Anti-Stokes Fluorescence of Cadmium Sulfide Quantum Dots Suspended in Condensed Media

Recently the first discovery of net laser cooling of a semiconductor has been announced: a group of Singapore researchers succeeded in cooling nanometer-thick belts of cadmium sulfide (CdS) grown by chemical vapor deposition on a silicon substrate with holes etched into it to permit optical access of a green laser beam onto the belts. This method of preparing the sample is laborious and not well suited to practical applications. A far cheaper method would be to suspend nanometer-sized quantum dots of CdS in a solid polymer host. Such polymers could be easily molded into different geometries depending on the desired applications. The present work demonstrates that commercially available 6-nm-diameter CdS dots can be dispersed in poly methyl methacrylate (commonly known as plexiglas or lucite). When optically pumped by the 514.5-nm line from a continuous-wave argon-ion laser, the quantum dots are found to emit at 475 nm, just as they do when suspended in liquid toluene. Assuming the dots decay entirely radiatively and that there is no absorption or energy transfer of the pump light by other modes or impurities, this anti-Stokes shift in the emission corresponds to an expected cooling coefficient of performance of 8% relative to the absorbed laser power. Future work should investigate whether a net temperature drop of a suitably optimized sample is in fact occurring.