Ultrafast dynamics in CdSSe-doped glasses

1 There has been considerable interest in nonlinear optical properties of semiconductor-doped glasses, since they have large optical nonlinearity with a fast response time The samples investigated were commercial CdSSe-doped filter glasses: Asahi R-64, R-66; Hoya R-64, R-66, R-68; Schott RG645, RG665; and Corning 2-58 and 2-64. CdS-doped glass L-42 and undoped glass Y-0 were also investigated for comparison. Thickness of samples was approximately 2.5 mm. The concentration of CdSSe was approximately 0.4 wt% nm The laser beam was divided into three parts: two pump beams and a probe beam. All beams were polarized in the same direction. Pump and probe beams were focused by a single lens on the surface of the sample. between pump and probe pulses follows closely that of the laser pulse. Note that the fast contribution is observed in glass without semiconductor (Y-0). Since signal intensity of the fast contribution in the undoped glass is the same as those in the doped glasses, the fast contribution is attributable to glass matrix. It is reported that the intensity of nearly degenerate frequency mixing signal in BK7 glass, which does not contain semiconductor, is comparable with those of CdSSe-doped glasses The slow contribution is observed in glasses with longer cut-off wavelength: R-64, R-66 etc. The relative intensity of the slow contribution increases with decrease in band-gap energy of semiconductor, E g . The band-gap energy corresponding to the absorption edge was derived from the measured cut-off wavelength. The value of E g is 1.97 eV for R-64, 1.91 eV for R-66 and 1.85 eV for R-68. Two-photon absorption occurs when ratio of photon energy of laser light to band-gap energy of semiconductor, h /E g , is larger than 1/2. When the value of h /E g approaches to one, two-photon absorption is enhanced. It is reported that two-photon absorption coefficient at 790 nm (h = 1.57 eV is 0.022 cm/GW for RG630 (E g = 2.00 eV) and 0.032 cm/GW for RG665 (E g = 1.89 eV) In order to confirm the assumption described above, we measured intensities of fast and slow contributions to the DFWM signal as a function of laser intensity for Hoya R-68. If carriers are generated by two-photon absorption, I 5 dependence should be observed, where I is the laser intensity (power density of laser light). I 5 dependence is typical for a fifth-order nonlinearity attributed to electron-hole pairs generated by two-photon absorption. However, isotropic media, such as semiconductor-doped glasses, have only odd-order nonlinearities. We consider that photodarkening Since surface states of semiconductor nanocrystals are located below the conduction band of semiconductor, filling of the surface states by one-photon absorption may occur, and it may cause nonlinearity. However, it should show I 3 dependence. This disagrees with the I 4 dependence shown in In order to compare the decay times of photogenerated carriers with those evaluated from another experimental method, we measured transient absorption in the infrared range. In this experiment, carriers are generated by inter-band transition from valence band to conduction band of semiconductor nanocrystals. Photogenerated carriers cause absorption in the infrared range by intra-band transition from the lowest state in the conduction band to higher states in the conduction band (or valence band). Therefore, the intensity of absorption is related to the population of photogenerated carriers, and the dynamics of absorption signal are indicative of carrier relaxation behaviou