Terahertz-wave induced non-linear optical effects in semiconductors

Recent development of ultrashort pulse technologies allows us to drive large amplitude motion of electron and ion coherently. The intense terahertz (THz) pulse resonant with the vibration frequency is promising to drive vibrations more directly and in coherent manner. In the case of semiconductors, one may coherently control the electronic system in the sub-level structures of quantum structures with intense THz waves. We recently performed the generation of intense terahertz pulse using tilted wave-front technique with a Ti:Sapphire regenerative amplifier. The maximum electric field is as large as 250 kV/cm, which is strong enough to induce nonlinear optical effects in solids. In this paper, we would like to focus THz nonlinear optical phenomena in semiconductor and molecular vibrations in solids. First, we will show strong spectral modulations in the heavy-hole and light-hole exciton absorption band in ZnSe/ZnMgSSe multiple-quantum-well (MQW) structure induced by intense THz electric field. The peak-shift of exciton absorption clearly indicates deviation from the Stark effect, which implies that the interaction between exciton and THz wave enters the non-perturbative regime. Similar effects have been confirmed in GaAs/AlGaAs MQWs. In this case, we clearly observed crossover from red shift to blue shift for the THz electric field in the heavy hole exciton absorption. These results indicate that AC Stark effect and dynamical Franz-Keldysh effect should be competed in this field strength. The appearance of these nonlinearities allows us to control optical response of semiconductors by terahertz pulses.