Efficient photoluminescence upconversion in porous Si

We report on a new phenomenon specific for a system of spatially interconne cted nanocrystal assemblies: efficient low temperature photoluminescence (P L) upconversion at resonant optical excitation of porous Si. The upconverte d photoluminescence (anti-Stokes FL) is observed at intensities as low as 0 .1 W/cm(2) and its intensity is as large as that of the Stokes PL band. The confirmation of the essence of connectivity between nanocrystals comes fro m the same type of studies performed on systems containing Si nanocrystals surrounded by a thick SiO2 shell where the anti-Stokes PL is completely abs ent for any excitation energy and intensity used while the properties of th e Stokes PL are very similar to those of porous Si.