Nonlinear absorption characteristics of carbon quantum dots

Abstract

ABSTRACT 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 satu rated 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 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 -13 m/W respectively. The experimental results show the signs of nonlinear absorptions for sample 1 and samp le 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 charact eristics 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 absorptio n characteristics. Keywords: carbon quantum dots; Z-scan; nonlinear absorption