Strong in-plane scattering of acoustic graphene plasmons by surface atomic steps

Surface-acoustic-wave-driven graphene plasmonic sensor for fingerprinting ultrathin biolayers down to the monolayer limit

Surface plasmon polaritons in graphene can enhance the performance of mid-infrared spectroscopy, which is key for the study of both the composition and the conformation of organic molecules via their vibrational resonances. In this paper, a plasmonic biosensor using a graphene-based van der Waals heterostructure on a piezoelectric substrate is theoretically demonstrated, where far-field light is coupled to surface plasmon-phonon polaritons (SPPPs) through a surface acoustic wave (SAW). The SAW creates an electrically-controlled virtual diffraction grating, suppressing the need for patterning the 2D materials, that limits the polariton lifetime, and enabling differential measurement schemes, which increase the signal-to-noise ratio and allow a quick commutation between reference and sample signals. A transfer matrix method has been used for simulating the SPPPs propagating in the system, which are electrically tuned to interact with the vibrational resonances of the analytes. Furthermore, the analysis of the sensor response with a coupled oscillators model has proven its capability of fingerprinting ultrathin biolayers, even when the interaction is too weak to induce a Fano interference pattern, with a sensitivity down to the monolayer limit, as tested with a protein bilayer or a peptide monolayer. The proposed device paves the way for the development of advanced SAW-assisted lab-on-chip systems combining the existing SAW-mediated physical sensing and microfluidic functionalities with the chemical fingerprinting capability of this novel SAW-driven plasmonic approach.

Polaritons in Van der Waals Heterostructures

2D monolayers supporting a wide variety of highly confined plasmons, phonon polaritons, and exciton polaritons can be vertically stacked in van der Waals heterostructures (vdWHs) with controlled constituent layers, stacking sequence, and even twist angles. vdWHs combine advantages of 2D material polaritons, rich optical structure design, and atomic scale integration, which have greatly extended the performance and functions of polaritons, such as wide frequency range, long lifetime, ultrafast all-optical modulation, and photonic crystals for nanoscale light. Here, the state of the art of 2D material polaritons in vdWHs from the perspective of design principles and potential applications is reviewed. Some fundamental properties of polaritons in vdWHs are initially discussed, followed by recent discoveries of plasmons, phonon polaritons, exciton polaritons, and their hybrid modes in vdWHs. The review concludes with a perspective discussion on potential applications of these polaritons such as nanophotonic integrated circuits, which will benefit from the intersection between nanophotonics and materials science.

Topological super-modes engineering with acoustic graphene plasmons

Acoustic graphene plasmons (AGPs) in a graphene-dielectric-metal structure possess extreme field localization and low loss, which have promising applications in strong photon-matter interaction and integrated photonic devices. Here, we propose two kinds of one-dimensional crystals supporting propagating AGPs with different topological properties, which is confirmed by the Zak phase calculations and the electric field symmetry analysis. Moreover, by combining these two plasmonic crystals to form a superlattice system, the super-modes exist because of the coupling between isolated topological interface states. A flat-like dispersion of super-modes is observed by designing the superlattice. These results should find applications in optical sensing and integrating photonic devices with plasmonic crystals.

Near-field probing of image phonon-polaritons in hexagonal boron nitride on gold crystals

Near-field mapping has been widely used to study hyperbolic phonon-polaritons in van der Waals crystals. However, an accurate measurement of the polaritonic loss remains challenging because of the inherent complexity of the near-field signal and the substrate-mediated loss. Here we demonstrate that large-area monocrystalline gold flakes, an atomically flat low-loss substrate for image polaritons, provide a platform for precise near-field measurement of the complex propagation constant of polaritons in van der Waals crystals. As a topical example, we measure propagation loss of the image phonon-polaritons in hexagonal boron nitride, revealing that their normalized propagation length exhibits a parabolic spectral dependency. Furthermore, we show that image phonon-polaritons exhibit up to a twice longer normalized propagation length, while being 2.4 times more compressed compared to the case of the dielectric substrate. We conclude that the monocrystalline gold flakes provide a unique nanophotonic platform for probing and exploitation of the image modes in low-dimensional materials.