The spectral range of solar radiation observed on the earth is approximately 295 to 2500 nm. How to widen the absorption band of the plasmonic absorber in this range has become a hot issue in recent years. In this paper, we propose a highly applicable refractory perfect absorber with an elliptical titanium nanodisk array based on a silica–titanium–silica–titanium four-layer structure. Through theoretical design and numerical demonstration, the interaction of surface plasmon resonance with the Fabry–Perot cavity resonance results in high absorption characteristics. Our investigations illustrate that it can achieve ultra-broadband absorption above 90% from a visible 550-nm wavelength to a near-infrared 2200-nm wavelength continuously. In particular, a continuous 712-nm broadband perfect absorption of up to 99% is achieved from wavelengths from 1013 to 1725 nm. The air mass 1.5 solar simulation from a finite-difference time domain demonstrates that this absorber can provide an average absorption rate of 93.26% from wavelengths of 295 to 2500 nm, which can absorb solar radiation efficiently on the earth. Because of the high melting point of Ti material and the symmetrical structure of this device, this perfect absorber has excellent thermal stability, polarization independence, and large incident-angle insensitivity. Hence, it can be used for solar cells, thermal emitters, and infrared detection with further investigation.

The miniaturization and integration of photonic devices are new requirements in the novel optics field due to the development of photonic information technology. In this paper, we report that a multifunctional layered structure of Au, SiO₂ and hexagonal nanodisk film is advantageous for ultra-narrowband filtering, near-perfect absorption and sensing in a wide refractive index (RI) region. This hexagonal nanostructure presented two remarkable polarization independent plasmon resonances with near-zero reflectivity and near-perfect absorptivity under normal incidence in the visible and near-infrared spectral ranges. The narrowest full width at half maximum (FWHM) of these resonances was predicted to be excellent at 5 nm. More notably, the double plasmon resonances showed extremely obvious differences in RI responses. For the first plasmon resonance, an evident linear redshift was observed in a wide RI range from 1.00 to 1.40, and a high RI sensitivity of 600 nm/RIU was obtained compared to other plasmonic nanostructures, such as square and honeycomb-like nanostructures. For the second plasmon resonance with excellent FWHM at 946 nm, its wavelength position almost remained unmovable in the case of changing RI surrounding nanodisks in the same regime. Most unusually, its resonant wavelength was insensitive to nearly all structural parameters except the structural period. The underlying physical mechanism was analyzed in detail for double plasmon resonances. This work was significant in developing high-performance integrated optical devices for filtering, absorbing and biomedical sensing.

In this paper, we present a dual-channel narrowband polarization absorber based on a metal-dielectric-metal structure, which consists of a top metallic nanorod array, a metal substrate, and an ultrathin middle dielectric spacer. The proposed structure can achieve high absorptance above 96% in a wide angular range of incidence around $\pm 20°$ at two remarkable absorption peaks for transverse magnetic polarization under normal incidence. Most significantly, the extremely highly confined enhancement of electromagnetic fields between Au film and nanorods has been observed by employing numerical simulation based on a finite element method, which is up to 110 times compared with the incident electric field. The underlying physics mechanism of a strong gap plasmon resonance is analyzed, and it is primarily attributed to simultaneous excitation of multiple localized electric dipole and magnetic dipole resonance modes in this film-coupled nanorods system. Additionally, we also investigate the dependence of dual resonance peaks on structural parameters as well as their sensitivities to the refractive index of media surrounding nanorods. The wavelength modulation and intensity modulation are also shown simultaneously. This structure is near-perfect absorbing, plasmonic refractive index sensing, and surface-enhanced Raman spectroscopy, all rolled into one. It will have great significance and potential in developing new miniaturized multifunctional photonics devices and their high integrations.