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.

But nonlinear materials, which can change as light passes through them, play by different rules.

“Without nonlinearity this asymmetry would not be possible,” says Giulio Casati, a physicist at the University of Insubria in Como, Italy.

A textbook pendulum, for instance, swings with a steady frequency that can be calculated from equations. But a nonlinear pendulum’s frequency changes over time in a way that can be worked out only with a computer.

Casati mathematically modeled the behavior of light passing through two layers of nonlinear material. The light changes the properties of the materials as it passes through, which in turn changes the behavior of light in complex ways.

Thanks to this back-and-forth dance, the frequency of light that can pass through these materials depends on the direction of the light, he reports in the April 22 Physical Review Letters.

“Other people have used nonlinearity, but they use it in a different way,” says Panayotis Kevrekidis, a mathematical physicist at the University of Massachusetts Amherst who wasn’t involved in the research.

Previous attempts to break down the reversibility of light used photonic crystals. Those materials can only block a portion of the light that has been boosted to twice its original frequency.

Casati’s initial simulations describe a way to transmit about 80 percent of the light traveling in one direction while blocking about 70 percent of the light coming from the opposite direction.

This selectivity could be useful for making wave diodes. Just as a traditional diode allows electrical current to flow only one direction in a piece of electronics, a wave diode could guide the flow of light in quantum or optical computing.

“This simple model may also apply to more general situations, like acoustics or different kinds of waves,” says Stefano Lepri, a physicist at the Italian National Research Council Institute for Complex Systems in Florence and a coauthor of the study. Materials that respond to sound waves nonlinearly could, he suggests, be useful for one-directional soundproofing.

Image: A proposed wave diode would allow light coming from the left to pass through (image on left) but reflect light coming from the other direction (right). (Stefano Lepri/Physical Review Letters)

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