Size effect of aluminum / silicon-dioxide / aluminum nanosandwich films for negative optical properties

In this work, three different aluminum (Al) / silicon-dioxide (SiO ₂) / aluminum (Al) nanosandwich films (SWFs) with different sizes are deposited using glancing angle deposition (GLAD) with continuous azimuthal rotation. The SWF comprises an SiO₂ layer that is sandwiched between Al nanopillars. The thickness of SiO₂ is fixed at 45nm. The thicknesses d of the top and bottom Al nanopillar is varied from 188nm to 233nm. The equivalent electromagnetic parameters of each film are derived from the reflection coefficients and transmission coefficients that are measured by walk-off and polarization interferometers. The equivalent optical parameters revealed that it has a negative real equivalent permittivity and a negative real equivalent permeability. The effect of size of the Al / SiO₂ / Al SWFs on their optical properties is also examined. As the thickness d increases from 188nm to 233nm, the equivalent refractive index is negative and its average magnitude decreases from -1.703 to -1.247. Similarly, the real part of the equivalent permittivity varies from -1.193 to -0.824 as the thickness increased. The SWFs are simulated to analyze the magnetic field in the SiO2 layer by finite-difference time-domain (FDTD) method. The result of the simulation shows that the negative permeability arises from the reversal of the magnetic resonance within the SiO2 layer. The reversed magnetic field becomes weaker as the thickness d increases.