A design method of broadband metalens using time-domain topology optimization

H Yasuda,S Nishiwaki

doi:10.1063/5.0048438

Abstract

Flat metalenses have attracted attention due to an increasing demand for compact electromagnetic devices. For such applications, broadband metalenses are highly desirable; however, conventional metalenses show relatively narrow band operation. Here, we propose a design method of free-form metalenses using topology optimization to operate with enhanced bandwidths. In contrast with preceding reports of topology optimization methods for metalenses, we developed a topology optimization method based on the time domain formulation to deal with broadband frequencies simultaneously. For this purpose, a group delay of optical pulses in the time domain, which is equivalent to the broadband phase matching condition in the frequency domain, is employed in the objective function. A level set based topology optimization method is applied to obtain a clear optimal configuration. To demonstrate the effectiveness of the proposed method, we provide design examples of metalens unit cells at millimeter frequency. We confirm that optimized unit cells of metalenses show superior performance compared to the conventional unit cells for both transmittance efficiency and phase error in broadband wavelength.

Highlights

Electromagnetic metamaterials are engineered materials that have extraordinary optical properties that cannot be found in nature.1. Such properties can be potentially applied for the development of novel optical devices, such as super-resolution microscopy and optical cloaking
To confirm the effectiveness of the proposed method, we provide design examples of metalens unit cells at millimeter frequency
We developed a design method of free-form metalens unit cells using time-domain topology optimization and provided the design example of metalens unit cells to operate with enhanced bandwidths at millimeter wavelength

Summary

INTRODUCTION

Electromagnetic metamaterials are engineered materials that have extraordinary optical properties that cannot be found in nature. Such properties can be potentially applied for the development of novel optical devices, such as super-resolution microscopy and optical cloaking. The development of innovative fabrication technology would be required, topology optimization design methods will be eventually applied in the optical domain, where strong demand for use in industrial applications exists. The proposed topology optimization methods of metalenses have been formulated as frequency domain methods. To consider the problem of designing a metalens for broadband operation in the frequency domain, a multi-objective formulation by selecting a set of frequencies is necessary. We proposed a level set based time-domain topology optimization method for the design of nanophotonic structures to control the spatiotemporal profile of optical pulses.. One advantage of time-domain analysis is that a band of frequencies can be effectively treated simultaneously, which is suitable for broadband optimization problems. To confirm the effectiveness of the proposed method, we provide design examples of metalens unit cells at millimeter frequency.

DESIGN METHOD OF METALENS

CONCLUSIONS