Observation of tunable nonlinear optical properties and quenching of photolu minescence emission in Cu~(2+) doped Zns quantum dots

摘要

Tunable photoluminescence emission (PL) and nonlinear optical (NLO) properties of chemically synthesised ZnS and Cu~(2+) doped (0.25 to 1%) ZnS (ZnS:Cu) quantum dots (QDs) of average sizes ～2.5nm are reported. It is found that the PL emission band from the samples is red shifted systematically and overall PL emission is quenched with increasing Cu~(2+) doping concentration in ZnS. The NLO properties of the QDs are measured by dispersing the powder samples in methanol and by using a open aperture (OA) and closed aperture (CA) z-scan set up with a visible laser radiation at 532nm which is obtained by the second harmonic generation from a Q-switched Nd:YAG laser pulsed radiation. By analysing OA z-scan data the value of three photon absorption (3PA) coefficients are extracted. It is found that the extracted values of 3PA coefficient of the synthesized samples are enhanced by the factor of ～10~9 than that of bulk ZnS in all the samples and for 0.75% Cu~(2+) doped ZnS QDs, 3PA coefficient is the highest having the value of 14.2×10~6 cm~3/GW~2 at a peak intensity of 0.60 GW/cm~2. The tuning of 3PA value in the doped samples has been attributed to the presence of variable defect states as confirmed by PL study. The nonlinear refraction coefficient of the samples are extracted by analysing the experimental CA z-scan data with the available analytical expressions and self focussing (i.e. positive) nonlinear refraction is observed for all the samples.