TWO-PHOTON ABSORPTION IN QUANTUM DOTS OF CdSxSe1-x USING OPEN APERTURE Z-SCAN AND FEMTOSECOND LASER

Two-photon optical nonlinear absorption has been studied in quantum dots of CdSxSe1-x grown in borosilicate glass matrix by two-step annealing technique. Femtosecond laser and open aperture Z-scan technique has been used for measuring the third-order nonlinear two-photon absorption coefficient (β(3)). Only the third-order and not the fifth-order nonlinear effects are observed at low intensities (1.6–3.2 GW cm-2) of laser used in the present experiment. At such low intensities, the variation of β(3) is found to be almost intensity-independent. For a given annealing duration of the quantum dots, the value of β(3) is found to be higher for sulfur-rich samples as compared to that for selenium-rich samples. This is attributed to the presence of shallow traps formed due to sulfur vacancies in the sulfur-rich samples. Further, the value of β(3) increases with the increase in the size of quantum dots and the rate of increase of β(3) with the increase of average radius is found to be higher for sulfur-rich samples.