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.

A Surface Plasmon with elliptical polarization would form under the antenna. The perpendicular component of the elliptical polarization would enable absorption of the incident light.

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.

In cases relevant to the proposal, the polarization states of the electric field are elliptical, characterized by major axes parallel to the interface on the metal side and perpendicular to the interface in the dielectric side. The contribution of the surface plasmon effect to perpendicular polarization would be augmented by the contribution of strong perpendicular-polarization components of the near field of the antenna. Presumably, designs could also be optimized to obtain resonant or broadband antenna structures to maximize coupling of light from free space into perpendicularly polarized plasmon modes.

This work was done by John Hong of Caltech for NASA’s Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

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Refer to NPO-43681, volume and number of this NASA Tech Briefs issue, and the page number.