Diego Correas-Serrano

Graphene-Based Plasmonic Tunable Low-Pass Filters in the Terahertz Band

We propose the concept, synthesis, analysis, and design of graphene-based plasmonic tunable low-pass filters operating in the terahertz 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 graphenes field effect. A synthesis procedure is presented to design filters with desired specifications in terms of cutoff 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 graphenes 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 terahertz communication systems.

Spatially Dispersive Graphene Single and Parallel Plate Waveguides: Analysis and Circuit Model

The propagation of surface waves along spatially dispersive graphene-based 2-D waveguides is investigated in detail. Graphene is characterized using a full-kρ 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.

Electrically and Magnetically Biased Graphene-Based Cylindrical Waveguides: Analysis and Applications as Reconfigurable Antennas

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.

Black phosphorus plasmonics: anisotropic elliptical propagation and nonlocality-induced canalization

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.

Nonreciprocal Graphene Devices and Antennas Based on Spatiotemporal Modulation

A new class of magnet-free nonreciprocal plasmonic devices operating at terahertz (THz) frequencies is introduced based on the spatiotemporal modulation of graphenes 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.

Nonlocal response of hyperbolic metasurfaces

We analyze and model the nonlocal response of ultrathin hyperbolic metasurfaces (HMTSs) by applying an effective medium approach. We show that the intrinsic spatial dispersion in the materials employed to realize the metasurfaces imposes a wavenumber cutoff on the hyperbolic isofrequency contour, inversely proportional to the Fermi velocity, and we compare it with the cutoff arising from the structure granularity. In the particular case of HTMSs implemented by an array of graphene nanostrips, we find that graphene nonlocality can become the dominant mechanism that closes the hyperbolic contour - imposing a wavenumber cutoff at around 300k(0) - in realistic configurations with periodicity L<π/(300k(0)), thus providing a practical design rule to implement HMTSs at THz and infrared frequencies. In contrast, more common plasmonic materials, such as noble metals, operate at much higher frequencies, and therefore their intrinsic nonlocal response is mainly relevant in hyperbolic metasurfaces and metamaterials with periodicity below a few nm, being very weak in practical scenarios. In addition, we investigate how spatial dispersion affects the spontaneous emission rate of emitters located close to HMTSs. Our results establish an upper bound set by nonlocality to the maximum field confinement and light-matter interactions achievable in practical HMTSs, and may find application in the practical development of hyperlenses, sensors and on-chip networks.

On the Influence of Spatial Dispersion on the Performance of Graphene-Based Plasmonic Devices

We investigate the effect of spatial dispersion phenomenon on the performance of graphene-based plasmonic devices at terahertz (THz). For this purpose, two different components, namely a phase shifter and a low-pass filter, are taken from the literature, implemented in different graphene-based host waveguides, and analyzed as a function of the surrounding media. In the analysis, graphene conductivity is modeled first using the Kubo formalism and then employing a full-kρ model that accurately takes into account spatial dispersion. Our study demonstrates that spatial dispersion upshifts the frequency response of the devices, limits their maximum tunable range, and degrades their frequency response. Importantly, the influence of this phenomenon significantly increases with higher permittivity values of the surrounding media, which is related to the large impact of spatial dispersion in very slow waves. These results confirm the necessity of accurately assessing nonlocal effects in the development of practical plasmonic THz devices.