  We propose an unconventional scheme of photoluminescence in a semiconductor
thin film, where the nonlocal correlation between an excitonic wave and a light
wave prominently enhances the exciton-light coupling beyond the long-wavelength
approximation (the so-called excitonic superradiance regime). On the basis of
the developed method extending input-output theory, we elucidate atypical
photoluminescence effects due to the strong wave-wave correlation. In
particular, the up-converted photoluminescence based on the coherent
superposition of excitons is found to be highly efficient, i.e., it can be
realized by weak pumping without auxiliary systems such as cavities or photonic
antennas.
