New Light and New Science: Terahertz-Induced Extreme Nonlinear Optical Transients in Semiconductor Quantum Wells

The optical response of semiconductor quantum-wells often displays excitonic resonances with a typical energy spacing of 1 to 100 meV. To directly access transitions between these resonances, one can apply electro�magnetic fields in the terahertz frequency range. The resulting quantum dynamics and associated nonlinear optical effects are of great interest. They both manifest fundamental physical processes, such as many-body interactions and Coulomb correlations; and also have broad applications for optoelectronic devices. We report an uncharted regime of THz excitations: quantum coherent transients of internal excitonic transitions in semiconductor QWs driven by ultrafast THz pulses. We employed a THz-pump and optical-probe technique to investigate time-resolved nonlinear optical effects induced by intense single-cycle THz pulses [1]. The most distinctive feature of this approach is that we can observe the THz-induced effects with the temporal resolution of 0.1 ps, limited by the femtosecond laser pulse duration.