The influence of self-absorption on the photoluminescence of thin film CdS demonstrated by two-photon absorption.

By means of two-photon excited photoluminescence, we demonstrate the influence of self-absorption on the emission properties of thin (1.5 microm) film CdS formed by laser ablation. The excitation of the sample is performed with 200 fs pulses at 804 nm (1.54 eV). The photoluminescence spectrum takes the form of a single peak centered at 510 nm (2.43 eV) at 300 K. The spectrum is shifted about 45 meV to lower energies with respect to the photoluminescence excited by one photon absorption. By fitting the photoluminescence spectra with the Roosbroeck-Shockley relation and Urbach's rule, it is shown by Beer's law that the shift is caused by self-absorption. The results further provide evidence of low impurity concentration and excellent surface quality. They also confirm the outstanding optical properties of thin film CdS formed by pulsed-laser deposition and suggest the application of the films for effective up-conversation materials in ultra-fast experiments.