  An electrically isolated quantum well (QW) island can be positively charged
by incoming infrared photon, because its electrons absorb photon energy via
intersubband transition and acquire enough energy to escape it. This process
has been used in a double QW photon-detector. Here, we present the observation
of so-called negative photon-response in such detector. Its origin is clarified
to be an electron mobility reduction phenomenon resulted from the photon
induced charges.
