Cadmium Sulfide Quantum Dot Lasing In Room Temperature Liquid Solution

Georgia Tech Research Corporation (Atlanta, GA) earned U. S.  Patent 7,778,301for a lasing complex comprising a room temperature solution containing cadmium sulfide (CdS) quantum dots. Optical gain has been observed in CdS nanocrystal quantum dots in strong confinement regime in toluene solution at room temperature using femtosecond transient absorption techniques.

According to inventors Mostafa A. El-Sayed, Wei Qian and Qusai Darugar the optical gain lifetime is measured to be 20 picoseconds under pump fluence of 0.77 mJ/cm². The relative lower gain threshold compared to that of CdSe quantum dots is attributed to the long lifetime of fluorescence and biexcitons and the relatively sharp photoluminescence linewidth. The CdS nanocrystals are excellent gain media for semiconductor quantum dot based blue lasers.

The Georgia Tech inventors developed nanoparticle lasing complexes that comprise a room temperature solution comprising quantum dots that each have an average diameter between 3 and 6 nanometers. All quantum dots in the solution are substantially the same size, and each quantum dot is capped with a capping material that passivates and protects the surface, maximizes the emission yield, reduces the line width of the emission from the quantum dot, and acts to suspend the quantum dot in the solution. The solution may preferably comprise toluene or water. The capping material may comprise a wide bandgap semiconductor material such as zinc sulfide, or an organic molecule such as tri-n-octylphosphine oxide (TOPO). 

The inventors also developed an exemplary laser apparatus comprising cadmium sulfide (CdS) quantum dots in a room temperature liquid solution. The exemplary laser apparatus is based upon experimental results of optical gain dynamics in strongly confined CdS quantum dots dissolved in a toluene solution at room temperature using femtosecond transient absorption technique. Because of the long lifetime of electrons and holes and narrow fluorescence band width, one can observe gain with a low threshold in the solution of CdS quantum dots at room temperature. Laser apparatus comprising a solution of cadmium sulfide (CdS) quantum dots at room temperature is also disclosed.

Optical gain dynamics was 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 nm and gain measurement was studied at the fluorescence maxima (440 nm and 460 nm). The optical gain lifetime was found to be as long as 20 picoseconds under pump fluence as low as 0.77 mJ/cm². The low threshold is the result of long lifetime of electrons and holes and narrow emission band width. These results suggest that CdS quantum dots in solution are excellent gain media for optically pumped high power blue lasers. 

 Semiconductor nanocrystal quantum dots have attracted great attention due to their tunable electronic and optical properties arising from three-dimensional quantum confinement effects. The advantages of using semiconductor quantum dots in the strong confinement regime as gain media in lasing devices (due to their predicted reduced lasing threshold and improved temperature stability) are the driving forces in the development of semiconductor quantum dots based lasers.