Jennifer N. Cha and colleagues have taken a natural approach to create a new material for lasing applications. Emulating composite materials such as shell and bone, whose properties result from the cooperative organization of organic and inorganic components, Cha et al. have used specifically tailored polypeptides to organize inorganic quantum dots into microscopic laser cavities (Nano Lett. doi:10.1021/nl034206k; 2003).

The structures are created by mixing a solution of a positively charged diblock copolypeptide with a solution of negatively charged quantum dots — nanoscopic particles of a semiconducting material (in this case, cadmium selenide). Hollow, micrometre-size spheres form, which Cha et al. attribute to charge matching between the components of the mixture. The addition of silica nanoparticles to the mixture stabilizes the spheres, giving the double-layer structure of an inner wall of quantum dots and an outer wall of silica nanoparticles that can be seen in optical-microscope images (such as the one shown here, left).

If the microspheres are excited by a laser pulse, the quantum dots emit light spontaneously; the spherical cavity acts as a resonator, modifying the resultant photoluminescence spectra. Although the microspheres are in contact with one another at their outer silica layers, the photoluminescence is confined to well-separated rings (right-hand image): the light emitted by the quantum dots does not spread out, but is confined to the quantum-dot layer within the silica wall of each microsphere. If the intensity of the laser pulse is increased above a certain threshold value, a sharp peak appears in the photoluminescence spectra of each microsphere, and this increases rapidly with the energy of the laser pulse — a distinctive signature of lasing action in the microspheres.

Microspheres make room-temperature microcavity lasing possible, without the need for mirrors or gratings. And, as the fabrication method is quite straightforward, it should be possible to tune the properties of both the quantum dots and the microcavity for particular lasing applications. The microspheres could also be assembled into arrays or more complex three-dimensional structures, their silica coating ensuring that individual spheres retain their functionality.