Numerical investigation of optimal structure for dynamic plasmonic color generated via photothermal deformation of a metal semi-shell structure

Introduction

The dyes and pigments widely used in coloring methods are often toxic, chemically unstable at high temperatures, prone to fading under ultraviolet rays, and cannot easily color objects of sub-micron size or smaller. Photonic crystals and dielectric multi-layers have limited spatial resolution, a low refractive index, and limited coloration. In order to solve these issues, we computationally demonstrate that dynamic wavelength tuning via the photothermal deformation of a metal semi-shell on a self-assembling nanoparticle can achieve a wide color gamut.

Methods

The calculation includes the shape of the structure, thickness of the metal of the semi-shell on a core, wavelength, angle of incidence, polarization, chromaticity coordinates, and evaluation index by area on the chromaticity diagram. From the reflectance spectra calculated using the discrete dipole approximation, the tristimulus values XYZ of the light are plotted in CIELuv space. The area enclosed by all coordinate points of all the tristimulus values is determined.

The core material is fused silicon oxide, the substrate material is aluminum, and the deposition material is silver, which is known as a thermally deformable plasmonic metal. The nanosphere diameter is 100 nm, the substrate thickness is 40 nm, and the deposition thicknesses are 15, 20, and 25 nm. The semi-shell capping angle varies from 30° to 90°.

Results

The color coordinates were calculated from the reflectance spectra of nanospheres with the silver semi-shells of 15, 20, and 25 nm in thickness, plotted in CIELuv space, and yielded the gamut area of 2.3x10^-2, 2.6x10^-2, and 2.2x10^-2, respectively, at the two-dimensional density of 19.8 μm^–2.

Conclusions

Silver semi-shells of 20 nm thickness showed the largest gamut area for nanospheres with 100 nm diameters at a density of 19.8 μm^–2 on Al substrate. Further investigations at various densities will be conducted to determine the optimal structure.