Laser-generated broadband antireflection structures for freeform silicon lenses at terahertz frequencies

We present a new flexible technology to generate broadband antireflection structures for the THz frequency range on planar and curved surfaces of silicon optics. Ultrashort laser pulses are used to ablate the surface in order to form subwavelength structures. Such structures effectively provide a gradual transition of the refractive index from air to silicon. We demonstrate antireflection structures on convex silicon lenses of both photoconductive emitter and detector devices. Thus, the THz pulse amplitude can be increased by about 28% and the spectral amplitude of single frequencies up to 58% in the range from 0.1 to 1.5 THz.