This paper summarizes the latest advances in THz optoelectronics and exploits this technology for two studies of THz nonlinearities that goes beyond the perturbative regime. In the first study, intense multi-terahertz fields are used to coherently promote optically dark and dense 1s para excitons in the Cu 2O semiconductor into the 2p state. By utilizing the ultrabroadband electro-optic sampling, the nonlinear field response of the intra-excitonic degrees of freedom is monitored in the time domain. In the second study, a semiconductor microcavity is employed to switch on ultrastrong vacuum Rabi splitting in the femtosecond scale without an external driving field. Intersubband cavity polaritons in a quantum well waveguide are photogenerated by 12-fs near-infrared pulses. Multi-THz transients trace the conversion of bare photons into cavity polaritons, a novel nonlinear optical phenomenon. Hence, this structure represents the first sub-cycle switching device of strong and ultrastrong light-matter coupling.
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