ABSTRACT
The nonlinear optical properties of Ag2S and PbS quantum dots were studied through a top-hat Z-scan technique under femtosecond laser pulses. The open Z-scan results show that Ag2S and PbS quantum dots both have a valley value, which indicates that they have strong reverse-saturation absorption characteristics. The nonlinear absorption coefficients of Ag2S and PbS quantum dots are 4.17 × 10−12 and 8.38 × 10−13 esu, respectively. The results of closed Z-scans show that Ag2S and PbS quantum dots all have self-focusing properties, and their nonlinear refractive index coefficients are 3.06 × 10−12 and 4.48 × 10−12 esu, respectively. Using transmittance curve fitting, the third-order nonlinear polarizabilities of Ag2S and PbS quantum dots are calculated as 1.15 × 10−13 and 1.07 × 10−13 esu, respectively. The nonlinear absorption characteristics of an Ag2S quantum dot are significantly better than those of a PbS quantum dot. The physical mechanism of the nonlinear optical response of Ag2S and PbS quantum dots is discussed. The results show that the nonlinear refraction of both samples is enhanced with an increase of excitation energy, but the difference is that the refractive coefficient of an Ag2S quantum dot increases with an increase of energy, while the nonlinear refractive coefficient of a PbS quantum dot decreases with an increase of energy. These two materials are of great significance for research into optoelectronics, biosensors, optical limiting and adaptive techniques.
