Ultrafast, broadband and tunable terahertz reflector and neutral density filter based on high resistivity silicon

We report THz transmission and reflection properties of an ultrafast optically excited highly resistive silicon wafer. Amplified Ti:Sapphire femtosecond laser pulses at 800 nm were used to create fluence-dependent carrier density on the front surface of the wafer which modifies the dielectric properties at the THz frequencies. Time-resolved experiments in the optical pump-THz probe configuration were conducted in which THz pulses reflected off from the surface at 0 degrees and 45 degrees angles of incidence make it possible to measure the pump-fluence dependent ultrafast evolution of the reflection and transmission coefficients in 0.5-6 THz range. An analytical model, where both the Drude contributions from the photo-excited electrons and holes account for the change of the dielectric constant of the photo-excited silicon, has been used to evaluate the THz reflection and transmission coefficients at steady state. Thus obtained results match well with the experimental results and demonstrate an all-optical means to convert a silicon wafer into an ultrafast, tunable and broadband neutral density filter or reflector in the THz frequency range. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Automated summary

This study reports the THz transmission and reflection properties of an ultrafast optically excited highly resistive silicon wafer. By using amplified femtosecond laser pulses, the carrier density on the front surface of the wafer was modified, leading to changes in its dielectric properties at THz frequencies. The experimental results and analytical model demonstrate the feasibility of converting a silicon wafer into an ultrafast, tunable, and broadband neutral density filter or reflector in the THz frequency range.