Super-octave longwave mid-infrared coherent transients produced by optical rectification of few-cycle 2.5-μm pulses

Femtosecond laser sources and optical frequency combs in the molecular fingerprint region of the electromagnetic spectrum are crucial for a plethora of applications in natural and life sciences. Here we introduce Cr2+-based lasers as a convenient means for producing super-octave mid-IR electromagnetic transients via optical rectification (or intra-pulse difference frequency generation, IDFG). We demonstrate that a relatively long, 2.5 mu m, central wavelength of a few-cycle Cr2+: ZnS driving source (20 fs pulse duration, 6 W average power, 78 MHz repetition rate) enabled the use of highly nonlinear ZnGeP2 crystal for IDFG with exceptionally high conversion efficiency (> 3%) and output power of 0.15 W, with the spectral span of 5.8-12.5 mu m. Even broader spectrum was achieved in GaSe crystal: 4.3-16.6 mu m for type I and 5.8-17.6 mu m for type II phase matching. The results highlight the potential of this architecture for ultrafast spectroscopy and generation of broadband frequency combs in the longwave infrared. (c) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement