Photoluminescence and optical gain in CuBr semiconductor nanocrystals

Size-dependent excitonic photoluminescence (PL) of CuBr nanocrystals is studied using both, resonant and band-to-band excitation. Narrow excitonic emission lines observed in big nanocrystals (mean radius >5 nm) transform into two wide emission bands in smaller nanocrystals. These two bands are not significantly narrowed under selective excitation within the Z(1,2) and Z(3) excitonic absorption bands. Measurements of PL intensity dependence and PL excitation spectra suggest that the emission is due to trapped excitons and biexcitons. Net optical gain of the order of 10 cm(-1) from trapped biexcitons can be detected by pump-and-probe transmission measurements as well as amplified spontaneous emission measurements using the variable stripe-length technique.