Credit: © 2008 ACS

Quantum dots are routinely used in imaging applications, especially in the life sciences, but researchers are still developing a more detailed understanding of their optical properties in order to make lasers based on quantum dots. One problem is that as the quantum dots get smaller, transitions that do not contribute to lasing become more likely. However, Lijun Wang and co-workers1 at the University of Erlangen and the Ludwig Maximilians University in Munich have shown that liquid droplets containing nanocrystal quantum dots can lase at densities some two orders of magnitude lower than previously reported or predicted by theory.

Wang and co-workers dissolved quantum dots with a cadmium selenide core and a zinc sulphide shell in water — with glycerine added to avoid evaporation — and then used a home-built device to generate droplets with diameters that ranged between 10 and 50 micrometres. The quantum dots had a core radius of 2.6 nm, which gave a peak in the photoluminescence at a wavelength of 625 nm. Because the droplets were charged, they could be trapped by electric fields, which allowed the Erlangen–Munich team to excite them with a laser and stimulate laser action.

Dots without a shell did not lase because, the authors argue, the ZnS coating reduced photo-oxidation of the CdSe core. The reasons for the system lasing at such a low quantum-dot density were not clear, although the authors suggest that it might be caused by the dots accumulating near the surface of the droplet, rather than being distributed uniformly throughout it.