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We demonstrate that a symmetric GaAs/GaAlAs quantum well photodetector with a peak responsivity centered at 8.1 μm shows a significant quadratic power response at 10.6 μm. This is due to intersubband two-photon transitions from the ground level to continuum states. The two-photon responsivity agrees with a theoretical prediction based on a perturbative approach and photoconductive gain estimations.
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Dupont et al. (1994) studied this question.
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