Nonlinear optical frequency conversion is an important technique for generation of mid-IR radiation from well-developed near-IR pumps like diode lasers and diode-pumped solid-state lasers. Zincblende semiconductor materials have excellent characteristics for optical frequency conversion including large nonlinearities and transparency through the mid-IR, but the absence of birefringence for phasematching has limited their application. Quasiphasematching provides an alternative solution to this phasematching problem, but requires techniques for inducing a modulation in the nonlinear susceptibility along the crystal. In this work we have developed all-epitaxial fabrication techniques for orientation-patterned semiconductor materials and have demonstrated that the resulting devices show excellent promise for mid-IR frequency conversion.
The two step fabrication technique requires making a template with the proper orientation pattern, which then serves as a substrate for orientation-patterned epitaxial growth. The template fabrication process utilizes polar-on-nonpolar molecular beam epitaxy (MBE) of a GaAs/Ge/GaAs heterostructure to achieve an epitaxial crystal inversion, after which the template is completed by a combination of photolithography and selective chemical etching. Orientation-patterned AlxGa1-xAs films grown on these templates exhibit ideal vertical antiphase domain propagation. Periods were demonstrated sufficient for quasiphasematching any collinear frequency mixing interaction in the transparency range of AlxGa1-xAs. Orientation-patterned AlxGa1-xAs waveguiding structures were fabricated and utilized for harmonic generation of 1.55 um laser radiation. The conversion efficiencies in these waveguide devices were limited by high modal propagation losses at both fundamental and harmonic wavelengths.
Thin GaAs orientation-patterned films were also used as templates for growth of much thicker (100s of um) GaAs films by the hydride vapor phase epitaxy (HVPE) technique for bulk- focused frequency conversion. The antiphase boundaries were again observed to propagate vertically through the GaAs film with aspect ratios 15:1. Periods sufficient to quasiphasematch para- metric amplification using pump lasers of wavelength longer than 1.5 um were demonstrated and excellent optical transmission was observed at both near-IR and mid-IR wavelengths. Frequency doubling of confocally-focused CO 2 laser radiation was performed in such an orientation-patterned film with efficiency close to theoretical.