Theodor Hänsch

Laser Spectroscopy & Quantum Physics

Ludwig-Maximilians-Universität München, Max Planck Institute of Quantum Optics

LMU | Faculty of Physics

Schellingstr. 4

80799 München

Tel. +49 89 2180 3212

t.w.haensch[at]physik.uni-muenchen.de

Group Webpage

Description

Research focus: quantum many-body physics, quantum optics, laser physics

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The Laser Spectroscopy Division of Professor Theodor W. Hänsch is developing tools to observe and manipulate quantum matter with light. Applications range from fundamental physics laws to nanoscopy of condensed matter quantum systems. The research in the Laser Spectroscopy Division is mainly organized along three principal lines: precision spectroscopy of simple atoms; molecular spectroscopy and imaging with laser frequency combs; and quantum optics with optical micro-cavities.

Precission laser spectroscopy

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Precision laser spectroscopy of hydrogen and other simple atomic systems enables accurate determinations of physical constants, stringent tests of fundamental theories and searches for possible drifts of the fundamental constants. Today, frequency measurements of the hydrogen 1S-2S resonance are reaching a precision of 15 decimal digits with the help of state-of-the-art tools including the laser frequency comb technique. A recent laser measurement of the Lamb shift of muonic hydrogen has yielded an independent value of the proton radius in strong disagreement with that derived from hydrogen spectroscopy. Improved measurements of different transitions in hydrogen have thus gained much in relevance. The development of laser frequency combs in the extreme ultraviolet is motivated by the prospect of precision spectroscopy of trapped hydrogen- or helium-like ions.

Laser frequency combs

Laser frequency combs are becoming compelling instruments for broadband molecular spectroscopy by dramatically improving the resolution and recording speed of Fourier spectrometers and by creating new opportunities for highly-multiplexed nonlinear spectroscopy and imaging. Advanced laser and photonic technologies, involving optical parametric oscillators, silicon photonics or high-quality factor micro-resonators, extend the spectral territory of frequency comb generators to the mid-infrared region, range of the fingerprints of molecules. Real-time coherent Raman comb-based hyperspectral imaging opens intriguing perspectives for microscopy of biological samples. Two-photon spectroscopy with two laser combs holds promise for Doppler-free spectroscopy of molecules.

Optical microcavities

Optical micro-cavities are powerful photonic devices with applications ranging from cavity quantum electro-dynamics with solid-state emitters to novel schemes for cavity-enhanced microscopy and spectroscopy. With laser-machined optical fibers, open-access cavities of very high finesse and small mode waist are harnessed to realize an efficient optical interface for color centers in nano-systems. This should lead to efficient single-photon sources and may provide a route towards the strong coupling regime in a cryogenic environment. Other experiments explore the potential of such micro-cavities for ultrasensitive spectroscopy of individual nano-scale objects, such as carbon nanotubes and gold nanoparticles.

Publications

Improved active fiber-based retroreflector with intensity stabilization and a polarization monitor for the near UV

V. Wirthl, L. Maisenbacher, J. Weitenberg, A. Hertlein, A. Grinin, A. Matveev, R. Pohl, T.W. Hänsch, T. Udem

Optics Express 29, 7024-7048 (2021).

Show Abstract

We present an improved active fiber-based retroreflector (AFR) providing high-quality wavefront-retracing anti-parallel laser beams in the near UV. We use our improved AFR for first-order Doppler-shift suppression in precision spectroscopy of atomic hydrogen, but our setup can be adapted to other applications where wavefront-retracing beams with defined laser polarization are important. We demonstrate how weak aberrations produced by the fiber collimator may remain unobserved in the intensity of the collimated beam but limit the performance of the AFR. Our general results on characterizing these aberrations with a caustic measurement can be applied to any system where a collimated high-quality laser beam is required. Extending the collimator design process by wave optics propagation tools, we achieved a four-lens collimator for the wavelength range 380-486 nm with the beam quality factor of M-2 similar or equal to 1.02, limited only by the not exactly Gaussian beam profile from the single-mode fiber. Furthermore, we implemented precise fiber-collimator alignment and improved the collimation control by combining a precision motor with a piezo actuator. Moreover, we stabilized the intensity of the wavefront-retracing beams and added in-situ monitoring of polarization from polarimetry of the retroreflected light. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

DOI: 10.1364/oe.417455

Two-photon frequency comb spectroscopy of atomic hydrogen

A. Grinin, A. Matveev, D. C. Yost, L. Maisenbacher, V. Wirthl, R. Pohl, T. W. Hänsch, T. Udem

Science 370, 1061 (2020).

Show Abstract

We have performed two-photon ultraviolet direct frequency comb spectroscopy on the 1S-3S transition in atomic hydrogen to illuminate the so-called proton radius puzzle and to demonstrate the potential of this method. The proton radius puzzle is a significant discrepancy between data obtained with muonic hydrogen and regular atomic hydrogen that could not be explained within the framework of quantum electrodynamics. By combining our result [f1S-3S = 2,922,743,278,665.79(72) kilohertz] with a previous measurement of the 1S-2S transition frequency, we obtained new values for the Rydberg constant [R∞ = 10,973,731.568226(38) per meter] and the proton charge radius [rp = 0.8482(38) femtometers]. This result favors the muonic value over the world-average data as presented by the most recent published CODATA 2014 adjustment.

DOI: 10.1126/science.abc7776

Photon-level broadband spectroscopy and interferometry with two frequency combs

N. Picque, T.W. Hänsch

Proceedings of the National Academy of Sciences of the United States of America 117 (43), 26688-26691 (2020).

Show Abstract

We probe complex optical spectra at high resolution over a broad span in almost complete darkness. Using a single photon-counting detector at light power levels that are a billion times weaker than commonly employed, we observe interferences in the counting statistics with two separate mode-locked femtosecond lasers of slightly different repetition frequencies, each emitting a comb of evenly spaced spectral lines over a wide spectral span. Unique advantages of the emerging technique of dual-comb spectroscopy, such as multiplex data acquisition with many comb lines, potential very high resolution, and calibration of the frequency scale with an atomic clock, can thus be maintained for scenarios where only few detectable photons can be expected. Prospects include spectroscopy of weak scattered light over long distances, fluorescence spectroscopy of single trapped atoms or molecules, or studies in the extreme-ultraviolet or even soft-X-ray region with comb sources of low photon yield. Our approach defies intuitive interpretations in a picture of photons that exist before detection.

DOI: 10.1073/pnas.2010878117

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