Final Report Summary - MULTICOMB (Multidimensional laser frequency comb spectroscopy of molecules) Laser frequency combs are providing powerful tools for laser spectroscopy. Mode-locked femtosecond lasers and related emerging miniaturized devices can produce a large number of precisely evenly spaced spectral lines. Almost two decades ago, such spectral combs were introduced as tools for optical frequency metrology, motivated by the challenges of precision laser spectroscopy of atomic hydrogen as tests for fundamental physics laws. Today, new applications of frequency combs are evolving far beyond the original purpose. This project has explored some of the novel opportunities for novel broadband molecular spectroscopy. Novel techniques in which a frequency comb directly interrogates the molecular sample show major improvements in terms of measurement times, precision, spectral span and/or sensitivity. Amongst such techniques, dual-comb spectroscopy, which measures the time-domain interference between two combs, has the distinguishing advantage of a multiplex instrument without moving parts. Two frequency combs of slightly different line spacing are employed. One or both are transmitted through the sample and they are then heterodyned on a single photodetector, yielding a down-converted radio-frequency comb containing spectral information on the absorption or dispersion experienced by the lines of the combs.Dual-comb spectroscopy still has to overcome challenges to provide a high signal-to-noise ratio in real-time spectra and to yield high-quality molecular line profiles. Much innovative research is currently stimulated by the search for simple solutions to realize the demanding requirements of mutual coherence of the two combs. Our group has explored new concepts and approaches to linear absorption dual-comb spectroscopy. With adaptive sampling, we are able to use unstabilized free-running femtosecond lasers without sacrificing performance. By generating proper clock signals, we compensate for laser short-term instabilities by electronic signal processing only.With frequency-agile lasers, we generate without mode-locked oscillators, two frequency combs of slightly different repetition frequencies and moderate, but rapidly tunable, spectral span. Such mutually coherent combs are combined in an interferometer. Unprecedented refresh rates and tuning speeds at high signal-to-noise ratio are achieved in the telecommunication near-infrared region and in the mid-infrared region, around 3 µm. Here, precise line parameters may be retrieved from spectra measured on the ms time scale. These unique capabilities hold much promise for trace gas sensing, a domain relevant to physics, biology, chemistry, industry or atmospheric sciences. Compared to conventional Michelson-based Fourier transform spectroscopy, recording times could be shortened from seconds to microseconds. The resolution improves proportionally to the measurement time. Therefore longer recordings allow high resolution spectroscopy of molecules with extreme precision, since the absolute frequency of each laser comb line can be known with the accuracy of an atomic clock.Moreover, our research has led to the proposal and the first implementations of nonlinear dual-comb spectroscopy, demonstrated with coherent Raman effects and two-photon excitation. Since laser frequency combs involve intense ultra-short laser pulses, nonlinear interactions can be harnessed. Nonlinear dual-comb spectroscopy is a novel approach to broadband spectroscopy, with applications as diverse as hyperspectral microscopic imaging or precision spectroscopy. We achieve coherent anti-Stokes Raman spectroscopy and spectro-imaging with two laser frequency combs. All spectral elements are simultaneously measured on a single photodetector within a short time on the microsecond scale. The spectral span is determined by the bandwidth of the ultrashort pulse lasers. The resolution is only limited by the physical width of the molecular bands. High resolution Raman spectra span the entire fingerprint region, more than 3000 cm-1, recorded within less than 80 µs. Such capabilities have been extended to hyperspectral imaging with an acquisition rate of 50 pixels/s. This opens up intriguing prospects for spectrally-resolved microscopy of biological samples.Another experiment explores the potential of two-photon excitation dual-comb spectroscopy. Direct frequency comb spectroscopy with a single frequency comb is only suitable for spectra composed of very few transitions, as any resonance can only be measured modulo the line spacing of the frequency comb. The new technique of dual-comb two-photon spectroscopy identifies each transition uniquely by the modulation imparted by the interfering excitations. It can combine sub-Doppler resolution with a free spectral range only limited by the spectral bandwidth of the laser frequency combs. Such multiplex technique with sub-Doppler resolution may enable broadband spectroscopy with unprecedented precision.
