Invisible Metals: Highly Opaque Metals Invisible to Infrared Radiation

Title: Extraordinarily Transparent Compact Metallic Metamaterials

Authors: Samuel J. Palmer, Xiaofei Xiao, Nicolas Pazos-Perez, Luca Guerrini, Miguel A. Correa-Duarte, Stefan A. Maier, Richard V. Craster, Ramon A. Alvarez-Puebla and Vincenzo Giannini

Year: 2019

Journal: Nature Communications

DOI: https://doi.org/10.1038/s41467-019-09939-8

Featured image and figures reprinted under a Creative Commons License (https://creativecommons.org/licenses/by/4.0/) from Palmer S. J. et. Al., Nature Communications, 2019, 10, 2118.

With great leaps in scientific progress over the last few decades, many functional parts have been improved by using novel materials. In this study, optical components are the primary focus.

One of the issues faced by optical components, especially lenses, is that they require high transparency and low dispersion. Additionally, it is desirable to have achromatic optical components to prevent the formation of various focal points as a result of differing wavelengths and refraction indices.

A recently discovered class of metallic metamaterials allows for the tuning of local refractive indices within a single lens. Metallic metamaterials are opaque arrays of densely packed metal nanoparticles which result in a material with high metal content (over 75%) while retaining infrared (IR) transparency. These materials are effective dielectrics and are more transparent than standard dielectrics, such as germanium. Additionally, the concentrated electric fields that are generated in the gaps between the metal particles result in enhanced regions, which can potentially boost readings over a range of wavelengths, significantly improving signals for infrared spectroscopy.


To understand the results of this study, it is essential to acknowledge the differences between the 3 materials that are discussed: metals, dielectrics and effective dielectrics.

Figure 1. Illustration of materials and their responses to an applied electric field.





Metals are comprised of an array of metal atoms, which have valence electrons that can freely move within the material, resulting in an induced electric field when a voltage is applied (Figure 1a). Dielectrics are similar, where they are made of a semiconducting material. However, in this material, the electrons are bound to their atoms, which only allows for polarization of the atoms in the presence of an electric field (Figure 1b). The third class, which is heavily featured in this paper, is effective dielectrics, or artificial dielectrics. This material comprises of a well ordered array of metal nanoparticles, which have free electrons. However, the movement of these electrons is restricted to the metal nanoparticle, hence behaving as an effective dielectric (Figure 1c). 

To optimize the material properties of effective dielectrics, Palmer et Al. investigated the effects of a number of factors such as: metal particle size, metal species, particle shape and orientation, and particle spacing. These were then fitted to various mathematical models to determine their fit. What they found was that in addition to the material being highly transparent, the response of the material could also be tuned. Using cylindrical nanoparticles, they noted that the effective index of the material could vary by as much as 50% by rotating the system or changing the particle size but retaining the particle positions. This is a significant result as these properties could be varied throughout the material, resulting in different effective indices at various locations in the material. As such, the nanoparticles could be adjusted to achieve a specified local effective index. On the other hand, the effective index is only affected by changes in wavelengths below 2┬Ám, indicating that the effective index is essentially the same for a broad range of frequencies, without a discernable upper bound.

This is a fascinating development in dielectrics, allowing for us to develop new uses and applications, with lower losses and higher transparencies than we could previously achieve. This is especially promising for the field of infrared spectroscopy, as these materials will result in improved enhancement and focusing of the signals, allowing for more precise results.

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