The performance of periodically poled lithium niobate (PPLN), along with that of other nonlinear optical materials such as periodically poled potassium titanyl phosphate (PPKTP) that rely on alternating the orientation of ferroelectric domains to maintain quasi-phase matching (QPM), drops off in the midinfrared (IR) beyond 4 μm due to multiphonon absorption. Researchers at the US Naval Research Laboratory (Washington, DC) are creating useful mid-IR nonlinear optical devices by taking another approach—alternating the actual crystal orientation of the semiconducting nonlinear material—to create potentially useful structures from gallium nitride (GaN) grown by hydride vapor phase epitaxy (HVPE).
Gallium nitride has a transparency window of 0.36 to 7 μm, a very high thermal conductivity of 220 W/m°C that eases heat removal, a second-order nonlinear susceptibility with similar magnitude to that of PPLN, and—when grown by HVPE—low dislocation densities on the order of 106 to 107 cm-2. The researchers grow a layer of aluminum nitride in stripes on an N-polar GaN substrate, then simultaneously grow GaN in polarities that alternate between the stripe and stripe-free regions. Thicknesses of up to about 80 μm have been grown this way; the aim is to grow even thicker structures for practical use, as well as to further improve material quality. Contact Jennifer Hite at [email protected].
