Boosting the Transparency of Thin Layers by Coatings of Opposing Susceptibility: How Metals Help See Through Dielectrics.

Mohammed Al Shakhs,Loïc Markley,Lucian Augusto,Kenneth J Chau

doi:10.1038/srep20659

Abstract

We propose a hypothesis that a very thin layer can be made more transparent by adding a thin coating with susceptibility of opposing sign. Two experimental tests backed by a theoretical model support this hypothesis. First, we show that the visible and near-infrared transmission through a semi-transparent silver film can be enhanced by up to ~70% and spectrally tailored depending on the type and thickness of the dielectric coating. Material types explored as dielectric coating layers include conventional metal oxides (titanium dioxide) and lesser-explored elemental semiconductors (undoped silicon, p-type silicon, and germanium). Second, and more surprisingly, we show that coating a 50-nm-thick silicon nitride membrane with a 10-nm-thick silver layer can modestly enhance the transmission by up to 6 ± 1% in the blue part of the spectrum. Transmission enhancements are observed for three silver-coated membranes in different configurations. Thinner silver coatings are theoretically capable of enhancement factors greater than 10%, but implementation is restricted by challenges in making smooth and continuous silver films below 10 nm in thickness. This study is important because it is the first demonstration of reciprocity with respect to the transmission enhancements achieved by combining thin metallic and dielectric layers.

Highlights

We derive a general condition based on Maxwell’s equations showing that a very thin layer composed of either dielectric or metal can be made more transparent by coating with a material having a susceptibility of opposing sign
Slight offset between the peaks of the calculated and measured transparency bands can be reasonably attributed to differences in the assumed optical constants of silver and TiO2 and their actual values, surface roughness of the films not described in the model, or a combination of both
We have proposed through a solution to Maxwell’s equations that the transmission through any optically thin layer can be boosted by adding another thin coating with a susceptibility of opposite sign

Summary

Introduction

We derive a general condition based on Maxwell’s equations showing that a very thin layer composed of either dielectric or metal can be made more transparent by coating with a material having a susceptibility of opposing sign. This is corroborated by experimental measurements of light transmission through dielectric-coated silver films and silver-coated silicon nitride membranes, which compare favorably to calculations. We begin by deriving an expression to show that transmission enhancements conferred by a coating layer to a base layer can occur when the layer susceptibilities have opposite sign. The optimal coating layer thickness to achieve maximal transmission satisfies the following tan(2φ2)

Methods

Results

Conclusion