Using Z-scan technique to investigate the nonlinear absorption characteristics of carbon quantum dots (CDs) under the picosecond laser pulse with the wavelength of 532 nm, which the CDs’ types were same particle size of different solvent and same solvent of different particle sizes. The experimental results showed that the nonlinear absorption of water-soluble CDs of 3.9 nm was saturated absorption, and oil-soluble CDs’ was reverse saturable absorption for the same particle size, their nonlinear absorption coefficients were -2.09×10-12 m/W and 6.9×10-12 m/W, respectively; the nonlinear absorption of water-soluble CDs of 4.8 nm was reverse saturable absorption, and oil-soluble CDs’ was saturated absorption for same particle size, their nonlinear absorption coefficients were 1.9×10-12 m/W and -1.3×10-12 m/W, respectively. The experimental results indicate that the nonlinear absorptions of CDs are mainly influenced by the solvent effect and quantum size effect in the same.
Based on Z-scan technique, the nonlinear absorption characteristics and nonlinear refraction characteristics of ZnO nanoparticles and ZnO crystal have been studied under the nanosecond (ns) laser pulses and the picosecond (ps) laser pulses respectively. The experimental results show that the nonlinear absorption characteristics of the ZnO nanoparticles and ZnO crystal are reverse saturable absorption mainly due to two-photon absorption under the ns laser pulses. ZnO nanoparticles don’t have nonlinear absorption, and the nonlinear absorption of the ZnO crystal is reverse saturable absorption under the ps laser pulses. The nonlinear refraction characteristics of ZnO nanoparticles are self-focusing under the ns and ps laser pulses respectively, and the nonlinear refractive indexs are 2.14×10-11 esu, 4.79×10-13 esu respectively because of bound electron. The nonlinear refraction characteristics of ZnO crystal are self-defocusing under the ns and ps laser pulses respectively, and the nonlinear refractive indexs are -2.2×10-8 esu, -2.91×10-11 esu respectively because of free carrier. The experimental results show that ZnO materials can influence the optical nonlinear characteristics, and laser pulse width has little influence on the optical nonlinear characteristics.
In this paper, we use Z-scan technique to investigate the nonlinear absorption characteristics of water-soluble carbon quantum dots (CDs) that fluorescence emission wavelengths are 450 nm (sample 1) and 550 nm (sample 2) respectively. The nonlinear absorptions of sample 1 and sample 2 are saturated absorption and reverse saturable absorption under the nanosecond (ns) and picosecond (ps) laser pulse. Their nonlinear absorption coefficients are -2.2×10-10 m/W, 2.1×10-10 m/W under the ns laser pulse and -2.09×10-12 m/W, 1.9×10-12 m/W under the ps laser pulse. The nonlinear absorptions of sample 1 and sample 2 are reverse saturable absorption and saturated absorption under the femtosecond (fs) laser pulse. Their nonlinear absorption coefficients are 8.5×10-13 m/W and -4.3×10-13m/W respectively. The experimental results show the signs of nonlinear absorptions for sample 1 and sample 2 are all opposite under the ns, ps and fs laser pulses respectively. The nonlinear absorption of sample 1 results from two-photon absorption. The nonlinear absorption of sample 2 results from two-photon absorption under the ns and ps laser pulses, and single photon absorption under the fs laser pulse. The analyses of experimental results show that the nonlinear absorption characteristics of CDs with different fluorescence emission wavelengths are different. The main reason dues to the different particle sizes of CDs, quantum size effects of CDs influence their nonlinear absorption characteristics.
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