Lifetime Measurement of Excitons in Si by Terahertz Time-domain Spectroscopy with High Spectral Resolution

We have developed an optical pump and terahertz (THz) probe spectroscopy scheme to study the photoexcited dynamics in solids ranging from sub-microsecond to a millisecond regime. We applied the developed scheme to measure the lifetime of long-lived indirect excitons in Si through the observation of intra-exciton transitions, resolving the fine structure of excitons with high spectral resolution in a spectral range from 0.5 to 7 THz (2 to 29 meV). We also performed the lifetime measurement of the lowest energy spin-forbidden dark excitons under the magnetic field. Through the observation of intra-exciton transitions, otherwise inaccessible spin-forbidden dark excitons were directly probed by the THz time-domain spectroscopy. By comparing with the photoluminescence spectroscopy, we revealed that the lowest energy dark excitons are accumulated in the crystal, whereas the recombination dynamics is governed by the nonradiative decay process.