Small-feature-size fan-out kinoform etched in GaAs

A binary fan-out kinoform for focusing and splitting an incident beam into a 4 × 4 spot array, with a largest deflection angle of 28°, was designed, fabricated in GaAs, and evaluated. The kinoform was defined in resist with electron-beam lithography and etched into GaAs with chemically assisted ion-beam etching. Light at wavelength 0.98 µm from a single-mode fiber was used to illuminate the kinoform. The efficiency was measured to be 34%, and the uniformity error for the 4×4 spots was 29%. Although the typical feature size of the kinoform is only roughly two wavelengths, we found that the scalar theory of diffraction can be used. A first kinoform was designed with the customary Fresnel-diffraction theory, which was found to be too coarse, resulting in a fan-out exhibiting some distortion. A second kinoform was designed with the more rigorous Fresnel-Kirchhoff expression, and its fan-out shows no distortion.