A unified theory for perfect absorption in ultra-thin absorptive films with constant tangential electric or magnetic fields

TL;DR

This paper derives conditions for ultra-thin films to achieve perfect absorption using reflectors, classifying film parameters into three groups and revealing extraordinary absorption with minimal losses.

Contribution

It provides a unified theoretical framework for designing ultra-thin perfect absorbers with various reflectors, identifying key parameter groups for optimal absorption.

Findings

Perfect absorption conditions derived for ultra-thin films with reflectors.

Classification of film parameters into three groups for absorption optimization.

Demonstration of extraordinary absorption with minimal material losses.

Abstract

The maximal absorption rate of ultra-thin films is 50% under the condition that the tangential electric (or magnetic) field is almost constant across the film in symmetrical environment. However, with certain reflectors, the absorption rate can be greatly increased, to even perfect absorption (100%). In this work, we explicitly derive the general conditions of the ultra-thin absorptive film parameters to achieve perfect absorption with general types of reflectors under the condition that the tangential electric (or magnetic) field is almost constant across the film. We find that the parameters of the film can be classified into three groups, exhibiting: 1) a large permittivity (permeability), 2) a near-zero permeability (permittivity), or 3) a suitable combination of the permittivity and the permeability, respectively. Interestingly, the latter two cases demonstrate extraordinary absorption in ultra-thin films with almost vanishing losses. Our work serves as a guide for designing ultra-thin perfect absorbers with general types of reflectors.