Inverse design and optical vortex manipulation for thin film absorption enhancement

TL;DR

This paper explores how optical vortices can be manipulated to significantly enhance absorption in thin films, using inverse design to optimize structures that generate high-circularity OVs for improved optical device performance.

Contribution

It introduces a novel inverse design approach for creating ultrathin absorbers with high OV circularity, leading to near-perfect absorption and high sensitivity to incident angles.

Findings

1D structure achieves 6.05-fold absorption enhancement.

Optimized 2D structures reach 99.90% absorption.

High OV circularity correlates with absorption efficiency.

Abstract

Optical vortices (OVs) have rapidly varying spatial phase and optical energy that circulates around points or lines of zero optical intensity. Manipulation of OV offers innovative approaches for various fields, such as optical sensing, communication, and imaging. In this work, we demonstrate the correlation between OVs and absorption enhancement in two types of structures. First, we introduce a simple planar one-dimensional (1D) structure that manipulates OVs using two coherent light sources. The structure shows a maximum of 6.05-fold absorption gap depending on the presence of OVs. Even a slight difference in the incidence angle can influence the generation/annihilation of OVs, which implies the high sensitivity of angular light detection. Second, we apply inverse design to optimize two-dimensional (2D) perfect ultrathin absorbers. The optimized free-form structure achieves 99.90% absorptance, and the fabricable grating structure achieves 97.85% at 775 nm wavelength. To evaluate OV fields and their contribution to achieving absorption enhancement, we introduce a new parameter, OV circularity. The optimized structures generate numerous OVs with a maximum circularity of 95.37% (free-form) and 96.14% (grating), superior to our 1D structure. Our study reveals the role of high-circularity localized OVs in optimizing nano-structured absorbers and devices for optical sensing, optical communication, and many other applications.