High-power sub-two-cycle mid-infrared pulses at 100â••MHz repetition rate

Powerful coherent light with a spectrum spanning the mid-infrared (MIR) spectral range is crucial for a number of applications in natural as well as life sciences, but so far has only been available from large-scale synchrotron sources1. Here we present a compact apparatus that generates pulses with a sub-two-cycle duration and with an average power of 0.1 W and a spectral coverage of 6.8–16.4 μm (at −30 dB). The demonstrated source combines, for the first time in this spectral region, a high power, a high repetition rate and phase coherence. The MIR pulses emerge via difference-frequency generation (DFG) driven by the nonlinearly compressed pulses of a Kerr-lens mode-locked ytterbium-doped yttrium– aluminium–garnet (Yb:YAG) thin-disc oscillator. The resultant 100 MHz MIR pulse train is hundreds to thousands of times more powerful than state-of-the-art frequency combs that emit in this range2–4, and offers a high dynamic range for spectroscopy in the molecular fingerprint region4–7 and an ideal prerequisite for hyperspectral imaging8 as well as for the time-domain coherent control of vibrational dynamics9–11. Over the past two decades, the development of coherent radiation sources in the MIR spectral region has been subject to intensive research. Most molecules exhibit fundamental vibrational modes in the range between 2 and 20 μm, which makes MIR spectroscopy a versatile tool for a variety of applications that range from fundamental science10, over security and environmental applications (for example, the detection of weak gas traces4–6) to medical diagnostics4,12. For all the spectroscopy schemes, a spatially coherent, laser-like beam is highly desirable as a result of the increased brightness and the ability to focus tightly. Another essential parameter for spectroscopy experiments is the average power P of the source, because the signal-to-noise ratio (SNR) contributions of the detector noise and of the shot noise scale with P and P √ , respectively7. Owing to the inferior noise figures of typical MIR detectors compared with visible detectors, a high spectral brightness is particularly important for MIR investigations. Finally, in the case of broadband pulsed radiation, the phase coherence of the pulses in combination with a repetition rate in the range of several tens of megahertz enables the resonant enhancement of the radiation in a passive optical cavity. This increases the interaction length with the investigated medium (for example, gas-phase volatile molecular components from living organisms) and, therefore, the sensitivity by a factor of 2FL/π (which can be of a few orders of magnitude), where F and L represent the cavity finesse and length, respectively4,5,13. A radiation source that combines all these properties will also benefit other applications beyond molecular spectroscopy. Such a source, for instance, could be used as a strong, ultrashort bias to study and control ultrafast charge transport in dielectrics9 or in semiconductors10. In the absence of suitable laser media, broadband coherent MIR sources rely on optical parametric processes to downconvert the frequency of ultrashort-pulsed, near-infrared (NIR) laser radiation4,14. In fact, recently parametric sources have enabled breakthroughs in many areas, including frequency comb spectroscopy4,13,15, the generation of ultrahigh harmonics with kiloelectronvolt photon energies16 and soft-X-ray absorption spectroscopy17. However, most of these sources employ oxide materials as nonlinear media, which are not transparent for wavelengths longer than ∼5 μm (ref. 18). Non-oxide nonlinear crystals broadly transparent in the MIR typically suffer from unfavourable thermomechanical properties and (linear or nonlinear) absorption when pumped with NIR highpower sources18. In addition, generating radiation at increasingly long wavelengths suffers from an ever-more unfavourable photonenergy ratio between the pump and signal. Most of the state-ofthe-art coherent sources in this spectral region are based on DFG in gallium selenide (GaSe), driven with wavelengths longer than 1 μm (refs 2,3,10). However, the driving laser sources either exhibit poor power scalability2,3 or reach high pulse energies at the expense of a reduced repetition rate with a complex set-up10. Recent alternatives to DFG demonstrate supercontinuum generation (SCG) in chalcogenide fibres, driven by optical parametric amplifiers19,20. However, at wavelengths longer than 5 μm these sources produce average powers of a few milliwatts at best. In this work, we present a source of coherent radiation that spans a spectrum from 6.8 to 16.4 μm (at −30 dB) and combines, for the first time, the desired properties of power scalability, high repetition rate (100 MHz) and phase coherence in this spectral region. The source is based on DFG driven by spectral components of the broadband NIR pulses of an Yb-based laser system in a chalcogenide crystal. Among the parametric processes, DFG allows for a convenient trade-off between efficiency and bandwidth4. In addition, the driving frequency components originate from one and the same pulse, so the carrier-envelope phase of the resulting pulses is inherently stabilized4. Furthermore, such a collinear intrapulse DFG exhibits the advantages of simplicity, compactness and reduced jitter21,22. In the prevailing context of coherent MIR sources, the power scalability in conjunction with a repetition rate