  We employ a silicon dielectric waveguide to confine and concentrate terahertz
pulses, and observe that the absorption saturates under strong terahertz
fields. By comparing the response between lightly-doped and intrinsic silicon
waveguides, we confirm the role of hot carriers in this saturable absorption.
We introduce a nonlinear dynamical model of Drude conductivity that, when
incorporated into a wave propagation equation, accurately reproduces the
observations and elucidates the physical mechanisms underlying this nonlinear
effect. The results are numerically confirmed by Monte Carlo simulations of the
Boltzmann transport equation, coupled with split-step nonlinear wave
propagation.
