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 tempera tures, and is absent in strongly oxidized porous Si and oxidized Si nanocry stals. These observations are explained by the resonant excitation of elect ron-hole pairs spatially separated in neighboring crystals. UPL results fro m the subsequent excitation of a second pair in the larger of the two cryst als and Auger ejection of a carrier into the smaller one, with the larger g ap.