We have demonstrated size-dependent radiative coupling in a Cu2O mesoscopic film, where the spatial matching between the center-of-mass exciton motion and light waves plays an essential role. The observed quadratic increase and oscillation of the photoluminescence intensity with varying film thicknesses (16-1000 nm) indicate a nonlocal response that is unique to mesoscopic systems. Optimally, a 104-fold enhanced radiative rate was found compared with bulk Cu2O. A new degree of freedom in the light-matter interaction is presented, which differs from the mode volume control in cavity quantum electrodynamics.
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