We obtain changes in amplitude and phase of the THz electric field in the course of the reflection and/or transmission processes, and determine optical constants directly from them. This gives plentiful information about the characteristics of free carriers, such as their density, effective mass, and life time. As a state-of-art technique, we are capable of the easy and reliable reflectivity measurement with the diffraction-limited probing area.
When photo-carriers are excited by the laser pulse, their collective motion leads to the surge current, and the THz wave can be emitted. There are numerous mechanisms for such surge current, such as the mobility difference between electron and hole (photo-Dember effect), the carrier acceleration in the surface depletion region, the linear momentum transfer from the photon (photon Drag effect), inverse spin Hall effect, and so on. By considering each THz emission mechanism, we therefore can study ultrafast photo-carrier dynamics in semiconductors, van der Waals materials, topological insulators, and so on.