Tailoring the Spectral Characteristics of Multilayered Chiral Mid-Infrared Metamaterials

Metamaterials, in particular chiral metamaterials, are of great interest in recent years as they offer potential for application in enhanced sensing techniques [1]–[3]. This is because the structures absorb incident radiation, and confine it to nanoscale regions, which can then be used to probe some analyte. Of particular interest are those materials which absorb mid-infrared (mid-IR) radiation, as the confined fields can then be used to probe the molecular vibrational frequencies of molecules, which also fall in this spectral region. Here, we present a simulation and experimental study into a particular class of chiral metamaterial, made up of twisted U-shape resonator elements [4]–[6], forming a multi-layered stack. First we scale the dimensions of the structures using simulations, to ensure the resonances fall in the important mid-IR region of the spectrum. Then, the structure is fabricated using standard lithographic techniques to produce gold resonators on an Si/SiO₂ substrate. By measuring the structure in an FTIR spectrometer we demonstrate high levels of circular dichroism (CD), a consequence of the chirality of the structure, in the mid-IR region, for three different configurations [7]. The three configurations discussed include a double layer structure, and two 4-layer structures. Finally, we demonstrate that the spectral position of the metamaterial resonances, and subsequent CD, can be tailored simply by changing the stacking configuration of the layers [7]. We explain this behaviour using a model of the induced magnetic dipoles at each resonance, caused by the coupling of adjacent twisted resonators, when the structure is excited.<br/>This study provides an important insight into the mechanisms of optical activity in an important class of metamaterial, particularly, how to tailor its spectral response. Demonstrating high levels of CD indicates that this structure is strongly optically active in the important mid-IR region, making it a possible candidate for enhanced spectroscopy applications in the future. Further work is underway to investigate the nature of the confined fields produced, how chiral they are, and how accessible they might be to a molecular analyte on the structures surface.<br/>References:<br/>[1] M. Schäferling, D. Dregely, M. Hentschel, and H. Giessen, “Tailoring enhanced optical chirality: Design principles for chiral plasmonic nanostructures,” <i>Phys. Rev. X</i>, vol. 2, no. 3, (2012).<br/>[2] O. Takayama, “Mid-infrared nanophotonics for biochemical sensing: A review,” <i>Romanian Reports in Physics</i>, vol. 72, no. 3., (2020).<br/>[3] X. Ma, M. Pu, X. Li, Y. Guo, P. Gao, and X. Luo, “Meta-chirality: Fundamentals, construction and applications,” <i>Nanomaterials</i>, vol. 7, no. 5. (2017).<br/>[4] N. Liu, H. Liu, S. Zhu, and H. Giessen, “Stereometamaterials,” <i>Nat. Photonics</i>, vol. 3, no. 3, , (2009).<br/>[5] X. Xiong <i>et al.</i>, “Construction of a chiral metamaterial with a U-shaped resonator assembly,” <i>Phys. Rev. B - Condens. Matter Mater. Phys.</i>, vol. 81, no. 7, (2010).<br/>[6] M. Decker, R. Zhao, C. M. Soukoulis, S. Linden, and M. Wegener, “Twisted split-ring-resonator photonic metamaterial with huge optical activity,” <i>Opt. Lett.</i>, vol. 35, no. 10, (2010).<br/>[7] H. R. Barnard and G. R. Nash, “Tailoring the spectral properties of layered chiral mid-infrared metamaterials,” <i>Appl. Phys. Lett.</i>, vol. 119, no. 24, (2021).