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.

We report a subharmonic (frequency-divide-by-2) optical parametric oscillator (OPO) with a continuous wavelength span of 3 to 12 µm ( − 37 d B level) that covers most of the molecular rovibrational “signature” region. The key to obtaining such a wide spectral span is the use of an OPO with a minimal dispersion—through the choice of intracavity elements, the use of all gold-coated mirrors, and a special “injector” mirror. The system delivers up to 245 mW of the average power with the conversion efficiency exceeding 20% from a 2.35 µm Kerr-lens mode-locked pump laser.

Second-order nonlinear interactions in disordered materials based on random phase matching suggest intriguing opportunities for extremely broadband frequency conversion. Here we present a quantitative realistic model for random phase matching in zinc-blende polycrystals (ZnSe, ZnS, GaAs, GaP, etc.) that takes into account effects of random crystal orientation, grain size fluctuations, and includes polarization analysis of the generated output. Our simulations are based on rigorous transformation of the second-order susceptibility tensor in randomly rotated coordinates − to account for random orientation of crystalline domains, and demonstrate a good agreement with our experimental data for ZnSe using a nanosecond λ = 4.7 µm source − in terms of variations of the strength and polarizations of the output fields. Also, it is revealed that random phase matching is most suitable for ultrafast (sub-100-fs) interactions with focused beams, e.g. second harmonic generation, sum and difference frequency generation, and optical parametric oscillation, that typically require short (< 1 mm) interaction lengths, where disordered polycrystals can be on par, in terms of conversion yield, with ideal quasi phase matched crystals.

Background-free methods have potentially superior detection sensitivity because of their ability to take advantage of the full laser power; they are therefore attractive to spectroscopists. We implement background-free Fourier transform spectroscopy based on coherent suppression of the background using an interferometer, whereby the central peak of the interferogram is suppressed without losing molecular absorption signatures. This results in the appearance of peaks rather than dips in the measured spectrum. The technique can be used with a variety of broadband spectroscopies and features advantages such as a reduction in the required detector dynamic range, the capability to perform quantitative measurements, and strongly enhanced sensitivity down to the quantum limit. We validated our method experimentally by performing mid-infrared dual-comb spectroscopy with a mixture of multiple molecular species over a broad wavelength range of 3-5 μm.

We demonstrated two liquid crystal diffractive waveplates: one optimized for near-infrared (1.06 µm), and another for mid-wave infrared (MWIR, 3~5 µm). By employing a low loss liquid crystal mixture UCF-M3, whose absorption loss is below 2% in the 4~5 µm spectral region, the grating achieves over 98% diffraction efficiency in a broad MWIR range. To switch the grating, both active and passive driving methods can be considered. In our experiment, we used a polymer-stabilized twisted nematic cell as the polarization rotator for passive driving. The obtained rise time is 0.2 ms and decay time is 10 ms.