Ultrafast stimulated emission due to quasidirect transitions in silicon nanocrystals

Abstract

We report on a detailed study of ultrafast stimulated emission arising from efficient quasidirect transitions in silicon nanocrystals under femtosecond pumping. In nanometer-sized silicon nanocrystals, the quasidirect transitions are rapidly cut off (on subpicosecond time scale) owing to surface trapping of carriers. Consequently, also the core-related radiative recombination undergoes ultrafast decay. We propose, theoretically describe, and experimentally demonstrate an extension of the commonly used variable stripe length (VSL) method, which enables us to measure transient stimulated emission even on the subpicosecond time scale. The extension consist in introducing a step-like or gradual pump pulse delay along the stripe length. By applying the extended VSL method in combination with the shifting excitation spot technique, we reveal the presence of room-temperature transient optical gain in silicon nanocrystals (lifetime $<$$1\phantom{\rule{0.222222em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$) at wavelengths $\ensuremath{\sim}$$590\phantom{\rule{0.222222em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ with peak values of the order of $100\phantom{\rule{0.222222em}{0ex}}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Finally, on the basis of our results we discuss possible ways of obtaining a laser source based on silicon nanocrystals.