Time-Varying Metasurfaces and Lorentz Non-Reciprocity

TL;DR

This paper introduces a novel approach using time-gradient metasurfaces to break the classical energy conservation constraint in optics, enabling non-reciprocal light manipulation and inelastic photon interactions for advanced photonic devices.

Contribution

It presents the theoretical development of time-gradient metasurfaces that relax energy conservation constraints, extending Snell's law and enabling non-reciprocal optical devices without magnetic fields.

Findings

Development of a generalized Snell's law with time-gradient phase discontinuity.

Demonstration of inelastic photon interactions and Doppler-like wavelength shifts.

Potential for creating magnetic-free optical isolators and advanced photonic applications.

Abstract

A cornerstone equation of optics, Snell's law, relates the angles of incidence and refraction for light passing through an interface between two media. It is built on two fundamental constrains: the conservation of tangential momentum and the conservation of energy. By relaxing the classical Snell law photon momentum conservation constrain when using space-gradient phase discontinuity, optical metasurfaces enabled an entirely new class of ultrathin optical devices. Here, we show that by eradicating the photon energy conservation constrain when introducing time-gradient phase discontinuity, we can further empower the area of flat photonics and obtain a new genus of optical devices. With this approach, classical Snell relations are developed into a more universal form not limited by Lorentz reciprocity, hence, meeting all the requirements for building magnetic-free optical isolators. Furthermore, photons experience inelastic interaction with time-gradient metasurfaces, which modifies photonic energy eigenstates and results in a Doppler-like wavelength shift. Consequently, metasurfaces with both space- and time-gradients can have a strong impact on a plethora of photonic applications and provide versatile control over the physical properties of light.