According to KAUST, optical-cavity designs have sought efficiencies of light either by trapping the electromagnetic wave, or confining light to the active region of the device to increase absorption. Most use micrometre-or nanometre-scale spheres in which the light propagates around in circles on the inside of the surface, a phenomenon called whispering gallery mode.
Former KAUST scientist Der-Hsien Lien, a postdoctoral researcher at the University of California, Berkeley, and his colleagues from China, Australia and the US have shown that a more complex geometry comprising convex nanoscale shells improves the performance of photodetectors by increasing the speed at which they operate and enabling them to detect light from all directions. The work is described in Advanced Materials.
Surface effects play an important role in the operation of some devices, said KAUST principal investigator, Jr-Hau He. Nanomaterials offer a way to improve performance because of their high surface-to-volume ratio. “However, although nanomaterials have greater sensitivity in light detection compared to the bulk, the light–matter interactions are weaker because they are thinner,” said He. “To improve this, we design structures for trapping light.”
KAUST said its researchers made their spherical multi-nanoshells from zinc oxide. They immersed solid carbon spheres into a zinc-oxide salt solution, coating them with the optical material. Heat treatment removed the carbon template and defined the geometry of the remaining zinc-oxide nanostructures, including the number of shells and the spacing between them. Lien and colleagues were reportedly able to engineer the interaction between outer and inner shells to induce a whispering gallery mode and light absorption near the surface of the nanomaterial.
The team incorporated their nanoshells into a photodetector. The symmetry of the spherical nanoshells meant that the whispering gallery mode could be excited with little dependence on the incident angle or the polarisation of the incoming light.
Previous photodetectors based on metal-oxide nanoparticles have been hindered by their lack of speed, with some devices taking as long as several hundred seconds to respond. Using zinc-oxide nanoshells, photodetectors are said to have responded in 0.8 milliseconds.
“This strategy can be applied to other work, such as solar cells and water-splitting devices,” said He. “In the future, we will look at different material systems and design structures that also improve device performance in these other applications.”
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