Nonlinear materials for the millimeter spectral region

Nonlinear materials offer great promise for use in devices such as phase shifters, modulators, switches, and frequency multipliers at frequencies above 50 GHz. State-of-the-art phase shifters (using ferrites) and power sources are inefficient at high frequencies. By contrast, the efficiencies for electro-optic phase shifting and harmonic generation increase with frequency, and devices based on these effects offer high-speed, low-loss operation over a wide spectral range. Ferroelectrics are particularly promising for nonlinear and electro-optic applications at millimeter wavelengths, because their large linear polarizability leads to large electro-optic and nonlinear coefficients. The major challenge in developing ferroelectric materials for these applications is reduction of absorption losses. In this work we have evaluated and measured both intrinsic and extrinsic contributions in our samples, and have shown that the intrinsic factors dominate. Perovskites were identified as the most promising class of materials, including BaTiO3, PbTiO3, KNbO3, and KTa1-xNbxO3(KTN). The material we chose to investigate is BaTiO3, with a few mole percent SrTiO3 added to stabilize the high temperature cubic phase. The resulting material, Ba1-xSrxTiO3 or BST, has properties which differ only slightly from BaTiO3.