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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 Cr 2-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 μm, central wavelength of a few-cycle Cr 2 :ZnS driving source (20 fs pulse duration, 6 W average power, 78 MHz repetition rate) enabled the use of highly nonlinear ZnGeP 2 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 μm. Even broader spectrum was achieved in GaSe crystal: 4.3-16.6 μm for type I and 5.8-17.6 μm 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.

We report a significant breakthrough in the development of fiber-pumped high-power CW laser systems based on Cr²⁺:ZnS and Cr²⁺:ZnSe gain media. We demonstrate output power levels of up to 140 W near 2500 nm, and 32 W at 2940 nm with corresponding optical efficiencies of 62% and 29%. Our novel approach is based on rapid simultaneous scanning of the collinear laser mode and pump beam across the Cr:ZnS/Se gain element which allows us to virtually eliminate thermal lensing effects and obtain unprecedented levels of output power with very high optical-to-optical efficiency.

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We demonstrate an optical parametric oscillator (OPO) based on random phase matching in a polycrystalline χ (2) material, ZnSe. The subharmonic OPO utilized a 1.5-mm-long polished ZnSe ceramic sample placed at the Brewster's angle and was synchronously pumped by a Kerr-lens mode-locked Cr:ZnS laser with a central wavelength of 2.35 µm, a pulse duration of 62 fs, and a repetition frequency of 79 MHz. The OPO had a 90-mW pump threshold, and produced an ultrabroadband spectrum spanning 3-7.5 µm. The observed pump depletion was as high as 79%. The key to success in achieving the OPO action was choosing the average grain size of the ZnSe ceramic to be close to the coherence length (~ 100 µm) for our 3-wave interaction. This is the first OPO that uses random polycrystalline material with quadratic nonlinearity and the first OPO based on ZnSe. Very likely, random phase matching in ZnSe and similar random polycrystalline materials (ZnS, CdS, CdSe, GaP) represents a viable route for generating few-cycle pulses and multi-octave frequency combs, thanks to a very broadband nonlinear response.