## What is it about?

Most materials we know and use everyday day act like a two-way street for light. That means that it does not really matter which side the light is coming from; it will be the same fraction of the incident light's intensity passing through. This is a core fundamental principle in Physics that we call "reciprocity". Even a one-way mirror is actually symmetric to the fraction of light shining through it; its name is rather deceptive. You can stand on one of the sides of a "one-way mirror" and remain unseen, only when you keep the lighting low in that side while having very bright lights on the other side. Only very special material and special conditions, like using a strong magnet, can break reciprocity, but these systems are either hard to implement or to scale for different wavelengths of light, like for visible or infrared light, or end-up being quite bulky. This work shows simple dielectric materials, like glass or silicon, if cleverly structured, can let different fractions of light pass through them depending on which side the light is coming from. How should this structure look like? There are two aspects that are important. Firstly, let's think of taking a dielectric material and creating a tiny object, quite tinier than the wavelength of light. This tiny object should have an asymmetric shape, namely it should look very different if we flip its upside down; like for example a cone or a pyramid. Secondly, we should then think of many such tiny objects placed next to each other but at a distance above a special threshold that depends on the wavelength of light and the angle with which light hits the structure. For a regular piece of material with no structuring or for a structured material but when the distance between the individual building-block objects is below such a threshold, light comes out of the structure only via one pathway. However, for a structure where the distance between these individual objects is above this special threshold, light comes out of structure via different pathways, that are called Bragg beams. It is these multiple Bragg beams together with the asymmetric shape of the structure that allow the light shining through the structure to be different depending on which side light comes from without violating the reciprocity principle.

## Why is it important?

This simple prototype system made of sculpted dielectric materials can be designed to operate at different wavelengths of light and can inspire practical implementations for photonic components that protect lasers from back-reflections or regulate light "traffic" in light-based circuits.

## Read the Original

This page is a summary of: Breaking Transmission Symmetry Without Breaking Reciprocity in Linear All-Dielectric Polarization-Preserving Metagratings, Physical Review Applied, February 2022, American Physical Society (APS),
DOI: 10.1103/physrevapplied.17.024064.
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