A novel way to control the amplitude and phase of terahertz (THz) emitters has now been demonstrated by Francesco Mezzapesa and co-workers from Italy. A quantum cascade laser (QCL) emitting at 3.93 THz served as a THz emitter. Patterned near-infrared laser light, made by passing light from an 832-nm laser through a spatial light modulator, was shone onto a 1-mm-thick n-type silicon slab. The patterned laser light induced a photocarrier distribution in the silicon altering its permittivity. This spatial variation in permittivity can be used modify the amplitude and phase of a THz beam that is shone onto the silicon from a nearby THz QCL and subsequently reflected. The approach may lead to the creation of phase modulators, switches and active hyperbolic media that operate in the THz spectral regime.