| Summary: | 碩士 === 高雄醫學大學 === 醫藥暨應用化學系碩士班 === 93 === Cuprous halides nanocrystals embedded in borosilicate glass are prepared by a melt and heat-treatment method. A large amount of these nanocrystals appear as dark ball shape with radius roughly ranging from 4 to 15 nm according to the TEM analysis. A Q-switched Nd:YAG laser delivery linearly polarized pulses at 355 nm wavelength ( ) with ~6 ns pulse width is employed as light source for the nonlinear absorption and the photoluminescence (PL) experiments.
The two photon absorption (TPA) coefficient and the free carrier absorption (FCA) cross section above the band gaps of these quantum dots are suggested to be attributed to the TPA, the linearly generated FCA, and the TPA generated FCA. The propagation equation is , where N is the instantaneous free carrier density given by = . Under the weak absorption limit (i.e. the fluence is less than the critical fluence defined as ), our experimental data of inverse transmission (1/T) as a function of incident fluence can be properly fitted with a formula derived from the above equations, while we obtain the values of and that are comparable to the literature values.
Upon excitation with 355 nm laser pulses in those cuprous halide doped glasses at room temperature, PL is observed, as we know, for the first time as the incident fluence is . This emission becomes very bright when . We also measure these PL specta at room temperature with a 0.5-m monochromator equiped by a PMT. The signal intensity is obtained with direct digital scanning from 350~800 nm. These spectra consist of a very broad band and centered at the visible region of about 520 nm ( ). Similar spectrum centered around 515 nm is also observed at low excitation irradiance. The intensity of PL at 520 nm is measured vs. the input intensity, and the results display that this emitted energy increases linearly with irradiance. We therefore suggested that the PL could be due to the radiative recombonation associated with traps or defects from impurities in the cuprous halide nanocrystals and their glassy surroundings.
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