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As a rapid, label-free, non-destructive analytical measurement requiring little to no sample preparation, Raman spectroscopy shows great promise for liquid biopsy cancer detection and diagnosis. We carried out Raman analysis and mass spectrometry of plasma and saliva from more than 50 subjects in a cohort of head and neck cancer patients and benign controls (e.g., patients with benign oral masses). Unsupervised data models were built to assess diagnostic performance. Raman spectra collected from either biofluid provided moderate performance to discriminate cancer samples. However, by fusing together the Raman spectra of plasma and saliva for each patient, subsequent analytical models delivered an impressive sensitivity, specificity, and accuracy of 96.3%, 85.7%, and 91.7%, respectively. We further confirmed that the metabolites driving the differences in Raman spectra for our models are among the same ones that drive mass spectrometry models, unifying the two techniques and validating the underlying ability of Raman to assess metabolite composition. This study bolsters the relevance of Raman to provide additive value by probing the unique chemical compositions across biofluid sources. Ultimately, we show that a simple data augmentation routine of fusing plasma and saliva spectra provided significantly higher clinical value than either biofluid alone, pushing forward the potential of clinical translation of Raman spectroscopy for liquid biopsy cancer diagnostics.

This brief presents a novel frequency reconfigurable microstrip non-reciprocal bandpass filter based on time-modulated microstrip $\lambda _{g}/2$ resonators. The modulation is achieved by loading both ends of the $\lambda _{g}/2$ transmission line resonators with time-modulated capacitors. The modulation voltage source is connected at the center of the resonator, where there is a natural voltage null, and a single inductor is used to further enhance the biasing isolation. The wideband nature of this isolation scheme enables the tuning of the devices over a wide frequency range. With more than 30-dB RF to modulation isolation, the proposed resonator structure also enables low insertion loss by eliminating RF signal leakage to the modulation ports. A 3-pole bandpass filter example is simulated and measured, showing a minimal insertion loss of 3.9dB, a 20-dB isolation bandwidth of 42MHz at 1.0GHz, and frequency tuning range of 885–1031MHz.

This work presents a novel low-pass filter based on using dielectric materials, in the critical areas of the structure, to prevent high-power space condition adverse effects, like multipactor breakdown. It is shown that the use of dielectric pieces in the inverters of the filter leads to increased gaps between metallic parts in the irises. In addition, by completely filling up the air space in critical gaps, the multipactor breakdown thresholds can be increased. Moreover, they also lead to a volume reduction, increasing compactness of the structure. To demonstrate the proposed concept, a prototype has been manufactured with an aluminum housing and a Teflon dielectric material, and tested for multipactor effect at ESA-VSC Laboratory. Tests confirmed that the manufactured filter has good electrical performance with cut-off frequency 13.6 GHz, insertion losses 0.32 dB, return losses better than 15 dB, and additionally does not show multipactor breakdown up to the maximum power tested in the experimental facility (5.5 kW). This research work has been financially supported by the Spanish Ministerio de Ciencia e Innovación in the frame of the project “Green an Efficient Technologies for Advanced Telecommunication Systems (GRETAS)” with Ref. TPID2019-103982RB-C42.

In this paper, we demonstrate, for the first time, an isolating bandpass filter with low-loss forward transmission and high reverse isolation by modulating its constituent resonators. To understand the operating principle behind the device, we develop a spectral domain analysis method and show that the same-frequency nonreciprocity is a result of the nonreciprocal frequency conversion to the intermodulation (IM) frequencies by the time-varying resonators. With appropriate modulation frequency, modulation depth, and phase delay, the signal power at the IM frequencies is converted back to the RF frequency and adds up constructively to form a low-loss forward passband, whereas they add up destructively in the reverse direction to create the isolation. To validate the theory, a lumped-element three-pole 0.04-dB ripple isolating filter with a center frequency of 200 MHz and a ripple bandwidth of 30 MHz is designed, simulated, and measured. When modulated with a sinusoidal frequency of 30 MHz, a modulation index of 0.25, and an incremental phase difference of 45°, the filter achieves a forward insertion loss of 1.5 dB and a reverse isolation of 20 dB. The measured nonmodulated and modulated results agree very well with the simulations. Such nonreciprocal filters may find applications in wideband simultaneous transmit and receive radio front ends.

Nanoscale manipulation and characterization of individual molecules is necessary to understand the intricacies of molecular structure, which governs phenomena such as reaction mechanisms, catalysis, local effective temperatures, surface interactions, and charge transport. Here we utilize Raman enhancement between two nanostructured electrodes in combination with direct charge transport measurements to allow for simultaneous characterization of the electrical, optical, and mechanical properties of a single molecule. This multi-dimensional information yields repeatable, self-consistent, verification of single-molecule resolution, and allows for detailed analysis of structural and configurational changes of the molecule in situ. These experimental results are supported by a machine-learning based statistical analysis of the spectral information and calculations to provide insight into the correlation between structural changes in a single-molecule and its charge-transport properties.

Drift-biased graphene enables strong nonreciprocal light-matter interactions. In this work, we show that applying two longitudinal DC bias orthogonal to each other permits to greatly manipulate the isofrequency contour of the surface in a nonreciprocal manner, allowing to dynamically steer and canalize one-way plasmons towards desired directions in the plane. Through a dedicated Green’s function formalism that takes graphene’s intrinsic nonlocality into account, we demonstrate that simply controlling two bias of just a few volts enables unprecedented flexibility to synthesize a large variety of nonreciprocal and broadband electromagnetic responses over a unique physical surface. Our findings may open intriguing applications in dynamic sub-diffractive imaging and sensing systems as well as in the routing and processing of surface plasmons.

Under adequate illumination of plane wave, polarizable nanoparticles near a plasmonic surface acquire out-of-plane polarization spin and experience lateral forces thanks to the recoil of directional surface plasmons excited on the surface. The strength and direction of such forces are mostly controlled by the power and direction of the incoming light. Here, we show that illuminating a nanoparticle near a drift-biased graphene metasurface excites unidirectional surface plasmons whose trajectory is fully determined by the drift-current and does not depend on the direction of incoming light. Due to momentum conservation, unidirectional lateral forces are exerted on the particle with a strength that can be controlled and even amplified over two orders of magnitude by the applied DC bias. Our findings may resolve delicate laser beam alignment issues and reduce the beam intensity required for the trapping and routing of nanoparticles.

This paper introduces a high-resolution dielectric sensor at 162GHz. The proposed system consists of a fundamental oscillator as the transmitter, a ring resonator based dielectric sensor, and a low noise super-regenerative receiver. A sinusoidal tone, generated by the transmitter, passes through the sensor and gets different attenuation due to the resonant frequency shift resulting from materials of different dielectric constants. The super-regenerative receiver then detects the resulting small signal difference caused by different dielectric constants to complete the sensing operation. The active transmitter and receiver are fabricated in CMOS technologies. Thanks to the ring resonator's high sensitivity and receiver's ultra-low noise, the sensor can achieve the best dielectric constant sensing resolution of 3.10×10−4 at the frequency range above 100GHz. The sensing system consumes total of 9mW DC power.

We propose the optical trapping of Rayleigh particles using tailored anisotropic and hyperbolic metasurfaces illuminated with a linearly polarized Gaussian beam. This platform permits to engineer optical traps at the beam axis with a response governed by nonconservative and giant recoil forces coming from the directional excitation of ultra-confined surface plasmons during the light scattering process. Compared to optical traps set over bulk metals, the proposed traps are broadband in the sense that can be set with beams oscillating at any frequency within the wide range in which the metasurface supports surface plasmons. Over that range, the metasurface evolves from an anisotropic elliptic to a hyperbolic regime through a topological transition and enables optical traps with distinctive spatially asymmetric potential distribution, local potential barriers arising from the momentum imbalance of the excited plasmons, and an enhanced potential depth that permits the stable trapping of nanoparticles using low-intensity laser beams. To investigate the performance of this platform, we develop a rigorous formalism based on the Lorentz force within the Rayleigh approximation combined with anisotropic Green's functions and calculate the trapping potential of nonconservative forces using the Helmholtz-Hodge decomposition method. Tailored anisotropic and hyperbolic metasurfaces, commonly implemented by nanostructuring thin metallic layers, enables using low-intensity laser sources operating in the visible or the IR to trap and manipulate particles at the nanoscale, and may enable a wide range of applications in bioengineering, physics, and chemistry., 39 pages - 8 figures

We propose the concept and design of a time-modulated antenna able to exhibit nonreciprocal polarization handedness during transmission and reception (for instance, the antenna radiates right-handed circularly polarized waves but receives left-handed circularly polarized waves) by exploiting the photonic Aharonov-Bohm effect. To demonstrate our approach, we combine a patch antenna fed from four symmetrical sides with an appropriate low-frequency modulation scheme to control orthogonal currents. We show that it is simultaneously possible to (i) obtain very high frequency conversion efficiency; (ii) independently tailor the phase of radiated fields that have orthogonal polarization with the phase of certain modulation signals, thus implementing any polarization state of the Poincare sphere; and (iii) achieve nonreciprocity at the polarization level with a single antenna element. We envision that the proposed time-modulated device may find exciting applications in the next generation of sensing, radar, and communication systems.

Excitons in monolayer transition metal dichalcogenides are formed at K and K′ points at the boundary of the Brillouin zone. They acquire a valley degree of freedom, which has been explored as an alternative information carrier, analogous to charge or spin. Two opposite valleys in transition metal dichalcogenides can be optically addressed using light with different helicity. Here, we demonstrate that valley-polarized excitons can be sorted and spatially separated at room temperature by coupling a MoS2 monolayer to a subwavelength asymmetric groove array. In addition to separation of valley excitons in real space, emission from valley excitons is also separated in photon momentum-space; that is, the helicity of photons determines a preferential emission direction. Our work demonstrates that metasurfaces can facilitate valley transport and establish an interface between valleytronic and photonic devices, thus addressing outstanding challenges in the field of valleytronics. Helicity of photons is exploited for preferential emission direction and sorting of valley-polarized excitons is achieved at room temperature.

This paper presents novel designs of frequency reconfigurable distributed non-reciprocal bandpass filter and diplexer based on spatio-temporally modulated microstrip λg/2 resonators. The modulation is achieved by loading both ends of the λg/2 transmission line resonators with time-modulated capacitors. To provide an inherent biasing isolation between the RF and the modulation signals, the modulation voltage source is connected at the center of the resonator, where there is a natural voltage null. A single inductor is used to further enhance such biasing isolation. The wideband nature of this isolation scheme enables the tuning of the devices over a wide frequency range. With more than 30-dB RF to modulation isolation, the proposed resonator structure also enables low insertion loss by eliminating RF signal leakage to the modulation ports. Two examples of a 3-pole bandpass filter and a diplexer are demonstrated with good agreement between the measurement and the simulation. The fabricated filter shows a minimal insertion loss of 3.9 dB, a 20-dB isolation bandwidth of 42 MHz at 1.0 GHz, and frequency tuning range of 885-1031 MHz. The measured diplexer has two non-reciprocal bandpass channels at 829 MHz and 997 MHz, respectively. The two channels can be independently reconfigured without affecting each other.

This contribution explores nonreciprocal and reconfigurable reflectarray unit-cells with full polarization control based on time-modulated resonators. To this purpose, a unit-cell operating in the X-band is designed to aperture-couple each polarization of the impinging fields through a patch into a substrate integrated waveguide (SIW). The SIWs are loaded with varactor diodes which are biased using coplanar waveguides printed on the back side of the ground plane. There, upon adequate modulation parameters, waves undergo an efficient conversion process towards a desired harmonic and are then radiated back to free space. The photonic Aharonov-Bohm effect is exploited on each cells through modulation signals to independently control the phase and amplitude of the fields radiated with each polarization, thus allowing to obtain any polarization state and achieve nonreciprocity at the polarization level. We envision that this technology will have important implications in wireless terrestrial and satellite communication systems.

A phased-array antenna is a device that generates radiation patterns whose shape and direction can be electronically controlled by tailoring the amplitude and phase of the signals that feed each element of the array. These devices provide identical responses in transmission and reception due to the constrains imposed by time-reversal symmetry. Here, we introduce the concept of nonreciprocal phased-array antennas and we demonstrate that they can exhibit drastically different radiation patterns when operated in transmission or in reception. The building block of the array consists of a time-modulated resonant antenna element that provides very efficient frequency conversion between only two frequencies: one associated to waves propagating in free-space and the other related to guided signals. Controlling the tunable nonreciprocal phase response of these elements with the phase of low-frequency modulation signals permits to independently tailor the transmission and reception radiation patterns of the entire array. Measured results at microwaves confirm isolation levels over 40 dB at desired directions in space with an overall loss below 4 dB. We believe that this concept can be extended across the electromagnetic spectrum provided adequate tuning elements are available, with important implications in communication, sensing, and radar systems, as well as in thermal management and energy harvesting., 26 pages; 16 figures; 1 table

This letter proposes a novel and compact nonreciprocal filtering antenna based on temporal modulation. The device is composed of a third-order filtering section integrated into a Yagi–Uda planar printed antenna. Strong nonreciprocity in transmission and reception is achieved at the same operation frequency by time-modulating the resonators of the filtering section. These resonators are implemented as quarter-wavelength microstrip lines terminated with varactors located on the ground plane. Such plane is also employed as a reflector for the Yagi–Uda antenna and to host the coplanar waveguides that feed low-frequency signals to the varactors, thus leading to a very compact design. A prototype is manufactured and successfully tested at 2.4 GHz, showing isolations greater than 20 dB between transmission and reception modes in both E- and H-planes for all directions in space and a gain drop of only 3.5 dB compared to a reference antenna. The proposed devices can easily be integrated with other electronic circuits and may find exciting applications in communication, radar, and sensing systems.

Author(s): Zang, JW; Correas-Serrano, D; Do, JTS; Liu, X; Alvarez-Melcon, A; Gomez-Diaz, JS | Abstract: We propose a paradigm to realize nonreciprocal wavefront engineering using time-modulated gradient metasurfaces. The essential building block of these surfaces is a subwavelength unit cell whose reflection coefficient oscillates at low frequency. We demonstrate theoretically and experimentally that such modulation permits tailoring the phase and amplitude of any desired nonlinear harmonic and determines the behavior of all other emerging fields. By appropriately adjusting the phase delay applied to the modulation of each unit cell, we realize time-modulated gradient metasurfaces that provide efficient conversion between two desired frequencies and enable nonreciprocity by (i) imposing drastically different phase gradients during the up/down conversion processes and (ii) exploiting the interplay between the generation of certain nonlinear surface and propagative waves. To demonstrate the performance and broad reach of the proposed platform, we design and analyze metasurfaces able to implement various functionalities, including beam steering and focusing, while exhibiting strong and angle-insensitive nonreciprocal responses. Our findings open an alternative direction in the field of gradient metasurfaces, in which wavefront control and magnetic-free nonreciprocity are locally merged to manipulate the scattered fields.

We explore the possibilities enabled by the spatiotemporal modulation of graphene’s conductivity to realize magnetic-free isolators at terahertz and infrared frequencies. To this purpose, graphene is loaded with periodically distributed gates that are time-modulated. First, we investigate plasmonic isolators based on various mechanisms such as asymmetric bandgaps and interband photonic transitions and we demonstrate isolation levels over 30 dB using realistic biasing schemes. To lessen the dependence on high-quality graphene able to support surface plasmons with low damping, we then introduce a hybrid photonic platform based on spatiotemporally modulated graphene coupled to high-Q modes propagating on dielectric waveguides. We exploit transversal Fabry-Perot resonances appearing due to the finite-width of the waveguide to significantly boost graphene/waveguide interactions and to achieve isolation levels over 50 dB in compact structures modulated with low biasing voltages. The resulting platform is CMOS-compatible, exhibits an overall loss below 4 dB, and is robust against graphene imperfections. We also put forward a theoretical framework based on coupled-mode theory and on solving the eigenstates of the modulated structure that is in excellent agreement with full-wave numerical simulations, sheds light in the underlying physics that govern the proposed isolators, and speeds-up their analysis and design. We envision that the proposed technology will open new and efficient routes to realize integrated and siliconcompatible isolators, with wide range of applications in communications and photonic networks.

We report a dramatic enhancement of the lateral optical forces induced on electrically polarizable Rayleigh particles near hyperbolic and extremely anisotropic metasurfaces under simple plane wave illumination. Such enhancement is enabled by the interplay between the increased density of states provided by these structures and the out-of-plane polarization spin acquired by the particle. The resulting giant lateral forces appear over a broad frequency range and may open unprecedented venues for routing, trapping, and assembling nanoparticles., 18 pages, 4 figures

We suggest and explore the possibility to realize broadband and unidirectional plasmonic hyperlensing over a graphene layer biased with two orthogonal drift-biases. This platform relies on the interplay between nonlocality and nonreciprocity to generate ultraconfined states in the direction aligned with the effective biasing current. The presence of these states shapes, elongates, and significantly flattens the isofrequency contour of the supported modes and enforces the canalization of surface plasmons along the two directions defined by the applied biases. Through a dedicated anisotropic Green's function formalism that takes graphene's intrinsic nonlocality into account, we show that this platform resolves the presence of two point-source emitters with a resolution larger than λ 0 / 500 over a broad frequency range ( ∼ 10 − 25 THz) using realistic biasing schemes. We also discuss how spin–orbit interactions can be exploited to construct unidirectional hyperlenses for near-field images with specific polarization distribution. Our findings may open exciting applications in dynamic, subdiffractive, and planar sensing and imaging systems and in the exciting, routing, and processing of surface plasmons.

In this Perspective, we discuss the physics and potential applications of planar hyperbolic metasurfaces (MTSs), with emphasis on their in-plane and near-field responses. After revisiting the governing dispersion relation and properties of the supported surface plasmon polaritons (SPPs), we discuss the different topologies that uniaxial MTSs can implement. Particular attention is devoted to the hyperbolic regime, which exhibits unusual features, such as an ideally infinite wave confinement and local density of states. In this context, we clarify the different physical mechanisms that limit the practical implementation of these ideal concepts using materials found in nature, and we describe several approaches to realize hyperbolic MTSs, ranging from the use of novel 2D materials such as black phosphorus to artificial nanostructured composites made of graphene or silver. Some exciting phenomena and applications are then presented and discussed, including negative refraction and the routing of SPPs within th...

We explore the unusual non-reciprocal and diffraction-less properties of surface plasmon polaritons propagating in drift-biased graphene-based metasurfaces. We show that applying a drift-current on a graphene sheet leads to extremely asymmetric in-plane modal dispersions from terahertz to infrared frequencies, associated with plasmons with low-loss (high-loss and ultra-high confinement) traveling along (against) the bias. Strikingly, truly unidirectional wave propagation is prevented by the intrinsic nonlocal response of a graphene, a mechanism that shapes the energy flow over the surface. We also show that highly-directive hyperbolic plasmons completely immune to backscattering propagate obliquely along the drift in nanostructured graphene. Finally, we discuss how spin-orbit interactions can be exploited in this platform to efficiently launch collimated plasmons along a single direction while maintaining giant non-reciprocal responses. Our findings open a new paradigm to excite, collimate, steer, and process surface plasmons over a broad frequency band.

A new class of wideband bandpass filters based on using thick metallic bars as microwave resonators, instead of common microstrip lines, is presented. These bars provide a series of advantages over fully planar printed technologies, including higher coupling levels between resonators, better unloaded quality factors QU, and larger bandwidths, implemented with more compact structures. Moreover, thick bar resonators can easily be coupled to an additional resonance excited in a box used for shielding, allowing to realize transversal topologies able to implement transmission zeros at desired frequencies. To illustrate the capabilities of this technology, two microwave filters with different bandwidths and one transmission zero have been designed. One of the filters has been manufactured and tested using copper bars inside an aluminum housing partially filled with Teflon. Measured data demonstrates a fractional bandwidth about FBW = 32%, spurious free range SFR > 50%, unloaded quality factor of QU = 1180 and return losses over 20 dB without requiring any post-tuning on the prototype, confirming the exciting performance of the proposed technology., 8 pages, 13 figuras, transaction

We propose a paradigm for the realization of nonreciprocal photonic devices based on time-modulated graphene capacitors coupled to photonic waveguides, without relying on magneto-optic effects. The resulting hybrid graphene-dielectric platform is low loss, silicon compatible, robust against graphene imperfections, scalable from terahertz to near-infrared frequencies, and it exhibits large nonreciprocal responses using realistic biasing schemes. We introduce an analytical framework based on solving the eigenstates of the modulated structure and on spatial coupled mode theory, unveiling the physical mechanisms that enable nonreciprocity and enabling a quick analysis and design of optimal isolator geometries based on synthetic linear and angular momentum bias. Our results, validated through harmonic-balance full-wave simulations, confirm the feasibility of the introduced low-loss ($l3$ dB) platform to realize large photonic isolation through various mechanisms, such as narrow-band asymmetric band gaps and interband photonic transitions that allow multiple isolation frequencies and large bandwidths. We envision that this technology may pave the wave to magnetic-free, fully integrated, and CMOS--compatible nonreciprocal components with wide applications in photonic networks and thermal management.

This contribution proposes to couple resonant patches with substrate integrated waveguides (SIW)through various slots to design reflective unit-cells with large bandwidth and very sharp frequency response. Such performance is achieved by appropriately combining the patch and slots resonances, while the required phase delay is obtained by adjusting the SIW length. The proposed cell is planar, easy to manufacture, low-loss, and can operate from micro to millimeter waves. A reflect array based on this unit-cell has been designed and analyzed at 9 GHz, exhibiting a 3 dB bandwidth of 18% and a sharp gain selectivity over 16 dB.

Leaky-wave antennas (LWAs) have received considerable attention in recent decades thanks to their exciting radiation and beam-scanning properties [1]. These structures are based on the energy that is gradually radiated to free-space by an electromagnetic wave that propagates trough a waveguide with a phase velocity larger than the speed of light. Even though the fundamentals of leaky-waves were unveiled by electrical engineers during the second half of last century [1], their application to other fields such as optics and physics has allowed to understand and correctly interpret many intriguing phenomena across the electromagnetic spectrum, including extraordinary transmission, electromagnetic induced transparency, Wood's anomaly, or Cherenkov radiation. In addition, leaky-wave antennas with exciting properties, including the ability to independently control their complex propagation constant, reconfigurability, or even non-reciprocity, have recently been put forward at microwaves, millimeter-waves, and optics. At terahertz, a frequency region that lies in the technological gap between the well-developed areas of photonics and electronics, the potential of leaky-waves has not yet been fully exploited due to immature state of technology and relatively large amount of losses that materials exhibit in this band. In a related context, graphene has recently led to a revolution in material science thanks to its outstanding thermal, electrical, and mechanical properties. In particular, this two-dimensional material supports the propagation of surface plasmon polaritons (SPPs) at terahertz and infrared frequencies that exhibit moderate loss, strong wave localization, and the exceptional property of being tunable by applying a modest DC bias. Such properties have recently been applied to realize a wide variety of plasmonic antennas, including resonant dipoles and reflectarrays, at terahertz. In addition to inherent reconfigurability, graphene antennas show high radiation efficiency thanks to their large intrinsic wave impedance that permits to easily match photomixers-one of the most common terahertz sources-with high input impedances $(\approx 10\text{k}\Omega)$ . On the contrary, common metallic antennas at terahertz, despite exhibiting lower loss, have a small input impedance (in the range of $100\text{s}\ \Omega$ ) that prevents the efficient coupling of terahertz waves and their subsequent radiation.

We propose a novel paradigm to achieve giant photonic non-reciprocity based on time-modulated graphene capacitors coupled to photonic waveguides, without reliance on any magneto-optic effects. The resulting hybrid graphene-dielectric platform is low-loss, silicon-compatible, robust against graphene's imperfections, scalable from terahertz to near infrared frequencies, and it exhibits very large nonreciprocal responses using realistic biasing schemes. Analytical frameworks based on solving the eigenstates of the modulated structure and on spatial coupled mode theory have been developed to unveil the underlying physics that dominate the proposed platform and to quickly design and analyze various isolators. Results, validated through harmonic balance full-wave simulations, confirm the flexibility of our low-loss (

A new class of magnet-free nonreciprocal plasmonic devices operating at terahertz (THz) frequencies is introduced based on the spatiotemporal modulation of graphene’s conductivity. The proposed components are based on graphene parallel-plate waveguides with double-gated electrodes, which allow independent manipulation of graphene properties in both space and time. We employ this structure for the design of plasmonic isolators and leaky-wave antennas at THz frequencies and study the effect of graphene and modulation parameters on their response. We envision that this technology may pave the way towards silicon-compatible fully planar nonreciprocal plasmonic components and antennas with enhanced functionalities at THz, with important applications in biosensing, imaging, and intra/interchip communications.

The propagation of surface waves along electrically and magnetically biased graphene-based cylindrical waveguides (GCWs) is investigated in detail. Analytical dispersion equations are derived for several GCW geometries, considering the presence of an inner metallic core and multiple (coaxial-like) graphene layers. The proposed formulation reveals a fundamental connection between surface plasmons found in GCWs/carbon nanotubes and planar graphene structures. Numerical results confirm the higher confinement of modes supported by GCWs compared with their planar counterparts, while keeping a similar level of losses. The proposed structure is applied to develop plasmonic reconfigurable dipole antennas in the low THz band, which provide higher radiation efficiency than current graphene-based radiators, without requiring the presence of bulky lenses. We envision that the proposed GCWs may find application in reconfigurable THz transceivers, near-field application, wireless interconnects, and sensing systems.

We report difference-frequency generation in nonlinear polaritonic metasurfaces based on coupling of electromagnetic nanocavity modes with intersubband nonlinearities in semiconductor heterostructures. Nonlinear susceptibility of 2.7×105 pm/V and the conversion efficiency of 0.13% were measured.

We investigate the use of black phosphorus thin films in infrared plasmonics and discuss spatial dispersion effects. For appropriate film thickness, black phosphorus exhibits a moderate bandgap suitable for operation in this frequency band, and the intrinsic anisotropy of its conductivity tensor provides exciting possibilities in metasurfaces/reflectarray design, such as polarization-dependent devices and unusual topological transitions for plasmons induced by electrical or mechanical bias. We study the limits on plasmon confinement and field enhancement imposed by spatial dispersion and demonstrate that these bounds are looser than in graphene. These findings may lead to infrared plasmonic devices enabling light-matter interactions beyond the capabilities of other plasmonic materials, with exciting applications in sensing, thermal imaging, and heat scavenging.

We propose the concept, synthesis, analysis, and design of graphene-based plasmonic tunable low-pass filters operating in the THz band. The proposed structure is composed of a graphene strip transferred onto a dielectric and a set of polysilicon DC gating pads located beneath it. This structure implements a stepped impedance low-pass filter for the propagating surface plasmons by adequately controlling the guiding properties of each strip section through graphene's field effect. A synthesis procedure is presented to design filters with desired specifications in terms of cut-off frequency, in-band performance, and rejection characteristics. The electromagnetic modeling of the structure is efficiently performed by combining an electrostatic scaling law to compute the guiding features of each strip section with a transmission line and transfer-matrix framework, approach further validated via full wave simulations. The performance of the proposed filters is evaluated in practical scenarios, taking into account the presence of the gating bias and the influence of graphene's losses. These results, together with the high miniaturization associated with plasmonic propagation, are very promising for the future use and integration of the proposed filters with other graphene and silicon-based elements in innovative THz communication systems., Comment: 14 pages, 15 figures, 4 tables, IEEE Transactions on Nanotechnology, 2014

The propagation of surface waves along spatially dispersive graphene based 2 D waveguides is investigated in detail. Graphene is characterized using a full kp conductivity model under the relaxation time approximation which allows to obtain analytical and closed formed expressions for the wavenumber of plasmons supported by sheets and parallel plate waveguides respectively. Per unit length equivalent circuits are introduced to accurately characterize the propagation in different waveguides and analytical relations between the effective TM mode circuit lumped elements and graphene conductivity are derived. The proposed circuits allow identifying the different mechanisms involved in spatially dispersive plasmon propagation explaining their connection with the intrinsic properties of graphene. Results demonstrate that spatial dispersion which significantly decreases the confinement and the losses of slow surface plasmons must be accurately assessed in the design of graphene based plasmonic components at millimeter waves and low terahertz frequencies. © 2013 IEEE.

The application of the Pancharatnam-Berry (PB) phase approach to the design of nonlinear metasurfaces has recently enabled subdiffractive phase control over the generated nonlinear fields, embedding phased array features in ultrathin structures. Here, we rigorously model, analyze, and design highly efficient nonlinear metasurfaces with advanced functionalities, including the generation of pencil beams steered in arbitrary directions in space, as well as vortex beams with polarization-dependent angular momentum, and we extend the PB approach to various nonlinear processes. To this purpose, we develop an accurate and efficient theoretical framework---inspired by the linear phase array theory---based on the effective nonlinear susceptibility method, thus avoiding the use of time-consuming numerical simulations. Our findings allow exploiting the flat nonlinear optics paradigm, enabling exciting applications based on subwavelength field control over flat and large-scale structures with giant nonlinear responses.

Extending the ‘flat optics’ paradigm to the nonlinear optics faces important challenges, since, for any practical situation, we are required to simultaneously achieve sub-diffraction phase control and efficient frequency conversion in metasurfaces of sub-wavelength thickness. Here, we experimentally demonstrate giant nonlinear response and continuous phase control of the giant nonlinear response in metasurfaces based on plasmonic nanoresonators coupled to intersubband transitions in semiconductor multi-quantum wells. Over 0.075% of second-harmonic power conversion efficiency is achieved experimentally in a 400-nm-thick metasurface using 10 microns wavelength pump with 20 kW/cm2 intensity.

Extending the ‘flat optics’ paradigm to the nonlinear optics faces important challenges, since, for any practical situation, we are required to simultaneously achieve sub-diffraction phase control and efficient frequency conversion in metasurfaces of sub-wavelength thickness. Here, we experimentally demonstrate giant nonlinear response and continuous phase control of the giant nonlinear response in metasurfaces based on plasmonic nanoresonators coupled to intersubband transitions in semiconductor multi-quantum wells. Over 0.075% of second-harmonic power conversion efficiency is achieved experimentally in a 400-nm-thick metasurface using 10 microns wavelength pump with 20 kW/cm2 intensity.

We investigate unusual surface plasmons polariton (SPP) propagation and light-matter interactions in ultrathin black phosphorus (BP) films, a 2D material that exhibits exotic electrical and physical properties due to its extremely anisotropic crystal structure. Recently, it has been speculated that the ultra-confined surface plasmons supported by BP may present various topologies of wave propagation bands, ranging from anisotropic elliptic to hyperbolic, across the mid- and near-infrared regions of the electromagnetic spectrum. By carefully analyzing the natural nonlocal anisotropic optical conductivity of BP, derived using the Kubo formalism and an effective low-energy Hamiltonian, we demonstrate here that the SPP wavenumber cutoff imposed by nonlocality prohibits that they acquire an arbitrary hyperbolic topology, forcing operation in the canalization regime. The resulting nonlocality-induced canalization presents interesting properties, as it is inherently broadband, enables large light-matter interactions in the very near field, and allows extreme device miniaturization. We also determine fundamental bounds to the confinement of BP plasmons, which are significantly weaker than for graphene, thus allowing a larger local density of states. Our results confirm the potential of BP as a promising reconfigurable plasmonic platform, with exciting applications, such as planar hyperlenses, optoelectronic components, imaging, and communication systems., 16 pages, 8 figures

Gradient metasurfaces, or ultrathin optical components with engineered transverse impedance gradients along the surface have recently been demonstrated to provide control of the phase of scattered fields over sub-wavelength scale, enabling a broad range of linear optical components. More recently, sub-wavelength-thin nonlinear metasurfaces with tailored nonlinear responses have provided new degrees of freedom in metamaterial design, with potential applications such as super-resolution imaging, frequency conversion with greatly relaxed phase-matching constraints, and in all-optical switching and memories at nanoscale. However for all practical purposes, the level of nonlinear responses enabled by traditional materials, such as metals and dielectric crystals, is too weak, and nonlinear metasurfaces with orders of magnitude larger nonlinear susceptibilities are necessary to realistically enable a new paradigm in nonlinear optics, based on efficient frequency mixing in sub-wavelength films with relaxed phase-matching conditions and local in-place phase and amplitude control of the nonlinear response. We have recently reported polaritonic nonlinear metasurfaces with record-high nonlinear optical responses for second-harmonic generation based on coupling of electromagnetic modes in plasmonic nanoresonators with quantum-engineered intersubband nonlinearities in multi-quantum-well (MQW) semiconductor heterostructures. In this work, we experimentally demonstrate continuous 360-degrees phase control of the nonlinear response for second-harmonic nonlinear optical process at the individual nanoresonator level with practical level of nonlinear optical responses using a generalization of the Pancharatnam-Berry (PB) phase approach to our MQW-based structures. Extending the ‘flat optics’ paradigm established with linear gradient metasurface, our result unveil a ‘flat nonlinear optics’ paradigm based on efficient frequency mixing and a complete control of the output wavefront with subwavelngth resolution.

Black phosphorus (BP) is an intrinsically anisotropic 2D material that possesses exotic electrical and physical properties, including a direct bandgap, the ability to support surface plasmons polaritons (SPPs) at infrared and optical frequencies, and electrical reconfiguration through applied electrostatic/magnetostatic fields and mechanical strain. Here, we investigate the use of BP to strongly enhance light-matter interactions by exciting anisotropic and hyperbolic ultra-confined SPPs on the structure. Our numerical results confirm the potential of black phosphorus as an interesting optical plasmonic platform, with applications in the development of optoelectronic devices, modulators, and communication systems.

We propose the design of a magnet-free non-reciprocal leaky-wave antenna at terahertz (THz) frequencies based on a spatiotemporally modulated graphene monolayer. Spatiotemporal modulation is achieved through time-varying voltages applied over a set of gate electrodes beneath the graphene monolayer, providing ample freedom to impart the required effective linear momentum to the supported mode. The nonreciprocal radiation properties of the antenna, in terms of transmission/reception gain patterns and frequency conversion, are discussed in detail. This class of graphene plasmonic devices may pave the way to the development of fully planar THz transceivers and advanced imaging and sensing systems.