High-efficiency subwavelength diffractive optical element in GaAs for 975 nm

We have fabricated subwavelength diffractive optical elements with binary phase profiles for operation at 975 nm. The individual surface-relief features of the elements are smaller than the wavelength of light in the material. By modulating the size and spacing of the features we form artificial, gradient, effective index-of-refraction surfaces. The blazed transmission gratings were designed with rigorous coupled-wave analysis and fabricated by direct-write electron-beam lithography and reactive ion-beam etching in GaAs. The gratings have minimum features 63 nm wide. Transmission measurements show 85% diffraction efficiency into the first order.