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 ${\mathit{a}}_{\mathit{B}}$ (∼0.7 nm), are considered here. Many structures have been observed in transmission and absorption spectra over the ${\mathit{Z}}_3$ exciton region. When L<30 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/${\mathit{n}}^2$ 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.