Total internal reflection-based extractor and collimator of terahertz radiation

An optical element for extracting terahertz radiation from the semiconductor substrate of a photoconductive antenna and its further collimation into a parallel beam is designed and modeled. The element is a truncated silicon paraboloid of a ∼1-2<i>c</i><i>m</i> length. The radiation is totally internally reflected by the paraboloid's convex surface and escapes the paraboloid through its wider cross-section as a beam of a ∼5-7<i>m</i><i>m</i> diameter with a flat wavefront. Geometrical optics calculations and finite-difference time-domain modeling show that the efficiency of the terahertz energy transmission through the paraboloid to free space can be as high as ≈70<i>%</i>, with ≈90<i>%</i> of the energy in the collimated beam.