Resonant two-photon photoemission in quantum-well infrared photodetectors

We report the nonlinear behavior of quantum-well infrared photodetectors with three energetically equidistant energy levels. The giant resonant nonlinearity leads to a quadratic power dependence of the photocurrent down to excitation power densities as low as 0.1 W/cm(2). Using these highly sensitive two-photon detectors, second-order autocorrelation measurements of ultrashort midinfrared laser pulses in the pJ regime are demonstrated. The dynamical behavior is studied by a numerical analysis of these measurements. At high bias voltages we observe a dominating linear contribution to the photocurrent arising from tunneling processes. (C) 2004 American Institute of Physics.