We report the development of terahertz intersubband photodetectors based on GaN/AlGaN quantum wells, covering the frequency range that is fundamentally inaccessible to existing III-V semiconductor devices due to Reststrahlen absorption. Two different approaches have been employed to mitigate the deleterious effects of the intrinsic polarization fields of nitride heterostructures: the use of suitably designed double-step quantum wells, and epitaxial growth on semipolar GaN substrates. Promising results are obtained with both approaches, which could be extended to other device applications as a way to utilize the intrinsic advantages of nitride semiconductors for THz intersubband optoelectronics.
A procedure is presented for the design of bilayer pellicle and bilayer-coated prism beam splitters that serve as the key optical element of the division-of-amplitude photopolarimeter (DOAP). The bilayer consists of two transparent thin films of sufficiently different refractive indices whose thicknesses are selected to achieve 50 to 50% split ratio and optimum ellipsometric parameters in reflection and transmission, such that the determinant of the DOAP instrument matrix is maximized at the proper angle of incidence. As a specific example, we present a Ge-coated nitrocellulose (NC) first-order pellicle beamsplitter for the near IR (2.0 to 2.2 µm). Visible designs include a GaP-coated NC fourth-order pellicle and a SiO2-GaP bilayer-coated glass prism at and near the 633-nm wavelength. Operation of these beamsplitters over a range of incidence angles, wavelengths, and in the presence of ±5% film thickness errors is also considered.