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Van der Waals (vdW) materials supporting phonon polaritons (PhPs) - light coupled to lattice vibrations - have gathered significant interest because of their intrinsic anisotropy and low losses. In particular, $\alpha$-MoO$_3$ supports PhPs with in-plane anisotropic propagation, which has been exploited to tune the optical response of twisted bilayers and trilayers. Additionally, various studies have explored the realization of polaritonic crystals (PCs) - lattices with periods comparable to the polariton wavelength -. PCs consisting of hole arrays etched in $\alpha$-MoO$_3$ slabs exhibit Bragg resonances dependent on the angle between the crystallographic axes and the lattice vectors. However, such PC concept, with a fixed orientation and size of its geometrical parameters, constrains practical applications and introduces additional scattering losses due to invasive fabrication processes. Here we demonstrate a novel PC concept that overcomes these limitations, enabling low-loss optical tuning. It comprises a rotatable pristine $\alpha$-MoO$_3$ layer located on a periodic hole array fabricated in a metallic layer. Our design prevents degradation of the $\alpha$-MoO$_3$ optical properties caused by fabrication, preserving its intrinsic low-loss and in-plane anisotropic propagation of PhPs. The resulting PC exhibits rotation of the Bloch modes, which is experimentally visualized by scanning near-field microscopy. In addition, we experimentally determine the polaritons momentum and reconstruct their band structure. These results pave the way for mechanically tunable nanooptical components based on polaritons for potential lasing, sensing, or energy harvesting applications. more...

We introduce a machine-learning approach (denoted Symmetry Seeker Neural Network) capable of automatically discovering discrete symmetry groups in physical systems. This method identifies the finite set of parameter transformations that preserve the system's physical properties. Remarkably, the method accomplishes this without prior knowledge of the system's symmetry or the mathematical relationships between parameters and properties. Demonstrating its versatility, we showcase examples from mathematics, nanophotonics, and quantum chemistry. more...

We present a scattering formalism to analyze the spin-momentum locking in structured holey plasmonic metasurfaces. It is valid for any unit cell for arbitrary position and orientation of the holes. The spin-momentum locking emergence is found to originate from the unit cell configuration. Additionally, we find that there are several breakdown terms spoiling the perfect spin-momentum locking polarization. We prove that this breakdown also appears in systems with global symmetries of translation and rotation of the whole lattice, like the Kagome lattice. Finally, we present the excitation of surface plasmon polaritons as the paramount example of the spin-momentum locking breakdown., Comment: 5 pages, 3 figures. 5 appendices: 3 pages, 1 table, 3 figures more...

We investigate the interaction between a monolayer of WS2 and a chiral plasmonic metasurface. WS2 possesses valley excitons that selectively couple with one-handed circularly polarised light. At the same time, the chiral plasmonic metasurface exhibits spin-momentum locking, leading to a robust polarisation response in the far field. Using a scattering formalism based on the coupled mode method, we analyse various optical properties of the WS2 monolayer. Specifically, we demonstrate the generation of circular dichroism in the transition metal dichalcogenide (TMD) by harnessing the excitation of surface plasmon polaritons (SPPs) in the metasurface. Moreover, we observe the emergence of other guided modes, opening up exciting possibilities for further exploration in TMD-based devices., Comment: 9 one-column pages, 6 figures. Preprint also submitted to Optica Open more...

With the advance of on-chip nanophotonics, there is a high demand for high refractive index, low-loss materials. Currently, this technology is dominated by silicon, but van der Waals (vdW) materials with high refractive index can offer a very advanced alternative. Still, up to now it was not clear if the optical anisotropy perpendicular to the layers might be a hindering factor for the development of vdW nanophotonics. Here, we studied WS2-based waveguides in terms of their optical properties and, particularly, in terms of possible crosstalk distance. Surprisingly, we discovered that the low refractive index in the direction perpendicular to the atomic layers improves the characteristics of such devices, mainly due to expanding the range of parameters at which single-mode propagation can be achieved. Thus, using anisotropic materials offers new opportunities and novel control knobs when designing the nanophotonic devices., Comment: 11 pages, 3 figures more...

Electrical conductivity of one-dimensional (1d) disordered solids decays exponentially with their length, which is a celebrated manifestation of the localization phenomenon. Here, we study the modifications of localized conductivity induced by placement of 1d semiconductors inside of single-mode electromagnetic cavities, focusing on the regime of non-degenerate doping. We use the Green's function technique modified for the non-perturbative account of cavity excited states, and including both coherent electron-cavity effects (i.e. electron motion in the zero-point fluctuating field) and incoherent processes of photon emission upon tunneling. The energy spectrum of electron transmission in the cavity acquires Fano-type resonances associated with virtual photon emission, passage along the resonant level, and photon re-absorption. The quality factor of the Fano resonance depends on whether the intermediate state is coupled to the leads, and reaches its maximum when this state is localized deep in the disorder potential. Coupling to the cavity also elevates the energies of the shallow bound states, bringing them to the conduction band bottom. Such an effect leads to the enhancement of the low-temperature conductance. more...

We revisit spin-orbit coupling in a plasmonic Berry metasurface comprised of rotated nanoapertures, which is known to imprint a robust far-field polarization response. We present a scattering formalism that shows how that spin-momentum locking emerges from the geometry of the unit cell without requiring global rotation symmetries. We find and confirm with Mueller polarimetry measurements that spin-momentum locking is an approximate symmetry. The symmetry breakdown is ascribed to the elliptical projection of circularly polarized light into the planar surface. This breakdown is maximal when surface waves are excited, and a new set of spin-momentum locking rules is presented for this case., Comment: 6 pages and 3 figures. 1 Supplemental material with 15 pages and 6 figures more...

The ability to control light polarization state is critically important for diverse applications in information processing, telecommunications, and spectroscopy. Here, we propose that a stack of anisotropic van der Waals materials can facilitate the building of optical elements with Jones matrices of unitary, Hermitian, non-normal, singular, degenerate, and defective classes. We show that the twisted stack with electrostatic control can function as arbitrary-birefringent wave-plate or arbitrary polarizer with tunable degree of non-normality, which in turn give access to plethora of polarization transformers including rotators, pseudorotators, symmetric and ambidextrous polarizers. Moreover, we discuss an electrostatic-reconfigurable stack which can be tuned to operate as four different polarizers and be used for Stokes polarimetry. more...

Vertical plasmonic coupling in double-layer graphene leads to two hybridized plasmonic modes: optical and acoustic plasmons with symmetric and anti-symmetric charge distributions across the interlayer gap, respectively. However, in most experiments based on far-field excitation, only the optical plasmons are dominantly excited in the double-layer graphene systems. Here, we propose strategies to selectively and efficiently excite acoustic plasmons with a single or multiple nano-emitters. The analytical model developed here elucidates the role of the position and arrangement of the emitters on the symmetry of the resulting graphene plasmons. We present an optimal device structure to enable experimental observation of acoustic plasmons in double-layer graphene toward the ultimate level of plasmonic confinement defined by a monoatomic spacer, which is inaccesible with a graphene-on-a-mirror architecture. more...

Integrating and manipulating the nano-optoelectronic properties of Van der Waals heterostructures can enable unprecedented platforms for photodetection and sensing. The main challenge of infrared photodetectors is to funnel the light into a small nanoscale active area and efficiently convert it into an electrical signal. Here, we overcome all of those challenges in one device, by efficient coupling of a plasmonic antenna to hyperbolic phonon-polaritons in hexagonal-BN to highly concentrate mid-infrared light into a graphene pn-junction. We balance the interplay of the absorption, electrical and thermal conductivity of graphene via the device geometry. This novel approach yields remarkable device performance featuring room temperature high sensitivity (NEP of 82 pW-per-square-root-Hz) and fast rise time of 17 nanoseconds (setup-limited), among others, hence achieving a combination currently not present in the state-of-the-art graphene and commercial mid-infrared detectors. We also develop a multiphysics model that shows excellent quantitative agreement with our experimental results and reveals the different contributions to our photoresponse, thus paving the way for further improvement of these types of photodetectors even beyond mid-infrared range. more...

Vibrational ultrastrong coupling (USC), where the light-matter coupling strength is comparable to the vibrational frequency of molecules, presents new opportunities to probe the interactions of molecules with zero-point fluctuations, harness cavity-enhanced chemical reactions, and develop novel devices in the mid-infrared regime. Here we use epsilon-near-zero nanocavities filled with a model polar medium (SiO$_2$) to demonstrate USC between phonons and gap plasmons. We present classical and quantum mechanical models to quantitatively describe the observed plasmon-phonon USC phenomena and demonstrate a splitting of up to 50% of the resonant frequency. Our wafer-scale nanocavity platform will enable a broad range of vibrational transitions to be harnessed for USC applications., Comment: 45 pages, 12 figures more...

Recently, it was demonstrated that a graphene/dielectric/metal configuration can support acoustic plasmons, which exhibit extreme plasmon confinement an order of magnitude higher than that of conventional graphene plasmons. Here, we investigate acoustic plasmons supported in a monolayer and multilayers of black phosphorus (BP) placed just a few nanometers above a conducting plate. In the presence of a conducting plate, the acoustic plasmon dispersion for the armchair direction is found to exhibit the characteristic linear scaling in the mid- and far-infrared regime while it largely deviates from that in the long wavelength limit and near-infrared regime. For the zigzag direction, such scaling behavior is not evident due to relatively tighter plasmon confinement. Further, we demonstrate a new design for an acoustic plasmon resonator that exhibits higher plasmon confinement and resonance efficiency than BP ribbon resonators in the mid-infrared and longer wavelength regime. Theoretical framework and new resonator design studied here provide a practical route toward the experimental verification of the acoustic plasmons in BP and open up the possibility to develop novel plasmonic and optoelectronic devices that can leverage its strong in-plane anisotropy and thickness-dependent band gap. more...

We explore the far-field scattering properties of anisotropic 2D materials in ribbon array configuration. Our study reveals the plasmon-enhanced linear birefringence in these ultrathin metasurfaces, where linearly polarized incident light can be scattered into its orthogonal polarization or be converted into circular polarized light. We found wide modulation in both amplitude and phase of the scattered light via tuning the operating frequency or material's anisotropy and develop models to explain the observed scattering behavior. more...

A beam of linearly polarized light transmitted through magnetically biased graphene can have its axis of polarization rotated by several degrees after passing the graphene sheet. This large Faraday effect is due to the action of the magnetic field on graphene's charge carriers. As deformations of the graphene membrane result in pseudomagnetic fields acting on the charge carriers, the effect of random mesoscopic corrugations (ripples) can be described as the exposure of graphene to a random pseudomagnetic field. We aim to clarify the interplay of these typically sample inherent fields with the external magnetic bias field and the resulting effect on the Faraday rotation. In principle, random gauge disorder can be identified from a combination of Faraday angle and optical spectroscopy measurements., Comment: 7 pages, 4 figures more...

The scattering of a flying photon by a two-level system ultrastrongly coupled to a one-dimensional photonic waveguide is studied numerically. The photonic medium is modeled as an array of coupled cavities and the whole system is analyzed beyond the rotating wave approximation using Matrix Product States. It is found that the scattering is strongly influenced by the single- and multi-photon dressed bound states present in the system. In the ultrastrong coupling regime a new channel for inelastic scattering appears, where an incident photon deposits energy into the qubit, exciting a photon-bound state, and escaping with a lower frequency. This single-photon nonlinear frequency conversion process can reach up to 50\% efficiency. Other remarkable features in the scattering induced by counter-rotating terms are a blueshift of the reflection resonance and a Fano resonance due to long-lived excited states, Comment: 5+4 pages more...

A theoretical study is presented on the scattering of graphene surface plasmons by defects in the graphene sheet they propagate in. These defects can be either natural (as domain boundaries, ripples and cracks, among others) or induced by an external gate. The scattering is shown to be governed by an integral equation, derived from a plane wave expansion of the fields, which in general must be solved numerically but it provides useful analytical results for small defects. Two main cases are considered: smooth variations of the graphene conductivity (characterized by a Gaussian conductivity profile) and sharp variations (represented by islands with different conductivity). In general, reflection largely dominates over radiation out of the graphene sheet. However, in the case of sharply defined conductivity islands there are some values of island size and frequency where the reflectance vanishes and, correspondingly, the radiation out of plane is the main scattering process. For smooth defects, the reflectance spectra present a single maximum at the condition $k_p a \approx \sqrt{2}$, where $k_p$ is the GSP wavevector and $a$ the spatial width of the defect. In contrast, the reflectance spectra of sharp defects present periodic oscillations with period $k_p' a$, where $k_p'$ is the GSP wavelength inside the defect. Finally, the case of cracks (gaps in the graphene conductivity) is considered, showing that the reflectance is practically unity for gap widths larger than one tenth of the GSP wavelength., Comment: 22 pages, 8 figures more...

We demonstrate that the interaction between two emitters can be controlled by means of the efficient excitation of surface plasmon modes in graphene. We consider graphene surface plasmons supported by either two-dimensional graphene sheets or one-dimensional graphene ribbons, showing in both cases that the coupling between the emitters can be strongly enhanced or suppressed. The super- and subradiant regimes are investigated in the reflection and transmission configurations. Importantly, the length scale of the coupling between emitters, which in vacuum is fixed by the free space wavelength, is now determined by the wavelength of the graphene surface plasmons that can be extremely short and be tuned at will via a gate voltage. more...

In this paper we analyze how Transformation Optics recipes can be applied to control the flow of surface plasmons on metal-dielectric interfaces. We study in detail five different examples: a cylindrical cloak, a beam shifter, a right-angle bend, a lens and a ground-plane cloak. First, we demonstrate that only the modification of the electric permittivity and magnetic permeability in the dielectric side can lead to almost perfect functionalities for surface plasmons. We also show that, thanks to the quasi two-dimensional character of surface plasmons and its inherent polarization, the application of conformal and quasiconformal mapping techniques allows the design of plasmonic devices in which only the isotropic refractive index of the dielectric film needs to be engineered., Comment: To be published in New Journal of Physics more...

A new strategy to control the flow of surface plasmon polaritons at metallic surfaces is presented. It is based on the application of the concept of Transformation Optics to devise the optical parameters of the dielectric medium placed on top of the metal surface. We describe the general methodology for the design of Transformation-Optical devices for surface plasmons and analyze, for proof-of-principle purposes, three representative examples with different functionalities: a beam shifter, a cylindrical cloak and a ground-plane cloak., Comment: 15 pages, 3 figures more...