Nonlinear Properties Of Water-soluble Ag2S And PbS Quantum Dots Under Picosecond Laser Pulses

In this paper, the water-soluble Ag2S and PbS quantum dots(QDs) are synthesized by aqueous phase synthesis. The nonlinear optical properties of Ag2S and PbS QDs were investigated by Top-hat Z-scan technique under ps laser pulses (λ = 532 nm, t = 21 ps). The open Z-scan results show that Ag2S and PbS QDs all have a valley value which indicate that they have strong reverse saturable absorption(RSA). The nonlinear absorption coefficient of Ag2S QDs is 6.72×10-13 esu, however, the valley value of PbS QDs at the focal point is similar to 1, so its nonlinear absorption coefficient is small, is 7.13×10-14 esu. The close Z-scan results show that Ag2S QDs has self-defocusing effect and PbS QDs has self-focusing effect, their nonlinear refractive index are on the order of -6.13×10-13 esu and 2.41×10-13 esu. After fitting calculation, the third order nonlinear polarizability of Ag2S and PbS QDs are 2.40×10-14 esu and 6.02×10-15 esu respectively. The physical mechanisms responsible for the nonlinear optical response of the QDs are discussed in detail. In summary, the nonlinear absorption and refraction properties of Ag2S QDs are better than that of PbS QDs, but both of these materials are of great significance for the research of photoelectric, biosensor, optical limiting, and self-adaptive technologies.