Optical properties of cadmium sulfide and cadmium telluride microcrystallites in glass and an optical associative memory.

Abstract

The linear and nonlinear properties of CdTe and CdS microcrystallites, or 'quantum dots', grown in glass were investigated. The CdTe quantum dots investigated, the first ever grown, demonstrated the most confinement peaks observed for quantum dots of this kind. The linear absorption for CdTe was modeled. The growth of the microcrystallites in glass was optimized using quantum confined behavior as the criteria. The temperature dependent red shift of the band gap, and the phonon broadening of the 1s-1s transition feature were measured. The fluorescence spectra for CdS and CdTe were taken and found to be different in some respects. The quantum confined Franz-Keldysh effect was modeled and measured for CdTe. Very good agreement with theory was found. The effect of trap state electric fields was considered. The Franz-Keldysh effect for bulk-like microcrystallites was observed. The results for CdS quantum dots were not similar to that of CdTe. The differential absorption for CdS and CdTe was measured with nanosecond time scale optical excitation. Typical differential signals were observed, and long lifetimes were measured. Microsecond time scale excitation was used for CdTe and exceptionally long lifetimes were seen. Differential signals were found to correlate to fluorescence lifetimes. The differential signals were attributed to trap state electric fields (coulombic interaction) with at least two lifetime components. Photodarkening was considered in the context of this trap state scheme.