  Photodetection is a process in which an incident field induces a polarization
current in the detector. The interaction of the field with this induced current
excites an electron in the detector from a localized bound state to a state in
which the electron freely propagates and can be classically amplified and
detected. The induced current can interact not only with the applied field, but
also with all of the initially unpopulated vacuum modes. This interaction with
the vacuum modes is assumed to be small and is neglected in conventional
photodetection theory. We show that this interaction contributes to the quantum
efficiency of the detector. We also show that in the Purcell enhancement
regime, shot noise in the photocurrent depends on the bandwidth of the the
vacuum modes interacting with the detector. Our theory allows design of
sensitive detectors to probe the properties of the vacuum modes.
