Measurement of the Frequency-Dependent Conductivity in Sapphire

Electron transport of photoexcited single-crystal sapphire (α-Al₂O₃) is characterized by terahertz time-domain spectroscopy. The complex conductivity displays a Drude-type frequency dependence, which yields carrier scattering rates and densities. Carrier scattering is dominated by interactions with acoustical and optical phonons at low and high temperatures, respectively, and follows Matthiessen's law over the measured temperature range of 40--350 K. The results, including low-temperature mobilities >10 000 cm²/V s, are compatible with a large-polaron description of the conduction electrons.