Plasmonics for Telecommunications Applications

Plasmonic materials, when properly illuminated with visible or near-infrared wavelengths, exhibit unique and interesting features that can be exploited for tailoring and tuning the light radiation and propagation properties at nanoscale dimensions. A variety of plasmonic heterostructures have been demonstrated for optical-signal filtering, transmission, detection, transportation, and modulation. In this review, state-of-the-art plasmonic structures used for telecommunications applications are summarized. In doing so, we discuss their distinctive roles on multiple approaches including beam steering, guiding, filtering, modulation, switching, and detection, which are all of prime importance for the development of the sixth generation (6G) cellular networks.


Plasmonic Resonances: Fundamentals
SPRs, the resonant coupling of the electric field component of light with free-electron excitations in metal surfaces, have the ability to enhance and squeeze light fields beyond the diffraction limit [28]. These resonances can involve propagating (guided) or localized (radiating) SPRs (LSPRs), depending on the geometrical properties of the metallic surface [29,30], thus providing a path for tailoring different nanoscale near-field optical phenomena [31][32][33]. Differences are not only observed in the propagation and radiation properties, but also in the way they are excited. Whereas LSPRs can be directly excited by freely-propagating light, SPRs need a proper matching mechanism to couple free-to-guided light modes, as schematized in Figure 1. The SPR wavevector for a thick metallic film, whose thickness is larger than the SPR decaying length, is written as k SPR = k 0 ε m ε d ε m +ε d ≥ k 0 , with k 0 = ω c being the free-space propagating wavevector, and ε m < 0 and ε d > 0 being the permittivities for the metal and dielectric media [34-36], respectively. Figure 1a,b show the widely known Kretschmann [37] and Otto [38] configurations, respectively, based on the attenuated total reflection (ATR) method. In these approaches, a transverse magnetic (TM) polarized light reaching the metallic surface from the prism has a wavevector component parallel to the surface, k x = k 0 √ ε P sin θ, which couples to the SPR mode under the wavevector matching condition where θ is the incidence angle and ε P is the permittivity of the prism. This latter condition can only be satisfied with ε P > ε d . The resonance angle, obtained from Equation (1) as θ SPR = sin −1 1 is very sensitive to dielectric properties of the surrounding media, which is extensively exploited for plasmonic sensing and biosensing purposes [34][35][36]. In telecommunications applications, on the other hand, the Kretschmann setup is used for photodetection and optical signal processing through the excitation and detection of Bloch surface waves (BSWs) [39,40]. Despite the high performance of these structures, the need to use prism couplers for SPR excitation hampers on-chip integration. Figure  1c, on the other hand, uses diffraction to couple light to SPR modes through a diffraction grating. In this case, light is split into a series of beams with k x = k 0 sin θ + mG, where G = 2π Λ , with m and Λ being an integer and the period length of the grating, respectively. The wavevector matching condition in this case becomes Analogous to the prism coupler approach, these plasmonic grating structures have also been used for sensing and biosensing applications [41][42][43][44]. In telecommunications, their applications range from transmission lines to optical modulators [45][46][47][48][49][50]. In contrast to SPR approaches, LSPRs are excited by freely propagating light impinging on metallic nanoparticles or other finite structures. In this case, the harmonic oscillation of the electric field of light produces oscillation of the free-electron-density in the metal surface, analogous to a forced harmonic oscillator. The maximum dipolar amplitudes are observed when the frequency of light matches the natural frequency of the electron density, known as the plasmon frequency (ω p ) [35]. These extremely localized and enhanced fields are commonly used for surface-enhanced-phenomena in plasmonic biosensing applications [51,52]. In the case of small and highly symmetric metallic nanoparticles, the electromagnetic response can be well-described through the quasi-static approximation [53]. In telecommunications applications, where LSPRs and SPRs are synergistically used [54] to convert free radiation into guided signals and vice versa, previous analytical expressions are useful for qualitative purposes, whereas proper theoretical analyses can only be made using specialized numerical methods [11,14,15].
Air gap

Telecommunications Applications
Plasmonic resonances, hybrid electronic-photonic modes, allow for on-chip devices seamlessly integrating the exceptional data rates of optical signals with the extreme miniaturization features of nanoelectronic circuits [55][56][57][58][59]. In particular, plasmonic nanoantennas, i.e., nanoscale analogous of RF antennas, has become a recent trend of research due to their incredible ability to convert localized into propagating far-field electromagnetic waves and vice versa [11][12][13][14][15]. However, at these high frequency regimes light penetrates the metals and ohmic losses become substantially higher, i.e., metals cannot be considered as perfect electric conductors (PEC), drastically changing the way they are designed in comparison to their RF counterparts. Other applications for proper implementation of plasmonic nanocircuits include plasmonic waveguides, modulators, photodetectors, filters, resonators, and switches among others, which we also survey here [60,61].

Plasmonic Nanoantennas
Plasmonic waveguides, structures able to confine and enhance electromagnetic fields at subdiffraction limits, are used for optical signal transmission among different nanophotonic circuit components [62]. These nanostructures can be developed in several different ways like, for example, flat planar multilayer slabs, nanoparticle arranges, metal-coated-fibers, metallic nanowires, and metal gratings, among others [63][64][65][66][67][68][69][70][71][72][73][74]. Although these systems allow for ultra-low interconnection delays with subwavelength scale devices, practical applications are hindered by energy dissipation and crosstalk between adjacent plasmonic waveguides [14,75]. In particular, ohmic losses in metals at optical frequencies limit propagation lengths up to only some few micrometers, stimulating the search for other propagation mechanisms. Wireless information transfer at nanoscale has recently emerged as a promising alternative [76][77][78][79]. In this approach, communication performance is strongly dependent on the design of the corresponding nanolinks, i.e., the nanoemitters and nanoreceivers, for which RF antenna theory is being actively used [11][12][13][14][15]. We will review here some of the most recent approaches for highly efficient nanoantenna designs.
The most simple nanoantenna one can imagine is a single or two-coupled metallic nanoscatterers (dipole nanoantenna) [80][81][82][83][84][85][86], as illustrated in Figure 2a. The scattering properties of these systems have been extensively investigated (from the numerical and experimental points of view) as a function of the nanoparticles geometry and the input impedance [82,85], demonstrating that it can be designed analogous to dipoles in RF domain [80,81,84]. These concepts have also opened up the possibility to design and develop optical nanocircuit elements analogous to radiation resistance, radiation efficiency, and conduction losses [83,87]. However, the lack of directionality hampers applications in optical signal transmission. An approach to circumvent this limitation uses plasmonic nanoparticle arrangements deposited over a properly designed multilayer substrate [88], which may result in a difficult and expensive mechanism. A recent alternative to control the directional radiation with nanoparticles, inspired by the well-known RF Yagi-Uda antenna design [89][90][91][92][93][94][95][96][97][98][99][100], considers an array of scatterers to achieve constructive interference of optical waves in one direction, whereas destructive in the opposite, as schematized in Figure 2b. This latter system efficiently links electron-based integrated computer chips to photon-based fiber networks for on-chip optical data transmission [100]. Importantly, these nanoantennas can be directly driven by optical or electrical signals [99,100]. Even though plasmonic Yagi-Uda nanoantennas show high directivity and ease of implementation, their performance can be severely limited due to their high sensitivity to the surrounding media [100]. In this regard, a waveguide-fed nanoscale analogous of the dipole aerial antenna was proposed [11] as a way to overcome the ohmic losses, when transmitting an optical signal, over several wavelengths at microscale level. Such a model was further improved [79] by placing a plasmonic nanoparticle adjacent to the dipole nanoantenna, as depicted in Figure 2c. Such a design hugely outperforms either the waveguide or dipole nanoantenna alternatives. In transmitting mode, the plasmonic nanoparticle works as a director, improving the directivity respect to their simplified dipole nanoantenna design [11]; whereas in the receiving mode, the field enhancement at the nanoparticle tips, enormously enhance the received signal for improved efficiency and longer distances. These properties led the researchers to develop a new plasmonic nanotransceiver [14] named Plantenna design, depicted in Figure 2d. These wireless optical data transfer/receiver links can be arranged with λ/2 separation in order to reach an efficient energy and data transfer up to millimeter distances [14]. Another waveguide-fed nanoscale analogue to a widely known RF model is the plasmonic horn nanoantenna [12,13,[101][102][103][104], illustrated in Figure 2e. This concept exploits the high directivity, low reflection, and planar far-field radiation pattern properties of conventional RF horn antennas at nanoscale levels. These systems can be developed by horn-like nanostructured [12,13,101] or slotted [102] metallic films for nanoscale beam steering. In comparison to the Plantenna design, the tunable high-directivity of plasmonic horn nanoantennas make them ideal candidates to work in the transfer mode, while the ability of Plantenna to amplify the received signal to operate in the receiving mode. This idea was numerically shown in a recent work [15], demonstrating that a hybrid impedance-matched horn-Plantenna optical nanolink can greatly outperform other recent proposals in terms of efficiency and communication distance. These optical nanolinks have potential in the future 6G telecommunication networks, envisioned to work at terabits per second (Tbps) data rates [105][106][107], for which optical nanoantennas are of fundamental importance in the design and development of ultracompact and ultrafast optical nanocircuits [108]. To make this a reality, seamless integration among THz links, inter-/intra-chip nanoelectronic functions, and the existing fiber-optic infrastructure is of crucial interest to avoid communication-performance bottlenecks [4,15]. In this regard, plasmonic nanoantennas also work as field detectors/modulators to mediate among optical, THz, and RF fields [109,110]. Figure 2f shows a plasmonic nanoantenna design consisting of two metallic arms, forming a plasmonic slot waveguide, filled with a second-order nonlinear organic material [110]. These systems can be used to accurately measure the amplitude and phase of THz signals if included as a plasmonic phase-shifter in a Mach-Zehnder interferometer (MZI) configuration [110], where optical probes are converted into SPRs and back to Si waveguides. Other approaches include the bowtie nanoantenna [111][112][113], i.e., a dipole-type nanoantenna built by two nearby metallic nanotriangles or nanopyramids separated by a small gap [114], as illustrated in Figure 2g. This configuration produces a very large near-field enhancement, with its EM field concentrated in the gap region [115][116][117]. A modified version of this dipole consists in the use of plasmonic cross nanoantennas [118,119], i.e., two dipoles placed perpendicularly to each other, as depicted in Figure 2h. Its shape not only allows for light enhancement and confinement but also to control the wave polarization [120], with applications for high-speed optoelectronic devices [121] and photodetectors [122]. Other geometrical approaches including J-pole, mirrored J-pole, and Vee, among others, have been numerically and experimentally studied [123][124][125].
The ability of these plasmonic structures for bending of light at-will stimulated several new trends including metasurfaces, LIDAR antenna arrays, and integration with microwave photonics for the development of faster high-performance optical-wireless communication [126][127][128][129][130].

Plasmonic Modulators
Researchers are seeking for ultrafast wireless THz links able to seamlessly integrate optical-to-THz (O/T) data carriers and vice versa in order to cope with the ever-growing need for higher data rates in wireless communication networks [4]. In addition to the enormous potential of plasmonic nanoantennas for this purpose [110], discussed in the previous section, there are also other plasmonic nanostructures offering high-performance and compactness to fit a coded data into the photonic domain through coherent detection of amplitude, phase, or both [131][132][133]. In this section, we will review some of the most promising approaches for signal modulation by plasmonic devices. Figure 3a shows an example of the Mach-Zender Modulator (MZM). This concept uses two gold stripe waveguides embedded in polymer, as depicted. One of the stripes is heated by electrical currents in order to introduce a phase-mismatch between the two MZM arms, which in turn changes the output amplitude. This modulation mechanism was successfully used for signal modulation at telecommunication wavelengths in the range of 1.51 µm to 1.62 µm [134]. Analogous structures have also been proposed recently [135,136]. The need for ultracompact silicon-compatible modulators, for on-chip all-optical and optoelectronic computational networks, has also stimulated other approaches. A metal-oxide-semiconductor (MOS) modulator, based on multimode interferometry in a plasmonic waveguide, exploiting the subwavelength strong SPR electromagnetic field enhancement for improved electro-optical nonlinearities has been proposed [137]. This plasmonic-based MOS modulator, named PlasMOStor, is schematized in Figure 3b. The PlasMOStor-based signal modulation is made by exploiting the fast modulation of accumulation conditions in the MOS capacitor, reaching modulation ratios around 10 dB. More recently, the unique optical properties of graphene [138][139][140] are also calling research attention to boost modulation performance, speeds, and optical bandwidths [141,142]. An approach for these graphene-based plasmonic modulators considers a capacitive graphene-insulator-graphene double layer in between two insulator-metal-insulator (IMI) platform [141], as depicted in Figure 3c. The chemical potential of graphene layers, working as capacitor and light absorber, is electrically controlled for efficient signal modulation.
Plasmonic waveguide modulation can also be reached by using a nearby nanoresonator filled with a gain medium (InGaAsP) [143]. A prototypical structure is illustrated in Figure 3d, consisting of a plasmonic metal-insulator-metal (MIM) waveguide, made by a silver-air-silver channel, with a side-coupled plasmonic rectangular resonator. In this approach, the cavity is electrically pumped to compensate the intrinsic ohmic losses of SPRs [144], also known as surface plasmon polaritons (SPPs), propagating in the waveguide. This waveguide-resonator system enables high-contrast modulation, allowing phase shifting up to 180 • , with bandwidths up to 100 GHz, and speeds in the order of 0.2 ns. Figure 3e, on the other hand, illustrates a 2D semiconductor-plasmonic heterostructure for an ultra-low switching energy plasmonic modulator [145]. This latter structure couples an SPP mode, propagating along the plasmonic waveguide, with excitons in the two-dimensional hBN-WSe 2 -hBN (hexagonal Boron Nitride-Tungsten Diselenide-hexagonal Boron Nitride) semiconductor. The system can work in the linear and nonlinear regimes, through proper modulating of the pump and probe beams, reaching modulation bandwidths around 1.5 THz. Another recent proposal considers a hybrid plasmonic modulator, combining silicon and electro-optic polymers (EOP) with silver, built by a straight waveguide coupled to a ring resonator [146]. The system is developed, from bottom to top, as silicon-EOP-silver-EOP-silicon on SiO 2 substrate, as depicted in Figure 3f. An externally applied voltage in the ring resonator, applied to change the EOP refractive index, is used for modulation of the plasmonic mode along the EOP/silver/EOP interfaces. An in-phase/quadrature (IQ) modulator [147], encoding information into the phase and amplitude of light, with attojoule per bit electrical energy consumption has been recently demonstrated [148]. The system, consisting of two imbalanced high-speed plasmonic MZMs integrated into a SiP Mach-Zehnder interferometer (MZI), is shown in Figure 3g. The phase delay in this platform can be adjusted either by a thermo-optic phase shifter (heater) or by tuning the wavelength, offering complex modulation up to 400 Gbps on a compact footprint.
A recent approach developed an Mach-Zehnder interferometer (MZI) all-plasmonic 116 Gbps electro-optical modulator from a single layer of gold using a substrate-independent process [149]. High-compactness, high-speed, and low-cost were reached through exploitation of the electro-optical Pockels effect [150,151] and the integration with a multicore optical fiber for SPR excitation [152] and polarization beam splitters [153][154][155][156]. An electro-optical modulator, based on a ring-resonator coupled to a buried low-loss silicon photonic waveguide, has also been recently proposed [157]. This latter proposal demonstrated an improved bandwidth and resilience to high temperature by replacing the conventionally used LiNbO 3 (lithium niobate) for BaTiO 3 (barium titanate) [158]. Other concepts include the use of hybrid plasmonic-polymer devices [47,159], liquid-crystal electro-optic plasmonic platforms [160,161], and carrier accumulation/epsilon-near-zero effect in transparent conductive oxides [162,163].

Plasmonic Filters, Switches, Routers, and Photodetectors
In addition to plasmonic waveguides, nanoantennas, and modulators, previously discussed, plasmonic filters are deserving attention for telecommunications applications, by virtue of their ability for frequency-selective absorption/transmission of optical signals, through proper design of metallic nanostructures [164][165][166]. Importantly, plasmonic notch filters inspired in tooth-based waveguides [167][168][169] have recently been developed at THz frequency range [170,171]. Figure 4a illustrates a plasmonic Ag cavity having two tooth-like cavities, filled with air, for rejection of wavelengths around 1 µm [171]. Previous approaches with metallic teeth-shape cavities on both sides, as illustrated in Figure 4b, have been demonstrated at microwave regime [172]. It is worth mentioning that these electromagnetic surface modes are named spoof surface plasmon polaritons (SSPP) as they mimic the optical SPR properties [173,174]. Although SSPPs are used in telecommunications for several applications like antennas [175][176][177] and transmission lines [178,179], they have shown an exceptional ability for highly efficient wavelength filtering [180,181]. Other filters' approaches include hexagonal [182,183] and ring resonators [184], as the one illustrated in Figure 4c, enabling easy tuning of rejection-band at mid-infrared frequencies by simply varying the ring's radius. In the case of hexagonal resonators, it can be easily coupled to a teeth-shape structure for single-mode filter applications [183]. Rectangular cavities, on the other hand, work as rejection-band filters [185] or wavelength demultiplexers [186]. Figure 4d schematically shows a rectangular cavity, with silver nanoblocks inside, for wavelength splitting and shifting [185]. Analogous platforms have been fed by a single input plasmonic waveguide and coupled to four output channels for wavelength mutiplexing/deplexing [187].
Plasmonic switches and routers are devices enabling selective propagation through different paths in a multiwaveguide system [188][189][190][191]. This selectivity allows for Boolean algebra [192], i.e., logical gates, and thus for optical computing with highly integrable and CMOS (Complementary Metal Oxide Semiconductor)-compatible devices [193][194][195][196]. Nonlinear optical phenomena, on the other hand, have also been recently demonstrated to be useful for plasmonic-based switching applications [197]. Of particular importance, symmetry-breaking features of plasmonic nanowires can also be used for polarization beam splitting, switching, and routing of light fields [198,199]. A SEM picture of a thick nanowire with adjacent nanoparticles is shown in Figure 4e. In this system, the output signal can be modulated/switched through the polarization angle of the input signals in port1 (1 in figure), port2 (2 in figure), or both. Chiral plasmonic structures [200], i.e., plasmonics systems whose opposite mirror images cannot be superimposed through symmetry operations, can also be used for routing of circularly polarized light (CPL) [201]. Figure 4f shows a SEM micrograph of a two-dimensional gold nanostructure used for polarization-based routing of the incident CPL. Magneto-optical (MO) properties of light have been used for externally controlled routing of SPP modes through two different paths [202], as depicted in Figure 4g. In this case, nanostructured plasmonic and MO materials is required. The intrinsic magnetization (M) of a ferromagnetic dielectric (MO material) can be alternated by an externally applied magnetic field in order to selectively route the SPP guided mode to travel along channel 1/channel 2 in the structure. Other proposals include hybrid photonic-plasmonic, Bragg-grating-based, and graphene-based router and switching devices [203][204][205]. Plasmonic photodetectors, on the other hand, enable fast and efficient detection and demodulation of optical signals [206,207]. In this context, plasmonic nanoantennas can also be used to downscale conventional semiconductor photodetectors-of high importance for THz and infrared detection [208,209]. A hybrid plasmonic silicon-graphene waveguide photodetector device [210], for light detection at 1.55 µm, is depicted in Figure 4h. These kinds of platforms allow broadband, low-footprint, and high-speed optical data reception, in addition to being CMOS compatible. Alternative graphene-based plasmonic photodetection devices have also been presented [211][212][213] (a) (b)

Outlook
The unparalleled ability of plasmonic nanostructures for light confinement and enhancement has been shown promising for future data networks. Indeed, at-will guiding and beam steering of optical pulses at nanoscale levels are envisaged as a key element for 6G communications [4], where data rates in the order of Tbps-in addition to an increasing number of end devices-are expected [15]. To this end, a seamless interconnection of terahertz signals from fiber networks, wireless cells, and inter-/intra-chip nanoelectronic functions must be ensured. Based on recent developments discussed in this review, we may foresee plasmonic elements as integral parts of future telecommunication networks. Such developments would benefit not only from the ability of plasmonics for optical signals propagation, but also from plasmonic-aided photovoltaics for energy efficient devices [214]. Plasmonic reconfigurable intelligent surfaces [215,216], on the other hand, have potential applications in the development of revolutionary wireless technologies for beam bending at THz frequencies [217]. We also envisage the application of the Pancharatnam-Berry