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Thick hydride vapor phase epitaxially grown orientation-patterned gallium phosphide (OPGaP) is a leading material for quasi-phase matching (QPM) frequency conversion in the mid- and longwave infrared (IR). This is due to its negligible two-photon absorption (2PA) in the convenient pumping range 1 – 1.7 μm, compared with the 2PA of some traditional QPM materials, such as GaAs. In this paper, we describe homo- and heteroepitaxial growth techniques aimed to produce hundreds of microns thick OPGaP on: 1) OPGaAs templates fabricated using an improved wafer-fusion process; 2) OPGaAs templates fabricated by using a molecular beam epitaxy (MBE) for sublattice polarity inversion, but one with and one without MBE regrowth after the inversion. Some of the advantages of the heteroepitaxial growth of OPGaP on OPGaAs templates include: 1) achieving good domain fidelity as a result of the significantly higher OPGaAs template quality; 2) eliminating the needs of using the poor quality commercially available GaP in the production of thick OPGaP material, and 3) suppression of the additional absorption band between 2 – 4 μm (which is due to incorporation of n-type impurities) and, in general, improvement of the IR transmittance in the entire IR region. Combining the advantages of the two most promising nonlinear materials, GaAs and GaP, will accelerate the development of high power, broadly tunable laser sources in the IR which, in addition, will be offered with higher device quality and at a reasonably lower unit cost.

Optical parametric oscillators (OPOs) producing longwave output from a much shorter pump wavelength suffer from low conversion efficiency into the idler due to the large quantum defect compared with similar devices operating in the 3 – 5 μm regime. One method to increase pump to idler conversion efficiency is to recycle the undesired and higher energy signal photons into additional idler photons in a second nonlinear stage. We present numerical simulation results showing the improvement in efficiency that can be obtained in a linear, two stage, cascaded orientation patterned gallium arsenide (OPGaAs) nanosecond OPO. It includes diffraction, crystal loss, phase mismatch, pump depletion, and back conversion; and it assumes monochromatic waves but it neglects group velocity dispersion. For a singly resonant oscillator (SRO) pumped by a 2.054 μm Tm:Ho,YLF laser with 45 ns pulse widths, the addition of the second crystal in the cavity increases idler generation by overall factor of two and exceeds the quantum defect limit. The model has been validated by comparison with SNLO for the case of a single-stage OPO, and suggests crystal and resonator parameters that will lead to an optimized cascaded OPO.

A series of nonlinear materials including GaAs, GaP, and ZnSe have been examined to determine their suitability for non-linear frequency conversion devices (FCD) and more specifically their use as high power, compact and broadly tunable IR and THz sources for defense and security applications. The more mature GaAs was investigated to reveal the causes for the optical losses that restrict achievement of higher conversion efficiency in quasi-phasematched FCD, while the efforts with GaP were oriented in developing simple, cost effective techniques for fabrication of orientation patterned (OP) templates and optimizing the subsequent thick HVPE growth on these templates. Thus, average growth rates of 50- 70 μm/h were achieved in up to 8-hour long experiments. High optical layer quality was achieved by suitable control of the process parameters. The optimal orientation of the pattern was determined and used as essential feedback aiming to improve the template preparation. This led to the production of the first 300-400 μm thick device quality OPGaP. Efforts to suppress the parasitic nucleation during growths with longer duration or to achieve thicker layers by a 2 step growth process were also made. The main challenge with the newer candidate, OPZnSe, was to establish suitable regimes for hydrothermal growth on plain (001) ZnSe seeds grown by chemical vapor deposition. Two different temperature ranges, 330-350 °C and 290-330 °C, were investigated. The mineralized concentration was also manipulated to accelerate the growth in (111) direction and, thus, to improve the growth in (001) direction. The next material in the line is GaN. The traditional HVPE approach will be combined with a growth at low reactor pressure. Growths will be performed in the next sequence: growth on thin GaN layers grown by MOCVD on sapphire wafers, growth on half-patterned GaN templates with different orientations and, finally, growth on OPGaN templates.

The U.S. Air Force is developing orientation-patterned GaAs (OPGaAs) for nonlinear frequency conversion in the 2-5 μm and 8-12 μm regions. We report recent progress in OPO device performance which reflects continued improvement in material quality. Seven new OPGaAs samples, representing four distinct growth regimes, were evaluated in terms of threshold, slope efficiency, and output spectral content using a Q-switched Tm,Ho:YLF laser operating at 2 μm as the pump source. The samples were identical in base template, grating period, length, and AR coating, permitting direct comparison of results to identify favorable growth conditions. As anticipated, performance varied significantly among the sample set, with slope efficiencies from 12% to 35% and thresholds from 9 μJ to 40 μJ. Less anticipated was the low level of uniformity across each sample and between samples grown in the same growth run. Significant variations in slope efficiency and threshold were not uncommon. Still, most of the samples performed manifestly better than previously grown material, indicating an overall improvement in OPGaAs quality. Research continues on understanding growth processes, optical loss mechanisms, and how these translate into device performance.