We present our latest work on the development of an all-optical (pump-probe) technique for estimating the quantum efficiency (QE) of silicon nanocrystals. Our experiment combines measurement of micro-photoluminescence and reflectivity when driven with an RF frequency modulated pump. By varying this frequency over a wide (5 decade) range, we are able to determine the dynamics of the free carrier concentration (multi-component in these systems) and thus retrieve the PL decay rate(s). Interestingly, we find that, in addition to the dominant (10’s-us) component (typically noted from direct measurement of the PL decay), a very slow component persists (in the ms regime), which we attribute to the recombination of ‘trapped’ charges. The technique provides a method for estimating the relative QE as a function of carrier concentration, providing clues as to the efficiency limiting processes in these materials. We present our most recent data for silicon nanocrystals formed in glass cover-slips after ion implantation and thermal annealing, and we discuss how the technique can be applied to luminescent materials more generally, e.g. to study the efficiency ‘droop’ mechanism observed at high carrier concentration in nitride based LED materials.