By applying a coating on a clear window, it has been possible to convert the window into a one-way reflective mirror. If applied correctly, the coating allows people from inside the room to see outside, but for those outside the room, the windowpanes act like mirrors, preventing them from looking in. To revert to the clear glass, it is necessary to peel off the coating. Clearly, this is not a reversible process.

A research team from the Imperial College London has now developed a process by which window panes can instantly turn reflective, or clear, as the user wishes. The material for the coating they use is made of an array of gold nanoparticles. This is different from the earlier chemical process, which, although based on nanoscopic systems that did alter the optical properties of the glass pane, was not reversible.

Using gold nanoparticles that are thousands of times smaller than the width of a single human hair, the researchers placed them in an array between two liquids that normally do not mix. On application of voltage, the nanoparticles assembled themselves into a new configuration of close dense formation. This made the surface reflective. Removal of voltage allowed the nanoparticles to drift apart, and the surface reverted to its transparent nature. The applied voltage modulated the density of the nanoparticle layer to allow or disallow light passing through the liquid layers.

According to Professor Joshua Edel, coauthor of the study, the team had achieved a really fine balance. For a long time, the applied voltage only served to form a clump of the nanoparticles as they assembled, rather than allowing them to space out evenly and accurately. The team had to build several models and conduct innumerable experiments to reach the point where they had a really tunable layer of nanoparticles.

Anthony Kucernak, a professor in the Department of Chemistry at Imperial, explains the phenomenon. The application of a specific voltage drives the nanoparticles, and they travel to an interface. The nanoparticles congregate here to form a mirror, reflecting the incident light, and not allowing it to pass through. Switching to a different voltage or removing the voltage allows the nanoparticles to move away from the interface, making the mirror transparent again.

Scientists have already been working with smart windows with the ability to adjust to sunlight falling on them. Such windows self-shade, allowing only a part of the sunlight falling on them to pass through. This helps in regulating the temperature of a building, and saving on expenditure on heating and cooling. Other developments turn windows into solar power generators, augmenting the power supply, and turning skyscrapers into potential solar farms.

The new window/mirror innovation will further advance the temperature control ability for windowpanes. However, this is not the only application for this technology. According to the research team, they can use this technology to create tunable optical filters for telescopes. This will not only help in astronomy, it will also make chemical sensors more sensitive. However, the team from Imperial College first wants to increase the response time of the nanoparticles.