Orientation-patterned GaAs (OPGaAs) films of 200 μm thickness have been grown by hydride vapor phase epitaxy (HVPE) on an orientation-patterned template fabricated by molecular beam epitaxy (MBE). Fabrication of the templates utilized only MBE and chemical etching, taking advantage of GaAs/Ge/GaAs heteroepitaxy to control the crystal orientation of the top GaAs film relative to the substrate. Antiphase domain boundaries were observed to propagate vertically under HVPE growth conditions so that the domain duty cycle was preserved through the thick GaAs for all domain periods attempted. Quasiphase-matched frequency doubling of a CO2 laser was demonstrated with the beam confocally focused through a 4.6 mm long OPGaAs film.

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The development of quasiphasematching (QPM) techniques in ferroelectric materials such as lithium niobate has made possible high-efficiency frequency conversion devices such as optical parametric oscillators (OPO) which take advantage of large nonlinear susceptibilities and operate in wavelength ranges previously unphasematchable.' Unfortunately these ferroelectric materials are only transparent out to 4-5 pm and so have limited potential for generation of mid-IR radiation from near-IR pump lasers. GaAs has many excellent characteristics for mid-IR frequency conversion including a large nonlinear susceptibility, transparency from 1 pm to 12 pm, and a large thermal conductivity, but some quasiphasematching technique must be employed since it has no birefringence. Wafer-bonding techniques have been used successfully to fabricate stacks of GaAs plates for frequency conversion applications? However, since this approach requires polishing GaAs plates to precise thickness and stacking them serially, it scales poorly for long crystal lengths and for the very short QPM periods (1 Os of pm) required for frequency conversion from the near-IR to the mid-IR. We have developed an alternative approach utilizing all-epitaxial processes which is capable of producing both long devices and short periods with great flexibility since the domain patterns are set using photolithography. Compared with previous orientationpattemed growth techniques, all wafer bonding is eliminated.' We have demonstrated orientation-pattemed GaAs films with domain periods short enough for phasematching near-IR to mid-IR frequency conversion and with sufficiently thick apertures for bulk-focused nonlinear optical interactions. photolithography and GaAs substrate i Figure 1 : fabrication process for orientation-pattemed GaAs. +/-indicates GaAs phase and sign of susceptibilityThe orientation-patterned GaAs fabrication process illustrated in figure 1 relies on an orientation template produced in two steps. First, we use polar-on-nonpolar GaAs/Ge/GaAs molecular beam epitaxy (MBE) to grow a layer of GaAs with inverted polarity (relative to the substrate) on top of a nonpolar Ge buffer4 This structure becomes an orientation template with a combination of photolithography and wet chemical etching down through the Ge layer to expose the original GaAs polarity across some regions of the surface. The template then undergoes

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