Highly nonlinear photoluminescence threshold in porous silicon

Porous silicon is excited using near-infrared femtosecond pulsed and continuous wave radiation at an average intensity of ${\text{∼10}}^6 {\mathrm{W/cm}}^{\mathrm{2}}{\text{\hspace{0.05em}(8×10}}^{10} {\mathrm{W/cm}}^{\mathrm{2}}$ peak intensity in pulsed mode). Our results demonstrate the presence of micron-size regions for which the intensity of the photoluminescence has a highly nonlinear threshold, rising by several orders of magnitude near this incident intensity for both the pulsed and continuous wave cases. These results are discussed in terms of stimulated emission from quantum confinement engineered intrinsic Si–Si radiative traps in ultrasmall nanocrystallites, populated following two-photon absorption.