Synopsis: Nanoweb Catches Light

Experiments show that normally transparent nanorods can absorb or reflect nearly $100%$ of light at a specific wavelength when the rods are arranged in a periodic two-dimensional array.

A good light-blocking shade doesn’t have any holes. Or so you’d think. Physicists have shown that an open weblike array of nanorods absorbs $25$ times more light at a specific wavelength than the same material spread out in a thin sheet. As reported in Physical Review Letters, similar geometric arrangements might inspire new optical filters or optomechanical devices that couple light to mechanical motion.

Nanostructures that interact strongly with light have many potential applications in biology and chemistry. Current research primarily uses metallic nanostructures, such as gold nanoparticles, whose electrons form surface plasmon resonances that strongly absorb or scatter light. But theory suggests that strong light interactions can also be produced by a periodic arrangement of free-standing dielectric nanostructures.

To focus on the effects of geometry, Petru Ghenuche of the Laboratory for Photonics and Nanostructures in Marcoussis, France, and his colleagues chose nominally transparent dielectric nanostructures—specifically silicon nitride nanorods—which don’t individually have any strong interaction with light. The team lined up the $500$-nanometer-wide nanorods in rows $3$ microns apart. They shined infrared light on this single-layer array from different angles and with various wavelengths and then measured the transmission and reflection. Most of the light passed through the nanorods, but light in a very narrow wavelength band (set by the spacing and angle of incidence) was almost $100%$ reflected. The nanorod arrangement looks like a very sparse diffraction grating (with the rods covering only $15%$ of the surface area), but the authors’ theoretical model showed that their nanostructure behaves more like a crystal, where the rods (acting like a monolayer of atoms) scatter light multiple times in between each other. – Michael Schirber

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