Light can behave in very unexpected ways when you squeeze it into small spaces. In a new paper in the journal Science, Mark Brongersma , a professor of materials science and engineering at Stanford University, and doctoral candidate Skyler Selvin describe the novel way they have used sound to manipulate light that has been confined to gaps only a few nanometers across – allowing the researchers exquisite control over the color and intensity of light mechanically.
The findings could have broad implications in fields ranging from computer and virtual reality displays to 3D holographic imagery, optical communications, and even new ultrafast, light-based neural networks.
The new device is not the first to manipulate light with sound, but it is smaller and potentially more practical and powerful than conventional methods. From an engineering standpoint, acoustic waves are attractive because they can vibrate very fast, billions of times per second. Unfortunately, the atomic displacements produced by acoustic waves are extremely small – about 1,000 times smaller than the wavelength of light. Thus, acousto-optical devices have had to be larger and thicker to amplify sound's tiny effect – too big for today's nanoscale world.
"In optics, big equals slow," Brongersma said. "So, this device's small scale makes it very fast."
Simplicity from the start
The new device is deceptively simple. A thin gold mirror is coated with an ultrathin layer of a rubbery silicone-based polymer only a few nanometers thick. The research team could fabricate the silicone layer to desired thicknesses – anywhere between 2 and 10 nanometers. For comparison, the wavelength of light is almost 500 nanometers tip to tail.
The researchers then deposit an array of 100-nanometer gold nanoparticles across the silicone. The nanoparticles float like golden beach balls on an ocean of polymer atop a mirrored sea floor. Light is gathered by the nanoparticles and mirror and focused into the silicone between – shrinking the light to the nanoscale.
To the side, they attach a special kind of ultrasound speaker – an interdigitated transducer, IDT – that sends high-frequency sound waves rippling across the film at nearly a billion times a second. The high‑frequency sound waves (surface acoustic waves, SAWs) surf along the surface of the gold mirror beneath the nanoparticles. The elastic polymer acts like a spring, stretching and compressing as the nanoparticles bob up and down as the sound waves course by.
The researchers then shine light into the system. The light gets squeezed into the oscillating gaps between the gold nanoparticles and the gold film. The gaps change in size by the mere width of a few atoms, but it is enough to produce an outsized effect on the light.
The size of the gaps determines the color of the light resonating from each nanoparticle. The researchers can control the gaps by modulating the acoustic wave and therefore control the color and intensity of each particle.
"In this narrow gap, the light is squeezed so tightly that even the smallest movement significantly affects it," Selvin said. "We are controlling the light with lengths on the nanometer scale, where typically millimeters have been required to modulate light acoustically."
Starry, starry sky
When white light is shined from the side and the sound wave is turned on, the result is a series of flickering, multicolored nanoparticles against a black background, like stars twinkling in the night sky. Any light that does not strike a nanoparticle is bounced out of the field of view by the mirror, and only the light that is scattered by the particles is directed outward toward the human eye. Thus, the gold mirror appears black and each gold nanoparticle shines like a star.
The degree of optical modulation caught the researchers off guard. "I was rolling on the floor laughing," Brongersma said of his reaction when Selvin showed him the results of his first experiments. "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."
The exceptional tunability, small form factor, and efficiency of the new device could transform any number of commercial fields. One can imagine ultrathin video displays, ultra-fast optical communications based on acousto-optics' high-frequency capabilities, or perhaps new holographic virtual reality headsets that are much smaller than the bulky displays of today, among other applications.
"When we can control the light so effectively and dynamically," Brongersma said, "we can do everything with light that we could want – holography, beam steering, 3D displays – anything."
