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(Nanowerk News) Rice University researchers have discovered a fundamentally different form of light-matter interaction in their experiments with gold nanoparticles.
They were not looking for it, but students at Rice chemist's lab, Stephan Link, discovered that exciting the microscopic particles unexpectedly produced an almost perfect modulation of the light they were broadcasting. This discovery could prove useful for the development of next-generation ultra-modern optical components for computers and antennas.
An article on research was published in the American Chemical Society ACS Nano ("Exploit evanescent field polarization for giant chiroptic modulation of achiral half-rings in gold").
The work results from the complex interactions between light and plasmonic metal particles that absorb and scatter light extremely efficiently. Plasmons are quasi-particles, collective excitations that move in waves on the surface of certain metals when excited by light.
Rice researchers studied the plasmonic structures of mill-like C-shaped gold nanoparticles to determine their reaction to circularly polarized light and its rotating electric field, particularly when the direction of rotation or the sense of rotation of the polarization was reversed. They then decided to study individual particles.
"We reinstated the simplest possible system in which we had only one arm of the reel, with only one direction of incident light," said Lauren McCarthy, a graduate student of Link Lab. "We did not expect to see anything. It was a total surprise when I placed this sample on the microscope and rotated my polarization from left to right. I thought, "Does it work and is it off?" This is not supposed to happen. "
She and her co-lead author Kyle Smith, a former Rice graduate, had to get to the bottom of things to understand why they saw this "giant modulation".
At first, they knew that bright polarized light at a particular angle to the surface of their sample of gold nanoparticles bonded to a glbad substrate would create an evanescent field, an oscillating electromagnetic wave that overlapped the surface of the glbad and imprisons the light in a parallel way. mirrors, an effect known as total internal reflection.
They also knew that circularly polarized light is composed of transverse waves. The transverse waves are perpendicular to the direction in which light travels and can be used to control the visible plasmon output of the particle. But when the light is confined, longitudinal waves also occur. Where the transverse waves rise and fall and from one side to the other, the longitudinal waves look like blobs pumped into a pipe (as shown by the shaking of a Slinky).
They discovered that the plasmonic response of C-shaped gold nanoparticles is dependent on the out-of-phase interactions between transverse and longitudinal waves in the evanescent field.
For the reel, the researchers found that they could alter the intensity of light by up to 50% by simply changing the direction of the hand of the circularly polarized light entry, thus changing the relative phase between the transverse and longitudinal waves.
When they divided the experiment into individual C-shaped gold nanoparticles, they discovered that the shape was important for the effect. Changing the manual nature of the polarized input has resulted in almost complete activation and deactivation of the particles.
Simulations of the effect made by Rice physicist Peter Nordlander, and his team confirmed the explanation of what the researchers observed.
"We knew we had an evanescent field and we could do something different, but we did not know exactly what," McCarthy said. "That did not happen until the simulations were complete, telling us what the light actually excited in the particles, and that it really corresponds to what the evanescent field looks like.
"This allowed us to realize that this can not be explained by the normal functioning of the light," she said. "We had to adjust our understanding of how light can interact with this type of structure."
The shape of the nanoparticle triggers the orientation of three dipoles (positive and negative charge concentrations) on the particles, McCarthy said.
"The fact that the half-ring has a radius of curvature of 100 nanometers means that the entire structure uses half a wavelength of light," she said. "We think it's important to excite the dipoles in this particular orientation."
Simulations have shown that the inversion of the incident polarization of light and the phase shift of the waves reversed the direction of the central dipole, greatly reducing the ability of the half-ring to scatter light under a single incident. The polarization of the evanescent field then explains the almost complete effect of activation and deactivation of the C-shaped structures.
"Interestingly enough, we've sort of made it through this work," said Link. "Flat metal surfaces also support surface plasmons such as nanoparticles, but they can be excited only by evanescent waves and do not disperse in the far field. We found here that excitation of nanoparticles of specific shape using evanescent waves produced plasmons with scattering properties different from those excited with free-space light. "
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