The functionality of an optical diode is simple to understand, as explained by MIT’s Caroline Ross, whose lab recently published a paper on the diode: “It lets light go one-way, but blocks it from going the other way.” In that sense, it’s no different from electrical diodes that have existed for decades. (While the electronics term is “diode,” the preferred term in photonics is “optical isolator.”)
But controlling photons presents challenges far more complex than controlling electrons. “You need to have a material where light propagating in one direction behaves differently from light propagating in the opposite direction,” says Ross. In order to achieve that, you need a transparent material that when magnetized creates an asymmetrical medium, which allows you to control the light’s direction.