Time Evolution of Excited States in CdSe Nanocrystals

Early theoretical work predicted that quantum dots would exhibit discrete, well separated optical excitations, with narrow linewidths, and uncommonly large excited state polarizabilities 1 . This unusual combination could result in some unusual nonlinear properties. Ever since there has been steady progress in the preparation and characterization of nanometer size semiconductor nanocrystals, with CdSe still remaining the prototypical direct band gap material 2 . During the past two years we have undertaken a systematic study of the size dependent photophysics in these materials. Colloidally prepared II-VI nanocrystals embedded in polymers and cooled to low T lend themselves readily to the application of many time and frequency resolved optical characterization techniques 3 . These include absorption, fluorescence, electric field modulation of the absorption and fluorescence, three pulse photon echoes, and resonance Raman scattering.