Credit: © 2007 Nature

Nanocrystal quantum dots have many useful optical properties and are widely used in various imaging applications. To date, however, they have not been exploited in laser applications because a variety of phenomena conspire to prevent nanocrystals from amplifying light. Now Victor Klimov and co-workers1 at the Los Alamos National Laboratory have overcome these problems.

The basic requirement for laser action in any material is that stimulated emission is stronger than absorption at the relevant wavelength, which happens when there are more electrons in the excited state than in the ground state. Klimov and co-workers study nanocrystals in which the ground state contains two electrons. When one of these is excited, it binds with the 'hole' it leaves behind to form an exciton. This single-exciton state cannot amplify light, however, because the emission and absorption essentially cancel each other out. And two-exciton states cannot amplify light either because non-radiative processes dominate over those that involve the emission of light.

The Los Alamos team get around this problem by working with nanocrystals that have a cadmium sulphide core and a zinc selenide shell. The electrons tend to reside in the core, whereas the holes prefer the shell, and the resulting charge imbalance creates an electric field that shifts the absorption spectrum. Laser action is now possible because the peaks of the absorption and emission spectra no longer occur at the same wavelength.