A team of researchers from the University of Stanford has just taken a key step in the control of light on a nanometric scale. Thanks to a device that is both simple and revolutionary, they managed to handle with extreme precision the color and intensity of light using … sound. This progress opens up fascinating perspectives in many areas, from holographic display to virtual reality, including optical neural networks.
Millimeter challenges for a light of the order of the nanometer
For decades, light has fascinated scientists, as much by its wave nature as by the difficulty in controlling it on a very small scale. In the world of optics, miniaturizing the components makes it possible to increase the speed, precision and efficiency of the devices. But modulate light on a nanometric scale – well below its own wavelength, which is around 500 nanometers – has so far been a major challenge.
The use of sound to modify light, called acousto-optic, has been known for a long time. However, it has always required relatively voluminous devices because the movements produced by acoustic waves are tiny: about a thousand times smaller than the wavelength of light. To amplify these effects, the old devices had to be large, therefore slow – a constraint incompatible with current and compactness needs.
An engineering of a misleading simplicity
It is here that the innovation of the Stanford team, led by Professor Mark Brongersma and doctoral student Skyler Selvin, changes the situation. Their device is based on an architecture as elegant as it is effective: a thin layer of polymer based on silicone, deposited on a gold mirror, itself surmounted by a network of gold nanoparticles.
The key lies in the thickness of the polymer layer, controlled with an accuracy of the order of a few nanometers (between 2 and 10 nm). This elastic film acts as a spring: when subjected to very high frequency acoustic waves (of the order of Gigahertz), it contracts and dilates imperceptibly. These tiny vibrations are enough to modify the spacing between the golden nanoparticles and the mirror – a tiny change, but with the spectacular effects on the light.
Unprecedented light-sound interaction
When a white light is projected in this system, the light is confined between nanoparticles and mirror. However, on this scale, even variations in a nanometer can radically modify the way in which light is diffused. By adjusting acoustic vibrations, researchers can thus vary the color and intensity of the light emitted by each nanoparticle.
The result is as aesthetic as it is fascinating: in the dark, nanoparticles shine like a starry sky, each sparkling of a different color. The golden mirror reflects the unrivaled light, strengthening the contrast and giving each light point a striking shine.
This level of optical modulation even surprised the researchers. “Jhe thought it would be a very subtle effect, but I was amazed by the extent of the change “Says Brongersma. The effectiveness of this system demonstrates how the interaction between mechanical and light waves can be exploited to so far inaccessible scales.
Credit: Pixabay/CC0 Public domain
Towards concrete applications in daily life
Beyond the demonstration of principle, the implications of this technology are considerable. It could allow the creation of new types of ultra-thin screens, capable of displaying images with an unequaled richness of color and depth. Virtual or more light, more precise and less energy -consuming virtual reality headsets could also benefit.
In the field of optical communications, this rapid and fine modulation of light could be used to transmit more information, faster and with increased energy efficiency. Finally, this technology opens the way to photonic neural networks, using light rather than electricity to make calculations – a promising track for the artificial intelligence of tomorrow.
A world to be reinvented on a nanometric scale
This advance marks a turning point in the way we can interact with light. Thanks to a compact, fast, and surprisingly simple to manufacture, Stanford researchers show that we can now manipulate the fundamental properties of light on scales of a few atoms.
It is no longer simply a question of miniaturizing the existing, but completely reinventing the way in which light can be controlled, modulated and exploited. A revolution in the making, made possible by the alliance of mechanics, optics and the science of materials.