Nonlinear materials allow directional discrimination of waves
Normally, a glass window doesn’t care where a ray of light came from. But special kinds of glass or plastic could be a bit pickier.
Nonlinear materials could distinguish between two rays of light coming from opposite directions, say two Italian physicists. Blocking a ray from one direction and allowing in a ray from the other could be useful for making a one-way street for light.
Textbook optics prohibits this kind of directional discrimination. Everyday linear materials are governed by the reciprocity theorem, which says that a beam of light coming from the left will pass through and reflect off a material in the same way as a beam of light coming from the right.