Microstrip antenna coupling for quantum-well infrared photodetectors

We computed the expected performance of microstrip antennas as optical coup lers in quantum-well infrared photodetectors (QWIPs) by the finite-differen ce time-domain technique. The microstrip antenna consisted of a metal squar e that is placed on one side of a thin QW stack, across from a continuous g round plane on the other side of the stack. The size of the square was one- half wavelength in the QW material, or similar to1.2 mum for a long-wavelen gth GaAs/AlGaAs QWIP. An array of these antennas can be used to couple to a n arbitrary-size pixel. Unlike the elements of a diffraction grating, each antenna in the array acts as a nearly independent coupler whose peak wavele ngth is determined by the dimensions of the antenna, not by the period of t he antenna array. Because of this local coupling, the array of microstrip a ntennas maintains absorption in small pixels much more effectively than do grating couplers. Furthermore, the microstrip antenna yields moderately hig h absorption over a useful spectral bandwidth with a very thin QW stack. Su ch a stack has a high photoconductive gain, which is useful for many applic ations. (C) 2001 Elsevier Science B.V. All rights reserved.