Surface plasmons—collective, light-driven oscillations of electrons in metal—have given us stained glass, flat lenses, and home pregnancy tests. Now they bring us the mirror–window, a liquid mirror whose reflectivity can be tuned, or eliminated altogether, with an applied voltage.
Developed by researchers led by Alexei Kornyshev, Anthony Kucernak, and Joshua Edel at Imperial College London, the device makes use of gold nanoparticles inside a cell filled with two immiscible electrolyte solutions—one aqueous, the other oily. Dispersed throughout one phase or the other, the nanoparticles interact negligibly with light, and the cell is transparent. But when the particles form a dense monolayer at the liquid–liquid interface, their plasmon resonances couple to each other and they become optically reflective.
To enable switching between those two states, Kornyshev and his coworkers charged their nanoparticles by tethering organic molecules to them. At equilibrium, the charged particles prefer the aqueous phase. But a few-hundred-millivolt potential...
