Recent reports on experimental observation of optical gain in silicon nanostructures in the visible region, performed at several laboratories all over the world, have triggered an extraordinary surge of interest in silicon lasing. However, attempts aimed at reproducing the red stimulated emission from "standard" silicon nanocrystals (sized 3-5 nm) at some other laboratories either failed or did not come to definite conclusions. Therefore, more detailed measurements of optical gain in a wider variety of samples containing Si nanocrystals are required in order to unravel whether or not the observation of optical gain is an intrinsic property of Si nanocrystals, etc.
We have performed a detailed study of optical gain in layers of densely packed Si nanocrystals using variable stripe length (VSL) method in combination with shifting excitation spot (SES) method. In selected samples we have observed a distinct difference in behaviour between VSL and SES curves, indicating the occurence of positive optical gain of ~24 cm^-1. Preliminary reports on transport and electroluminescence measurements in thin films of SiO₂ doped with porous silicon grains, prepared by spin-coating technique, will also be discussed.

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