Abstract:We report a systematic investigation of both three-photon absorption (3PA) spectra and wavelength dispersions of Kerr-type nonlinear refraction in wide-gap semiconductors. The Z-scan measurements are recorded for both ZnO and ZnS with femtosecond laser pulses. While the wavelength dispersions of the Kerr nonlinearity are in agreement with a two-band model, the wavelength dependences of the 3PA are found to be given by (3E photon /E g -1)5/2 (3E photon /E g ) -9 . We also evaluate higher-order nonlinear optical effects including the fifth-order instantaneous nonlinear refraction associated with virtual three-photon transitions, and effectively seventh-order nonlinear processes induced by three-photon-excited free charge carriers. These higher-order nonlinear effects are insignificant with laser excitation irradiances up to 40 GW/cm 2 . Both pump-probe measurements and three-photon figures of merits demonstrate that ZnO and ZnS should be a promising candidate for optical switching applications at telecommunication wavelengths. 1951-1955 (1997

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Auger recombination, quantized Auger rate, and intraband absorption of two-photon-excited carriers in colloidal CdSe quantum dots have been investigated systematically with femtosecond Z scans and transient absorption measurements. The Auger constant is revealed to be on the order of 10 −30 cm 6 s −1 , while the intraband absorption cross sections are found to be in the range of 10 −18-10 −17 cm 2. The authors' experimental evidence demonstrates that the Auger recombination or the intraband absorption takes place under the condition that the average electron-hole pair per quantum dot is in excess of the unity.

We report on nonlinear upconversion photoluminescence (PL) of CdS nanocrystals (NCs) embedded in a polymeric film. The CdS NCs of 2 nm radius are synthesized by an ion exchange method and highly concentrated in the two layers near the surfaces of a freestanding Nafion film. We observed the differences between the two-photon and one-photon excited PL emissions. The two-photon absorption cross section and quantum yield of CdS NCs were measured with femtosecond laser pulses at 800 nm excitation. The origin of PL, optical nonlinearity, and the figure of merit for optical switching have been discussed.