Study of THz surface waves (TSW) on bare and coated metal surface

Abstract

Scope and Method of Study: The focus of the research was to investigate the propagation characteristics (such as field distribution, attenuation and group velocity) of terahertz surface waves on bare and dielectric coated metal surface. The experiment was carried out on a modified standard terahertz time-domain spectroscopy system. Surface waves were coupled into the metal surface using the parallel plate waveguide coupling mechanism. The picosecond terahertz pulses were generated and detected using the Grischkowsky photo-conductive transmitter and antenna driven by a femtosecond laser.

Findings and Conclusions: Surface waves at microwave and terahertz frequencies are weakly guided on bare metal surface due to the high metal conductivity. Detailed wave coupling analysis and experiment has shown that on a bare metal surface, the majority of energy remains to be uncoupled freely propagating waves. The spatial extent of the terahertz surface wave collapses two orders of magnitude upon the addition of the sub-wavelength dielectric layer on the metal surface. Simple theory in terahertz range gives an accurate explanation to this effect. Direct experimental measurements of the terahertz surface wave on an aluminum surface covered with a 12.5 Mm thick dielectric layer have completely characterized the wave. The measurements of the frequency-dependent exponential fall-off of the evanescent wave from the surface agree well with theory.