Power-dependent photoluminescence decay kinetics of silicon nanocrystals under cw and pulsed excitation
Power-dependent photoluminescence (PL) decay kinetics of silicon nanocrystals (Si NCs) in solid and liquid samples are studied under cw and pulsed excitation. The lifetime distribution and, consequently measured PL kinetics is shown to depend on the excitation pulse duration until it is not suffciently short (pulsed limit) or long (cw limit). These two excitation limits, however, are proven to excite different distributions of lifetime components and cannot be directly compared. We derive and experimentally confirm the equality of lifetimes averaged over amplitude and intensity for cw and pulsed excitation, accordingly. Absorption cross section (ACS) of Si NCs in solid and liquid samples is assessed and compared by two approaches under cw-excitation based on treatment of power-modulated PL kinetics or PL amplitude saturation curves under low and moderate excitation powers, respectively. The discrepancy in extracted ACS values as well as long-time debated phenomena of incomplete PL saturation of matrix-embedded Si NCs is explained by proposed model that is based on saturation of various components in ensemble distribution at different excitation powers. The model finally allows us to explain the mystery of average decay lifetime dependence on varied excitation powers in non-linear power regime. By varying the excitation from cw to pulsed, we show the reduction of average decay lifetime in later case and attribute this to increased contribution of fast lifetime components that results in at least one order of magnitude lower values of ACS. Finally, exciting the solid sample with very high excitation powers we detected PL power decrease region that allowed us to extract Auger lifetime which is of 170 ns.