Spoof Surface Plasmon Polaritons Waveguide Research Articles

Terahertz broadband spoof surface plasmon polaritons using high-order mode developed from ultra-compact split-ring grooves.

Terahertz (THz) broadband spoof surface plasmon polaritons (SSPPs) using new structure of ultra-compact split-ring grooves are proposed. The high-order mode propagation is highly concentrated around the proposed structure with lower radiation loss implying improved operating bandwidth. More importantly, a size reduction of 83.5% can be realized as compared to the traditional grounded SSPP structure with the same high-order asymptotic frequency. To further verify the proposed idea, a similar structure in microwave regime is designed and measured, where the excitation is easily achieved by directly connecting the microstrip line to the proposed SSPP waveguide. The gradient transition section, such as flaring ground, can be avoided, which decreases the waveguide's longitudinal and transversal lengths and simplifies the design procedure. The measured results of the microwave prototype illustrate that it has good lowpass filtering performance, in which the reflection coefficient is better than -10 dB up to 13 GHz, with the smallest and worst insertion losses of 0.5 dB and 4.5 dB, respectively. To the best of the authors' knowledge, this work presents THz high-order broadband SSPP propagation for the first time, having significant potential for plasmonic integrated circuits application at microwave/THz frequencies.

Rejection of Spoof SPPs Using the Second Resonant Mode of Vertical Split-Ring Resonator

In this letter, the studies of the vertical split-ring resonator (VSRR) at its second resonant mode are carried out for the first time. The VSRR can be regarded as an electric resonator at such resonant mode, which is proven by its resonant current distribution. Then, a new type of rejection plasmonic filter is constructed by adding the VSRR on the planar spoof surface plasmon polariton (SPP) waveguide using double-side corrugated metallic strips. The second resonance of the VSRR is excited by the $z$ -component of the electric field of the spoof SPP mode, near which the SPP propagation is prevented. Utilizing this principle, a plasmonic filter with broadband rejection is designed and fabricated. The measurement result is presented to confirm the highly efficient filtering performance of the proposed filter, which makes it possible to be applied in the advanced integrated plasmonic devices and circuits at microwave and terahertz frequencies.

Properties of spoof plasmon in thin structures.

Spoof surface plasmon polariton (SSPP) is an exotic electromagnetic state that confines light at a subwavelength scale at a design-specific frequency. It has been known for a while that spoof plasmon mode can exist in planar, thin structures with dispersion properties similar to that of its wide three-dimensional structure counterpart. We, however, have shown that spoof plasmons in thin structures possess some unique properties that remain unexplored. Our analysis reveals that the field interior to SSPP waveguide can achieve an exceptional hyperbolic spatial dependence, which can explain why spoof plasma resonance incurs red-shift with the reduction of the waveguide thickness, whereas common wisdom suggests frequency blue-shift of a resonant structure with its size reduction. In addition, we show that strong confinement can be achieved over a wide band in thin spoof plasmon structure, ranging from the spoof plasma frequency up to a lower frequency considerably away from the resonant point. The nature of lateral confinement in thin SSPP structures may enable interesting applications involving fast modulation rate due to enhanced sensitivity of optical modes without compromising modal confinement.

Flexibly designed spoof surface plasmon waveguide array for topological zero-mode realization.

We propose a flexibly designed photonic system based on ultrathin corrugated metallic "H-bar" waveguide that supports spoof surface plasmon polariton (SPP) at microwave frequencies. Five designs were presented, in order to demonstrate flexibility according to varying height, period, core width, rotation, and shifting on the "H-bar" unit of the waveguide. The propagation constant between two hybrid designs of period and height structure was then shown in order to study the coupling effect. Next, we constructed a coupled waveguide array that followed the Su-Schrieffer-Heeger (SSH) model. This model was constructed by a hybrid design with the identical propagation constant of each waveguide, except it had dimerized spacing. The propagation feature of topological zero mode was then observed as theoretically expected in the dimerized array. Our proposed spoof SPP waveguide array has great flexibility to be used as a powerful experiment platform, particularly in photonic simulation of the quantum or topological phenomena described by Schrödinger equation in condensed matters.

Super Subwavelength Guiding and Rejecting of Terahertz Spoof SPPs Enabled by Planar Plasmonic Waveguides and Notch Filters Based on Spiral-Shaped Units

We numerically simulate novel planar plasmonic waveguides and notch filters with excellent guiding and rejection of terahertz (THz) waves with super subwavelength confinement. Our design is based on spoof surface plasmon polaritons—surface plasmon polaritons with a frequency that has been tuned using patterned conductive surfaces. We find that by using patterns of periodically arranged spiral-shaped units, the dispersion characteristics can be engineered at will by tuning the parameters of the spirals. We find that the resulting plasmonic waveguides have much lower asymptotic frequencies and much tighter terahertz field confinement when compared with conventional rectangular-grooved plasmonic waveguides. We show it is possible to design a structure with lateral dimensions that are only 25% the size of the conventional spoof surface plasmon polariton waveguides but with the same asymptotic frequency. Finally, we combined this architecture with broadband couplers to design an ultrawideband low-pass filter with sharp roll-off (cut-off frequency at 1.29 THz) and low insertion loss (<3 dB). Furthermore, by introducing double ring resonators based on spiral-shaped units, a planar plasmonic notch filter with rejection of more than 17 dB between 0.97 and 0.99 THz is demonstrated. The proposed waveguides and notch filters may have great potential applications in the promising terahertz integrated plasmonic circuits and systems.

Wide-Angle Frequency-Controlled Beam-Scanning Antenna Fed by Standing Wave Based on the Cutoff Characteristics of Spoof Surface Plasmon Polaritons

A wide-angle frequency-controlled beam-scanning antenna fed by the standing wave of spoof surface plasmon polaritons (SPPs) is proposed. The antenna is constructed by placing two rows of circular patches at both sides of an SPP waveguide. The waveguide is composed of two ultrathin corrugated metal strips with two levels of depth, which generate two cutoff frequencies. In the working band between two cutoff frequencies, circular patches are fed by standing waves. Simulated and measured results validate that the proposed antenna has a much wider scanning angle of 83° from 9.2 to 16 GHz than that in the published results. The measured gains of the proposed antenna are greater than 10.3 dBi over the whole band. The proposed beam-scanning antenna has advantages in radar and wireless communication system because of its low cost, compact structure, and wide beam-steering.

Broadband high-order mode of spoof surface plasmon polaritons supported by compact complementary structure with high efficiency.

In this work, spoof surface plasmon polaritons (SSPPs) supported by microstrip lines with T-shaped complementary grooves are proposed. Compared with the traditional SSPP structure based on single-conductor transmission lines, a broadband high-order mode of SSPPs can be predicted from the dispersion diagrams. Besides, a decrease in transverse size of 75% can be realized under the same asymptotic frequency. Then, a smooth and simple transition, which is composed of gradient complementary grooves, is designed to realize high-efficiency excitation and support practical applications of the high-order mode of SSPPs. As a result, a seamless connection with microstrip lines is easily achieved without using flaring ground, which simplifies the design procedure. Based on the proposed structure, a prototype of a SSPP waveguide is designed and fabricated. Both numerical and experimental results validate efficient excitation and broadband propagation (up to 12GHz) of the high-order mode of SSPPs. This work will greatly accelerate the development of advanced plasmonic integrated circuits at microwave frequencies.

High‐efficiency circularly polarised leaky‐wave antenna fed by spoof surface plasmon polaritons

A circularly polarised (CP) periodic leaky-wave antenna excited by spoof surface plasmon polaritons (SPPs) is proposed. It is based on an SPP transmission line loaded by periodic rectangular patches on both sides with 90° phase shift, which converts the SPPs mode to CP propagating waves in the air. Due to the dispersion nature of SPPs waveguide, this leakywave antenna exhibits a beam scanning feature. Both simulated and measured results demonstrate that the proposed antenna has an impedance bandwidth of 33% for |S 11 | <; - 10dB and a 3 dB axial ratio bandwidth of 15%. The average gain and radiation efficiency are about 10 dBic and 87% from 5.5 to 6.4 GHz, respectively.

Force distribution inside the spoof surface plasmon polariton waveguide

The Maxwell stress tensor for the field inside a spoof surface plasmon polariton (SSPP) waveguide and the electromagnetic force distribution acting on a dielectric particle are investigated. We show that, for particles made of material with the dielectric function slightly different from that of the medium filling the waveguide, the force distribution is fully described by the SSPP field in the absence of the particle. The spatial profile of the field strongly depends on the relation between the operating frequency and the SSPP resonance frequency. We show that varying the characteristic frequencies introduces a switching effect, when the direction of the force along the waveguide axis changes (from toward the grooves to away from them). We propose a pumping mechanism based on force acting on a small particle inside the SSPP waveguide.

Spoof Surface Plasmon Polaritons Power Divider with large Isolation

Periodic corrugated metal structure is designed to support and propagate spoof surface plasmon polaritons (SSPPs) wave in the microwave frequencies. In this paper, firstly a plasmonic waveguide consisting of oval-ring shaped cells is proposed with the performance of high transmission efficiency in a wide frequency range. The coplanar waveguides (CPWs) with 50 Ω impedance are adopted to feed the energies or extract signals at both ends of the plasmonic waveguide. Then a well-isolated power divider is constructed based on the SSPPs waveguides aiming to equally split the energy of the SSPPs wave into two equal parts. The stepped-impedances are co-designed with the three input/output ports of the power divider to achieve the impedance-matching between the SSPPs waveguides and the coplanar waveguides. Besides, a single resistor is placed in the middle of two symmetrical half oval-rings to realize the isolation between the two output ports over the spectrum of 4.5–7.5 GHz. Finally, both plasmonic waveguide and the power divider are fabricated and tested to verify the predicted characteristics.

Spoof plasmonic waveguide developed from coplanar stripline for strongly confined terahertz propagation and its application in microwave filters.

A novel planar terahertz (THz) plasmonic waveguide developed from coplanar stripline (CPS) is proposed for the first time to achieve strongly confined THz propagation performance based on the concept of spoof surface plasmon polaritons (SSPP). Guided-wave characteristics of the proposed plasmonic waveguide are theoretically investigated by eigen-mode simulation technique and finite-difference time-domain solutions. It is found that the waveguide propagation characteristics can be directly manipulated by designing the SSPP unit cells, which exhibit flexible tuning ability of the asymptotic frequency and strong THz field confinement. The idea has been validated through fabricated filter experiments in microwave frequency regime by scaling up the geometry size of the proposed structure. The measured results illustrate high performance of the ultra-wideband filter, in which the reflection coefficient is better than -10 dB from 3 to 13.1 GHz with the smallest and worst insertion losses of 2.2 dB and 5.6 dB, respectively. This work presents a new SSPP waveguide developed from CPS to realize the THz-wave propagation with strong field confinement, which may have promising potential applications in various integrated THz plasmonic devices.

Microwave Vortex‐Beam Emitter Based on Spoof Surface Plasmon Polaritons

AbstractSince orbital angular momentum (OAM) being investigated intensively in the optical region, there have been growing interests in employing OAM to solve the problem in wireless communications as a new method. It is found that the independence between different OAM modes is crucial to wireless communications. Motivated by the tremendous potential of OAM in communication systems, a novel method to generate vortex beams by spoof surface plasmon polariton (SPP) is proposed. A looped double‐layer spoof SPP waveguide is applied to realize the transmission of electromagnetic waves. Beam emitting is accomplished through a series of circular patches, whose role is not only the radiation units but also resonators giving rise to the phase shifts required by the vortex beam. The proposed method is validated by both numerical simulation and experiment. The measured results show that spoof SPP are radiated by the circular patches and vortex beams carrying different OAM modes are observed at different frequencies. The proposed method possesses smaller size and is much easier to be integrated into systems and integrated circuits. The simple structure and design procedure make the proposed method promising in future wireless communication systems.

Terahertz spoof surface-plasmon-polariton subwavelength waveguide

Surface plasmon polaritons (SPPs) with the features of subwavelength confinement and strong enhancements have sparked enormous interest. However, in the terahertz regime, due to the perfect conductivities of most metals, it is hard to realize the strong confinement of SPPs, even though the propagation loss could be sufficiently low. One main approach to circumvent this problem is to exploit spoof SPPs, which are expected to exhibit useful subwavelength confinement and relative low propagation loss at terahertz frequencies. Here we report the design, fabrication, and characterization of terahertz spoof SPP waveguides based on corrugated metal surfaces. The various waveguide components, including a straight waveguide, an S-bend waveguide, a Y-splitter, and a directional coupler, were experimentally demonstrated using scanning near-field terahertz microscopy. The proposed waveguide indeed enables propagation, bending, splitting, and coupling of terahertz SPPs and thus paves a new way for the development of flexible and compact plasmonic circuits operating at terahertz frequencies.

Spatial accumulation of phase difference in spoof plasmon based Mach–Zehnder Interferometer

An active beam steering structure comprising the spoof surface plasmon polariton (SSPP) waveguide is presented. The proposed structure consists of two stages: the two arms structure as a Mach–Zehnder interferometer (MZI), and free space propagation stage. The SSPP waveguide includes periodic grooves, which are filled with a thin layer of doped semiconductor like GaAs. By applying electric voltage at the electrodes on top of the waveguide, the depletion layer thickness and the effective refractive index of the dielectric inside the grooves can be changed. The effective refractive index of the dielectric inside the groove is derived analytically and the phase change of the propagation wave along the single sided SSPP waveguide is calculated as a function of input voltage. The relation between the input signal and the angle of maximum radiation of output beam is investigated and the linearity is improved. We have shown that the MZI beam steering section has the capability to bend the electromagnetic beam about 42 degrees. At the end, we presented an application of the proposed beam steering device as a spatially accumulated phase difference structure that can be utilized in devices such as digital-to-analog converter (DAC).

Terahertz particle-in-liquid sensing with spoof surface plasmon polariton waveguides

We present a highly sensitive microfluidic sensing technique for the terahertz (THz) region of the electromagnetic spectrum based on spoof surface plasmon polaritons (SPPs). By integrating a microfluidic channel in a spoof SPP waveguide, we take advantage of these highly confined electromagnetic modes to create a platform for dielectric sensing of liquids. Our design consists of a domino waveguide, that is, a series of periodically arranged rectangular metal blocks on top of a metal surface that supports the propagation of spoof SPPs. Through numerical simulations, we demonstrate that the transmission of spoof SPPs along the waveguide is extremely sensitive to the refractive index of a liquid flowing through a microfluidic channel crossing the waveguide to give an interaction volume on the nanoliter scale. Furthermore, by taking advantage of the insensitivity of the domino waveguide’s fundamental spoof SPP mode to the lateral width of the metal blocks, we design a tapered waveguide able to achieve further confinement of the electromagnetic field. Using this approach, we demonstrate the highly sensitive detection of individual subwavelength micro-particles flowing in the liquid. These results are promising for the creation of spoof SPP based THz lab-on-a-chip microfluidic devices that are suitable for the analysis of biological liquids such as proteins and circulating tumour cells in buffer solution.

Modelling of the effective dielectric constant of planar spoof surface plasmon polariton waveguides

The modelling of the effective dielectric constant of a planar spoof surface plasmon polariton waveguide for X-band frequencies is presented. The waveguide has a single conductor structure that is corrugated on both sides. A methodology to extract the effective dielectric constant of waveguides with complex patterns and a novel closed-form formula based on complex analysis of the structure and empirical methods are suggested. The suggested formula is a function of the corrugation depth, aperture, periodicity, thickness of the substrate material and permittivity. The worst case error between the formula and measurement result is 8.4%, which is much lower than the previously reported coplanar stripline formulation approach having an error of 14.5%.

Spoof surface plasmonic waveguide devices with compact length and low-loss

A spoof surface plasmon polaritons (SSPPs) waveguide structure with periodic T-grooves is proposed and investigated in this paper. Compared to the conventional plasmonic waveguide with periodic rectangular-grooves, the proposed waveguide support the guided SSPPs wave with enhanced propagation confinement. Moreover, the waveguide with T-grooves can achieve 39.4% decrease in length compared with the rectangular-grooves waveguide. The propagation properties of the SSPPs with enhanced confinement are able to improve by adjusting the shape dimensions of T-grooves. The effects of bend radius of 90° T-grooves waveguide on propagation performance are analyzed, and the good transmission characteristics of waveguides with small bend radius are verified. The experimental verifications of the T-grooves plasmonic waveguide filter and 3 dB power divider in microwave region are realized, S21-parameter of waveguide filter is very close to 0 dB from 0 GHz to 5.6 GHz. And 3 dB power divider achieves very good power equipartition from 0 GHz to 5.5 GHz.

Microwave metamaterials—from passive to digital and programmable controls of electromagnetic waves

Since 2004, my group at Southeast University has been carrying out research into microwave metamaterials, which are classified into three catagories: metamaterials based on the effective medium model, plasmonic metamaterials for spoof surface plasmon polaritons (SPPs), and coding and programmable metamaterials. For effective-medium metamaterials, we have developed a general theory to accurately describe effective permittivity and permeability in semi-analytical forms, from which we have designed and realized a three dimensional (3D) wideband ground-plane invisibility cloak, a free-space electrostatic invisibility cloak, an electromagnetic black hole, optical/radar illusions, and radially anisotropic zero-index metamaterial for omni-directional radiation and a nearly perfect power combination of source array, etc. We have also considered the engineering applications of microwave metamaterials, such as a broadband and low-loss 3D transformation-optics lens for wide-angle scanning, a 3D planar gradient-index lens for high-gain radiations, and a random metasurface for reducing radar cross sections. In the area of plasmonic metamaterials, we proposed an ultrathin, narrow, and flexible corrugated metallic strip to guide SPPs with a small bending loss and radiation loss, from which we designed and realized a series of SPP passive devices (e.g. power divider, coupler, filter, and resonator) and active devices (e.g. amplifier and duplexer). We also showed a significant feature of the ultrathin SPP waveguide in overcoming the challenge of signal integrity in traditional integrated circuits, which will help build a high-performance SPP wireless communication system. In the area of coding and programmable metamaterials, we proposed a new measure to describe a metamaterial from the viewpoint of information theory. We have illustrated theoretically and experimentally that coding metamaterials composed of digital units can be controlled by coding sequences, leading to different functions. We realised that when the digital state of a coding unit is controlled by a field programmable gate array, the programmable metamaterial, which is capable of manipulating electromagnetic waves in real time, can generate many different functions.

Negative-index dispersion and accidental mode degeneracy in an asymmetric spoof–insulator–spoof waveguide**Project supported by the National Basic Research Program of China (Grant No. 2013CBA01702) and the National Natural Science Foundation of China (Grant Nos. 61377016, 61575055, 10974039, 61307072, 61308017, and 61405056).

It has been recently demonstrated that negative-index dispersion and mode degeneracy can be achieved by manipulating a spoof–insulator–spoof (SIS) waveguide. In this paper, we propose a new SIS waveguide, which is composed of two spoof surface plasmon polaritons (SSPPs) waveguides drilled with periodic rhomboidal grooves. Both the symmetric and asymmetric cases are investigated. Our simulation results show that the asymmetric SIS waveguides are more significant. By tailoring the tilt of the rhomboidal grooves, the negative-index dispersion can be achieved and the microwave band gap (MBG) can be effectively modulated. At a critical tilt, there appears an accidental mode degeneracy at the edge of the first Brillouin zone. The excitation and propagation of the two coupled modes sustained by the asymmetric SIS waveguides are also demonstrated.

A Multi-Layer Spoof Surface Plasmon Polariton Waveguide With Corrugated Ground

Due to the unique properties of field confinement and enhancement, spoof surface plasmon polaritons (SPPs) are considered as special modes to break many challenges in traditional electronic and microwave engineering. Ultrathin corrugated metallic structure offers an easy fabrication method to realize spoof SPP waveguides on substrate, and has been proved to have many merits in recent years. Lately, a programmable and coding spoof SPP waveguide loaded by active elements was presented, which makes it possible to control SPPs in real time. However, this programmable SPPs waveguide suffers from the limited loadable space and hence can hardly achieve more complex controlling functions. On the other hand, the existing methods to excite spoof SPPs require large conversion structures, which are unadoptable in modern integrated circuits. In this paper, we propose a new type of spoof SPP waveguide composed of a metallic strip and corrugated ground with an open loadable space for large-scale and complex controlling networks. A new conversion structure is presented to achieve high-efficiency transition from the traditional microstrip to the SPP waveguide without using extra space. Both numerical simulations and experiments indicate the outstanding performance of the new spoof SPP waveguide. The proposed structure is also convenient to connect with active devices due to its multi-conductor nature. Hence, the proposed structure may find wide applications in SPP-based integrated circuits and systems in the future.