Numerical Investigation of Surface Lattice Plasmonic Modes, Amplified in the Ultraviolet Spectral Regions, for Improved Ag@Al Core–Shell Periodic Nanostructures

Abstract

The present work deals with the study of the plasmonic modes of core–shell nanostructures (silver–aluminum nanocylinders) using the finite-difference time-domain (FDTD) numerical method. The nanocylinders are placed in a two-dimensional square periodic array under normal radiation in the spectral range of 200–800 nm. This study has tried to reduce radiation losses and create strong surface lattice resonances (SLR) in the ultraviolet (UV) region by using a suitable combination of materials and nanostructures. In this regard, it is demonstrated that by changing the dimensions and geometry of the cores of nanocylinders, as well as the characteristics of incident light radiation, one can obtain a suitable optical response in the near-UV spectral region. The calculations show that a new SLR peak related to the hybrid nanostructure is formed in the near-UV region under s-polarized radiation, and for the core with a circular cross-section, in addition to the primary modes related to an array composed of individual nanoparticles. This peak, with a quality factor of 41 at the wavelength of 372 nm, is close to the diffraction modes of the dielectric substrate. Also, by reducing the height of the core, another peak is formed with a quality factor of 12 at the wavelength of 260 nm under the shell’s plasmonic effects and close to the environment’s diffraction modes. These modes (SLRs) provide the possibility of achieving high-energy spectral regions with a suitable quality factor, which is not usually possible in other nanostructures. By changing the polarization of the incident light to p-polarization, depending on the period of the array, the principle resonance peak is formed with a shift to higher wavelengths in the visible region with a quality factor of 20 at the wavelength of 452 nm, as compared to s-polarization. This demonstrates the different spectral responses of nanocylinders under the influence of changes in the polarization of the incident light. The effects of the refractive index of the substrate on the plasmonic modes are also studied. The results indicate that the location and intensity of the core–shell modes (372 and 260 nm) are highly dependent on the substrate refractive index. The findings of the present study suggest the use of nanocylinders (core–shell) as a suitable option for application in sensors, nanolasers, and other optoelectronic devices.