One-way mirror

Schematic of NIST's one-way metamaterial. Forward travelling green light (left) or red light passes through the multilayered block and comes out at an angle due to diffraction off of grates on the surface of the material. Light travelling in the opposite direction (right) is almost completely filtered by the metamaterial and can't pass through. Credit: Xu/NIST
New NIST Metamaterial Gives Light a One-Way Ticket