  Relativistically moving dielectric perturbations can be used to manipulate
light in new and exciting ways beyond the capabilities of traditional nonlinear
optics. Adiabatic interaction with the moving front modulates the wave
simultaneously in both space and time, and manifests a front-induced transition
in both wave vector and frequency yielding exotic effects including
non-reciprocity and time-reversal. Here, we introduce a technique called
SLIPSTREAM, Spacetime Light-Induced Photonic STRucturEs for Advanced
Manipulation. The technique is based on the creation of relativistic fronts in
a semiconductor-filled planar waveguide by photoexcitation of mobile charge
carriers. Here we demonstrate the capabilities of SLIPSTREAM for novel
manipulation of THz light pulses through relativistic front-induced
transitions. In the sub-luminal front velocity regime, we generate temporally
stretched THz waveforms, with a quasi-static field lasting for several
picoseconds tunable with the front interaction distance. In the super-luminal
regime, the carrier front outpaces the THz pulse and a time-reversal operation
is performed via a front-induced intra-band transition. We anticipate our
platform will be a versatile tool for future applications in the THz spectral
band requiring direct and advanced control of light at the sub-cycle level.
