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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/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., F.H.L.K. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R& D (SEV-2015-0522), support by Fundacio Cellex Barcelona, Generalitat de Catalunya through the CERCA program, and the Agency for Management of University and Research Grants (AGAUR) 2017 SGR 1656. Furthermore, the research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement no. 785219 and no. 881603 Graphene Flagship for Core2 and Core3. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). K.J.T. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 804349. R.H. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (national project RTI2018-094830-B-100 and the project MDM-2016-0618 of the Marie de Maeztu Units of Excellence Program) and the Basque Government (grant No. IT1164-19). S.C. acknowledges financial support from the Barcelona Institute of Science and Technology (BIST), the Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement de la Generalitat de Catalunya and the European Social Fund (L’FSE inverteix en el teu futur)—FEDER. D.E. acknowledges partial support from the Army Research Office MURI “Ab-Initio Solid-State Quantum Materials” Grant No. W911NF18-1-0431. J.G. was supported by the ARL-MIT Institute for Soldier Nanotechnologies (ISN). T.S. and L.M.M. acknowledge support by Spain’s MINECO under Grant No. MAT2017-88358-C3-1-R and the Aragon Government through project Q-MAD.

We present a very-compact bicomponent-permanent-magnet design capable of generating 1.25 T in a small volume, significantly above the 0.6 T available from a single uniformly magnetized permanent magnet. In addition to the enhanced maximum field, our design drastically limits the stray field present around a standard permanent magnet. These features make it suitable for retrofitting existing experiments with a substantial magnetic field, in particular, scanning probes and optical, Raman, and photoemission spectroscopy, in diverse environments, from ambient to ultrahigh vacuum and over a wide temperature range., This work is financed by the EU Project “Graphene Flagship” (CNECT-ICT604391) and UNIGE/Unitec via the Innogap program.

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 GPs 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., The authors acknowledge support from the European Commission under the Graphene Flagship (GrapheneCore1, grant no. 696656) and the Spanish Ministry of Economy and Competitiveness (projects MAT2014-53432-C5-1-R and MAT2014-53432-C5-4-R).

We discuss plasmons of biased twisted bilayer graphene when the Fermi level lies inside the gap. The collective excitations are a network of chiral edge plasmons (CEP) entirely composed of excitations in the topological electronic edge states that appear at the AB-BA interfaces. The CEP form a hexagonal network with a unique energy scale ${\ensuremath{\epsilon}}_{p}=({e}^{2})/({\ensuremath{\epsilon}}_{0}\ensuremath{\epsilon}{t}_{0})$ with ${t}_{0}$ the moir\'e lattice constant and $\ensuremath{\epsilon}$ the dielectric constant. From the dielectric matrix we obtain the plasmon spectra that has two main characteristics: (i) a diverging density of states at zero energy, and (ii) the presence of a plasmonic Dirac cone at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\sim}{\ensuremath{\epsilon}}_{p}/2$ with sound velocity ${v}_{D}=0.0075c$, which is formed by zigzag and armchair current oscillations. A network model reveals that the antisymmetry of the plasmon bands implies that CEP scatter at the hexagon vertices maximally in the deflected chiral outgoing directions, with a current ratio of $4/9$ into each of the deflected directions and $1/9$ into the forward one. We show that scanning near-field microscopy should be able to observe the predicted plasmonic Dirac cone and its broken symmetry phases.

The paper shows that the Faraday rotation angle can be strongly enhanced by straining graphene in the presence of small magnetic fields (easily reachable with permanent magnets). Strain provides a large pseudo-magnetic gauge field, while the external magnetic field produces the breaking of time-reversal symmetry needed to obtain any non-reciprocal effect, such as the Faraday rotation.

Faraday rotation is a fundamental property present in all nonreciprocal 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 designed metasurfaces, magnetoplasmons can be used to circumvent this limitation. We find excellent agreement between theory and experiment and provide physical insights and predictions on these phenomena. Finally, we demonstrate strong tunability in these metasurfaces using electric and magnetic field biasing., This work has been financially supported by the Swiss National Science Foundation (SNSF) under GrantsNo. 133583 and No. 168545, the Hasler Foundation under Project No. 11149, and the European Commission under Graphene Flagship (Contract No. CNECT-ICT-604391).

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., This research was supported by the EU Graphene Flagship (Contract No. CNECT-ICT-604391 and 696656) and by the Swiss National Science Foundation (Grant No. 200020-156615). A.Y.N., T.M.S. and L.M.-M. acknowledge support from the Spanish Ministry of Economy and Competitiveness (projects MAT2014-53432-C5-1-R and MAT2014-53432-C5-4-R). J.F. acknowledges the ERC Grant ‘MUSIC’.

The rigorous excitation equations are used to analyze wave propagation in a rectangular waveguide section with sharp periodic corrugations whose height is much greater than the period. It has been shown that the Brillouin diagram of the corrugated and regular waveguides in the case of strong gradient scattering is characterized by qualitatively the same frequency dependence of the longitudinal wavenumber. The only difference is that the cutoff frequencies of the corrugated waveguide modes increase with decreasing the corrugation period, and it is always possible to implement the single-mode regime within an arbitrary frequency range lying higher than the first cutoff frequency. The effect can be used to suppress the mode competition in microwave amplifiers, as well as in narrow-band oscillators provided that the latter involve the Bragg reflectors as a feed-back.

The nonlinear response of a layered superconductor to symmetric (in magnetic field) electromagnetic excitation has been theoretically investigated. An ambiguous dependence of the phase of the reflected signal on the amplitude H0 of the irradiating wave is predicted. This causes hysteresis jumps in the dependence of the surface reactance of the superconductor on H0. If the frequency of the irradiating field is close to the Josephson plasma frequency, this unusual nonlinear effect can be observed when the amplitudes of the ac field are not very strong. The conditions for the appearance of hysteresis are obtained. Expressions for the phase shift of the reflected wave are derived, using the coupled sine-Gordon equations. Moreover, a class of solutions of these equations that are discontinuous in the coordinate are studied that correspond to a continuous spatial distribution of the magnetic field in the superconductor. Such solutions result in the appearance of additional branches in the dependence of the phas...

For layered superconductors, we study the specific nonlinear Josephson plasma waves (NJPWs) propagating along thin superconducting slabs and damping away from them. Two cases are considered, when the superconductor is surrounded by either vacuum or metals. The magnetic field of the NJPW is distributed symmetrically with respect to the middle of the sample and can change its sign inside the slab. The impedance ratio of the tangential electric and magnetic field amplitudes for NJPWs can be of the order of unity. For the case of a superconductor surrounded by the vacuum, this results in a non-monotonic dispersion relation, ω ( k ), strongly sensitive to the NJPW amplitudes. Therefore, the “stopping light” phenomenon can be observed at frequencies where d ω ( k )/d k = 0. Resonance excitations of the NJPWs should produce anomalies in the amplitude dependence of the reflectivity and transmissivity of the incident THz waves, which could be useful for THz devices. Animations illustrating the results presented here, are available online at dml.riken.go.jp/nonlinear .

It is predicted that the time dependence of the electric field at the surface of a superconducting slab carrying a transport current will have jumps that occur as a result of the interaction of the current with the field of the electromagnetic wave. A theoretical analysis of the phenomenon is carried out in a simple model for arbitrary monotonic dependence of the critical current density on the magnetic induction. The dependence of the value of the jump of the electric field on the amplitude of the wave and the strength of the transport current is investigated. The predicted effect can be observed experimentally by measuring the emf of a coil wound on the slab in the plane perpendicular to the external ac magnetic field.

et al., 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 ultranarrow gaps. It also demonstrates that tip-enhanced near-field microscopy is a powerful contactless tool to examine nanoscale defects in graphene. © 2013 American Chemical Society., R.H. acknowledges support by the ERC Starting Grant 258461 (TERATOMO) and the National Project MAT2012-36580 from the Spanish Ministerio de Ciencia e Innovación. M.L.N., A.Y.N., T.M.S., and L.M.M. acknowledge the Spanish Ministry of Science and Innovation Grant MAT2011-28581-C02. S.T. and F.J.G.A. acknowledge support from the Spanish MEC (contract no. MAT2010-14885). The work of I.C. and A.B.K. was supported by Swiss National Science Foundation (Grant 200020-140710). F.H.L.K. acknowledges support by the Fundacicio Cellex Barcelona, the ERC Career integration grant 294056 (GRANOP) and the ERC starting grant 307806 (CarbonLight). We acknowledge support by the E.C. under Graphene Flagship (contract no. CNECT-ICT-604391).

We analytically study the excitation of surface Josephson plasma waves (SJPWs) propagating across the junctions in layered superconductors in the presence of external dc magnetic field. Both the attenuated total reflection and the modulation of the superconducting parameters methods of the SJPWs excitation are considered. We show that the reflection of the incident electromagnetic wave can be substantially decreased due to the resonance excitation of SJPWs, for certain angles and frequencies of the incident wave when changing the magnetic field. Moreover, we find physical conditions guaranteeing the total suppression of the specular reflectivity. The analytical results are supported by the numerical simulations.

We analytically examine the excitation of surface Josephson plasma waves (SJPWs) in periodically-modulated layered superconductors. We show that the absorption of the incident electromagnetic wave can be substantially increased, for certain incident angles, due to the resonance excitation of SJPWs. The absorption increase is accompanied by the decrease of the specular reflection. Moreover, we find the physical conditions guaranteeing the total absorption (and total suppression of the specular reflection). These conditions can be realized for Bi2212 superconductor films., 17 pages, 3 figures

We consider here a layered superconductor subject to an externally applied moderately-strong electromagnetic field. We predict hysteretic jumps in the dependence of the surface reactance of the superconductor on the amplitude of the incident electromagnetic wave. This very unusual nonlinear phenomenon can be observed in thin superconducting slabs at not very strong ac amplitudes, if the frequency of the irradiating field is close to the Josephson plasma frequency. Using the set of coupled sine-Gordon equations, we derive the expression for the phase shift of the reflected wave and obtain the conditions for the appearance of hysteresis., Comment: 15 pages, 5 figures

This is a theoretical study of the resonant suppression of the specular reflection of terahertz waves in layered superconductors due to the excitation of surface Josephson plasma waves (SJPWs). Here, we consider in detail the specific case of SJPW excitations by evanescent electromagnetic waves via the attenuated total reflection of incident waves in a dielectric prism. We also derive the dispersion relation for surface waves propagating along the vacuum-superconductor interface parallel to the $\mathbf{a}\mathbf{b}$ plane. We show that, due to the SJPW excitation, the reflectivity of the incident wave depends resonantly on both its frequency and incident angle. We find the optimal conditions for the best matching of the incident wave and SJPWs, as well as for the total suppression of the specular reflection.

It is shown that nonlinear Josephson plasma waves (NJPWs) can propagate along a thin slab of layered superconductor surrounded by metals. These waves have no analogs in other systems consisting of the conducting materials only. The NJPWs are stable for extremely anisotropic superconductors in a certain interval of wave amplitudes.

In this paper the surface Josephson plasma wave (SJPW) excitation on the periodically-modulated layered superconductor is considered. It is shown that the absorption of the incident electromagnetic wave can be substantially increased at certain incident angles due to the resonance excitation of the surface waves. The optimal combinations of the parameters of the problem (modulation amplitude, wavelength, incident angle) corresponding to maximum SJPW excitation and the effect of the total suppression of specular reflection (TSSR) are found for a real medium (Bi2212).

The detailed theoretical analysis of the resonance phenomena caused by excitation of the surface Josephson plasma waves (SJPW) in layered superconductors is made. It is shown that the surface waves at the boundary between the vacuum and layered superconductor can be excited by attenuated total reflection (ATR). The dispersion relation for the SJPW and the intensity of the reflected wave are investigated. The parameters of the problem (optimal thickness of vacuum layer, optimal wavelength) corresponding to the total suppression of specular reflection (TSSR) are found.

arXiv:1307.0310v1, An analytical method for diffraction of a plane electromagnetic wave at a periodically modulated graphene sheet is presented. Both interface corrugation and a periodic 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 bandgap in the vicinity of the first Brillouin zone edge are 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-principles numeric simulations. © 2013 IOP Publishing Ltd.