Spontaneous and Stimulated Emission by Self-Trapped Excitons in Cadmium Iodide

Abstract

The spontaneous and the stimulated emission of CdI2 have been studied in the temperature range from 10 to 200°K, under low and high intensity of excitation by means of a nitrogen laser (Imax= 10 MW/cm2, τ= 5 nsec), whose photon energy was 3.68 eV, i.e. larger than the energy gap of the ionic crystals of CdI2. Our samples showed a 4H-polytypism as confirmed by the spontaneous Raman spectroscopy measurements performed by means of a 500 mW Argon ion laser excitation. Our crystals appeared unintentionally doped with Pb++, as confirmed by the spontaneous emission spectra at T=20°K obtained under dye laser (h ν =3.25 eV) excitation in resonance with Pb levels. The absorption due to these impurity levels strongly appears even at RT in the photoacustic spectra we have carried out. The spontaneus emission spectra at low excitation show a worth e-volution with the sample temperature of the competitive broad bands in the near ultraviolet and visible optical regions. The line shape of these bands, extracted from the total spectrum generated from their superimposing, results to be gaussian. The energy scheme to assign the nature of these bands could be that of self-trapped excitons typical of ionic crystals like CdI2. At high intensity the spectrum consist only in a very broad band, whose halfwidth of about 100 nm is independent of the sample temperature. The emission comes out from the whole sample surface even if the excitation is localized in a small spot. The stimulated emission shows the same feature of the spontaneous spectra obtained under high excitation. On the contrary the optical gain spectra show several main peaks that at 80°K are localized at 2.81, 2.53, 2.37 and 2 eV respectively. These gain peaks are well consistent with the maxima of the deconvolved bands observed in spontaneous spectra under low excitation. The values of the optical gain coefficient are surprisingly high (g ≧104 cm−1) for all the peaks at 80°K whereas at temperatures either lower or higher than LNT the optical gain shows a sharp drop. The integrated external efficiency of the stimulated emission from CdI2 at 80°K excited by N2 laser results of about 0.1%. The very high value of the optical gain coefficient of CdI2 at 80°K can be explained by taking into account that the self-trapped excitons levels diagram looks like the typical four levels laser scheme. Anyway the population inversion threshold up to-day can be overcome only by means of high intensity optical pumping(1).