# Black phosphorus-based anisotropic absorption structure in the mid-infrared

## TL;DR

This paper presents a BP-based anisotropic absorption structure that achieves near-perfect absorption for TM polarization in the mid-infrared, with tunable resonance properties and potential applications in polarization-sensitive photonic devices.

## Contribution

It introduces a simple, tunable, and highly polarization-dependent absorption structure using black phosphorus sandwiched with polymers and dielectrics.

## Key findings

- Achieves 99.9% absorption for TM polarization at mid-infrared wavelengths.
- Exhibits strong polarization dependence with minimal TE absorption.
- Resonance wavelength can be tuned via doping, geometry, and incident angle.

## Abstract

Black phosphorus (BP), an emerging two-dimensional (2D) material with intriguing optical properties, forms a promising building block in optics and photonics devices. In this work, we propose a simple structure composed of BP sandwiched by polymer and dielectric materials with low index contrast, and numerically demonstrate the perfect absorption mechanism via the critical coupling of guided resonances in the mid-infrared. Due to the inherent in-plane anisotropic feature of BP, the proposed structure exhibits highly polarization-dependent absorption characteristics, i.e., the optical absorption of the structure reaches 99.9$\%$ for TM polarization and only 3.2$\%$ for TE polarization at the same wavelength. Furthermore, the absorption peak and resonance wavelength can be flexibly tuned by adjusting the electron doping of BP, the geometrical parameters of the structure and the incident angles of light. With high efficiency absorption, the remarkable anisotropy, flexible tunability and easy-to-fabricate advantages, the proposed structure shows promising prospects in the design of polarization-selective and tunable high-performance devices in the mid-infrared, such as polarizers, modulators and photodetectors.