Tunable metasurfaces offer a way to control the light that is used to power many types of applications, from computer and virtual reality (VR) displays to 3D holographic imagery, optical communications, and neural networks. However, the ability to manipulate the optical resonances of metasurfaces at speeds exceeding gigahertz (GHz) frequencies, allowing precise control of light, remains a challenge. To achieve this goal, researchers at Stanford University developed a nanodevice that uses electrically-driven surface acoustic waves (SAWs) and the extreme light concentration provided by plasmon gaps to electrically manipulate metasurface response. To build the nanodevice, the researchers created a nanoparticle-on-mirror configuration that combines plasmonic and soft materials. Professor Mark Brongersma and researcher Skyler Selvin developed a nanoscale device that uses high-frequency surface acoustic waves (SAWs) to manipulate light at the nm scale. Courtesy of Mark Brongersma and Skyler Selvin. Acoustic waves vibrate very fast, but they produce extremely small atomic displacements. To amplify the sound produced by acoustic waves, acousto-optical devices are typically large and bulky. “In optics, big equals slow,” professor Mark Brongersma said. “So, this device’s small scale makes it very fast.” The scientists placed an array of 100-nm gold nanoparticles on a mirror and layered ultrathin, squeezable polymer spacers in between. The polymer layers are a few nm thick and can be fabricated to thicknesses between 2-10 nm. They then used an interdigitated transducer (IDT) to send high-frequency SAWs across the mirror (metasurface) and shift its resonant response. In the nanodevice, light is confined to nm-sized plasmon gaps between the mirror and the nanoparticles. When light is shined on the device, it is squeezed into the plasmon gaps and shrunk to the nanoscale. The size of the plasmon gap is controlled by modulating the SAWs. The gap size determines the color of the light resonating from each nanoparticle, so by modulating the acoustic wave, the researchers can control the color and intensity of each particle. A change in the size of the gaps of just a few atoms is enough to produce an outsized effect on the light. “In this narrow gap, the light is squeezed so tightly that even the smallest movement significantly affects it,” researcher Skyler Selvin said. “We are controlling the light with lengths on the nanometer scale, where typically millimeters have been required to modulate light acoustically.” The researchers found that the SAWs produced mechanical deformations in the polymer, resulting in nonlinear mechanical dynamics that caused unexpectedly large levels of strain and spectral tuning. The degree of optical modulation surprised the team. “I thought it would be a very subtle effect, but I was amazed how much nanometer changes in distance can change the light scattering properties so dramatically,” Brongersma said. The team used the nanodevice to tune light scattering at speeds approaching the giagahertz regime, to realize fast, dynamic control of metasurface response. The exceptional tunability, small form factor, and efficiency of the device could make it a valuable tool for developing ultrathin video displays, ultrafast optical communications based on high-frequency acousto-optics, and holographic VR headsets that are much smaller than today’s bulky displays, among other applications. “When we can control the light so effectively and dynamically, we can do everything with light that we could want — holography, beam steering, 3D displays — anything.” Brongersma said. The research was published in Science (www.doi.org/10.1126/science.adv1728).
