Creating a material that can alter its optical properties in real time has proved challenging for scientists, but a team at Imperial College London has finally achieved a major breakthrough by developing a material that can transition from clear to reflective and back again using an array of gold nanoparticles.

As well as overcoming the difficulty in creating a tunable material on such a tiny scale, the big innovation here was using electrical currents as the trigger for the shift in optical properties. Previous nanoscopic systems have successfully altered optical properties through chemical processes, but this was not reversible.

The team utilized an array of gold nanoparticles, localized between two different liquids. The density of the nanoparticle layer could then be modulated by applying a small electrical voltage. This either made the nanoparticles cluster together densely and create a reflective mirror, or separate apart enough to allow light to pass through and make the material transparent.

"It's a really fine balance – for a long time we could only get the nanoparticles to clump together when they assembled, rather than being accurately spaced out," says co-author of the study, Professor Joshua Edel. "But many models and experiments have brought us to the point where we can create a truly tuneable layer."

The novel reversibility of the process allows for, not only a future of funky alternating mirror-windows in your house, but an assortment of practical applications, including tunable optical filters for telescopes or chemical sensors.

Take a look at the material switch from transparent to reflective in the video below.

The research was published in the journal Nature Materials.

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