[Paper Review] Genetic-algorithm-aided ultra-broadband perfect absorbers using plasmonic metamaterials

Mayer, Alexandre, Bi, Hai+6 more|arXiv (Cornell University)|Aug 20, 2021

Metamaterials and Metasurfaces Applications28 citations

TL;DR

This study employs a genetic algorithm to optimize ultra-broadband plasmonic metamaterial absorbers based on periodic arrays of truncated pyramidal structures with alternating metal/dielectric layers. By exploring over 10^17 configurations using Ni, Ti, Cr, and W with PMMA dielectric, it identifies designs achieving >99% integrated absorptance from 420–1600 nm, demonstrating high robustness and low angular dependence, while showing noble metals like Au and Ag do not yield optimal performance for this application.

ABSTRACT

Complete absorption of electromagnetic waves is paramount in today's applications, ranging from photovoltaics to cross-talk prevention into sensitive devices. In this context, we use a genetic algorithm (GA) strategy to optimize absorption properties of periodic arrays of truncated square-based pyramids made of alternating stacks of metal/dielectric layers. We target ultra-broadband quasi-perfect absorption of normally incident electromagnetic radiations in the visible and near-infrared ranges (wavelength comprised between 420 and 1600 nm). We compare the results one can obtain by considering one, two or three stacks of either Ni, Ti, Al, Cr, Ag, Cu, Au or W for the metal, and poly(methyl methacrylate) (PMMA) for the dielectric. More than 10^17 configurations of geometrical parameters are explored and reduced to a few optimal ones. This extensive study shows that Ni/PMMA, Ti/PMMA, Cr/PMMA and W/PMMA provide high-quality solutions with an integrated absorptance higher than 99% over the considered wavelength range, when considering realistic implementation of these ultra-broadband perfect electromagnetic absorbers. Robustness of optimal solutions with respect to geometrical parameters is investigated and local absorption maps are provided. Moreover, we confirm that these optimal solutions maintain quasi-perfect broadband absorption properties over a broad angular range when changing the inclination of the incident radiation. The study also reveals that noble metals (Au, Ag, Cu) do not provide the highest performance for the present application.

Motivation & Objective

• To develop ultra-broadband perfect electromagnetic absorbers (PEAs) for visible to near-infrared wavelengths.
• To identify optimal metal/dielectric stack configurations that achieve quasi-perfect absorption across 420–1600 nm.
• To evaluate the performance and robustness of these absorbers under varying geometrical parameters and incident angles.
• To determine whether noble metals (Au, Ag, Cu) outperform other metals in broadband absorption performance.

Proposed method

• A genetic algorithm (GA) is used to optimize 13.9 trillion parameter combinations of geometrical variables: lateral size (L1, L2, L3), thickness (t1, t2, t3), and periodicity (P) of metal/dielectric layers.
• The GA encodes variables in Gray code, uses rank-based selection, binary and real-valued crossover, and adaptive mutation with isotropic mutation option.
• Fitness evaluation is based on absorptance (A) over the 420–1600 nm range, with convergence accelerated via quadratic approximation near the best individual.
• Finite-element simulations with rigorous coupled-wave analysis (RCWA) are used to compute absorptance, with validation using 21×21 plane wave grids to ensure numerical stability.
• Robustness is assessed by testing sensitivity to parameter variations and by comparing results across different plane wave counts (11×11 vs. 21×21).
• Field maps and angular response are analyzed to evaluate localization and angular insensitivity of the optimal absorbers.

Experimental results

Research questions

• RQ1 Which metal/dielectric combinations yield the highest broadband absorptance in pyramidal metamaterial structures across 420–1600 nm?
• RQ2 How does the performance of noble metals (Au, Ag, Cu) compare to base metals (Ni, Ti, Cr, W) in achieving ultra-broadband absorption?
• RQ3 To what extent do the optimal absorber designs remain robust under geometric parameter variations?
• RQ4 How does the absorptance vary with incident angle, and what is the angular bandwidth of quasi-perfect absorption?
• RQ5 Can numerical convergence and solution reliability be assessed through plane wave count sensitivity?

Key findings

• Ni/PMMA, Ti/PMMA, Cr/PMMA, and W/PMMA structures achieve integrated absorptance exceeding 99% across the 420–1600 nm range.
• The optimal solutions maintain absorptance above 99% for incident angles up to 60°, demonstrating strong angular robustness.
• Noble metals such as Au, Ag, and Cu do not yield the highest performance; instead, Ni, Ti, Cr, and W provide superior broadband absorption.
• Solutions identified via the GA are numerically stable, as confirmed by convergence tests using 21×21 plane waves, with minimal deviation from 11×11 results for robust designs.
• Solutions sensitive to parameter variations show significant deviation under higher plane wave counts, confirming their instability and justifying their rejection.
• Local absorption maps confirm strong field confinement at the metal/dielectric interfaces, indicating excitation of localized surface plasmons as the dominant absorption mechanism.