Enabling all-to-circular polarization upconversion by nonlinear chiral metasurfaces with rotational symmetry

TL;DR

This paper demonstrates a novel chiral metasurface design with high rotational symmetry that efficiently converts various polarized light into circularly polarized nonlinear signals, advancing nonlinear optics and metasurface engineering.

Contribution

It introduces a stacking strategy for chiral metasurfaces with high rotational symmetry enabling efficient polarization conversion and nonlinear upconversion, a significant advancement over previous designs.

Findings

Circularly polarized pump converts to nonlinear signals with high efficiency.

The intensity ratio of circular signals scales with the fourth and second power of local field enhancement.

The design responds solely to one circular polarization, enabling selective nonlinear responses.

Abstract

We implement a stacking strategy in designing chiral metasurfaces with high rotational symmetry, enabling quasi-bound-in-the-continuum (quasi-BIC) resonances characterized by absolute chirality. The rotational symmetry allows a circularly polarized pump to be converted into a circularly polarized nonlinear signal. Meanwhile, our bilayered metasurface can be engineered to respond solely to one selected circular polarization. Consequently, integrating resonant chiral response and rotational symmetry endows a unique category of metasurfaces to upconvert any linear or unpolarized pump into a circularly polarized nonlinear signal. Our results reveal that when such a metasurface is subjected to a linearly polarized pump, the intensity ratio of the resultant circularly polarized signals varies with the order of the nonlinear process. Counterintuitively, this ratio scales as the fourth power of the local field enhancement in the second harmonic process and the second power in the third harmonic process. Our work offers a comprehensive theoretical description of the nonlinear processes in chiral structures with rotation and provides universal guidelines for designing nonlinear all-dielectric metasurfaces with a strong chiral response.