Heat-assisted nonreciprocity

One-way light transmission can be realized by embracing, rather than mitigating, thermal effects in carefully engineered metasurfaces.
Making light travel in one direction only is difficult owing to the principle of Lorentz reciprocity. Simply put, Lorentz reciprocity states that if light flows from left to right, it must also be able to flow from right to left. This makes it hard to protect lasers from back reflections. Indeed, light emitted from a laser can be reflected to the laser itself, causing damage. To prevent that from happening, an element that breaks reciprocity is necessary. Nonreciprocal elements typically exploit magnetic effects which, unfortunately, are weak. As a result, those elements are bulky and difficult to integrate with other photonic technologies. Alternative methods have therefore received considerable interest in recent years.