Terahertz nonlinear detection and quantum optical studies by resonant two-photon transitions in semiconductor quantum wells

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The aim of this project is exploiting two-photon transitions for nonlinear optical spectroscopy in the Terahertz(THz) regime, and to study second-order coherence and nonlinear susceptibility at frequencies in the range 1 – 7 THz. Our approach makes use of resonant two-photon absorption involving three energetically equidistant conduction subbands in GaAs/AlGaAs quantum wells. This optical nonlinearity is by six orders of magnitude more efficient as compared to bulk crystals and allows us to realize extremely sensitive quadratic detectors, suitable for autocorrelating extremely weak THz pulses. These detectors will be used for autocorrelation studies of photoconductive THz emitters pumped by femtosecond Ti: Sapphire lasers, and for quantum optical investigations of photon bunching of incoherent THz radiation from synchrotron-pumped infrared beamlines. The ultrashort correlation time of these radiation sources will allow us to determine intrinsic time constants of the detectors, thus gaining new information on intersubband dynamics, in particular on relaxation and dephasing well below the optical phonon frequencies. Another goal will be the study of phonon-polariton resonances of the nonlinear susceptibility in III-V semiconductors.