Publications

2025

1. arXiv

   Nodal lines in a honeycomb plasmonic crystal with synthetic spin

   Sang Hyun Park, E. J. Mele, and Tony Low

   arXiv, Feb 2025

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   We analyze a plasmonic model on a honeycomb lattice of metallic nanodisks that hosts nodal lines protected by local symmetries. Using both continuum and tight-binding models, we show that a combination of a synthetic time-reversal symmetry, inversion symmetry, and particle-hole symmetry enforce the existence of nodal lines enclosing the {}mathrm{K} and {}mathrm{K}’ points. The nodal lines are not directly gapped even when these symmetries are weakly broken. The existence of the nodal lines is verified using full-wave electromagnetic simulations. We also show that the degeneracies at nodal lines can be relieved by introducing a Kekul}’e distortion that acts to mix the nodal lines near the {}mathrm{K},}mathrm{K}’ points. Our findings open pathways for designing novel plasmonic and photonic devices without reliance on complex symmetry engineering, presenting a convenient platform for studying nodal structures in two-dimensional systems.

2024

1. Phys. Rev. B

   Helical boundary modes from synthetic spin in a plasmonic lattice

   Sang Hyun Park, Michael Sammon, E J Mele, and 1 more author

   Phys. Rev. B, Apr 2024

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   Artificial lattices have been used as a platform to extend the application of topological band theory beyond electronic systems. Here, using the two-dimensional Lieb lattice as a prototypical example, we show that an array of disks which each support localized plasmon modes gives rise to an analog of the quantum spin-Hall state enforced by a synthetic time-reversal symmetry. We find that the plasmonic modes naturally possess a synthetic spin degree of freedom which leads to a spin-dependent second-neighbor coupling mechanism mediated by interorbital coupling. This interaction introduces a nontrivial 𝑍2 topological order and gaps out the Bloch spectrum. A faithful mapping of the plasmonic system onto a tight-binding model is developed and shown to capture its essential topological signatures. Full wave numerical simulations of graphene disks arranged in a Lieb lattice confirm the existence of propagating helical boundary modes in the nontrivial band gap.

2023

1. Nat. Commun.

   Achieving near-perfect light absorption in atomically thin transition metal dichalcogenides through band nesting

   Seungjun Lee, Dongjea Seo, Sang Hyun Park, and 10 more authors

   Nat. Commun., Jul 2023

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   Near-perfect light absorbers (NPLAs), with absorbance, $\mathcalA A , of at least 99%, have a wide range of applications ranging from energy and sensing devices to stealth technologies and secure communications. Previous work on NPLAs has mainly relied upon plasmonic structures or patterned metasurfaces, which require complex nanolithography, limiting their practical applications, particularly for large-area platforms. Here, we use the exceptional band nesting effect in TMDs, combined with a Salisbury screen geometry, to demonstrate NPLAs using only two or three uniform atomic layers of transition metal dichalcogenides (TMDs). The key innovation in our design, verified using theoretical calculations, is to stack monolayer TMDs in such a way as to minimize their interlayer coupling, thus preserving their strong band nesting properties. We experimentally demonstrate two feasible routes to controlling the interlayer coupling: twisted TMD bi-layers and TMD/buffer layer/TMD tri-layer heterostructures. Using these approaches, we demonstrate room-temperature values of \mathcalA$ A =95% at λ =2.8 eV with theoretically predicted values as high as 99%. Moreover, the chemical variety of TMDs allows us to design NPLAs covering the entire visible range, paving the way for efficient atomically-thin optoelectronics.

2. Light Sci. Appl.

   Non-Hermitian chiral degeneracy of gated graphene metasurfaces

   Soojeong Baek, Sang Hyun Park, Donghak Oh, and 9 more authors

   Light Sci. Appl., Apr 2023

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   Non-Hermitian degeneracies, also known as exceptional points (EPs), have been the focus of much attention due to their singular eigenvalue surface structure. Nevertheless, as pertaining to a non-Hermitian metasurface platform, the reduction of an eigenspace dimensionality at the EP has been investigated mostly in a passive repetitive manner. Here, we propose an electrical and spectral way of resolving chiral EPs and clarifying the consequences of chiral mode collapsing of a non-Hermitian gated graphene metasurface. More specifically, the measured non-Hermitian Jones matrix in parameter space enables the quantification of nonorthogonality of polarisation eigenstates and half-integer topological charges associated with a chiral EP. Interestingly, the output polarisation state can be made orthogonal to the coalesced polarisation eigenstate of the metasurface, revealing the missing dimension at the chiral EP. In addition, the maximal nonorthogonality at the chiral EP leads to a blocking of one of the cross-polarised transmission pathways and, consequently, the observation of enhanced asymmetric polarisation conversion. We anticipate that electrically controllable non-Hermitian metasurface platforms can serve as an interesting framework for the investigation of rich non-Hermitian polarisation dynamics around chiral EPs.

2022

1. Nat. Commun.

   Plasmonic gain in current biased tilted Dirac nodes

   Sang Hyun Park, Michael Sammon, Eugene Mele, and 1 more author

   Nat. Commun., Dec 2022

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   Surface plasmons, which allow tight confinement of light, suffer from high intrinsic electronic losses. It has been shown that stimulated emission from excited electrons can transfer energy to plasmons and compensate for the high intrinsic losses. To-date, these realizations have relied on introducing an external gain media coupled to the surface plasmon. Here, we propose that plasmons in two-dimensional materials with closely located electron and hole Fermi pockets can be amplified, when an electrical current bias is applied along the displaced electron-hole pockets, without the need for an external gain media. As a prototypical example, we consider WTe 2 from the family of 1T $^\prime $ ′ -MX 2 materials, whose electronic structure can be described within a type-II tilted massive Dirac model. We find that the nonlocal plasmonic response experiences prominent gain for experimentally accessible currents on the order of mA μ m −1 . Furthermore, the group velocity of the plasmon found from the isofrequency curves imply that the amplified plasmons are highly collimated along a direction perpendicular to the Dirac node tilt when the electrical current is applied along it.

2. Nature

   Observation of chiral and slow plasmons in twisted bilayer graphene

   Tianye Huang, Xuecou Tu, Changqing Shen, and 13 more authors

   Nature, Dec 2022

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   Moiré superlattices have led to observations of exotic emergent electronic properties such as superconductivity and strong correlated states in small-rotation-angle twisted bilayer graphene (tBLG)1,2. Recently, these findings have inspired the search for new properties in moiré plasmons. Although plasmon propagation in the tBLG basal plane has been studied by near-field nano-imaging techniques3–7, the general electromagnetic character and properties of these plasmons remain elusive. Here we report the direct observation of two new plasmon modes in macroscopic tBLG with a highly ordered moiré superlattice. Using spiral structured nanoribbons of tBLG, we identify signatures of chiral plasmons that arise owing to the uncompensated Berry flux of the electron gas under optical pumping. The salient features of these chiral plasmons are shown through their dependence on optical pumping intensity and electron fillings, in conjunction with distinct resonance splitting and Faraday rotation coinciding with the spectral window of maximal Berry flux. Moreover, we also identify a slow plasmonic mode around 0.4 electronvolts, which stems from the interband transitions between the nested subbands in lattice-relaxed AB-stacked domains. This mode may open up opportunities for strong light–matter interactions within the highly sought after mid-wave infrared spectral window8. Our results unveil the new electromagnetic dynamics of small-angle tBLG and exemplify it as a unique quantum optical platform.

2021

1. ACS Photonics

   Accessing the Exceptional Points in a Graphene Plasmon–Vibrational Mode Coupled System

   Sang Hyun Park, Shengxuan Xia, Sang-hyun Oh, and 2 more authors

   ACS Photonics, Nov 2021

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   The coupling of plasmons and vibrational modes has been routinely observed in graphene and other 2D systems in both near-field and far-field spectroscopy. However, the relation between coupling strength and modal losses, and exceptional point physics has not been discussed. Here we apply a non-Hermitian framework to a model system of molecular layers on graphene and show that the transition point between strong and weak coupling regimes coincides with the exceptional point of non-Hermitian physics. We further show that the exceptional point can be conveniently located by changing the incident angle of the light and the graphene Fermi energy. Finally, we show that enhanced spectral sensitivity is obtained from small changes in molecular film thickness when the system is tuned to the exceptional point.

2020

1. Nanophotonics

   Observation of an exceptional point in a non-Hermitian metasurface

   Sang Hyun Park, Sung Gyu Lee, Soojeong Baek, and 6 more authors

   Nanophotonics, Nov 2020

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   Exceptional points (EPs), also known as non-Hermitian degeneracies, have been observed in parity-time symmetric metasurfaces as parity-time symmetry breaking points. However, the parity-time symmetry condition puts constraints on the metasurface parameter space, excluding the full examination of unique properties that stem from an EP. Here, we thus design a general non-Hermitian metasurface with a unit cell containing two orthogonally oriented split-ring resonators (SRRs) with overlapping resonance but different scattering rates and radiation efficiencies. Such a design grants us full access to the parameter space around the EP. The parameter space around the EP is first examined by varying the incident radiation frequency and coupling between SRRs. We further demonstrate that the EP is also observable by varying the incident radiation frequency along with the incident angle. Through both methods, we validate the existence of an EP by observing unique level crossing behavior, eigenstate swapping under encirclement, and asymmetric transmission of circularly polarized light.