Control of chirality and directionality of nonlinear metasurface light source via twisting

TL;DR

This paper demonstrates how twisting multilayer nonlinear metasurfaces enables in situ, multidimensional control over light's polarization and directionality, including singularities and unidirectional emission, advancing nonlinear optical source design.

Contribution

It introduces the concept of twisted multilayer nonlinear metasurfaces for dynamic, in situ tuning of nonlinear susceptibilities and polarization states, a significant advancement over traditional static tuning methods.

Findings

Existence of nonlinear susceptibility singularities in twist angle and interlayer gap.

Ability to achieve unidirectional emission with full polarization control.

Demonstration of polarization singularities like C points and V points in nonlinear metasurfaces.

Abstract

Metasurfaces have revolutionized nonlinear and quantum light manipulation in the past decade, enabling the design of materials that can tune polarization, frequency, and direction of light simultaneously. However, tuning of metasurfaces is traditionally achieved by changing their microscopic structure, which does not allow \emph{in situ} tuning and dynamic optimization of the metasurfaces. In this Letter, we explore the concept of twisted bilayer and tetralayer nonlinear metasurfaces, which offer rich tunability in its effective nonlinear susceptibilities. Using gold-based metasurfaces, we demonstrate that a number of different singularities of nonlinear susceptibilities can exist in the parameter space of twist angle and interlayer gap between different twisted layers. At the singularities, reflected/transmitted light from the nonlinear process (such as second-harmonic generation) can either become circularly polarized (for C points), or entirely vanish (for V points). By further breaking symmetries of the system, we can independently tune all aspects of the reflected and transmitted nonlinear emission, achieving unidirectional emission with full-Poincar\'e polarization tunability, a dark state (V-V point), or any other bidirectional output. Our work paves the way for multidimensional control of polarization and directionality in nonlinear light sources, opening new avenues in ultrafast optics, optical communication, sensing, and quantum optics.