Optical selection rule and oscillator strength of confined exciton system in CuCl thin films

Transmission and absorption spectra of CuCl epitaxial films are studied. Many structures are observed over the ${\mathit{Z}}_3$ exciton region, corresponding to the quantization condition ${\mathit{K}}_{\mathit{n}}$=nπ/L with odd n (${\mathit{K}}_{\mathit{n}}$: wave number). The absence of the states with even n is interpreted in terms of the parity selection rule in the confined exciton system. The value of the oscillator strength for the nth exciton is found to be proportional to L/${\mathit{n}}^2$ for odd n and zero for even n, in accordance with theoretical expectation. As a polariton system, this belongs to a simple regime of long wavelength approximation.