Efficient photoluminescence upconversion in porous Si

We report on a new phenomenon specific for a system of spatially interconnected nanocrystal assemblies: efficient low temperature photoluminescence (PL) upconversion at resonant optical excitation of porous Si. The upconverted 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 from the same type of studies performed on systems containing Si nanocrystals surrounded by a thick SiO2 shell where the anti-Stokes PL is completely absent for any excitation energy and intensity used while the properties of the Stokes PL are very similar to those of porous Si.