# Freeform Mode-Engineered Metasurfaces

## TL;DR

This paper introduces a new inverse design framework for creating nanophotonic structures with tailored optical properties for advanced applications.

## Contribution

A novel inverse design framework for freeform mode engineering in nanophotonic metasurfaces.

## Key findings

- The framework enables optimization of Mie-type modes in dielectric nanophotonic structures.
- Freeform nonlocal metasurfaces with bound states in the continuum are demonstrated.
- Applications in sensing, nonlinear optics, and quantum information processing are highlighted.

## Abstract

Nanophotonic technologies inherently rely on tailoring
light–matter
interactions through the excitation and interference of deeply confined
optical resonances. However, existing concepts in optical mode engineering
remain heuristic and are challenging to extend toward complex and
multifunctional resonant phenomena. We introduce an inverse design
framework that optimizes near-field distributions, ideally suited
to tailoring Mie-type modes within dielectric nanophotonic structures,
and we demonstrate its application to the discovery of new classes
of nonlocal metasurfaces. We show that freeform nonlocal metasurfaces
supporting accidental bound states in the continuum can be readily
optimized for tailored illumination conditions, modal properties,
and quality factors. We further generalize the framework to higher-order
and multifunctional mode engineering and experimentally demonstrate
freeform planar nonlocal multiwavelength and chiral metasurfaces.
Our versatile framework for freeform mode engineering has applications
in broad high-quality-factor nanophotonic platforms relevant to sensing,
nonlinear optics, optomechanics, and quantum information processing.