Interest in nonlinear optical (NLO) materials for mid-infrared frequency conversion has exploded in recent years due to the emergence of new laser applications ranging from frequency-comb-based spectroscopy to high harmonic generation. NLO semiconductors such as ZnGeP2 (ZGP) and orientation-patterned GaAs (OP-GaAs) offer transparency deep into the mid-infrared and THz with d-coefficients 5-10 times higher than those of oxides. ZGP and OP-GaAs must be pumped at wavelengths ≥ 1.9 mm, but new analogs for these materials – cadmium silicon phosphide (CdSiP2, or CSP) and orientation-patterned GaP (OP-GaP) – can be pumped with widely-available 1-mm and 1.5-mm lasers.
ZnGeP2 remains the NLO material of choice for frequency conversion between 2 and 8 microns. Continued efforts to improve ZGP crystal quality and aperture size for high-energy laser applications are ongoing, enabling record-level peak- and average-output powers in the mid-IR. ZGP, however, still has two main limitations: 1) its transparency and phase-matching range make it incompatible with 1- and 1.5-micron laser pumping; and 2) its usefulness for generating output in the 8-12 micron atmospheric window is limited by severe multi-phonon absorption.
CdSiP2 (CSP) is a new bulk birefringent chalcopyrite analog of ZGP grown by horizontal gradient freeze growth in a transparent furnace. Its larger band gap (512 nm) and birefringence (-0.05) allows for 1- and 1.5-mm pumping, and its nonlinear coefficient (d14=85 pm/V) and thermal conductivity (13 W/mK) are dramatically higher than existing materials (AgGaS2, AgGaSe2, and PPLN) that can be pumped at these wavelengths. In addition, CSP's lower absorption losses make it an attractive alternative to ZGP for power-scaling 2-micron-based devices.
OP-GaAs and OP-GaP are quasi-phasematched (QPM) NLO semiconductors grown by all-epitaxial processing : first, polar-on-nonpolar MBE produces GaAs (GaP) with an inverted orientation inverted with respect to the substrate which is photo-lithographically patterned, etched with the desired grating structure, and re-grown by hydride vapor phase epitaxy (HVPE) at rates up to 200 mm/hr to produce thick (> 1mm), low-loss (< 0.01cm-1) QPM layers for in-plane laser pumping. OP-GaAs has the highest gain among all QPM materials (d14 = 94 pm/V), and can be pumped at 2-mm to generate 8-12 mm output and beyond, whereas OP-GaP is a low-loss QPM ZGP analog than can be pumped with 1-mm lasers. Numerous OP-GaP device demonstrations have been achieved based on 1-, 1.5- and 2-mm laser pumping, and highly parallel grating propagation during growth promises to extend aperture sizes well beyond 1 mm.
