A mirror that reverses how light travels in time

Scientists have long theorized about time reflections, where a signal passing through a time "Interface" would act like it was traveling backward in time. Now a new study for the first time demonstrates time reflections with light waves. Previously, researchers knew of no way to change a material's optical properties in a fast, strong, and uniform enough way to create a photonic time interface that could generate time reflections for electromagnetic waves. A time reflection for light can occur if a material's optical properties change much faster than the variations in time of the signals involved, says study senior author Andrea Alù, an electrical engineer at the City University of New York's Graduate Center. The new time interfaces can stretch or compress the light signals in time. In contrast, time interfaces can enable time reversal very quickly and with little energy, Alù says. The new time interfaces may help scientists develop exotic new ways to control light, such as "Photonic time crystals." A normal crystal is a structure of many atoms arranged into a regular pattern in space, whereas a conventional photonic crystal possesses features smaller than the wavelengths of light it is designed to deal with.