THz quantum optics with dark excitons in Cu2O: from stimulated emission to nonlinear population control

Fundamental quantum optical processes in photogenerated excitons in Cu2O are demonstrated using few‐cycle terahertz pulses and field‐sensitive electro‐optic sampling: (i) In a dilute ensemble of 3p excitons, stimulated terahertz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10‐14 cm2. (ii) An optically dark, dense, and cold 1s‐paraexciton gas is prepared by two‐photon generation of electron‐hole pairs and subsequent phonon cooling. Intense multi‐terahertz fields of order MV/cm coherently promote 70% of the quasiparticles from the 1s to the 2p state via a partial internal Rabi oscillation. Electro‐optic detection monitors the Larmor precession of the Bloch vector directly in the time domain, with femtosecond resolution. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)