Low-temperature photoluminescence upconversion in porous Si

We report efficient low-temperature upconverted photoluminescence (UPL) at resonant excitation of the porous Si photoluminescence band. The UPL has a linear dependence on the excitation intensity, quenches at elevated temperatures, and is absent in strongly oxidized porous Si and oxidized Si nanocrystals. These observations are explained by the resonant excitation of electron-hole pairs spatially separated in neighboring crystals. UPL results from the subsequent excitation of a second pair in the larger of the two crystals and Auger ejection of a carrier into the smaller one, with the larger gap.