  Recent experiments on K$_3$C$_{60}$ and layered copper-oxide materials have
reported substantial changes in the optical response following application of
an intense THz pulse. These data have been interpreted as the stimulation of a
transient superconducting state even at temperatures well above the equilibrium
transition temperature. We propose an alternative phenomenology based on the
assumption that the pulse creates a non-superconducting, though non-equilibrium
situation in which the linear response conductivity is negative. The negative
conductivity implies that the spatially uniform pre-pulse state is unstable and
evolves to a new state with a spontaneous electric polarization. This state
exhibits coupled oscillations of entropy and electric charge whose coupling to
incident probe radiation modifies the reflectivity, leading to an apparently
superconducting-like response that resembles the data. Dependencies of the
reflectivity on polarization and angle of incidence of the probe are predicted
and other experimental consequences are discussed.
