The Design of Optical Circuit-Analog Absorbers through Electrically Small Nanoparticles

Alessio Monti,Andrea Alù,Alessandro Toscano,Filiberto Bilotti

doi:10.3390/photonics6010026

Abstract

In the last few years, the perfect absorption of light has become an important research topic due to its dramatic impact in photovoltaics, photodetectors, color filters and thermal emitters. While broadband optical absorption is relatively easy to achieve using bulky devices, today there is a strong need and interest in achieving the same effects by employing nanometric structures that are compatible with modern nanophotonic components. In this paper, we propose a general procedure to design broadband nanometer-scale absorbers working in the optical spectrum. The proposed devices, which can be considered an extension to optics of microwave circuit-analog absorbers, consist of several layers containing arrays of elongated nanoparticles, whose dimensions are engineered to control both the absorption level and the operational bandwidth. By combining a surface-impedance homogenization and an equivalent transmission-line formalism, we define a general analytical procedure that can be employed to achieve a final working design. As a relevant example, we show that the proposed approach allows designing an optical absorber exhibiting a 20% fractional bandwidth on a thickness of λ/4 at the central frequency of operation. Full-wave results confirming the effectiveness of the analytical findings, as well as some considerations about the experimental realization of the proposed devices are provided.

Highlights

The design of devices able to dissipate all the energy of an impinging electromagnetic wave into heat or other forms of energy, i.e., absorbers, has a long history that can be dated back to the World War II [1]
It is known that when the size of a plasmonic nanoparticle is smaller than the mean free path of electrons, its optical losses increase compared to the ones of the bulk material due to the additional scattering of electrons at the nanoparticle boundaries [22]
As we show in the following, arrays of plasmonic nanoparticles can be engineered to behave both as a resistive and a reactive layer and, optical

Summary

Introduction

The design of devices able to dissipate all the energy of an impinging electromagnetic wave into heat or other forms of energy, i.e., absorbers, has a long history that can be dated back to the World War II [1]. The constants of each transmission line segment, its characteristic impedance ηi as it will be clear later, the strong frequency-dispersion of the nanoparticle-based optical resistive and propagation constant βi, can be written in the following form: sheets limits the applicability of this simple approach in our scenario. An equivalent transmission-line representation of the device, such as the one shown, is more convenient for our purposes In this formalism, the i-th dielectric spacer is represented by a transmission line with length di and characteristic impedance η i , whereas each reactive layer is modeled through a shunt lumped impedance Zsi. The secondary constants of each transmission line segment, i.e., its characteristic impedance η i and propagation constant βi , can be written in the following form:. By engineering and minorofaxes the nanoparticles, is possible to obtain anthat optical possible to make these structures quasi-isotropic by properly arranging different replicas of the nanoparticles [20], we remark that such a feature can be considered an advantage in several applications, such as the design of optical polarizers or polarization-sensitive devices

Results

Alternative Layout and Sensitivity Analysis

Findings

Conclusions