Researchers observe chirality and slow plasmons in twisted bilayer graphene

A multi-institutional team of scientists have reported two new characteristics of plasmons residing in the moiré superlattice of twisted bilayer graphene (tBLG). They have identified plasmons bearing chiral signatures, as well as a slow plasmonic mode in specially prepared graphene samples. The details of the findings are published in Nature in an article titled, “Observation of chiral and slow plasmons in twisted bilayer graphene.” The observations open up new opportunities for light-matter interactions in the mid-wave infrared spectrum, and reveal tBLG as a unique quantum optical platform. Informed by the theoretical work of ECE’s Paul Palmberg Professor of Electrical and Computer Engineering Tony Low, the experiment was led by Professor Xiaomu Wang (School of Electronic Science and Engineering of Nanjing University, China). 

The presence of moiré superlattices in tBLG introduces changes in the electronic properties of its structure, which could potentially mean the discovery of new electromagnetic oscillations. In fact, the existence of novel properties at the level of plasmons in this superlattice structure in graphene was theoretically predicted in a series of papers authored by Low and collaborators, including Tobias Stuaber and Xiao Lin, published in Physical Review Letters of the American Physical Society (details of papers at the end of this article). However, the experimental observation of two such properties brings us closer to harnessing these characteristics for optic and nanophotonic level uses. Using a novel method to prepare high quality bilayer graphene, the present team of scientists have revealed key findings, which have far reaching impact and hold the potential for a wide range of applications down the road in fields as varied as healthcare, chemistry, and novel device development. 

Commenting on the significance of their observations Professor Low says, "Plasmons in this material reside in the mid-infrared spectrum, an interesting spectrum range which coincides with vibrational modes of molecules. Imbuing these plasmons with chirality and reduced speed brings us a step closer to the future device realization of molecular spectral fingerprinting. 

Significance of chirality and slow plasmon mode

The first major outcome of the team’s work is the detection of chiral plasmons within the lattice structure of tBLG. Although predicted previously, this is the first time that scientists have been able to experimentally observe plasmons bearing a chiral characteristic. Chirality is a geometric asymmetry where an object and its mirror image cannot be superimposed on each other. Its presence introduces a handedness to the overall structure, i.e. the structure is either left handed or right handed. Because of the presence of a twist angle in a moiré superlattice of tBLG, the plasmons in such a structure inherit the chiral nature. Significantly, the handedness of the plasmons in tBLG makes them sensitive to the presence of chirality in molecular structures, which can be important for chemical characterization. 

The second crucial finding presented by the team is the presence of a slow plasmon mode. Typically a plasmon is about 50 times slower than the speed of light. But in the current experiment, the team has successfully slowed the plasmon to be roughly 500 times slower than the speed of light. Slow plasmons can be particularly useful for sensitivity and characterization applications as the interaction time between the plasmon and the subject molecules increases. 

Professor Xiaomu Wang points to the value of moving from theory to demonstration: "Chiral plasmon combined electron interactions and Berry curvature were conceptually predicted to arise in conceived anomalous Hall metals, but have not been observed in the laboratory. Here we report the first realization of new chiral and slow plasmon modes in real twisted bilayer graphene. Our findings exemplify a fundamental characteristic of interacting topological excitations at zero magnetic field."

Besides the two critical observations, the team’s research has been particularly significant in the method used to set up the experiment. Typically tBLG samples are made by a ‘tear and stack’ method, but that alone can yield samples of inconsistent quality, thereby impacting study results. In the current investigation, Wang’s team prepared the graphene samples using the chemical vapor deposition (CVD) method. Two layers from the same crystal produced by the CVD method were stacked with a slight misorientation, and the prepared sample was then inspected using Raman spectroscopy. Checking the samples allowed the team to proceed with the experiment using only high-quality, defect free tBLG. In fact the consistent sample quality was instrumental to the observation of the two new plasmon modes.  

A plasmon, the outcome of a photon’s coupling with the electronic excitation within a material, because of its considerably shorter infrared wavelength at about 100 nanometers has many useful applications. The reduced wavelength (as compared to 10 microns in the case of photons) makes it more sensitive to molecular signatures. The increased sensitivity also makes plasmons critical for biosensing applications. The experimental observation of the two new properties in plasmons supports a deeper understanding of these quasiparticles, and will eventually lead to diverse applications. Chiral plasmons can be used for the study of topological materials, and can enable a variety of nanophotonic devices. Additionally, the discovery of the slow plasmon mode could help with understanding and harnessing plasmonics in the sought after spectral range of 3-5 μm (micrometer) for novel infrared devices. The discovery of these new properties of plasmonics in tBLG paves the way for the future exploration of chiral nanophotonics and sensors.

The research was supported by several research entities in China (see Acknowledgements in the Nature article) as well as the Paul Palmberg Professorship in ECE

Prediction papers published in Physical Review Letters:
Chiral Plasmons with Twisted Atomic Bilayers published in August 2020
Chiral Response of Twisted Bilayer Graphene published in January 2018
Novel Midinfrared Plasmonic Properties of Bilayer Graphene published in March 2014