Relativistic Doppler reflection of terahertz light from a moving plasma front in an optically pumped Si wafer

Abstract

The relativistic Doppler reflection of terahertz (THz) light from a photoinduced moving plasma front in a silicon wafer irradiated by an optical pump light was investigated. The intensity and the phase shift spectra of the reflected THz light were investigated by THz time-domain spectroscopy with broadband electro-optic sampling in a thin GaP crystal. The frequency up-shift factor, which is a ratio of averaged frequencies from after to before Doppler reflection, was measured to be $\mathrm{\ensuremath{\Gamma}}=2.02$ with a pump energy density of $3.3\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{J}/\mathrm{m}{\mathrm{m}}^{2}$ at a wavelength of 800 nm. The frequency up-shift increased as optical pump energy density increased. The frequency up-shift was reproduced by evaluating the moving plasma front whose spatial carrier density distribution depended on the pump energy density.