Microstrip antenna coupling for quantum-well infrared photodetectors

Abstract

We computed the expected performance of microstrip antennas as optical couplers in quantum-well infrared photodetectors (QWIPs) by the finite-difference time-domain technique. The microstrip antenna consisted of a metal square that is placed on one side of a thin QW stack, across from a continuous ground plane on the other side of the stack. The size of the square was one-half wavelength in the QW material, or ∼1.2 μm for a long-wavelength GaAs/AlGaAs QWIP. An array of these antennas can be used to couple to an arbitrary-size pixel. Unlike the elements of a diffraction grating, each antenna in the array acts as a nearly independent coupler whose peak wavelength is determined by the dimensions of the antenna, not by the period of the antenna array. Because of this local coupling, the array of microstrip antennas maintains absorption in small pixels much more effectively than do grating couplers. Furthermore, the microstrip antenna yields moderately high absorption over a useful spectral bandwidth with a very thin QW stack. Such a stack has a high photoconductive gain, which is useful for many applications.