We demonstrated the superradiance-to-polariton crossover of the blue excitons in Cu2O by varying the sample thicknesses instead of controlling the cavity quality factor. The crossover behavior was compared with unprecedented calculations based on the nonlocal optical response theory with the inclusion of three exciton resonances. The crossover thickness, found to be 177±2 nm, was smaller than the predicted value for a single resonance by a factor of 5. The fact that there was much larger longitudinal-transverse splitting (40±5 meV) than in the bulk implies a surprisingly fast radiative recombination even without a cavity structure.
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