Ultrafast, broadband and tunable THz reflector based on high resistivity silicon

The evolution of the THz reflectivity of a high resistivity silicon wafer upon excitation by a femtosecond pulse centered at 800 nm is studied. Upon optical excitation, the THz reflectivity of silicon is increased within less than 1 ps and it can be tailored even beyond 90 % over a broad THz frequency range. Such variation is well accounted for by the pump pulse induced carriers within a thin layer on the surface of silicon.