Photo-induced variation of dielectric constant of silicon in the far infrared: applications to light-controllable photonic band-gap crystals in the terahertz frequency range

The far infrared behavior of doped semiconductors is dominating by the free carrier response and it is well described by the simple Drude model. As the free carriers can be photogenerated in high resistivity semiconductors, the electromagnetic response of these semiconductors can be changed, via laser excitation, from a dielectric behavior to a metallic one. Using terahertz time-domain spectroscopy, we have checked this Drude-like behavior with several silicon wafers of different doping, and with a high resistivity silicon sample photo-excited by a Ti:Sa laser beam. Then we benefited of this effect to photo-modify the transmission of a photonic band-gap crystal in the terahertz range. The photonic band-gap crystal is a woodpile structure, which exhibits a complete forbidden band around 265 GHz. Silicon-made defects are introduced in interstitial sites of the lattice of the photonic crystal in order to create a defect mode inside the photonic band-gap of the crystal. This defect mode then acts as a narrow bandpass filter at the frequency of 253 GHz. Under 400-mW laser excitation power, the resonance peak associated to the defect mode completely vanishes.