Spoof Surface Plasmon Polaritons Wave Research Articles

A low-profile high-isolation endfire MIMO antenna based on spoof surface plasmon polaritons for X-band applications

This work proposes a three-port, low-profile, coplanar endfire multiple-input-multiple-output (MIMO) antenna based on spoof surface plasmon polaritons (SSPPs). The proposed antenna comprises a coplanar waveguide (CPW)-fed SSPPs transmission line (TL). A transition to transform the CPW quasi-TEM mode to the SSPPs mode. The radiator section with asymmetric discontinuities converts the bounded SSPPs waves into radiating waves. The proposed MIMO antenna exploits the inherent feature of high isolation in the SSPPs TL due to its strong field confinement competency. Wider beamwidths for ample azimuth and elevation coverage with a stable pattern in the endfire direction are achieved in the 9.52–11.23 GHz operating band. In the operating band, the MIMO antenna has an average realized gain of 6 dBi. Mainly, it has high isolation below -35 dB (measured) in the operating range, a maximum envelope correlation coefficient (ECC) of 1.5 × 10−3, and a total active reflection coefficient (TARC) value below 0.3. The measured results are closely matched with the simulated ones in terms of S-parameters, radiation pattern, ECC, TARC, and CCL.

Efficient Amplification of Large‐Signal Spoof Surface Plasmon Polaritons in a Subwavelength Size

AbstractThe advancement of communication systems demands ever‐smaller sizes, leading to the integration of more circuits in compact forms. A promising solution lies in Spoof Surface Plasmon Polaritons (SSPPs), which enable strong confinement, low interference, and a significant wave vector, thereby facilitating smaller sizes. However, the amplification of large plasmonic signals in a compact size remains a daunting scientific challenge for the further development of SSPPs communication systems. In this study, this challenge is tackled by selecting GaN High Electron Mobility Transistors (HEMTs) and load/source‐pull technologies. To ensure efficient operation, impedance‐matching networks are designed, resulting in a remarkably small amplifier structure. The approach involves utilizing the Bayesian optimization (BO) algorithm to fine‐tune the matching network, leading to exceptional amplification performance. The results obtained in the frequency range of 1–3 GHz are highly promising, with both simulated and measured results demonstrating an impressive output power exceeding 40 dBm, a power added efficiency (PAE) surpassing 60%, and a gain exceeding 10 dB. These remarkable achievements underscore the viability of amplifying large‐signal SSPPs and outputting high‐power SSPPs waves within a subwavelength size, opening exciting possibilities for SSPPs communication, sensing, and imaging systems.

Broadside radiation antenna based on tightly coupled spoof surface plasmon polaritons

A spoof surface plasmon polariton (SSPP) is a hybrid mode of a surface wave in the air and electron oscillation within artificial electromagnetic (EM) media, with the dispersion curve lying below the light line, giving rise to many unique EM properties. In this paper, we investigate the EM properties of tightly coupled SSPP (TC-SSPP) modes and propose a slender broadside antenna based on the TC-SSPP. With the comb structure (CS), dispersion properties of different SSPP modes are firstly analyzed under plane wave illumination, and then we introduce a modified SSPP structure, the broadside-coupled (BC)-CS, to excite and guide TC-SSPP modes. Since TC-SSPP modes can be confined tightly around the slender BC-CS, EM waves can be manipulated within a much more compact space, which is favorable for miniaturization, compactness and the light weight of microwave devices. To demonstrate this, we propose a slender broadside radiation antenna based on the TC-SSPP, using a probe as the feed and a finite-length BC-CS as the radiator. Due to near-field magnetic coupling, the SSPP wave can be excited on the BC-CS. By varying the geometric parameters, the antenna can radiate in the broadside direction. A prototype was designed, fabricated and measured. Both the simulated and measured results verify our design.

Bandpass Filter With Reconfigurable Rejection and Deep-Upper-Wideband Harmonics Suppression Using Spoof Surface Plasmon Polaritons of Hollowed-Bow-Tie Cells

A bandpass filter with a reconfigurable rejection is designed and experimentally validated by exploiting spoof surface plasmon polaritons (SPPs) of hollowed-bow-tie (HBT) cells. As a novel structural discovery, the dispersion curve of the proposed C-loaded HBT cell splits into two modes comparing with the traditional bow-tie cell. With properties of propagating on metal–dielectric interfaces and the exponential increasing propagation constant, SPPs waves show advantages of low crosstalk and tightly confined fields, and experimental results verify that the harmonics suppression level of the transmission characteristic is above 20 dB from 6 to 16 GHz. Moreover, integrated with lumped elements, direct current (dc) bias circuits, and varactor diodes, the designed bandpass filter also has a low cutoff frequency and a reconfigurable rejection. By regulating the bias voltage, the rejection level, frequency, and bandwidth of the stopband are tuned accordingly. The cell dispersion relationships, equivalent circuits, and the near-field distribution are given for theory analyses, and all the full-wave simulations validate well with measurements. The proposed bandpass filter lays a ramming foundation for complex reconfigurable integrated microwave systems, electronically controlled antenna feed networks, and adjustable anti-interference systems.

Realization of broadband coherent perfect absorption of spoof surface plasmon polaritons

In this work, we propose and demonstrate a scheme to realize broadband coherent perfect absorption of spoof surface plasmon polaritons (SPPs) using an ultrathin conductive film vertically placed on a plasmonic metasurface that supports spoof SPPs. When the conductive film possesses an appropriate sheet resistance, two incident coherent beams of spoof SPP waves can be simultaneously absorbed over a broad spectrum. By tuning the phase difference between the two incident beams from 0 to π, the absorption can be gradually changed from near-100% to near-zero, as verified in full-wave simulations and microwave experiments. Our work extends the theory of coherent perfect absorption from propagating waves to spoof SPPs with a significant advantage of broad working bandwidth.

Reconfigurable Spoof plasmonic Coupler for Dynamic Switching between Forward and Backward Propagations

AbstractA reconfigurable coupler constructed using two spoof surface plasmon polariton (SSPP) waveguides and a spoof localized surface plasmon (SLSP) resonator to achieve switching between the forward and backward coupling of SSPP waves is proposed. By changing the bias voltage applied to the varactor diodes loaded in the SSPP waveguide, the dispersion of the plasmonic structure is adjusted to achieve a reversed group velocity. Consequently, the direction of the coupled SSPP waves can be dynamically controlled at the same operating frequency. The tunability of this coupler can be further improved by incorporating varactor diodes into the SLSP resonator, where forward–backward coupling can be achieved at multiple frequencies.

Passive amplitude-phase modulations and sensing based on Mach–Zehnder interferometer of spoof surface plasmon polaritons

We numerically and experimentally demonstrate an ultrathin and compact Mach–Zehnder interferometer (MZI) for sensing and amplitude/phase modulations of spoof surface plasmon polariton (SSPP) waves. For a specific frequency, the magnitude and phase of the far-field transmission are modulated when a dielectric sample is loaded on one of the MZI arms. The phase difference between the SSPP waves propagating along the sensing and reference arms causes the outputs from both arms to interfere, allowing small perturbations in the sensing arm to be detected. Our study shows that the sensitivity of the proposed SSPP MZI is significantly higher than that of the single-armed SSPP waveguide, and also better than that of the conventional MZI formed with the traditional microstrip lines. The dependence of the sensitivity on the structural parameters such as geometrical parameters of the detected material and the arm length of MZI is discussed, revealing the possibility of realizing miniaturized MZI with high sensitivity.

An ultra‐wideband out‐of‐phase power divider based on odd‐mode spoof surface plasmon polariton

In this article, a feasible design of an ultra-wideband power divider (PD) based on spoof surface plasmon polariton (SSPP) is proposed. A quasi T-junction transmission structure is composed of the sub-wavelength corrugated grooves drilled on the up metal layer, realizing the wideband and low loss for the propagation of the odd-mode SSPP wave. The consistency of the dispersion curves between the H-shaped and the U-shaped unit cells under the same dimension parameters ensures the smooth transition at the bifurcation point and out-of-phase property further. The odd-mode SSPP is effectively excited by using the microstrip-to-slotline convertor. Some design details like gradient corrugations and stepped slotlines are employed to achieve high-coupling efficiency from the input electromagnetic energy. The simulated and measured results show that the proposed PD achieves 180° ± 5° phase difference over an ultra-wideband from 3.3 to 9.8 GHz (a relative bandwidth of 99.24%) with high-transmission efficiency. The ultra-wideband out-of-phase property of this design has potential applications in the differential feed network.

A customizable linearly polarized transmission line based on microstrip spoof surface plasmon polaritons with tilted grooves

In this paper, an ultrathin linearly polarized transmission line (TL) based on tilted microstrip spoof surface plasmon polaritons (SSPPs) is proposed. The proposed structure of the TL consists of an ultrathin corrugated metallic strip with periodic tilted groove arrays and metallic ground. Compared with the reported traditional SSPPs, the proposed SSPPs structures with tilted grooves can support the SSPPs wave with a larger propagation constant, which means a stronger field confinement in the microstrip SSPPs structure and effectively lower the dimension in transverse direction of the TL. Besides, the simulation demonstrates that the wave confinement performance can be well maintained when the grooves tilt from 0 to 60°, and the electric field vectors of the tilted grooves undergo a rotation correspondingly, resulting in a flexible polarization. Therefore, the polarization of different angles can be easily realized when the proposed SSPPs is employed as radiating or feeding structure. To verify characteristic of the proposed structure at microwave frequencies, we simulate, fabricate and measure a back-to-back transition sample with 45°-titled grooves. Both simulations and measured results have clearly validated the good characteristic of the proposed designs.

Integrated multi-scheme digital modulations of spoof surface plasmon polaritons

The future wireless communications require different kinds of modulation functions to be integrated in a single intelligent device under different scenarios. Here, we propose a multi-scheme digital modulator to achieve this goal based on integrated spoof surface plasmon polaritons (SPP) in different frequency bands. By constructing switchable spoof SPP units, the propagating wave in the proposed spoof SPP waveguide can be manipulated in amplitude domain, frequency domain, and phase domain. As a proof of concept, the integrated multi-scheme digital modulator is experimentally verified to achieve at least three kinds of modulations, including amplitude shift keying, phase shift keying, and frequency shift keying, in a single digital spoof plasmonic waveguide. The simulated and measured results show that the modulator has excellent property of field confinement and is capable of frequency-domain modulation. Hence, the multi-scheme modulation property makes the proposed SPP digital modulator be an effective and reliable candidate for efficient manipulations of SPP waves and for advanced modulation technology

Microwave-Vortex-Beam Generation Based on Spoof-Plasmon Ring Resonators

Vortex beams carrying orbital angular momentum (OAM) have recently attracted great interest due to their capability for providing a new method to increase the communication channels in wireless communications. It is expected that the unbounded eigenstates of OAM can bring a large number of channels to enhance the transmission capacity. Here, we propose an effective strategy to generate vortex beams by using spoof surface plasmon polaritons (SSPPs) in the microwave region. A periodically modulated SSPP ring resonator is applied to convert near-field SSPP waves into radiating a vector vortex beam. The emitted beam can possess different OAM modes at different frequencies through rational design. In addition, circularly polarized vortex beams are demonstrated. Both numerical simulations and experimental characterizations confirm the theoretical predictions. Owing to the deep subwavelength characteristic of SSPP, the proposed device can be readily integrated into miniaturized circuits and systems. We anticipate that our design will enable a wide range of applications for future wireless communication technologies.

Programmable Multifunctional Device Based on Spoof Surface Plasmon Polaritons

The spoof surface plasmon polariton (SPP) waveguide, which could support well-confined surface waves along an ultrathin corrugated metallic strip, is one of the most advantageous candidates in integrated circuits due to the remarkable properties of low crosstalk, low loss, and packaging minimization. Many devices based on the spoof SPP waveguide have emerged, but it is still a challenge to manipulate the SPP waves freely in real-time. Here, we propose a programmable multifunctional spoof SPP device by loading varactor diodes between adjacent metallic teeth of SPP waveguides. By programming the bias voltages of the varactor diodes dynamically, the coupling waves propagating on two SPP waveguides can be freely controlled. Simulations and experiments demonstrate that the programmable SPP device can realize high-efficiency SPP through transmission, unequal coupling, 3 dB direction coupling, and crossover transmission with better isolation effect. Different functions and operational frequency ranges can be reconfigured in real-time. The proposed method lays the groundwork for applications in large-scale microwave integrated circuits and digital communication systems.

HR-Si prism coupled tightly confined spoof surface plasmon polaritons mode for terahertz sensing.

We report a high-resistivity silicon (HR-Si) prism coupled terahertz (THz) spoof surface plasmon polaritons (SSPPs) on flat subwavelength metasurface. Using a high refractive index prism as an external coupler, a more tightly confined SSPPs mode can be excited in a smaller resonant cavity, leading to strong light-matter interaction. Besides, theoretical analysis and experimental results have both indicated that the SSPPs resonance response to the filling patterns of analyte in the resonant cavity are quite different. In particular, we have found that the interaction between analyte and SSPPs wave can be maximized when the analyte filled with the whole resonant cavity and a higher sensitivity for THz sensing can be obtained. A high sensitivity varied from 0.31 THz/RIU to 0.85 THz/RIU is predicted. Furthermore, these SSPPs modes exhibit high Q-factor, and characteristic spectra of water caused by surface plasmon resonance (SPR) are observed, which is significant in promoting the THz-SPR sensing of polar liquids or aqueous analytes with THz metasurfaces.

Flexible and Dynamically Tunable Attenuator Based on Spoof Surface Plasmon Polaritons Waveguide Loaded With Graphene

This paper reports on a flexible and dynamically tunable spoof surface plasmon polariton (SSPP) attenuator for the first time. It consists of an SSPP waveguide and a graphene sandwich structure (GSS). Due to the flexibility of both the GSS and the SSPP waveguide, the attenuator yields a good flexible feature. The GSS is spread on the SSPP waveguide along the propagation direction to dissipate the SSPP wave. By applying the proper bias voltage to the GSS, the surface impedance of the GSS can be tuned while accordingly modifying the attenuation of the attenuator. Moreover, to well understand the attenuation mechanism, a theoretical analysis of SSPP waveguide based on Poynting power flow is proposed according to the structural characteristics of SSPP waveguide. All the theoretical, simulated, and measured results have proved that the proposed attenuator in this paper can achieve broadband attenuation tuning range, low return loss, and 90° bendability without any obvious change in the attenuation.

Miniaturized Spoof Surface Plasmon Polaritons Low-Pass Filter With a Novel Transition Structure

In this letter, a miniaturized spoof surface plasmon polaritons (SSPPs) low-pass filter with a novel transition structure is proposed. The novel transition structure is designed by two rectangular strips with an inclination of 17° and right-angled triangle patches. In this letter, we analyzed the normal and azimuthal components of the electric field perpendicular to the plane of the filter at different locations, verifying the effectiveness of the transitional structure. Besides, efficient conversion between quasi-transverse electromagnetic (TEM) waves and transverse magnetic (TM) polarized SSPPs waves are achieved by loading the novel transition structures. The designed filter can suppress unnecessary radiation loss and increase transmission efficiency. More importantly, the filter size is 47.52 mm $\times7.47$ mm and its length is reduced by at least 61% compared with the other low-pass filters with the same cutoff frequency. This letter has promoted the development and application of SSPPs low-pass filter and related equipment in microwave frequencies.

Ultrathin and flexible directional coupler with arbitrary coupling level using s-shaped spoof surface plasmon polariton coupled-line

Here, we study the coupling effect between two identical ultrathin plasmonic waveguides and present an ultra-flexible directional coupler by printing a pair of corrugated metallic strips with mirror symmetry on the top of an ultrathin dielectric substrate. The proposed coupler supports the transmission and coupling of spoof surface plasmon polariton waves efficiently. We experimentally demonstrate that the coupler possesses excellent flexibility, arbitrary coupling level and wide bandwidth simultaneously. Hence, the proposed coupler paves the way to develop flexible plasmonic devices, conformal circuits, human body sensors and wearable devices in microwave and terahertz frequencies.

Scattering of spoof surface plasmon polaritons in defect-rich THz waveguides

We report on the first observation of ‘Spoof’ Surface Plasmon Polariton (SPP) scattering from surface defects on metal-coated 3D printed, corrugated THz waveguiding surfaces. Surface defects, a result of the printing process, are shown to assist the direct coupling of the incident free-space radiation into a spoof SPP wave; removing the need to bridge the photon momentum gap using knife-edge or prism coupling. The free space characteristics, propagation losses and confinement of the spoof SPPs to the surface are measured, and the results are compared to finite-difference time domain simulations. Angular resolved THz spectroscopy measurements reveal the scattering patterns from surfaces and are compared with Mie theory, taking into account the shortened wavelength of the photons in their bound SPP state compared to their free space wavelength. These results confirm yet another similarity between the properties of THz spoof SPPs and their natural, non-spoof, counterparts at optical and infrared frequencies which also, unexpectedly, adds functionality to the structures.

A compact leaky‐wave antenna using a planar spoof surface plasmon polariton structure

This article presents and validates a leaky-wave antenna by using the spoof surface plasmon polariton (SSPP) technique. By properly designing the proposed SSPP unit, the SSPP wave can be switched between the confinement and radiation modes. A large radiation efficiency can be achieved by properly designing the modulation depth, which ensures that a compact SSPP leaky-wave array can be realized by using a small number of SSPP radiation units. To verify the design, a prototype which consists of a SSPP feeding network and a 4 by 4 SSPP radiation units has been fabricated by using a low cost FR-4 substrate. A good agreement between simulated and measured results has been obtained. The proposed array antenna shows the promising capability of the SSPP technique for leaky-wave antenna applications.

The engineering way from spoof surface plasmon polaritons to radiations

In recent years, spoof surface plasmon polaritons (SPPs) have been investigated at microwave and THz frequencies for engineering purpose. Due to momentum mismatch, the SPP mode cannot be directly converted from the spatial mode, and vice versa. Stimulating schemes have been developed to transform spatial waveguide modes to SPP modes with high efficiency. On the other hand, the question may arise that, is it possible to transform the propagating SPP waves to directive radiating waves for wireless communication? In view of this, this paper introduces the new-concept antennas based on spoof SPPs at microwave frequencies. Methods of transforming SPP modes to radiating modes are studied, whilst a series of antenna designs are presented and discussed. Feeding networks for antenna arrays using SSPP TLs are also investigated. Most works reviewed in this paper are fulfilled at Southeast University in China.

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.