Researchers have made a reversible electrotunable filter that can change between a reflective ‘mirror’ and a transmissive ‘window’. This discovery could help scientists create metamaterials with optical properties that could be changed in real time, making these materials useful for a range of applications, from tunable optical filters to miniature chemical sensors. Researchers from Imperial College London began by creating conditions for allowing 16 nanometer plasmonic nanoparticles to localize at the interface between two immiscible electrolyte solutions. Researchers applied a small voltage across this interface and demonstrated a tunable nanoparticle layer that could be dense or sparse, allowing for switching between a reflective mirror and a transparent surface. Experiments showed that optical properties such as reflectivity and spectral position of the absorption band could be varied within ±0.5 V. According to the team, this observed effect is in excellent agreement with theoretical calculations corresponding to the change in average interparticle spacing. By finely tuning the distance between nanoparticles in a single layer, researchers have made a filter that can change between a mirror and a window. Courtesy of Imperial College London. “Finding the correct conditions to achieve reversibility required fine theory; otherwise it would have been like searching for a needle in a haystack. It was remarkable how closely the theory matched experimental results,” said professor Alexei Kornyshev. In order to tune the optical properties of a single layer of nanoparticles, the spaces between them need to be precise and uniform. The distance between the nanoparticles determines whether the layer permits or reflects different wavelengths of light. At one extreme, all the wavelengths are reflected, and the layer acts as a mirror. At the other extreme, where the nanoparticles are dispersed, all wavelengths are permitted through the interface, and the layer acts as a window. “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. But many models and experiments have brought us to the point where we can create a truly tunable layer,” said professor Joshua Edel. In contrast to previous nanoscopic systems that used chemical means to change the optical properties, the team’s electrical system is reversible. This study opens a route toward realization of fully tunable metamaterials for applications ranging from new classes of sensors to superlenses, among others. The research is published in Nature Materials (doi:10.1038/nmat4969). This video shows the system in action. The layer first acts as a window onto a £10 note below, and then reflects the £1 coin above when a voltage is applied. Courtesy of Imperial College London.