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In this work, we investigate magnetic responses in various Ag-SiO2-Ag nanosandwich structures at
visible wavelengths. The two electric resonant modes corresponding to the in-phase (symmetric) and
anti-phase (asymmetric) electric dipole on the top and the bottom nanopillars are observed by the finite
difference time domain (FDTD) simulation. In the asymmetric resonant mode, the phases of electric
fields oscillating in the top and bottom pillars have opposite directions, leading to a virtual current loop
that induces the magnetic field reversal. The nanosandwich structure produces a large enhancement of
the magnetic field as the thickness of SiO2 nanopillar is much smaller than wavelength. By increasing
the diameter of nanopillars from 150 nm to 250 nm, the inverse magnetic response wavelength shifts
from 532 nm to 690 nm. On account of the magnetic field reversal caused by the anti-phase electric
dipole coupling, the real part of the equivalent permeability of the film is negative. Therefore, the
wavelength range associated with the intensity of inverse magnetic response is tunable by varying the
size of Ag-SiO2-Ag nanosandwich structure. The equivalent electromagnetic parameters of the
Ag-SiO2-Ag nanosandwich thin film prepared by glancing angle deposition are derived from the
transmission and the reflection coefficients measured by walk-off interferometers. The measured
results indicate that film exhibit double negative properties and lead to negative values of the real parts
of equivalent refractive indices -0.854, -1.179, and -1.492 for λ = 532 nm, 639 nm, and 690 nm,
respectively. Furthermore, the real part of permeability is negatively enhanced to be -4.771 and the
maximum value of figures of merit (FOM) recorded being 6.543 for p-polarized light at λ = 690 nm.
Finally, we analyze the admittance loci for our nanosandwich thin film. This analysis can be applied to
interpret extraordinary optical properties such as negative index of refraction from Ag-SiO2-Ag
nanosandwich films.
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