Optical Properties of Nanocomposite Thin-Films

Abstract

This paper aims at developing numerically validated models for predicting the through-plane effective index of refraction and absorption index of nanocomposite thin-films. First, models for the effective optical properties are derived from previously reported analysis applying the volume averaging theory (VAT) to the Maxwell's equations. The transmittance and reflectance of nanoporous thin-films are computed by solving the Maxwell's equations and the associated boundary conditions at all interfaces using finite element methods. The effective optical properties of the films are retrieved by minimizing the root mean square of the relative errors between the computed and theoretical transmittance and reflectance. Nanoporous thin-films made of SiO2 and TiO2 consisting of cylindrical nanopores and nanowires are investigated for different diameters and various porosities. Similarly, electromagnetic wave transport through dielectric medium with embedded metallic nanowires are simulated. Numerical results are compared with predictions from widely used effective property models including (1) Maxwell-Garnett Theory, (2) Bruggeman effective medium approximation, (3) parallel, (4) series, (5) Lorentz-Lorenz, and (6) VAT models. Very good agreement is found with the VAT model for both the effective index of refraction and absorption index. Finally, the effect of volume fraction on the effective complex index of refraction predicted by the VAT model is discussed. For certain values of wavelengths and volume fractions, the effective index of refraction or absorption index of the composite material can be smaller than that of both the continuous and dispersed phases. These results indicate guidelines for designing nanocomposite optical materials.