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Peter Schunemann: Optical materials - an industry perspective

Interest in nonlinear optical (NLO) materials for mid-infrared frequency conversion has exploded in recent years due to the emergence of new ultrafast laser applications ranging from infrared countermeasures, multi-photon microscopy, frequency-combbased spectroscopy for breath analysis, and THz generation for semiconductor inspection. Well-established solid-state lasers operate at just a handful of discrete wavelengths. In order to meet the demands of countless applications across the electromagnetic spectrum, nonlinear optical crystals are needed to shift the laser output wavelength into the region of interest. 

This lecture will introduce some of the basic requirements of NLO frequency conversion in crystals, summarize the state-of-the-art of widely used materials in the ultraviolet and visible, and then focus on the development of new, highly-efficient crystals for infrared generation to address the application-rich “molecular fingerprint region” extending from 6-20 microns. These new materials include zinc germanium phosphide - ZnGeP2 (ZGP), cadmium silicon phosphide - CdSiP2 (CSP), orientationpatterned gallium arsenide - OP-GaAs, and orientation-patterned gallium phosphide - OP-GaP. 

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 peakand average-output powers in the mid-IR. ZGP, however, still has two main limitations: 
1) its transparency and phasematching 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-um 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 200um/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-um to generate 8-12 um output and beyond, whereas OP-GaP is a low-loss QPM ZGP analog than can be pumped with 1-um lasers. Numerous OP-GaP device demonstrations have been achieved based on 1-, 1.5- and 2-um laser pumping, and highly parallel grating propagation during growth promises to extend aperture sizes well beyond 1 mm.