Quantization of excitons in CuCl epitaxial thin films: Behavior between a two-dimensional quantum well and the bulk

A study of quantized excitons in CuCl epitaxial thin films is reported. Film thicknesses (L) that are much smaller than the photon wavelength ({λ}) in the medium ({~}160 nm), but larger than the exciton Bohr radius aB ({~}0.7 nm), are considered here. Many structures have been observed in transmission and absorption spectra over the Z₃ exciton region. When L30 nm, these structures correspond well to the confined exciton levels with odd quantum number n. The oscillator strength for the nth quantized exciton is found to be proportional to L/n² for odd n and zero for even n. The absence of the states with even n in the optical spectra is interpreted in terms of the parity selection rule in the confined exciton system where the long-wavelength approximation holds. The measured optical spectra are compared with the additional-boundary condition-free response theory. Polariton-interference fringes given by the theory demonstrate a good agreement with the experimental results both for thick and thin films. By analyzing the absorption line shape, the dependence of nonradiative damping of the quantized exciton on quantum number n and film thickness L is discussed.