Flare coupled metal parallel-plate waveguides for high resolution terahertz time-domain spectroscopy

Abstract

We report on a new coupling scheme for high resolution terahertz spectroscopy of microcrystalline films using parallel-plate waveguides. Metal flares are used to couple the terahertz radiation into and out of the waveguide. Very good coupling ratios as high as 35% at 1 THz from a collimated free-space beam into a subwavelength gap are obtained. This microwave approach is compared in terms of coupling ratio and spectral characteristics to the established technique of quasioptic coupling to parallel-plate waveguides using silicon lenses. Various samples at room and cryogenic temperatures are measured to show the capabilities of flare coupling for high resolution terahertz spectroscopy.

High Resolution Waveguide Terahertz Time-Domain Spectroscopy

“…Machined tapers [68] or metal flares [20] are used. The latter ones even offer a superior coupling ratio compared to the silicon lens optics and were already applied for high-resolution spectroscopy of polycrystalline molecular films [44].…”

“…In the waist of the collimated beam, the parallel-plate waveguides are measured between the two parabolic mirrors. Here a parallel-plate waveguide is shown with silicon lens coupling (reprinted with permission from [44]. © 2010 American Institute of Physics).…”

A Review on Thin-film Sensing with Terahertz Waves

“…Typical amplitude transmission through entire PPWG systems is around 20% normalized to that of a free-space TDS system. A more recent method of coupling free-space terahertz waves to the TEM mode via flared waveguides has improved this to 35% [65], mainly by eliminating Fresnel losses at the numerous Si lens boundaries. Such continued improvements can critically benefit sensing in low-power THz systems.…”

Terahertz sensing with waveguides

“…In this context, one-dimensional guiding of terahertz waves in parallel-plate waveguides for spatial confinement and enhancement of the electromagnetic fields has shown some promise. As a major advantage, parallelplate waveguides offer negligible dispersion, moderate loss as well as efficient coupling of free-space terahertz radiation to the TEM-like fundamental waveguide mode [1][2][3][4][5] . Yet, terahertz waves rapidly diverge in the unguided dimension which limits the maximally achievable terahertz field intensity.…”