels (LL) in both K/K' valleys. This manifests itself in a many-body state. Our experiments demonstrate that the exciton in monolayer semiconductors is only a single particle boson close to charge neutrality. We find that away from charge neutrality it smoothly transitions into polaronic states with a distinct spin-valley flavor that is defined by the LL quantized spin and valley texture.

Wave mixing is an archetypical phenomenon in bosonic systems. In optomechanics, the bidirectional conversion between electromagnetic waves or photons at optical frequencies and elastic waves or phonons at radio frequencies is building on precisely this fundamental principle. Surface acoustic waves (SAWs) provide a versatile interconnect on a chip and thus enable the optomechanical control of remote systems. Here we report on the coherent nonlinear three-wave mixing between the coherent fields of two radio frequency SAWs and optical laser photons via the dipole transition of a single quantum dot exciton. In the resolved sideband regime, we demonstrate fundamental acoustic analogues of sum and difference frequency generation between the two SAWs and employ phase matching to deterministically enhance or suppress individual sidebands. This transfer between the acoustic and optical domains is described by theory that fully takes into account direct and virtual multiphonon processes. Finally, we show that the precision of the wave mixing is limited by the frequency accuracy of modern radio frequency electronics. (C) 2021 Optical Society of America under the temis of the OSA Open Access Publishing Agreement

Layered, two-dimensional (2D) materials are promising for next-generation photonics devices. Typically, the thickness of mechanically cleaved flakes and chemical vapor deposited thin films is distributed randomly over a large area, where accurate identification of atomic layer numbers is time-consuming. Hyperspectral imaging microscopy yields spectral information that can be used to distinguish the spectral differences of varying thickness specimens. However, its spatial resolution is relatively low due to the spectral imaging nature. In this work, we present a 3D deep learning solution called DALM (deep-learning-enabled atomic layer mapping) to merge hyperspectral reflection images (high spectral resolution) and RGB images (high spatial resolution) for the identification and segmentation of MoS2 flakes with mono-, bi-, tri-, and multilayer thicknesses. DALM is trained on a small set of labeled images, automatically predicts layer distributions and segments individual layers with high accuracy, and shows robustness to illumination and contrast variations. Further, we show its advantageous performance over the state-of-the-art model that is solely based on RGB microscope images. This AI-supported technique with high speed, spatial resolution, and accuracy allows for reliable computer-aided identification of atomically thin materials.

We demonstrate the on-demand creation and positioning of photon emitters in atomically thin MoS2 with very narrow ensemble broadening and negligible background luminescence. Focused helium-ion beam irradiation creates 100s to 1000s of such mono-typical emitters at specific positions in the MoS2 monolayers. Individually measured photon emitters show anti-bunching behavior with a g(2)(0) similar to 0.23 and 0.27. From a statistical analysis, we extract the creation yield of the He-ion induced photon emitters in MoS2 as a function of the exposed area, as well as the total yield of single emitters as a function of the number of He ions when single spots are irradiated by He ions. We reach probabilities as high as 18% for the generation of individual and spectrally clean photon emitters per irradiated single site. Our results firmly establish 2D materials as a platform for photon emitters with unprecedented control of position as well as photophysical properties owing to the all-interfacial nature.

We present spectroscopic experiments and theory of a quantum dot driven bichromatically by two strong coherent lasers. In particular, we explore the regime where the drive strengths are substantial enough to merit a general nonperturbative analysis, resulting in a rich higher-order Floquet dressed-state energy structure. We show high-resolution spectroscopy measurements with a variety of laser detunings performed on a single InGaAs quantum dot, with the resulting features well explained with a time-dependent quantum master equation and Floquet analysis. Notably, driving the quantum dot resonance and one of the subsequent Mollow triplet sidepeaks, we observe the disappearance and subsequent reappearance of the central transition and transition resonant with detuned laser at high detuned-laser pump strengths and additional higher-order effects, e.g., emission triplets at higher harmonics and signatures of higher-order Floquet states. For a similar excitation condition but with an off-resonant primary laser, we observe similar spectral features but with an enhanced inherent spectral asymmetry.

Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of additional features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential nondestructive, phase- and composition-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temperatures, near and off resonance. By combining polarization-resolved Raman data with calculations of the phonon dispersion and using symmetry considerations, we identify the four first-order Raman modes and higher-order two-phonon modes. Moreover, we observe defect-activated phonon processes, which provide a route toward a quantitative assessment of the defect concentration and, thus, the crystal quality of the materials. Our work establishes a solid background for future research on material synthesis, study, and application of Janus TMD monolayers.

We demonstrate electrostatic switching of individual, site-selectively generated matrices of single photon emitters (SPEs) in MoS2 van der Waals heterodevices. We contact monolayers of MoS2 in field-effect devices with graphene gates and hexagonal boron nitride as the dielectric and graphite as bottom gates. After the assembly of such gate-tunable heterodevices, we demonstrate how arrays of defects, that serve as quantum emitters, can be site-selectively generated in the monolayer MoS2 by focused helium ion irradiation. The SPEs are sensitive to the charge carrier concentration in the MoS2 and switch on and off similar to the neutral exciton in MoS2 for moderate electron doping. The demonstrated scheme is a first step for producing scalable, gate-addressable, and gate-switchable arrays of quantum light emitters in MoS2 heterostacks.

Monolayer semiconducting transition metal dichal-cogenides are a strongly emergent platform for exploring quantum phenomena in condensed matter, building novel optoelectronic devices with enhanced functionalities. Because of their atomic thickness, their excitonic optical response is highly sensitive to their dielectric environment. In this work, we explore the optical properties of monolayer thick MoSe2 straddling domain wall boundaries in periodically poled LiNbO3. Spatially resolved photoluminescence experiments reveal spatial sorting of charge and photogenerated neutral and charged excitons across the boundary. Our results reveal evidence for extremely large in-plane electric fields of similar or equal to 4000 kV/cm at the domain wall whose effect is manifested in exciton dissociation and routing of free charges and trions toward oppositely poled domains and a nonintuitive spatial intensity dependence. By modeling our result using drift-diffusion and continuity equations, we obtain excellent qualitative agreement with our observations and have explained the observed spatial luminescence modulation using realistic material parameters.

We investigate the degree of indistinguishability of cascaded photons emitted from a three-level quantum ladder system,. in our case the biexciton-exciton cascade of semiconductor quantum dots. For the three-level quantum ladder system we theoretically demonstrate that the indistinguishability is inherently limited for both emitted photons and determined by the ratio of the lifetimes of the excited and intermediate states. We experimentally confirm this finding by comparing the quantum interference visibility of noncascaded emission and cascaded emission from the same semiconductor quantum dot. Quantum optical simulations produce very good agreement with the measurements and allow us to explore a large parameter space. Based on our model, we propose photonic structures to optimize the lifetime ratio and overcome the limited indistinguishability of cascaded photon emission from a three-level quantum ladder system.

We present diffraction-limited photocurrent (PC) microscopy in the visible spectral range based on broadband excitation and an inherently phase-stable common-path interferometer. The excellent path-length stability guarantees high accuracy without the need for active feedback or post-processing of the interferograms. We illustrate the capabilities of the setup by recording PC spectra of a bulk GaAs device and compare the results to optical transmission data.

Atomistic van der Waals heterostacks are ideal systems for high-temperature exciton condensation because of large exciton binding energies and long lifetimes. Charge transport and electron energy-loss spectroscopy showed first evidence of excitonic many-body states in such two-dimensional materials. Pure optical studies, the most obvious way to access the phase diagram of photogenerated excitons, have been elusive. We observe several criticalities in photogenerated exciton ensembles hosted in MoSe2-WSe2 heterostacks with respect to photoluminescence intensity, linewidth, and temporal coherence pointing towards the transition to a coherent many-body quantum state, consistent with the predicted critical degeneracy temperature. For this state, the estimated occupation is approximately 100% and the phenomena survive above 10 K.Y

Janus crystals represent an exciting class of 2D materials with different atomic species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an electric field and leads to a wealth of novel properties, such as large Rashba spin-orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe2 and via plasma stripping followed thermal annealing of MoS2. However, the high processing temperatures prevent growth of other Janus materials and their heterostructures. Here, a room-temperature technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low-energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room-temperature method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.

Ultrathin InAs nanowires (NW) with a one-dimensional (1D) sub-band structure are promising materials for advanced quantum-electronic devices, where dimensions in the sub-30 nm diameter limit together with post-CMOS integration scenarios on Si are much desired. Here, we demonstrate two site-selective synthesis methods that achieve epitaxial, high aspect ratio InAs NWs on Si with ultrathin diameters below 20 nm. The first approach exploits direct vapor-solid growth to tune the NW diameter by interwire spacing, mask opening size and growth time. The second scheme explores a unique reverse-reaction growth by which the sidewalls of InAs NWs are thermally decomposed under controlled arsenic flux and annealing time. Interesting kinetically limited dependencies between interwire spacing and thinning dynamics are found, yielding diameters as low as 12 nm for sparse NW arrays. We clearly verify the 1D sub-band structure in ultrathin NWs by pronounced conductance steps in low-temperature transport measurements using back-gated NW-field effect transistors. Correlated simulations reveal single- and double degenerate conductance steps, which highlight the rotational hexagonal symmetry and reproduce the experimental traces in the diffusive 1D transport limit. Modelling under the realistic back-gate configuration further evidences regimes that lead to asymmetric carrier distribution and breakdown of the degeneracy depending on the gate bias.

Resonance fluorescence has played a major role in quantum optics with predictions and later experimental confirmation of nonclassical features of its emitted light such as antibunching or squeezing. In the Rayleigh regime where most of the light originates from the scattering of photons with subnatural linewidth, antibunching would appear to coexist with sharp spectral lines. Here, we demonstrate that this simultaneous observation of subnatural linewidth and antibunching is not possible with simple resonant excitation. Using an epitaxial quantum dot for the two-level system, we independently confirm the single-photon character and subnatural linewidth by demonstrating antibunching in a Hanbury Brown and Twiss type setup and using high-resolution spectroscopy, respectively. However, when filtering the coherently scattered photons with filter bandwidths on the order of the homogeneous linewidth of the excited state of the two-level system, the antibunching dip vanishes in the correlation measurement. Our observation is explained by antibunching originating from photon-interferences between the coherent scattering and a weak incoherent signal in a skewed squeezed state. This prefigures schemes to achieve simultaneous subnatural linewidth and antibunched emission.

Precisely positioned and scalable single-photon emitters (SPEs) are highly desirable for applications in quantum technology. This Perspective discusses single-photon-emitting atomistic defects in monolayers of MoS2 that can be generated by focused He-ion irradiation with few nanometers positioning accuracy. We present the optical properties of the emitters and the possibilities to implement them into photonic and optoelectronic devices. We showcase the advantages of the presented emitters with respect to atomistic positioning, scalability, long (microsecond) lifetime, and a homogeneous emission energy within ensembles of the emitters. Moreover, we demonstrate that the emitters are stable in energy on a timescale exceeding several weeks and that temperature cycling narrows the ensembles' emission energy distribution.

Inter-layer excitons (IXs) in hetero-bilayers of transition metal dichalcogenides (TMDs) represent an exciting emergent class of long-lived dipolar composite bosons in an atomically thin, near-ideal two-dimensional (2D) system. The long-range interactions that arise from the spatial separation of electrons and holes can give rise to novel quantum, as well as classical multi-particle correlation effects. Indeed, first indications of exciton condensation have been reported recently. In order to acquire a detailed understanding of the possible many-body effects, the fundamental interactions between individual IXs have to be studied. Here, we trap a tunable number of dipolar IXs (N-IX 1-5) within a nanoscale confinement potential induced by placing a MoSe2-WSe2 hetero-bilayer (HBL) onto an array of SiO2 nanopillars. We control the mean occupation of the IX trap via the optical excitation level and observe discrete sharp-line emission from different configurations of interacting IXs. The intensities of these features exhibit characteristic near linear, quadratic, cubic, quartic and quintic power dependencies, which allows us to identify them as different multiparticle configurations with N-IX 1-5. We directly measure the hierarchy of dipolar and exchange interactions as N-IX increases. The interlayer biexciton (N-IX = 2) is found to be an emission doublet that is blue-shifted from the single exciton by Delta E = (8.4 +/- 0.6) meV and split by 2J = (1.2 +/- 0.5) meV. The blueshift is even more pronounced for triexcitons ((12.4 +/- 0.4) meV), quadexcitons ((15.5 +/- 0.6) meV) and quintexcitons ((18.2 +/- 0.8) meV). These values are shown to be mutually consistent with numerical modelling of dipolar excitons confined to a harmonic trapping potential having a confinement lengthscale in the range l approximate to 3 nm. Our results contribute to the understanding of interactions between IXs in TMD hetero-bilayers at the discrete limit of only a few excitations and represent a key step towards exploring quantum correlations between IXs in TMD hetero-bilayers.

Nanowires (NWs) hold great potential in advanced thermoelectrics due to their reduced dimensions and low-dimensional electronic character. However, unfavorable links between electrical and thermal conductivity in state-of-the-art unpassivated NWs have, so far, prevented the full exploitation of their distinct advantages. A promising model system for a surface-passivated one-dimensional (1D)-quantum confined NW thermoelectric is developed that enables simultaneously the observation of enhanced thermopower via quantum oscillations in the thermoelectric transport and a strong reduction in thermal conductivity induced by the core–shell heterostructure. High-mobility modulation-doped GaAs/AlGaAs core–shell NWs with thin (sub-40 nm) GaAs NW core channel are employed, where the electrical and thermoelectric transport is characterized on the same exact 1D-channel. 1D-sub-band transport at low temperature is verified by a discrete stepwise increase in the conductance, which coincided with strong oscillations in the corresponding Seebeck voltage that decay with increasing sub-band number. Peak Seebeck coefficients as high as ≈65–85 µV K−1 are observed for the lowest sub-bands, resulting in equivalent thermopower of S2σ ≈ 60 µW m−1 K−2 and S2G ≈ 0.06 pW K−2 within a single sub-band. Remarkably, these core–shell NW heterostructures also exhibit thermal conductivities as low as ≈3 W m−1 K−1, about one order of magnitude lower than state-of-the-art unpassivated GaAs NWs.

Substrate, environment, and lattice imperfections have a strong impact on the local electronic structure and the optical properties of atomically thin transition metal dichalcogenides. We find by a comparative study of MoS2 on SiO2 and hexagonal boron nitride (hBN) using scanning tunneling spectroscopy (STS) measurements that the apparent bandgap of MoS2 on SiO2 is significantly reduced compared to MoS2 on hBN. The bandgap energies as well as the exciton binding energies determined from all-optical measurements are very similar for MoS2 on SiO2 and hBN. This discrepancy is found to be caused by a substantial amount of band tail states near the conduction band edge of MoS2 supported by SiO2. The presence of those states impacts the local density of states in STS measurements and can be linked to a broad red-shifted photoluminescence peak and a higher charge carrier density that are all strongly diminished or even absent using high quality hBN substrates. By taking into account the substrate effects, we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about 0.53 eV on SiO2 and 0.44 eV on hBN. Published under license by AIP Publishing.

Excitons in atomically thin semiconductors interact very strongly with electromagnetic radiation and are necessarily close to a surface. Here, we exploit the deep-subwavelength confinement of surface plasmon polaritons (SPPs) at the edge of a metal-insulator-metal plasmonic waveguide and their proximity of 2D excitons in an adjacent atomically thin semiconductor to build an ultracompact photodetector. When subject to far-field excitation we show that excitons are created throughout the dielectric gap region of our waveguide and converted to free carriers primarily at the anode of our device. In the near-field regime, strongly confined SPPs are launched, routed and detected in a 20 nm narrow region at the interface between the waveguide and the monolayer semiconductor. This leads to an ultracompact active detector region of only similar to 0.03 mu m(2) that absorbs 86% of the propagating energy in the SPP. Due to the electromagnetic character of the SPPs, the spectral response is essentially identical to the far-field regime, exhibiting strong resonances close to the exciton energies. While most of our experiments are performed on monolayer thick MoSe2, the photocurrent-per-layer increases super linearly in multilayer devices due to the suppression of radiative exciton recombination. These results demonstrate an integrated device for nanoscale routing and detection of light with the potential for on-chip integration at technologically relevant, few-nanometer length scales.

Quantum light sources in solid-state systems are of major interest as a basic ingredient for integrated quantum photonic technologies. The ability to tailor quantum emitters via site-selective defect engineering is essential for realizing scalable architectures. However, a major difficulty is that defects need to be controllably positioned within the material. Here, we overcome this challenge by controllably irradiating monolayer MoS2 using a sub-nm focused helium ion beam to deterministically create defects. Subsequent encapsulation of the ion exposed MoS2 flake with high-quality hBN reveals spectrally narrow emission lines that produce photons in the visible spectral range. Based on ab-initio calculations we interpret these emission lines as stemming from the recombination of highly localized electron-hole complexes at defect states generated by the local helium ion exposure. Our approach to deterministically write optically active defect states in a single transition metal dichalcogenide layer provides a platform for realizing exotic many-body systems, including coupled single-photon sources and interacting exciton lattices that may allow the exploration of Hubbard physics.

We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickest and that even minute deviations of the perfect hexagonal shape strongly perturb the commonly assumed 6-fold symmetric ground state.

Photonic quantum technologies call for scalable quantum light sources that can be integrated, while providing the end user with single and entangled photons on demand. One promising candidate is strain free GaAs/A1GaAs quantum dots obtained by aluminum droplet etching. Such quantum dots exhibit ultra low multi-photon probability and an unprecedented degree of photon pair entanglement. However, different to commonly studied InGaAs/GaAs quantum dots obtained by the Stranski-Krastanow mode, photons with a near-unity indistinguishability from these quantum emitters have proven to be elusive so far. Here, we show on-demand generation of near-unity indistinguishable photons from these quantum emitters by exploring pulsed resonance fluorescence. Given the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots, we show single photon indistinguishability with a raw visibility of V-raw = (95.0(-6.1)(+5.0))%, without the need for Purcell enhancement. Our results represent a milestone in the advance of GaAs quantum dots by demonstrating the final missing property standing in the way of using these emitters as a key component in quantum communication applications, e.g., as quantum light sources for quantum repeater architectures.