Let There Be Light

The light emitted by a display literally could not be any smaller or faster moving. As a fundamental point-like particle, the photon is considered to be massless, and the speed of light is the absolute cosmological speed limit. So when it comes to building displays that are thinner and with higher refresh rates, light itself is never the limiting factor. Rather, it is the LEDs and other supporting electronics that add the physical bulk and slow down the refresh rate in modern displays.

So if we ever want to have ultrathin displays, super-lightweight virtual reality headsets with rapid refresh rates, or even blazing fast optical communications technologies, we will need to take a different approach when it comes to generating or manipulating light. One such approach was recently proposed by a group of researchers at Stanford University. They have exploited some of the more exotic behaviors of light by squeezing it into nanometer-sized gaps in a substrate and vibrating it at billions of cycles per second with acoustic waves.

Lighting the path forward

This highly unusual acousto-optical system allows for precise control of the color and intensity of tiny points of light on an ultrathin surface. Because the frequency of the surface acoustic waves that modulate the light is so high, it also enables the device to have an extremely high refresh rate.

The device is built around a surprisingly simple structure. At its base is a thin gold mirror, which reflects and traps light. On top of that, the researchers deposit a film of silicone-based polymer, just a few nanometers thick. To put this scale into perspective, visible light has a wavelength around 500 nanometers, which is many times thicker than the polymer layer.

Sprinkled across this polymer film are 100-nanometer gold nanoparticles, like tiny metallic beach balls floating on a rubbery ocean. When light is shined into this structure, it becomes trapped and focused into the nanoscale gaps between each gold nanoparticle and the gold mirror beneath it. This confinement causes the light to behave in unusual ways, amplifying its interaction with the surrounding materials.

Good vibrations

Using a device called an interdigitated transducer, the researchers generate surface acoustic waves that ripple across the gold mirror. These sound waves vibrate at nearly a billion times per second. As they pass under the nanoparticles, the elastic polymer stretches and compresses slightly, causing the nanoparticles to bob up and down.

These vertical motions, though only on the scale of a few atoms, are enough to change the gap between each nanoparticle and the mirror. Since the size of that gap directly affects how light resonates within it, even these minuscule changes cause significant shifts in the color and intensity of light emitted from each point. By dynamically adjusting the acoustic waves, the researchers can precisely modulate the light (color, brightness, and timing) with nanometer-level control.

When viewed from the side with white light illumination, the activated system produces a vivid display of twinkling, multicolored dots against a jet-black background that looks like stars flickering in the night sky. Light that does not interact with a nanoparticle is absorbed or reflected away by the gold mirror, making only the actively modulated light visible to the observer.

At present, the work is still in the early stages. But with additional refinement, the team believes their technology could be useful in everything from traditional to holographic displays in the future.