Molecular fingerprinting with bright, broadband infrared frequency combs

Spectroscopy in the molecular fingerprint spectral region (6.7–20 μm) yields critical information on material structure for physical, chemical, and biological sciences. Despite decades of interest and effort, this portion of the electromagnetic spectrum remains challenging to cover with conventional laser technologies. In this paper, we present a simple and robust method for generating super-octave, optical frequency combs in the fingerprint region through intra-pulse difference frequency generation in an orientation-patterned gallium phosphide crystal. The attainable brightness from this tabletop source reaches the same level achievable by infrared synchrotron radiation with a bandwidth spanning from 4 to 12 μm. We demonstrate the utility of this unique coherent light source for high-precision, dual-frequency-comb spectroscopy of methanol and ethanol vapor. These results highlight the potential of laser frequency combs for a wide range of infrared molecular sensing applications from basic molecular spectroscopy to nanoscopic imaging.