Quantum size-dependent luminescence and nonlinear optical properties of silicon quantum dots/SiO2 multilayer

The luminescence and nonlinear optical properties of silicon quantum dots (Si QDs)/SiO2 multilayers with different QDs diameters (2.5, 3 and 4 nm) were investigated by utilizing continue wave (325 nm) and picosecond laser (tp = 25 ps, λ = 1064 nm), respectively. It was found that both the linear and nonlinear optical properties of Si QDs/SiO2 multilayers strongly depended on the QDs size. The photoluminescence intensity could be enhanced by ∼32-fold, and about 17 times increase in the saturable absorption (SA) coefficient was achieved, which could be ascribed to the controllable density of interface-states as well as their coupling effect with the Si QDs core states. Moreover, a transformation from reverse saturable absorption (RSA) characteristic through two-step absorption process into the SA behavior due to the interface-state filling effect was observed when increasing the incident laser intensity. We proposed that the ultrafast free carries trapping process by the interface states would greatly influence the self-defocusing effect. In addition, the free carries absorption and SA effect should be considered in accounting for the variation of nonlinear parameters as changing the pump intensity. Our results suggest that the widely tunable SA feature through controlling the Si QDs size provides great potential to develop high-efficient Si QDs-based nonlinear optical devices such as passive mode-locking or Q-switching laser.