Two photon absorption in quantum dot-in-a-well infrared photodetectors

Two photon absorption processes in InAs/In01.5Ga0.85As/GaAs quantum dot-in-a-well photodetectors are studied using free electron laser excitation. Two photon induced, normal incidence photocurrent, observed in the range of 20-30 mu m, arises from sequential near-resonant two-step transitions involving electron ground to first excited states in the dot, to quantum well final states. We find a two photon absorption coefficient of beta similar to 1x10(7) cm/GW at 26.5 mu m (47 meV) and 0.8 V applied bias. Second-order autocorrelation measurements exhibit two characteristic time constants of similar to 3 and similar to 40 ps. The latter is associated with the intermediate state electron lifetime, whereas the short decay is explained by the involvement of acoustic phonon assisted transitions. (c) 2008 American Institute of Physics.