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

Abstract

Transmission and absorption spectra of CuCl epitaxial films are studied. Many structures are observed over the Z3 exciton region, corresponding to the quantization condition K" ^nnlL with odd n (K 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 Lin 1 for odd n and zero for even «, in accordance with theoretical expectation. As a polariton system, this belongs to a simple regime of long wavelength approximation.PACS numbers: 73.20.Dx, 68.55.Bd, 71.35,+z, 71.36.+C Quantum confinement effects of excitons in semiconductor systems with reduced dimensions have attracted considerable attention because of their potential application to nonlinear optical devices, and as a means of exploring the basic physics of the quantized excitonic states. The qualitative behavior of the exciton in confined systems can be characterized by the ratio of a film thickness L to the Bohr radius as of the exciton. In a quantum well with Lias ^ 1, the electron and hole levels are separately quantized in discrete subbands, leading to increases in both the binding energy and the oscillator strength of the exciton. In this confinement regime, the oscillator strength is independent of the electron and the hole quantum numbers (n e ,n n ), and the excitonic transitions obey the selection rule of An s=: n e -nh = 0.These properties of the exciton in quantum wells have been extensively studied in III-V compound systems both in theory and experiment [l]. For thin films outside the quantum well regime, the center-of-mass (cm.) motion of the exciton is quantized, while the relative motion is essentially identical to that in the bulk except for a possible distortion near the surfaces [2]. When long wavelength approximation (LWA) holds, L<£X (X: photon wavelength), this confined system can be regarded as an assembly of local oscillators characterized by the quantized exciton energy E n and its ^-dependent oscillator strength f n , where n is the quantum number of the exciton. Optical transitions are expected to obey the parity selection rule that only the transitions with odd quantum number n are allowed. The properties of cm. confinement have been studied in heterostructures of II-VI [3-6] and III-V [2,7] compounds. Alternating strengths of the resonant structures have been observed in luminescence excitation spectra [3-6] and luminescence spectra [7]. These spectra were analyzed in terms of the interference between the upper and lower polaritons (UP, LP) [8,9], but an analysis in terms of {/"} has not been done, probably because of the ambiguity in obtaining {f n } from luminescence measurements.Compared to the III-V and II-VI compounds, CuCl has a large binding energy (-190 meV) and a small Bohr radius (~7 A) of the exciton. The electron-hole Coulomb interaction will be strong enough to maintain the bulklike electron-hole relative motion, down to surprisingly small film thic...

Transient reflectivity response with negative time delay caused by femtosecond pulse propagation in GaAs thin films

“…The notations of n=X HH and LH indicate the Xth confined heavy-hole (HH) and light-hole (LH) excitons, respectively. In the exction confinement regime, which the excitonic characteristics are described by the long wavelength approximation and the transition selection rule has odd parity usually [9]. However, in the case of the film thickness of 110 nm, only excitons with even quantum number are allowed [10,11].…”

Dramatic improvement of excitonic photoluminescence in metal halide films

“…For example, high-quality CuCl films are successfully grown on (1 0 0) NaCl [65], CaF 2 [66,67], (0 0 1) MgO [68], (0 0 0 1) Al 2 O 3 [69,70], (11 0)TiO 2 [71] and (1 0 0) and (111) Si [72]. Although the films exhibit enhanced excitonic PL, however, its intensity still seems very weak as compared with the cases of improved metal halides such as CsPbCl 3 mentioned in Section 1.…”

Radiative coupling of A and B excitons in ZnO

“…Although there have been no experimental reports on the quantization of the excitonic CM motions for ZnO, it can be observed as the other materials 41,42 if the samples have sufficient quality with small nonradiative damping constants. In a thin sample, the distortion of wavefunctions near the surface generally affects the energy structures of exciton.…”