Study of metal nanostructure array for near-UV structural color and near-IR hyperbolic metasurface

Abstract

This research contains a study to broaden the operating wavelength of structural color and hyperbolic metasurface to a near-ultraviolet range and near-infrared range, respectively. We first present near-ultraviolet structural color generation using aluminum nanodisk arrays. The plasmonic resonance of the aluminum nanodisk on a quartz substrate produces a vivid reflective structural color based on the strong backward scattering. As changing the diameter of the nanodisk and the period of the square lattice, the resonance wavelength (λres) in the reflectance spectrum gradually moves over the near-ultraviolet region. Even though the filling ratio and thickness of the nanodisk arrays are only ~11% and ~35 nm, a high reflectance value of ~35% on average can be achieved at the resonant condition. Typical full-width at half-maximum of the resonance peak is ~λres/5, which is narrow enough to generate vivid structural color. The numerical simulations employing the finite-difference time-domain (FDTD) method successfully reproduce the experimental results. We also demonstrated ultraviolet structural color pixels with different resonance wavelengths and examined their performances by measuring bright-field microscope images under various illumination conditions with different center wavelengths. Secondly, we present hyperbolic metasurface operating in broad wavelength from visible to near-infrared range using gold nanowire array. The transverse electric mode, the electric field of which is highly confined in the gap, exhibits a hyperbolic dispersion, because of the strong near-field coupling between individual gold nanowire modes. The modal properties and dispersion relations of the metasurface-supporting modes are calculated by the finite-element method (FEM). Using the FDTD method, we theoretically demonstrated that the hyperbolic metasurface on a silicon nitride slab waveguide supports the negative refraction for the transverse electric incidence. The correlation between the hyperbolic dispersion and negative refraction is successfully identified. The loss compensation of the hyperbolic dispersion mode by employing a gain medium is also examined.