# Design and optimization of a polarization-insensitive Ti/TiO2 metamaterial absorber using particle swarm optimization for broadband solar–thermal applications

## TL;DR

This paper presents a highly efficient, broadband metamaterial absorber made of titanium and titanium dioxide for solar-thermal applications.

## Contribution

The design achieves the broadest bandwidth and highest efficiency among Ti-based absorbers using particle swarm optimization.

## Key findings

- The absorber achieves 98.99% average absorption across a 3533 nm bandwidth (467–4000 nm).
- It maintains over 90% absorption for TE polarization and 80% for TM polarization at angles up to 60°.
- The device shows 98.17% solar absorption and 99.26% thermal emission efficiency under AM 1.5G conditions.

## Abstract

In this paper, a polarization-insensitive and ultra-broadband metamaterial absorber composed of titanium (Ti) and titanium dioxide (TiO2) resonators is proposed. The multilayer architecture integrates three square resonators and one disk resonator, which are optimized to maximize absorption across the 0.25–4 μm spectral range. Particle swarm optimization (PSO) yields an average absorption of 98.99% and a bandwidth of 3533 nm (467–4000 nm). The structure maintains absorption above 80% for transverse magnetic (TM) polarization and above 90% for transverse electric (TE) polarization for wavelengths above 600 nm, even at incidence angles up to 60°. Under the Air Mass 1.5 Global spectrum (AM 1.5G) solar spectrum, the device achieves a solar absorption efficiency of 98.17% and a thermal emission efficiency of 99.26% at 1800 K. This performance is a result of the combined effects of surface plasmon polaritons, magnetic plasmons, and localized surface plasmon resonances. The absorber further demonstrates robustness against structural variations, maintaining over 90% absorption across the wavelength range of 250–4919 nm (BW = 4669 nm). The designed absorber exhibits a compact electrical size, with a total thickness of approximately 0.2λ and periodicity of 0.1λ, calculated at the longest operating wavelength (4 μm), confirming its subwavelength characteristics and suitability for planar integration. In comparison to existing Ti-based absorbers, this design provides both the broadest operational bandwidth and the highest efficiency, indicating strong potential for solar energy harvesting applications.

The online version contains supplementary material available at 10.1038/s41598-025-31966-3.