Extreme optical and electronic nonlinearities in GaP induced by an ultrastrong Terahertz field

Researchers from the SwissFEL laser group have succeeded in using intense Terahertz radiation to dramatically change the optical properties of a semiconductor on a sub-cycle timescale. In their experiment the material Gallium Phosphide (GaP) was illuminated by an extremely strong THz electric field with up to 50 MV/cm in strength.

The instantaneous interaction gives rise to a transient modification of the optical nonlinearities in the material which results in a spectacular broadening of the probe spectrum by more than 500 %. The physical origin of this broadening is THz-induced nonlinear cross-phase modulation between the THz pump and the optical probe beam through the combination of the Pockels and Kerr effect. The magnitude of the effect presented here overcomes by far previous works and shows for the first time the potential of using intense THz pulses to instantaneously modify the optical properties which has been used for ultrafast spectral shaping of the laser pulses.

In addition to the spectral effects the researchers observed an ultrafast nonlinear transient modification of the optical conductivity induced by the extreme THz field. The demonstrated ultrafast control of the electronic properties by an intense THz field may enable applications towards novel high-speed electronics in near future. The results have been published in Physical Review Letters. The presented results have become possible thanks to the recent Terahertz source developments at PSI. The cutting-edge THz source used in this study will also play an important role at SwissFEL for Terahertz pump x-ray probe experiments.