Studies on third-harmonic generation in chemically grown ZnS quantum dots

ZnS quantum dots having a narrow size distribution are chemically synthesized and characterized as regards their linear and non-linear optical properties. Quantum-size effects are revealed by the size-dependent blue-shift observed in the optical absorption spectra. The third-order optical non-linearity of a colloidal solution of ZnS quantum dots is studied by a third-harmonic-generation technique at the fundamental wavelength of 1.06 mu m. The third-harmonic signal is size dependent and increases with particle size over the range 7 to 21 Angstrom. The third-harmonic signal of ZnS dots is also dependent on the choice of embedding medium (for example, DMF and acetonitrile). The results are qualitatively explained on the basis of the local field effect.