Giant Modulation of Visible Light

Researchers studying the effect of light on pinwheels and single C-shaped gold nanoparticles have found an unknown effect on single particles. Stimulating the particles just right produced a near-perfect modulation of the light they scatter via their plasmonic response. The discovery may become useful in the development of chips for next-generation optical components for computers and antennas. (Source: Link Research Group, Rice U.)

Rice University researchers have discovered a funda­mentally different form of light-matter inter­action in their experiments with gold nano­particles. They weren’t looking for it, but students in the lab of Rice chemist Stephan Link found that exciting the micro­scopic particles just right produced a near-perfect modu­lation of the light they scatter. The discovery may become useful in the develop­ment of next-gene­ration, ultra­small optical components for computers and antennas.

The work springs from the compli­cated inter­actions between light and plasmonic metal particles that absorb and scatter light extremely effi­ciently. Plasmons are quasi­particles, collective exci­tations that move in waves on the surface of some metals when excited by light. The Rice researchers were studying pinwheel-like plasmonic structures of C-shaped gold nano­particles to see how they responded to circu­larly polarized light and its rotating electric field, especially when the handedness, or the direction of rotation of the polari­zation, was reversed. They then decided to study indi­vidual particles.

“We stripped it back into the simplest possible system where we only had a single arm of the pinwheel, with a single incident light direction,” said Lauren McCarthy, a graduate student in the Link lab. “We weren’t expecting to see anything. It was a complete surprise when I put this sample on the micro­scope and rotated my polari­zation from left- to right-handed.” She and Kyle Smith had to go deep to figure out why they saw this giant modu­lation.

At the start, they knew shining polarized light at a particular angle onto the surface of their sample of gold nano­particles attached to a glass substrate would create an evanescent field, an oscil­lating electro­magnetic wave that rides the surface of the glass and traps the light like parallel mirrors, an effect known as a total internal reflection. They also knew that circu­larly polarized light is composed of transverse waves. But when the light is confined, longi­tudinal waves also occur. Where trans­verse waves move up and down and side to side, longi­tudinal waves look something like blobs being pumped through a pipe.

They disco­vered the plasmonic response of the C-shaped gold nano­particles depends on the out-of-phase interactions between both transverse and longi­tudinal waves in the evanescent field. For the pinwheel, the researchers found they could change the inten­sity of the light output by as much as 50 percent by simply changing the handed­ness of the circu­larly polarized light input, thus changing the relative phase between the transverse and longi­tudinal waves.

When they broke the experiment down to indi­vidual, C-shaped gold nano­particles, they found the shape was important to the effect. Changing the handedness of the polarized input caused the particles to almost completely turn on and off. Simu­lations of the effect by Rice physicist Peter Nordlander and his team confirmed the expla­nation for what the researchers observed.

“We knew we had an evanescent field and we knew it could be doing something different, but we didn’t know exactly what,” McCarthy said. “That didn’t become clear to us until we got the simu­lations done, telling us what the light was actually exciting in the particles, and seeing that it actually matches up with what the evanescent field looks like. It led to our realization that this can’t be explained by how light normally operates,” she said. “We had to adjust our under­standing of how light can interact with these sorts of structures.”

The shape of the nano­particle triggers the orientation of three dipoles on the particles, McCarthy said. “The fact that the half-ring has a 100-nanometer radius of curva­ture means the entire structure takes up half a wave­length of light,” she said. “We think that’s important for exciting the dipoles in this particular orien­tation.”

The simu­lations showed that reversing the incident-polarized light handed­ness and throwing the waves out of phase reversed the direction of the center dipole, drama­tically reducing the ability of the half-ring to scatter light under one-incident handedness. The polari­zation of the evanescent field then explains the almost complete turning on and off effect of the C-shaped structures.

“Interes­tingly, we have in a way come full circle with this work,” Link said. “Flat metal surfaces also support surface plasmons like nano­particles, but they can only be excited with evanescent waves and do not scatter into the far field. Here we found that the excitation of speci­fically shaped nano­particles using evanescent waves produces plasmons with scattering pro­perties that are different from those excited with free-space light.” (Source: Rice U.)

Reference: K. W. Smith et al.: Exploiting Evanescent Field Polarization for Giant Chiroptical Modulation from Achiral Gold Half-Rings, ACS Nano 1211657 (2018); DOI: 10.1021/acsnano.8b07060

Link: Laboratory for Nanophotonics, Rice University, Houston, USA

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