Abstract

Quantum semiconductor crystallites (typically 20-60 A in diameter) show a 3-D quantum size effect. In zero order they exhibit discrete excited electronic states, even though their crystal structure is the same as that of bulk materials. In this paper I consider electronic structure, coupling of excited states to crystallite vibrations, and the picosecond time scale relaxation dynamics. The crystallites are made by chemical synthetic techniques that chemically terminate the surface with organic molecules. Such surface passivation prevents crystallite fusion into bulk material. Photophysical hole burning and resonance Raman experiments on 40-Å diam CdSe crystallites show that the coupling of the 1s—1s lowest excited state to LO phonons, via the Frohlich interaction, is far weaker than in bulk material. However, coupling to low frequency modes is stronger than in bulk material and is surface sensitive. The very large oscillator strengths present cause nonlinear optical phenomena—population saturation, AC stark effects, and χ₃ coherent coupling of pump and probe beams—to be prominent in the picosecond transient optical spectra.

© 1989 Optical Society of America