Nonlinear optics of quantum confined semiconductor nanocrystals

An overview is given of research results for semiconductor crystallites from a few up to several tens of nanometers in size and a wide range of volume concentrations, artificially grown in a glass matrix by a thermal diffusion process. The continuum of valence-to-conduction-band transitions of the bulk material is replaced by a series of discrete localized transitions whose spectral positions depend on the crystallite size. Two mechanisms have been singled out to explain the broadening of these transitions. One is due to the homogeneous electron-photon coupling and the other is due to the unavoidable inhomogeneous size distribution of the crystallites. Recent time-resolved hole burning experiments give evidence of strong electron-phonon coupling in II-VI semiconductor nanocrystals. The third-order susceptibility for the optical Kerr effect, χ⁽³⁾, shows a pronounced enhancement near the quantum confined resonances, and so does the figure of merit χ⁽³⁾/α, where α is the absorption coefficient. When a static electric field is applied to a quantum-confined nanocrystal the absorption is modified in accordance with the expected field-induced modifications of the quantum-confined spectrum.