Subwavelength, binary lenses at infrared wavelengths

We describe the nanofabrication of subwavelength, binary lenses in GaAs for operation in the infrared. Subwavelength surface relief structures create an artificial material with an effective index of refraction determined by the fill factor of the binary pattern and can be designed to yield high-efficiency diffractive optical elements. In this work, we designed and fabricated a circular-aperture, off-axis lens with a deflection angle of 20°, focal length of 110 μm, and diameter of 80 μm, for operation at 975 nm. The off-axis lens design has a theoretical efficiency of 92% and the fabricated lens exhibits a diffraction efficiency into the first order of 72% and 59% of the transmitted power for TE and TM polarization, respectively. A significant advantage of these subwavelength structures is that fabrication requires only a single-lithography-and-etch-step process, in this case, electron-beam lithography and reactive-ion-beam etching.