A multilayer that comprises ultra-thin metal and dielectric films has been investigated and applied as a layered metamaterial. By arranging metal and dielectric films alternatively and symmetrically, the equivalent admittance and refractive index can be tailored separately. The tailored admittance and refractive index enable us to design optical filters with more flexibility. The admittance matching is achieved via the admittance tracing in the normalized admittance diagram. In this work, an ultra-thin light absorber is designed as a multilayer composed of one or several cells. Each cell is a seven-layered film stack here. The design concept is to have the extinction as large as possible under the condition of admittance matching. For a seven-layered symmetrical film stack arranged as Ta2O5 (45 nm)/ a-Si (17 nm)/ Cr (30 nm)/ Al (30 nm)/ Cr (30 nm)/ a-Si (17 nm)/ Ta2O5 (45 nm), its mean equivalent admittance and extinction coefficient over the visible regime is 1.4+0.2i and 2.15, respectively. The unit cell on a transparent BK7 glass substrate absorbs 99% of normally incident light energy for the incident medium is glass. On the other hand, a transmission-induced metal-dielectric film stack is investigated by using the admittance matching method. The equivalent anisotropic property of the metal-dielectric multilayer varied with wavelength and nanostructure are investigated here.
The substrate cooling method was used in glancing angle deposition to grow a slanted silver nanorod array (NRA). Liquid nitrogen was allowed to flow under the substrate during deposition, and we compared the morphologies of Ag NRAs deposited with and without cooling. The cooling reduced the average width of the nanorods. A Z-shaped nanostructure array composed of three sections of silver NRAs was then deposited under the same cooling conditions. The average tilt angle of the nanorods from the surface normal was varied from the bottom section to the top section with the nanorods of the top section made to lie almost parallel to the substrate. Under normal illumination, the rods in the top section exhibit distinct longitudinal and transverse plasmon modes that cause strong polarization-dependent transmittance and reflectance.
A traditional high-reflection optical coating was applied to enhance the directional radiation of nanoantennas. A highly reflective multilayer upon the top lateral side of a horizontally lying silver nanorod enhances the forward scattering when an optical wave is incident on the bare bottom side of the nanorod. Enhanced forward scattering can thus be observed from a glancing deposited silver nanorod array (NRA). An effective method for coupling the energy of incident light into a NRA involves arranging the NRA in a prism coupling. The highly efficient light coupling effect over a broadband and a wide range of angles results in extra-strong forward light scattering.
In this work, the equivalent Herpin index and phase thickness of a symmetrical film stack that consists of a dielectric
film D and a metal film M are analyzed using the film matrix method. Five-layered symmetrical MDMDM film stacks in
which the thickness of each film is less than 1/10 of the incident wavelength are utilized. The positive real part of the
equivalent Herpin index and the negative real part of the phase thickness result in a negative real part of the equivalent
refractive index. The range of refractive indices of D and M that lead to a negative refractive index of the overall
material is developed as a procedure. When a p-polarized light wave obliquely propagates into the material with the
negative refractive index, negative refraction and backward wave propagation occur. To reduce the loss in the negative
index metamaterial, a porous metal film is introduced as a substitute for the metal film M in MDMDM to increase the
feasibility of the use of the metamaterial as an optical coating.
A slanted silver nanorod array (NRA) deposited with glancing angle of deposition around 89°. By controlling the
deposition angle, SiO2 and Ta2O5 grow on sliver rods in different morphologies. The multilayer designed as high
reflective multilayer by arranging SiO2 and Ta2O5 alternatively on a cylindrical silver rod with diameter of 80 nm and a
length of 200 nm would enhance the local field intensity and scattering when the rod is illuminated by s-polarized and ppolarized
light waves. In this work, the reflective multilayer is designed at wavelengths of 450nm and 750nm that are
associated with transverse plasmonic mode and longitudinal plasmonic mode, respectively. It is demonstrated
experimentally that the intensity of light scattering from the capped NRA is enhanced due to the local field confinement
around silver rod.