We employ tip-enhanced infrared near-field microscopy to study the plasmonic properties of epitaxial quasi-free-standing monolayer graphene on silicon carbide. The near-field images reveal propagating graphene plasmons, as well as a strong plasmon reflection at gaps in the graphene layer, which appear at the steps between the SiC terraces. When the step height is around 1.5 nm, which is two orders of magnitude smaller than the plasmon wavelength, the reflection signal reaches 20% of its value at graphene edges, and it approaches 50% for step heights as small as 5 nm. This intriguing observation is corroborated by numerical simulations, and explained by the accumulation of a line charge at the graphene termination. The associated electromagnetic fields at the graphene termination decay within a few nanometers, thus preventing efficient plasmon transmission across nanoscale gaps. Our work suggests that plasmon propagation in graphene-based circuits can be tailored using extremely compact nanostructures, such as ultra-narrow gaps. It also demonstrates that tipenhanced near-field microscopy is a powerful contactless tool to examine nanoscale defects in graphene.

The magnetic circular dichroism and the Faraday rotation are the fundamental phenomena of great practical importance arising from the breaking of the time reversal symmetry by a magnetic field. In most materials, the strength and the sign of these effects can be only controlled by the field value and its orientation. Furthermore, the terahertz range is lacking materials having the ability to affect the polarization state of the light in a non-reciprocal manner. Here we demonstrate, using broadband terahertz magneto-electro-optical spectroscopy, that in graphene both the magnetic circular dichroism and the Faraday rotation can be modulated in intensity, tuned in frequency and, importantly, inverted using only electrostatic doping at a fixed magnetic field. In addition, we observe strong magneto-plasmonic resonances in a patterned array of graphene antidots, which potentially allows exploiting these magneto-optical phenomena in a broad THz range.

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 approach yields remarkable device performance featuring room temperature high sensitivity (NEP of 82 pW$$/\sqrt{{\bf{Hz}}}$$ / 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 very good 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.

Abstract. An analytical method for diffraction of a plane electromagnetic wave at periodically-modulated graphene sheet is presented. Both interface corrugation and periodical change in the optical conductivity are considered. Explicit expressions for reflection, transmission, absorption and transformation coefficients in arbitrary diffraction orders are presented. The dispersion relation and decay rates for graphene plasmons of the grating are found. Simple analytical expressions for the value of the band gap in the vicinity of the first Brillouin zone edge is derived. The optimal amplitude and wavelength, guaranteeing the best matching of the incident light with graphene plasmons are found for the conductivity grating. The analytical results are in a good agreement with first-principle numeric simulations.

Graphene plasmons (GPs) exhibit extreme confinement of the associated electromagnetic fields. For that reason, they are promising candidates for controlling light in nanoscale devices. However, despite the ubiquitous presence of surface corrugations in graphene, very little is known on how they affect the propagation of GPs. Here we perform a comprehensive theoretical analysis of GP scattering by both smooth and sharp corrugations. For smooth corrugations, we demonstrate that scattering of GPs depends on the dielectric environment, being strongly suppressed when graphene is placed between two dielectrics with the same refractive indices. We also show that sharp corrugations can act as effective GP reflectors, even when their dimensions are small in comparison with the GP wavelength. Additionally, we provide simple analytical expressions for the reflectance of GP valid in an ample parametric range. Finally, we connect these results with potential experiments based on scattering scanning near-field optical microscopy (s-SNOM) showing how to extract the GP reflectance from s-SNOM images.Graphene plasmons (GPs) -collective oscillations of free Dirac charge carriers in graphene coupled to electromagnetic fields -have an extremely short in-plane wavelength and are strongly confined to graphene sheet [1][2][3][4][5]. In addition, the GP wavelength depends on the Fermi level in graphene, and therefore can be manipulated by electrostatic gating. Lately, a great deal of attention has been devoted to the scattering characteristics of GPs by different inhomogeneities, as this is of particular importance for analyzing and controlling the GP propagation. GP efficient reflection has been already proved at graphene edges [6,7], grain boundaries [8,9], nanogaps in SiC terraces [10], boundaries introduced by ion beams [11], and at one-dimensional electrostatic barriers arising from a line of charges [12]. All previous cases can be related to conductivity inhomogeneities. Additionally, graphene also presents relief defects. In fact, free standing graphene is not flat. But neither it is graphene placed on a substrate (supported graphene), which has a tendency to form corrugations due to either imperfections of the substrate [13 -15] or to the formation of graphene wrinkles (characterized by widths between one and tens of nm, heights below 15 nm and lengths above 100 nm) [16 -21], ripples (which are corrugations with comparable height and width and smaller than wrinkles) [22] or bubbles (out-of-plane graphene deformations, with different shapes and sizes from tens to hundreds of nm in diameter and tens of nm in height, which accumulate air or other gas residuals between graphene and the substrate) [23 -28].The propagation of GPs can be strongly affected by the presence of these corrugations. However, very little is known about the scattering process, with the notable exception of ref. [28], which describes how a field hotspot can be formed by launching GPs in nano-bubbles.

We discuss propagation of symmetric and antisymmetric Josephson plasma waves in a slab of layered superconductor clad between two identical dielectrics. We predict two branches of surface waves in the terahertz frequency range, one above and another below the Josephson plasma frequency. Apart from this, there exists a discrete set of waveguide modes with electromagnetic fields oscillating across the slab thickness and decaying exponentially away from the slab. We consider the excitation of the predicted waves by means of the attenuated-total-reflection method.It is shown that for specific set of the parameters of the structure the excitation of the waveguide modes is accompanied by total suppression of specular reflection.

Faraday rotation is a fundamental property present in all non-reciprocal optical elements. In the THz range, graphene displays strong Faraday rotation; unfortunately, it is limited to frequencies below the cyclotron resonance. Here we show experimentally that in specifically design metasurfaces, magneto-plasmons can be used to circumvent this limitation. We find excellent agreement between theory and experiment and provide new physical insights and predictions on these phenomena. Finally, we demonstrate strong tuneability in these metasurfaces using electric and magnetic field biasing. PACS numbers:Graphene is considered a very promising material for non-reciprocal magneto-optical applications at microwave, terahertz and infrared frequencies [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Two of the most common non-reciprocal devices are isolators and circulators, and both are realizable starting from a Faraday rotator [9,11,17,20]. Faraday rotation (FR) observed in uniform graphene typically exhibits a maximum at low frequency (< 1 THz), and is barely present at higher frequencies, apparently precluding applications above 3 THz. [3,4,12]. It was experimentally found that the magneto-optical response in transmission is enhanced at the plasmonic resonance frequency, in structures such as graphene dots [8], antidots [21], and ribbons [22]. The effect of magneto-plasmonic resonance on the FR are currently experimentally unexplored even if it was numerically demonstrated that such plasmonic structures should also induce a blue-shifting of the Faraday rotation maximum [17,23]. In continuous graphene, the impedance of the continuous monolayer is given for the two opposite circular polarisations by [5] :where ω is the photon frequency, τ the carriers' scattering time, σ DC = e 2 τ |E F |(π 2 ) −1 is the low-temperature lowfrequency limit of graphene's conductivity for no mag- * Electronic address: mtamagnone@seas.harvard.edu † Electronic address: Jean-Marie.Poumirol@unige.ch netic bias, and ω c = ev 2 f B|E F | −1 is the semi-classical cyclotron frequency. In such a system the maximum FR always appear at energies bellow the cyclotron resonance. To go even further, when considering the highly doped (and/or low magnetic field regime) where τ −1 is dominant over ω c , the Faraday rotation will peak at zero frequency and will not extend above a cutoff frequency given by ω = τ −1 . Explaining the experimentally observed strong reduction of FR above 3 THz [3,4,12].Trying to circumvent these limitations, in this paper we have studied both experimentally and numerically the behaviour of the FR in three different patterned structures: a periodic array of graphene square dots (GSD), a graphene square anti-dot lattice (GSA), and a hybrid metal-graphene patterned structure (HMG). We have confirmed for all structures the presence of nonreciprocal magneto-plasmons blue shifting the Faraday rotation above the cyclotron resonance and compared the relative merits of the different geometries.All the samples measured ...

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