Enhancement of optical gain in semiconductor nanocrystals through energy transfer

The development of optical gain in semiconductor nanocrystals requires a high concentration of electronic excitation energy. Generating and maintaining such a high excitation density is difficult because of fast Auger relaxation channels that operate under such conditions. In this article, the use of energy transfer within nanocrystal clusters is proposed as a means for concentrating excitation energy in a subset of optically active particles. Numerical simulations of the dynamic evolution of the system under moderate pumping intensity have been carried out to evaluate the potential efficacy of this strategy. Using realistic values for relevant rate constants, these simulations predict that energy transfer in nanocrystal clusters can reduce the effective gain threshold by 3- to 4-fold and can increase the gain lifetime by an order of magnitude as compared with those of samples of noninteracting nanocrystals. Strong performance enhancements are realized even if the energytransfer rate is slower than the Auger relaxation process that normally hinders the development of gain. The proposed scheme is entirely compatible with other nanocrystal lasing strategies, such as the use of type II heteronanocrystals and electrical pumping, and should contribute substantially to the ultimate goal of nanocrystal lasing under practical pumping conditions.