Holographic focusing elements for far-IR radiation

The authors present a method for designing and recording computer-generated holographic optical elements (HOES) for far-infrared radiation. The design method is based on minimising the mean-squared difference of the propagation vectors between the actual output wavefronts and the desired output wavefronts. This minimisation yields an analytic solution for the optimal grating vector. The design method is illustrated by recording a reflective off-axis focusing element with a laser scanner and lithographic techniques. The element is then tested, and the results indicate that diffraction-limited performance for a relatively large range of incidence angles can be obtained.