| Nov. 29, 2018 |
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( Nanowerk News ) Researchers at Rice University have discovered a fundamentally different form of light-matter interaction in their experiments with nanoparticles. 39; gold.
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They were not looking for it, but chemistry laboratory students Rice, Stephan Link, discovered that exciting the microscopic particles just occasionally 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.
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| Researchers at Rice University study the effect of light on nanoparticles (left) and simple C-shaped (right) have unknown effect on individual particles. The just stimulation of the particles allowed an almost perfect modulation of the light that they diffuse via their plasmonic response. This discovery could prove useful for the development of chips for next-generation optical components for computers and antennas. (Image: Link Research Group / Rice University)
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An article on this research was published in the American Chemical Society ACS Nano ("Exploiting Evanescent Field Polarization for Giant Chiroptic Modulation from Achir Gold Half Rings") .
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The work stems 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.
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Rice researchers investigated the plasmonic structure of mill-like C-shaped gold nanoparticles to determine how they responded to circularly polarized light and its rotating electric field, particularly when the direction of rotation or direction of rotation of the polarization were reversed. . They then decided to study individual particles.
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"We integrated it into 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 from the Link laboratory. "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. "
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She and her co-lead author Kyle Smith, a recent Rice graduate, had to dig deeper to understand why they saw this "giant modulation".
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At first, they knew that a bright polarized light at a particular angle to the surface of their sample of gold nanoparticles attached to a glbad substrate would create an evanescent field, an electromagnetic wave oscillating that would overlap the surface of the glbad and imprison the light like parallel mirrors, an effect known as total internal reflection.
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They also knew that circularly polarized light was 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 somehow resemble the formation of blobs pumped into a pipe (as shown by the shaking of a Slinky).
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| Circularly polarized light delivered at a particular angle compared to C-shaped gold nanoparticles producing a different plasmonic response, according to researchers at Rice University. When the incident polarized light is pbaded from the left hand (blue) to the right hand (green) and vice versa, the light of the plasmons is turned on and off almost completely. (Image: Link Research Group / Rice University)
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They found that the plasmonic response of C-shaped gold nanoparticles depends on the out-of-phase interactions between transverse and longitudinal waves in the evanescent field.
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With regard to the windmill, the researchers found that they could modify the intensity of light by up to 50% by simply changing the handling factor of the circularly polarized light input, thus modifying the relative phase between the transverse and longitudinal waves.
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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.
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The simulations of the effect carried out by the rice physicist, Peter Nordlander, and his team confirmed the explanation of what the researchers observed.
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"We knew we had an evanescent field and we knew he could do something different, but we did not know exactly what," McCarthy said. "It did not come to us until we had finished the simulations, telling us what the light really excited in the particles, and that it really corresponds to what the evanescent field looks like.
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"This led us to understand that this was not 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."
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The shape of the nanoparticle triggers the orientation of three dipoles (positive and negative charge concentrations) on the particles, McCarthy said.
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| As seen under the lens of a microscope, researchers at Rice University discovered that circularly polarized light – polarization Right-polar circular (RCP) is shown here – though the possibility of significantly altering the plasmonic efficiency of C-shaped gold nanoparticles. The light input triggered the modification by shifting the phase relationship of transverse and longitudinal waves in an evanescent field exciting the particle. This then controlled the level of plasmonic response. The letter k represents the direction of entry of light onto the particle after pbading through a prism. (Image: Link Research Group / Rice University)
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"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."
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Simulations have shown that inversion of incident polarization and phase shift of waves reverse 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.
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"Interestingly enough, we have sort of closed the loop with this work," Link said. "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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