Observation of optical gain in solutions of CdS quantum dots at room temperature in the blue region

The optical gain dynamics has been studied for two CdS quantum dot samples dispersed in toluene at room temperature. This was carried out by using femtosecond transient absorption technique with an excitation at $400\mathrm{nm}$ and gain measurement was studied at the fluorescence maxima (440 and $460\mathrm{nm}$ ). The optical gain lifetime was found to be as long as $20\mathrm{ps}$ under pump fluence as low as $0.77\mathrm{mJ}∕{\mathrm{cm}}^2$ . The low threshold is the result of long lifetime of electrons and holes and narrow emission bandwidth. These results suggest that CdS quantum dots in solution are excellent gain media for optically pumped high power blue lasers.