The optical properties of semiconductor structures at the nanoscale are governed by excitons—electron-hole pairs held together by their mutual electrostatic attraction. Excitons are created when a light beam excites a semiconductor, and after a short time —known as lifetime —they recombine producing light emission.

Excitons are also of interest for their potential in the study of fundamental quantum phenomena resulting from their coherent and collective behaviour. An example is Bose-Einstein condensation, which has been observed in atoms, and that according to theoretical studies should also occur in collections of excitons. Now, scientists at Zhongshan (Sun Yat-Sen) University in China and colleagues at the University in Hong Kong,1 have observed the occurrence of quantum phenomenon known as super-radiance, due to excitons in a semiconductor nanostructure.

In an exciton, an electron and corresponding hole are spatially separated, giving rise to an electric dipole. Dipoles of different excitons are usually oriented randomly, but at sufficiently high concentrations their mutual interaction can lead to the creation of a dipole extended over large distances. In particular, if light excitation directly creates excitons in this collective state forming a large dipole, then the light emitted is known as “super-radiance”.

Fig. 1: Scanning electron micrograph of a typical tetrapod investigated.

He-Zhou Wang and colleagues studied super-radiance in a single ZnO tetrapod. They monitored the time evolution of light emission after excitation with an ultraviolet laser pulse (Fig.1). For low laser intensity—low exciton concentration—they observed an exponential decay in fluorescence having a lifetime of a few hundred picoseconds. For intensities above a given threshold, the light was emitted in very short pulses, with a width limited by the resolution of the experimental set-up. This observation confirmed that the excitons were super-radiant.

For He-Zhou Wang, the importance of the observation lies in the fact that it was observed in a nanostructure. “Because the diameter of nano-crystal is generally smaller than the half wavelength of light, it is possible that superradiance of exciton in single nano-crystal is the major collective emission phenomenon,” says Wang.

The findings could be crucial in the development of nanodevices and possibly quantum information processing and quantum communication devices.