Nuno M. R. Peres

The boundary conditions imposed by confined dipoles with arbitrary orientation on surfaces are presented, extending the conventional in-plane (IP) and out-of-plane (OOP) treatments, here applied for planar and cylindrical sheets. Examples include van der Waals heterostructures, thin films of molecular aggregates, and metal–dielectric interfaces. For large dipole strengths, the reflectance peak associated with the dipole oscillation frequency splits into two, revealing the presence of oblique dipoles. The loss function for the dipole sheet reveals pairs of polaritonic resonances originating from the IP and OOP dipole components, accessible through near-field probes. The point-dipole model for s-SNOM shows two distinct peaks, revealing higher sensitivity to dipole obliqueness than reflectance experiments. We apply the model to monolayer WSe ₂ , showing that the oblique dipole formulation with a dipole angle of 5.7° yields significant improvements in the qualitative and quantitative agreement with an experiment reported in the literature. This work proposes a unified language for the description of two-dimensional materials, thin films, and interfaces with anisotropic dipolar responses and shows that near-field methods, sensitive to high in-plane momenta, are suitable for measuring such oblique dipoles.

In this article, we analyze the quantum and topological properties of graphene-based plasmonic systems. We consider the following plasmonic materials: single-layer graphene, twisted bilayer graphene, and other graphene stackings, as well as the following architectures: graphenebased gratings, grids, chains of graphene disks, and the kagomé lattice.

We present a general electromagnetic description for dipoles confined to surfaces with oblique dipole moment orientations, extending the conventional in-plane (IP) and out-of-plane (OOP) treatments. This description is useful for describing localized polarization in, \textit{e.g.}, van der Waals heterostructures, thin films of molecular aggregates, and metal-dielectric interfaces. The theory is suitable for any material with vanishingly thin thickness relative to the light wavelength, independent of the geometry of the material and the media interfacing it. We apply the formalism to a uniaxial excitonic sheet, covering a large number of two-dimensional (2D) materials and organic thin films. Our theory reveals pairs of polaritonic resonances originating from the IP and OOP components of the excitonic dipole moment. The formalism suggests experimentally accessible signatures of dipole moment orientation, enhanced by near-field probes. This work proposes a unified language for the description of 2D materials, thin films and interfaces with anisotropic dipolar responses.

Coreshell nanoparticles can combine optical features of different materials in a single nanostructure, which makes them interesting for many applications from biomedicine to energy harvesting. On the other hand, periodic arrays of plasmonic nanoparticles can exhibit topological phenomena such as topological edge states. Here, we study periodic chains of Si@Ag coreshell nanoparticles. For this task, we combine the hybridization of surface plasmonic modes in complex nanostructures with the coupled electric dipole formalism employed for modeling the scattering of light by periodic arrays of small plasmonic nanoparticles. We propose treating a coreshell nanoparticle as several coupled electric dipoles instead of just one and show how this is a more appropriate framework to study arrays of coreshell nanoparticles. This method allows us to build a one-to-one connection between the resonant modes of the nanoparticles and the dispersion bands of the system. Within this formalism, we show that a quasi-static Su–Schrieffer–Heeger chain of coreshell Si@Ag nanoparticles hosts multiple topological edge states at different bandgaps with tunable frequencies.

Excitons in biased bilayer graphene are electrically tunable optical excitations residing in the mid‐infrared (MIR) spectral range, where intrinsic optical transitions are typically scarce. Such a tunable material system with an excitonic response offers a rare platform for exploring light–matter interactions and optical hybridization of quasiparticles residing in the long wavelength spectrum. In this work, it is demonstrated that when the bilayer is encapsulated in hexagonal‐boron‐nitride (hBN)—a material supporting optical phonons and hyperbolic‐phonon‐polaritons (HPhPs) in the MIR—the excitons can be tuned into resonance with the HPhP modes. It is found that the overlap in energy and momentum of the two MIR quasi‐particles facilitate the formation of multiple strongly coupled hybridized exciton‐HPhP states. Using an electromagnetic transmission line model, the dispersion relations of the hybridized states are derived and showed that they are highly affected and can be manipulated by the symmetry of the system, determining the hybridization selection rules. These results establish a general tunable MIR platform for engineering strongly coupled quasiparticle states in biased graphene systems, opening new directions for studying and controlling light–matter interactions in the long‐wavelength regime.

We present a hydrodynamic model, grounded in Madelung’s formalism, to describe collective electronic motion in anisotropic materials. This model incorporates nonlocal contributions from the Thomas–Fermi quantum pressure and quantum effects arising from the Bohm potential. We derive analytical expressions for the magnetoplasmon dispersion and nonlocal optical conductivity. To demonstrate the applicability of the model, we examine electrons in the conduction band of monolayer phosphorene, an exemplary anisotropic two‐dimensional electron gas. The dispersion of plasmons derived from our hydrodynamic approach is closely aligned with that predicted by ab initio calculations. Then, we use our model to analyze few‐layer black phosphorus, whose measured infrared optical response is hyperbolic. Our results reveal that the incorporation of nonlocal and quantum effects in the optical conductivity prevents black phosphorus from supporting hyperbolic surface plasmon polaritons. We further demonstrate that the predicted wavefront generated by an electric dipole exhibits a significant difference between the local and nonlocal descriptions for the optical conductivity. This study underscores the necessity of moving beyond local approximations when investigating anisotropic systems capable of hosting strongly confined plasmon‐polaritons.

Coreshell nanoparticles can combine optical features of different materials in a single nanostructure, which makes them interesting for many applications from biomedicine to energy harvesting. On the other hand, periodic arrays of plasmonic nanoparticles can exhibit topological phenomena such as topological edge states. Here we study periodic chains of Si@Ag coreshell nanoparticles. For this task, we combine the hybridization of surface plasmonic modes in complex nanostructures with the coupled electric dipole formalism employed for modelling the scattering of light by periodic arrays of small plasmonic nanoparticles. We propose treating coreshell nanoparticle as several coupled electric dipoles instead of just one and show how this is a more appropriate framework to study arrays of coreshell nanoparticles, which allows to build a one-to-one connection between the resonant modes of the nanoparticles and the dispersion bands of the system. Within this formalism, we show that a Su-Schrieffer-Heeger (SSH) chain of coreshell Si@Ag nanoparticles host multiple topological edge states pinned at the resonant frequencies of the coreshell nanoparticles.

Terahertz polarizers are needed for advanced spectroscopic systems, but they have drawbacks such as low transmission, short bandwidths, and low extinction ratios. A method for the development of ultrabroadband THz polarizers based on the nanoimprint lithography technique is reported here, in which high performance is achieved for a double-wire-grid polarizer (DWGP) structures on cyclic olefin copolymer (COC) substrates. Over the 0.1–25 THz frequency range, the polymer DWGPs exhibited more than twice the TM-polarized transmittance of their silicon-based counterparts. The degree of polarization was greater than 98% in the 0.1–16 THz range, and the extinction ratio was greater than 65.4 dB at 4.2 THz. THz time-domain spectroscopy (THz-TDS) and Fourier-transform infrared spectroscopy (FTIR) were also employed to characterize the optical properties of materials over the frequency ranges of 0.1–40 THz and 0.9–20 THz, respectively. The nanofabricated polymer DWGP showed better optical properties than the Si DWGP in terms of enhanced TM transmittance and reduced TE leakage. In addition, the prepared COC polarizers exhibited cost-effectiveness, scalability, and durability and can be considered environmentally friendly alternatives to conventional Si-based polarizers. This study opens up the possibility of using polymer-based DWGPs as important high-performance components in THz imaging and sensing applications and in wireless communication systems.