Semiconductor saturable absorbers for ultrafast terahertz signals

We demonstrate saturable absorber behavior of n-type semiconductors GaAs, GaP, and Ge in theterahertz (THz) frequency range at room temperature using nonlinear THz spectroscopy. Thesaturation mechanism is based on a decrease in electron conductivity of semiconductors at highelectron momentum states, due to conduction band nonparabolicity and scattering into satellitevalleys in strong THz fields. Saturable absorber parameters, such as linear and nonsaturabletransmission, and saturation fluence, are extracted by fits to a classic saturable absorber model.Further, we observe THz pulse shortening, and an increase in the group refractive index of thesamples at higher THz pulse peak fields