Time-resolved terahertz spectroscopy in a parallel-plate waveguide

Time-resolved THz spectroscopy is a powerful tool to investigate photoconductivity dynamics in a wide variety of materials with sub-picosecond resolution, all without applying contacts to the material. This technique uses coherently detected and broadband pulses of far-infrared light, known as THz pulses, as a probe pulse following photoexcitation of a material with a femtosecond optical pulse. In the usual geometry, the THz pulse probes the sample in free space by normal incidence transmission. We have devised a new technique for performing these measurements on materials embedded inside a parallel plate waveguide, termed time-resolved THz waveguide spectroscopy. The interaction of the THz radiation with the sample is then enhanced by travelling along the material over lengths of cm's instead of through it where the length can be nm's. We highlight the performance of this technique by examining photocarriers induced in Si and show a spin-off technique where we can use the photocarriers to control the propagation of the THz pulse within the waveguide.