Energy transfer in dye impregnated porous silicon

Two kinds of porous silicon layers (fresh and oxidized) were impregnated with laser dyes solutions before drying. An original technique of diffuse reflectance allowed us to give an estimation of the concentration of the dye molecules in the pores, which appears to be higher for oxidized samples than for fresh ones. Selective excitation of the red photoluminescence of the silicon crystallites with a pulsed ultraviolet laser was performed and it showed a rapid luminescence of the laser dye. This emission is more important for fresh samples than for oxidized ones, although the dye concentration is lower in fresh samples. This indicates that the luminescence of the dye molecules does not come from direct excitation of the laser but that it is provided by an energy transfer from the porous silicon crystallites. In as far as the coupling between the crystallite and the dye molecule theoretically decreases when the distance between them increases, a lowering of the transfer on oxidized samples for whom the distance is larger is expected and is experimentally observed. (C) 1997 American Institute of Physics.