Fabrication of Diffractive GaAs Microlenses

A planar array of microscopic diffractive optical elements that resemble macroscopic Fresnel lenses can be fabricated as an array of continuous relief patterns on a GaAs substrate by a procedure that includes the transfer of a corresponding array of patterns formed on a surface layer of poly(methyl methacrylate) (PMMA) by electron-beam lithography. The diffractive optical elements could be, for example, microlenses on back-side-illuminated imaging arrays of GaAs-based quantum-well infrared photodetectors (these microlenses are used to concentrate incident infrared light into sub-pixel-size active device areas). The fabrication procedure is not limited to arrays of microlenses; it can also be adapted to making other optical elements (for example, holograms or diffractive macrolenses) in GaAs.