Nanorod web to block light

nanorods are one morphology of nanoscale objects with dimensions ranging from 1–100 nm. produced by direct chemical synthesis from metals or semiconducting materials. A combination of ligands acts as shape control agents bonding to different facets of the nanorod growing at different rates to produce elongated objects with desired strengths. The nanorods find application in MEMS, in energy harvesting and light emitting devices, as tunable photoluminescence and particularly in display technologies due to property that the reflectivity of the rods can be changed by changing their orientation with an applied electric field.
To make a new generation of optical devices such as light filters, sensors and other applications it is desirable to have a property by which it can be easily tuned with the interaction of light.
Web-like structure
Researchers at the Laboratory for Photonics and Nanostructures in Marcoussis, France have shown that a web-like structure made up of an array of evenly spaced nanorods can to block almost 100% of a specific light wavelength. The photonic nanoweb consists of transparent freestanding dielectric silicon nitride nanorods around 500 nm thick covering only 15% of the surface area and lined up in a single layer of rows 3 µm apart. The researchers found that the two dimensional array can allow a broad range of light wavelengths to pass through it but light that has a wavelength of exactly 3.2 µm of infrared range cannot pass through and is almost totally reflected due to a change in the  optical response in a very narrow spectral range. But light with a wavelength of 3 µm is transmitted through the web while that at 3.2 µm sharply drops.
The nanoweb resembles a very sparse diffraction grating and behaves more like a crystal with the rods acting like a monolayer of atoms that multiply scatter light. The incident light is first scattered by each nanorod and then some of this scattered light impinges on the other nanorods and is scattered again due to which a constructive interference of light waves build up from this multiple scattering process in the plane of the scatterers and, finally, the sum of the scattered light is emitted in both the forward and backward directions.
In the forward direction, the light waves scattered by the rods and transmitted through the rods are cancelled out by destructive interference, leading to perfect optical extinction and complete light reflection.
No Bragg diffraction
In the nanoweb almost all photons interact with the scatterers in a single lattice plane, where as in Bragg diffraction the constructive interference involves a large number of planes.
Until now, research on nanostructures that interact strongly with light in this way was confined to metallic nanostructures, such as gold nanoparticles, in which collective oscillations of surface electrons (or so-called surface plasmon resonances) strongly absorb or scatter light. The new study shows that these strong interactions can also be produced by a periodic arrangement of freestanding dielectric structures, like silicon nitride nanorods.
It can be used to make a new generation of optical devices, including light filters, sensors and light-sensing applications. The finding will result in a structure resembling a type of diffraction grating made of nanorods regularly ordered in a 2D array that would perfectly reflect light of a specific wavelength.


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