Mid-infrared Plasmonic Circular Dichroism Generated by Graphene Nanodisk Assemblies

TL;DR

This paper demonstrates that chiral graphene nanodisk assemblies can generate strong mid-infrared circular dichroism, enabling applications in thermal bio-imaging and vibrational spectroscopy of biomolecules.

Contribution

It introduces a novel design of chiral graphene nanodisk assemblies that produce significant mid-infrared CD signals through plasmon-plasmon coupling, expanding plasmonic chiral spectroscopy.

Findings

Strong mid-infrared CD signals from graphene assemblies.

CD spectra overlap with biomolecular vibrational modes.

Potential applications in thermal imaging and biosensing.

Abstract

It is very interesting to bring plasmonic circular dichroism spectroscopy to the mid-infrared spectral interval, and there are two reasons for this. This spectral interval is very important for thermal bio-imaging and, simultaneously, this spectral range includes vibrational lines of many chiral biomolecules. Here we demonstrate that graphene plasmons indeed offer such opportunity. In particular, we show that chiral graphene assemblies consisting of a few graphene nanodisks can generate strong circular dichroism (CD) in the mid-infrared interval. The CD signal is generated due to the plasmon-plasmon coupling between adjacent nanodisks in the specially designed chiral graphene assemblies. Because of the large dimension mismatch between the thickness of a graphene layer and the incoming light's wavelength, three-dimensional configurations with a total height of a few hundred nanometers are necessary to obtain a strong CD signal in the mid-infrared range. The mid-infrared CD strength is mainly governed by the total dimensions (total height and helix scaffold radius) of the graphene nanodisk assembly, and by the plasmon-plasmon interaction strength between its constitutive nanodisks. Both positive and negative CD bands can be observed in the graphene assembly array. The frequency interval of the plasmonic CD spectra overlaps with the vibrational modes of some important biomolecules, such as DNA and many different peptides, giving rise to the possibility of enhancing the vibrational optical activity of these molecular species by attaching them to the graphene assemblies. Simultaneously the spectral range of chiral mid-infrared plasmons in our structures appears near the typical wavelength of the human-body thermal radiation and, therefore, our chiral metastructures can be potentially utilized as optical components in thermal imaging devices.