Two-photon QWIPs for quadratic detection of weak mid-infrared laser pulses

We report on QWIPs optimized for nonlinear detection using a bandstructure design with three energetically equidistant subbands. The double resonant character of the two-photon transition from the ground state into the second excited state results in quadratic dependence of the photocurrent on the incident power down to excitation densities as low as 0.1 W/cm2. We have used these two-photon QWIPs as quadratic detectors in second-order interferometric autocorrelation measurements allowing for the separation of coherent and incoherent contributions to the photocurrent. We have studied the carrier dynamics and the influence of the QW doping on both phase and energy relaxation times. The measurements also demonstrate that these devices provide an ultra-sensitive tool with sub-ps response time for the characterization of ultrashort (about ps) mid-infrared pulses in the pJ-regime such as those generated by modelocked quantum cascade lasers.