Molecules are the building blocks of our world. They are many-body quantum systems and all but the simplest molecules still defy ab-initio calculations. Spectroscopy provides important information about the structure and dynamics of molecules. Sensitive spectroscopic detection of molecules can diagnose diseases and monitor the safety of our environment. Rapid spectral analysis is important for many tasks, from combustion diagnostics to label-free microscopic imaging of selected proteins. Advancing tools of laser science, photonics, opto-electronics, and digital data processing make it possible to invent and explore new approaches to molecular spectroscopy which far exceed existing techniques in speed and sensitivity.
Can we create molecular spectroscopy laboratory on a chip? How can we achieve fast, sensitive spectral acquisition over a broad spectral range? What is the best way to access the mid-infrared molecur fingerprint region? How can we replace optical and mechanical complexity by digital signal processing? What are the ultimate quantum limits? What new insights can we contribute to molecular science?
In recent years, laser frequency combs have revealed an exciting potential for new approaches to 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.
Novel approaches to dual-comb spectroscopy
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 pro les. 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 sacri cing performance. By generating proper clock signals, we compensate for laser short-term instabilities by electronic signal processing only. We record spectra spanning the full bandwidth of near-infrared bre lasers, around 1.55 μm, with Doppler- limited line pro les highly suitable for measurements of concentrations or line intensities.
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 (80Figure 5: Time-domain interference signal of the beats between the two frequency-agile laser combs and part of the resulting spectrum of the CO2 molecule around 1570 nm.kHz) and tuning speeds (10 nm s-1) at high signal-to-noise ratio are achieved (Fig. 5) in the telecommunication near- infrared region. The technique of frequency-agile dual-comb spectroscopy has just been extended to the mid-infrared region, around 3 μm. Here, precise line parameters, including positions (relative accuracy: 10-8) and intensities (2%), may be retrieved from such mid-infrared Doppler-limited 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.
Nonlinear dual-comb spectroscopy
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. The femtosecond pulses of one comb periodically excite low- lying vibrational levels in a two-photon Raman process. After each excitation, a pulse of a second comb probes, at a linearly increasing time delay, the sample. An alternately blue- and red-shifted beam is thus generated. Measuring the intensity modulation of the blue-shifted light behind a spectral filter yields a radio-frequency signal encoded at the excited vibrational frequencies. The Fourier transform of this time-domain interferogram reveals the Raman spectrum. 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 (4 cm-1) Raman spectra span the entire ngerprint region, more than 3200 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 identi es 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. Proof-of-principle demonstration has been achieved with the 5S-5D two-photon transitions of rubidium. Such multiplex technique with sub-Doppler resolution may enable broadband spectroscopy with unprecedented precision.
Frequency comb generators based on electro-optic modulators
Frequency combs based on electro-optic modulators are nding new applications in broadband molecular spectroscopy. In a collaboration with the Basic Research Laboratories of Nippon Telegraph and Telephone (Japan), we have explored the potential of cascaded phase and intensity electro-optic modulators of large line spacing (25 GHz) for near-infrared vibrational spectroscopy. With the Laboratoire Interdisciplinaire Carnot de Bourgogne (France), we use intensity modulators and a nonlinear optical bre to produce two mutually coherent frequency combs suited for gas phase spectroscopy in the near and mid-infrared region. More than a thousand evenly spaced infrared spectral lines are generated with a remarkably at intensity distribution. Line spacing and spectral position can be selected quickly and freely by simply dialling a knob. Such frequency-agile optical combs offer unprecedented freedom when interrogating a molecular spectrum via the powerful technique of multiplexed dual-comb spectroscopy.
Mid-infrared frequency combs
The mid-infrared spectral region (2-20 μm) contains strong fundamental vibrational transitions of most molecules, as well as two atmospheric transmission windows. Photonics instrumentation is not as developed in the mid-infrared as in the visible region, and this technically challenging area of research is currently under active development. Many applications in spectroscopy, material science, security and industry process control, or chemical, biological and medical sensing would straightforwardly take advantage of mid- infrared photonics devices of higher performance.
In particular new strategies to mid-infrared frequency comb generation would strongly bene t molecular sciences. However, the laser systems and optical materials suitable for low-threshold octave-spanning phase-coherent spectral broadening remain scarce and dif cult to engineer.
A new approach to mid-infrared frequency comb generation has been successfully explored. In a collaboration with ICFO (Spain), mid-infrared synchronously-pumped optical parametric oscillators (OPO) have been generated, with as few as 3.7 optical cycles at 2682 nm and a tunability from 2179 to 3732 nm. CMOS-compatible highly nonlinear dispersion-engineered silicon nanophotonic waveguides on a silicon-on-insulator chip, developed at the University of Ghent and IMEC (Belgium), have been used to spectrally broaden the output of the OPO. We were able to record phase-coherent octave-spanning (1500–3300 nm) comb spectra (Fig. 6), with a coupled input pulse energy as low as 16 pJ. Importantly, the waveguides are chemically stable and no modi cations of the characteristics of the generated supercontinuum have been observed over several months. With further system development, silicon technology has the potential to provide a room-temperature-operating platform for supercontinuum generation extending deeper in the mid-infrared, up to 8500 nm. On a longer term, such miniaturized wires could be part of on-chip frequency-comb spectrometers for chemical sensing.