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.
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 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 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.