In a proposed scheme for coupling light into a quantum-well infrared photodetector (QWIP), an antenna or an array of antennas made of a suitable metal would be fabricated on the face of what would otherwise be a standard QWIP (see figure). This or any such coupling scheme is required to effect polarization conversion: Light incident perpendicularly to the face is necessarily polarized in the plane of the face, whereas, as a matter of fundamental electrodynamics and related quantum selection rules, light must have a non-zero component of perpendicular polarization in order to be absorbed in the photodetection process. In a prior coupling scheme, gratings in the form of surface corrugations diffract normally incident light to oblique angles, thereby imparting some perpendicular polarization. Unfortunately, the corrugation- fabrication process increases the overall nonuniformity of a large QWIP array. The proposed scheme is an alternative to the use of surface corrugations.
For a given QWIP, the metal and the size and shape of the antenna would be chosen so that the combination of the antenna and the adjacent surface dielectric layer of the QWIP would support surface plasmon states at wavelengths of interest. The interface between a dielectric and a metal can support a surface electromagnetic wave if the permittivity of the metal, expressed as a complex number, has a negative real component. The distribution of amplitude in a plasmon peaks at the metal/dielectric interface and decays exponentially with distance from the interface into the metal or the dielectric.