Numerical Study on Metal Cavity Couplers for Terahertz Quantum-Well Photodetectors

Metal cavities composed of metal gratings, layered semiconductor structures, and bottom metal layers are proposed as light couplers for terahertz quantum-well photodetectors (QWPs). Due to a strong waveguide effect, the coupling properties of the metal cavities are significantly changed in comparison with the case of metal-grating-coupled terahertz QWPs. Numerical calculations show that there are two resonant coupling peaks for the sub-wavelength metal cavity couplers, with one located at the lower side of the first grating diffraction mode and the other at the upper side. A simple ray propagation picture explains the behaviors of the two resonant coupling peaks on a qualitative level. Our numerical data show that the coupling efficiencies of metal cavity couplers are several tens of times higher than that of the 45-degree facet coupling scheme.