Fresnel reflection in optical components and detectors for the terahertz frequency band

Abstract

Plane-parallel dielectric and semiconductor windows, beam splitters and radiation absorbers, as well as reflecting metal surfaces, are important components used in experiments with monochromatic terahertz radiation. The wavelengths of this radiation are longer than the wavelengths of visible light; therefore, multibeam interference becomes important in relatively transparent components with a thickness of the order of 1 mm, which is typical for optical systems. This interference causes the characteristics of optical components and detectors to be dependent on the wavelength. It has been found that bolometric power meters with a crystal used as an absorber of terahertz radiation and pyroelectric detectors with a germanium entrance window demonstrate individual periodic variations in sensitivity under variations in the wavelength. Basic characteristics of the windows and beam splitters manufactured from germanium, polypropylene, and Dacron (the materials that are of the greatest interest for terahertz-band spectroscopy) are calculated in the wave band 30–300 µm. The phase shift of s- and p-polarized terahertz waves reflected from metal surfaces are calculated as functions of the wavelength and the angle of incidence. Using the Fresenel formulas for processing experimental data on the reflection coefficient of terahertz radiation incident onto a K8 glass specimen, it is shown that, in the terahertz frequency band, this coefficient is equal to 16% at normal incidence and the absorption coefficient is equal to 860 cm−1.