Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors

TLDR

Using time‑domain spectroscopy, we measured far‑infrared absorption and dispersion from 0.2 to 2 THz in crystalline dielectrics (sapphire, quartz, fused silica) and semiconductors (silicon, GaAs, germanium). The measurements confirm earlier low‑frequency data for sapphire, quartz, and fused silica, reveal excess absorption in sapphire above 1 THz, show that high‑resistivity silicon is exceptionally transparent with a flat index, resolve weak GaAs features at 0.4 and 0.7 THz, and demonstrate that germanium’s absorption and dispersion follow Drude behavior due to intrinsic carriers.

Abstract

Using the method of time-domain spectroscopy, we measure the far-infrared absorption and dispersion from 0.2 to 2 THz of the crystalline dielectrics sapphire and quartz, fused silica, and the semiconductors silicon, gallium arsenide, and germanium. For sapphire and quartz, the measured absorptions are consistent with the earlier work below 0.5 THz. Above 1 THz we measure significantly more absorption for sapphire, while for quartz our values are in reasonable agreement with those of the previous work. Our results on high-purity fused silica are consistent with those on the most transparent fused silica measured to date. For the semiconductors, we show that many of the previous measurements on silicon were dominated by the effects of carriers due to impurities. For high-resistivity, 10-kΩ cm silicon, we measure a remarkable transparency together with an exceptionally nondispersive index of refraction. For GaAs our measurements extend the precision of the previous work, and we resolve two weak absorption features at 0.4 and 0.7 THz. Our measurements on germanium demonstrate the dominant role of intrinsic carriers; the measured absorption and dispersion are well fitted by the simple Drude theory.