Efficient Waveguide Mode Conversions by Spoof Surface Plasmon Polaritons at Terahertz Frequencies

In search of new technologies for optimizing the performance and space requirements of electronic and optical micro-circuits, the concept of spoof surface plasmon polaritons (SSPPs) has come to the fore of research in recent years. Due to the ability of SSPPs to confine and guide the energy of electromagnetic waves in a subwavelength space below the diffraction limit, SSPPs deliver all the tools to implement integrated circuits with a high integration rate. However, in order to guide SSPPs in the terahertz frequency range, it is necessary to carefully design metasurfaces that allow one to manipulate the spatio-temporal and spectral properties of the SSPPs at will. Here, we propose a specifically designed cut-wire metasurface that sustains strongly confined SSPP modes at terahertz frequencies. As we show by numerical simulations and also prove in experimental measurements, the proposed metasurface can tightly guide SSPPs on straight and curved pathways while maintaining their subwavelength field confinement perpendicular to the surface. Furthermore, we investigate the dependence of the spatio-temporal and spectral properties of the SSPP modes on the width of the metasurface lanes that can be composed of one, two or three cut-wires in the transverse direction. Our investigations deliver new insights into downsizing effects of guiding structures for SSPPs.

Interconnects have become a major obstacle in chip scaling. Spoof surface plasmon polariton (SSPP) modes have gained attention for their ability to manipulate light beyond diffraction limits at a given frequency, leading to SSPP interconnects. This article investigates the transmission characteristics of SSPP interconnect pairs placed side by side in the terahertz frequency range with comprehensive performance analysis. The proposed SSPP waveguide pair exhibits a maximum transmission coefficient of around −0.05 dB in the −3 dB band in the terahertz frequency range. Due to field confinement near the metal–dielectric interface, energy remains confined for the designed SSPP interconnect pair system. The proposed SSPP structure shows several bands in the terahertz frequency range, whereas conventional interconnects shows almost zero transmission at such frequencies. Additionally, the effect of geometric parameters on transmission coefficients (S21) and coupling coefficients (S41) has been investigated. Moreover, it has been shown that the bandwidth, as well as the upper cutoff frequency, can be tuned by varying the geometric parameters such as groove height, groove width and groove density. Since global interconnects undergo bending in actual circuits during distant data transmission on chips, geometric mismatches may occur between adjacent pairs of SSPP interconnects. Hence, it has also been examined how bending and mismatches affect transmission and coupling coefficients. Several SSPP schemes have been simulated, among which the best performance is obtained with 2 μm mismatch in groove height. For this optimized design, two corrugated metal interconnects are considered with groove heights of 20 μm and 22 μm, respectively, a groove width of 3 μm, a period of 20 μm, and the number of grooves at 50. For this particular configuration, an ultra-wide passband is found having a bandwidth of almost 400 GHz, with a signal reflection of below −12 dB and little insertion loss of ∼−1.43 dB.

Spoof surface plasmon polaritons(SSP) on the corrugated metallic surfaces can find many interesting applications from microwave to terahertz band. In this paper, we propose a new excitation method of high-order SSP mode on a lamellar grating by injected electron beam. The dispersion characteristics and field confinement of SSP modes on the structure are presented by theoretical and numerical methods at terahertz frequencies. Besides, the excitation of high-order SSP mode by injected electron beam are modeled and implemented by numerical simulations based on FDTD algorithm. It is shown that the needed beam energy is much higher than the previously shown fundamental SSP mode excitation with metallic grating. Thus, it is very challenging to excite higher-order SSP mode by electron beam in the structure. Also, the output performances of the excited high-order SSP mode are also analyzed. The presented studies on the excitation of high-order SSP mode by electron beam pave the ways for the development of novel terahertz radiation source which can operate at higher frequency band and/or multi-frequency spectrum.

Spoof Surface Plasmon Polariton (SSPP) transmission line is a novel type of surface wave structure in microwave and terahertz frequency range. For limited electromagnetic field between corrugated metal and dielectric, SSPP transmission lines and components have the advantages of wideband adjustment, miniaturization and high integration. In this paper, the influence of several geometric parameters on the dispersion curves of the oral-ring SSPP unit is analyzed. And then, the attenuation constant of the oval-ring transmission line is calculated. Finally, the SSPP splitters with oval-ring unit cell are reviewed.

In this presentation, I will review recent advances of spoof surface plasmons in the microwave, millimeter wave, and terahertz frequencies based on ultrathin metallic strictures. In details, I will introduce an ultrathin transmission line of spoof surface plasmon polaritons (SPPs) on corrugated metallic strip, which can be printed on ultrathin and flexible dielectric film to support conformal surface plasmons; an ultrathin corrugated metallic disk to support spoof localized surface plasmons (LSPs); passive SPP devices; active SPP devices for amplifying SPP signals and generating the second harmonics of SPPs; and the combined SPP and LSP circuits. Experimental results are presented for the SPP and LSP devices and circuits.

High-order spoof surface plasmon (SSP) mode on corrugated metallic surfaces can find many interesting applications, such as in imaging, sensing, transmission and enhanced radiation source, etc. In this paper, an efficient excitation method of the high-order SSP mode by using an injected electron beam on the uniform rectangular metallic grating is proposed and investigated numerically. Based on the matched wave momentum between the SSP mode and the electron beam, both the fundamental and high-order SSP modes can be excited on the structure by using a single injected electron beam. Numerical simulation results indicate that the maximum electric field intensity of the generated high-order SSP mode is about two orders higher than that of the fundamental SSP mode. In addition, the peak power of the excited high-order SSP mode is almost two times that of the fundamental SSP mode power by the same energy electron beam, which demonstrates the obvious advantage of the high-order SSP electronic excitation approach compared to the previous fundamental SSP mode excitation on the structure. The central working frequency of high-order SSP power spectrum is about three times that of the fundamental SSP power spectrum. Moreover, the influences of the injected electron beam energy on the excited SSP power spectrum are analyzed specifically. It is shown that the generated SSP power spectrum demonstrates a blue shift with the decreased working voltage of the electron beam simultaneously, with its peak power increasing. However, the working bandwidth is narrowed with decreased beam voltage, which further reveals its working mechanism of presented electronic excitation of the SSP mode. The presented studies provide a new method to excite a high-order SSP mode on the metallic grating, which can find some potential applications in high-sensitivity sensing, deep sub-wavelength waveguide, and many others in terahertz spectra.

We propose a duplexer based on spoof surface plasmon polaritons (SSPPs) transmission lines (TLs). The duplexer consists of two domino SSPP TLs with different lower cut-off and higher cut-off (asymptotic) frequencies, forming the two frequency band channels. The low frequency band channel is the domino SSPP TL with higher unit height. For the branch of the high frequency band channel, which is based on a SSPP TL with lower unit height, multiple SSPP TL sections functioning as band-stop structures are placed aside the SSPP TL to realize satisfying out-of-band rejection below the lower cut-off frequency. The two branches are fed through a common rectangular waveguide. Different transition length of the two branches as well as a shifted placement from the center is designed to achieve efficient transition for both branches at respective bands.

A highly confined spoof surface plasmon (SSP) based sensor to test the dielectric materials in microwave and terahertz (THz) frequency region is presented. The proposed sensor comprised of the main coplanar waveguide (CPW) line and the defected T-shaped SSP line at the top and bottom of the substrate, respectively. The defect in the SSP line provides stop band, and hence localization of the transmission energy at the defect location. The resonance frequency and localization of the transmission energy can primarily be tuned by varying the height of the defect and the periodicity of the SSP strip. The proposed sensor is numerically optimized and tested in the microwave and THz frequency region using the full wave electromagnetic solver (CST-MWS). The designed sensor conforms the planar geometry, which facilitates its ease of integration to the modern radio frequency (RF) and THz frequency systems. The sensor prototype is fabricated and experimentally tested in the microwave frequency region using a number of standard solid and liquid samples. It is found that the test samples with closely spaced dielectric constant can easily be distinguished using the developed SSP sensor owing to its considerably high quality factor (~165) and hence high resolution. Additionally, the fabricated compact sensor prototype has adequate sensitivity (2.84%) and small sensing area, as a result it helps to test small volume of liquid specimen (order of few microliter), which is generally the case with the biological samples due to their low volume availability.

In this paper, two novel waveguides with ELC like-shaped and Z-shaped grooves have been proposed to achieve highly efficient and strongly confined spoof surface plasmon polaritons (SSPPs) propagation. Low-dispersion bands can be realized by such structures with tight field confinement of SSPPs, resulting in size miniaturization of the proposed waveguides. Specifically, our method yields deep physical insight into the effect that the geometrically induced modifications of the supporting structure has on the dispersion properties and field confinement capabilities of SSPPs. In comparison, SSPP waveguide results presented in Aziz (2021 Results Opt. 5 100116) are given which is regarded to have stronger SSPPs field confinement as compared to previously reported different grooves shapes based SSPP waveguides. It is found that the dispersion and waveguide propagation characteristics can be directly manipulated by varying the geometrical parameters of the horizontal and slanted slits of Z-shaped and ELC-like shaped plasmonic waveguides without increasing the lateral dimension of the waveguides. Based on this waveguiding scheme, the proposed waveguides exhibit much lower asymptotic frequency of the dispersion relation and even tighter SSPPs field confinement than I-shaped plasmonic waveguide. Then, broadband transitions with a tapered metallic strip and an array of graded height ELC-like and Z-shaped units with good impedance matching and high mode conversion efficiency are designed. Fully controlled field enhancement functionality has been performed, by using such metamaterial particles in the form of grooves, decorated in SSPP waveguide. Simulated results have demonstrated that the proposed SSPP waveguides have much stronger field confinement than the highly efficient I-shaped grooves based SSPP waveguide. The proposed waveguides can be a significant contribution towards the advancement of plasmonic functional devices and integrated circuits in microwave frequencies.

Accurate permittivity characterization at terahertz frequencies is important for material analysis and device design, yet it remains challenging for small-volume samples and compact test structures. In this work, a terahertz permittivity sensor based on a spoof surface plasmon polariton (SSPPs) transmission line coupled to a backside split-ring resonator (SRR) is proposed and numerically studied. The SSPPs line is patterned on the top side of the substrate, while the SRR is etched on the backside, with the sample loaded into the SRR gap. The SSPPs mode penetrates through the substrate and excites the SRR, producing a pronounced transmission notch. Changes in the sample permittivity modulate the effective capacitance of the resonator, resulting in a monotonic shift in the notch center frequency. For relative permittivities from 1 to 8, the notch center frequency decreases from 152.1 GHz to 117.8 GHz, corresponding to a total shift of 34.3 GHz and an average sensitivity of about 4.90 GHz/εr. The minimum S21 remains within approximately −23.80 to −21.56 dB, while the Q-factor stays in the range of 94.33–108.23, indicating good spectral readability. Tolerance analysis further shows that the resonance frequency is sensitive to critical structural dimensions and layer alignment, and practical implementation is therefore more suitable for single-device calibrated frequency-shift sensing. These results demonstrate the feasibility of the proposed dual-layer SSPPs–SRR configuration for compact permittivity sensing in the terahertz regime.

The concept of chip-to-chip information propagation by spoof surface plasmon polariton (SSPP) meta-surface at terahertz frequency has been introduced of late, that promises data transfer with high bandwidth, low crosstalk, and low energy consumption. As the exotic electromagnetic properties of the metasurface derive from its designed geometric pattern and periodicity, any possible variation of fabrication process parameters may affect the design pattern and consequently the information capacity of SSPP interconnects. In this work, we have investigated the extent of performance degradation of SSPP interconnect with the statistical variation of geometric pattern of the metasurface. We also described the technique of the design of appropriate analog circuit so that the loss of signal integrity incurred by the process variation can be recuperated in real time.

This paper presents an approach to develop spoof surface plasmon polaritons (SSPPs) in substrate integrated waveguides (SIWs) with simple geometries (arrays of rectangular holes). Dispersion properties of both SSPPs based on a SIW with a rectangular hole and the traditional SSPPs based on a single-conductor metallic strip with the same hole are compared. Moreover, a smooth transition built by stepped impedance transmission lines with gradient holes is designed for high-efficiency mode conversion. Consequently, an SSPP waveguide is constructed by a SIW with uniform holes and the proposed transition, and the frequency responses of the waveguide are simulated for validation. The simulated bandwidth can reach as up to 100.1% covering a range from 4.34 to 13.05 GHz, which agrees well with the dispersion diagram.

At terahertz (THz) frequencies, spoof surface plasmon polaritons (SPPs) are guided along a metal surface with an array of holes or gratings. We have proposed metal disctype splitters with a center hole and radially placed gratings for spoof SPP waves. The three-dimensional FDTD method in cylindrical coordinates has been used to show the frequency splitting behavior. In this article, the power propagating in each direction is evaluated using the Poynting vector calculations. At 1 and 1.5 THz, the propagating power is discussed with the illustration of the spoof SPP waves guided along the disc.

Spoof surface plasmon polaritons (SSPPs) are developed in a rectangular waveguide (RWG) to realize a controllable band-pass propagation feature for millimeter-wave applications. The proposed SSPP structure consisting of a single row of corrugated metallic blocks, is embedded inside the closed structure of an RWG. The dispersion property of the RWG-based SSPP (RWG-SSPP) has tunable lower and upper cut-off frequencies, and a band-pass filter with flexible bandwidth has been designed using an RWG-SSPP structure. The proposed band-pass filter is based on an RWG-SSPP with an all-metal structure, which is of great potential for millimeter-wave applications due to small losses.

In this paper, squeezing and tunneling of spoof surface plasmon polaritons (SSPPs) are realized by introducing an effective epsilon-near-zero (ENZ) material channel between two plasmonic waveguides constructed from dielectric filled rectangular waveguide etched with deep sub-wavelength periodical transverse slots on the upper wall. The tunneling frequency can be flexibly tuned by changing the relative permittivity of the effective ENZ material and the SSPPs can tunnel efficiently through a straight or an arbitrarily bent effective ENZ channel with ultra-low loss. This simple design holds great promise in significantly increasing the propagation length or arbitrarily tuning the propagation direction of the SSPPs in the microwave and terahertz frequencies.