Terahertz Time-domain Spectroscopy of Ultra-high Reflectance Photonic Crystal Mirrors

Unprecedented ultrahigh-reflectance (R > 0.9995 and root R > 0.9998) in a one-dimensional photonic crystal terahertz supermirror was experimentally demonstrated. The terahertz supermirror had three air gaps and four high-resistivity float-zone silicon layers, prepared by a high-precision chemical and mechanical polishing machine. The reflectance spectra, measured by terahertz time-domain spectroscopy, showed that the terahertz supermirror has a higher peak reflectance than metallic mirrors, such as Cu, An, and Al mirrors, over a wide frequency range. The effects of material absorption, surface roughness, and thickness variation on the reflectance were also analyzed.