  In an electromagnetic cavity, photons can be created from the vacuum state by
changing the cavity's properties with time. Using a simple model based on a
massless scalar field, we analyze resonant photon creation induced by the
time-dependent conductivity of a thin semiconductor film contained in the
cavity. This time dependence may be achieved by irradiating periodically the
film with short laser pulses. This setup offers several experimental advantages
over the case of moving mirrors.
