Terahertz Spoof Surface Plasmon Polaritons Research Articles

Reconfigurable terahertz stretchable spoof surface plasmon polariton waveguide for filter applications

In this paper, a reconfigurable terahertz spoof surface plasmon polariton (SPP) waveguide is proposed on a stretchable polydimethylsiloxane (PDMS) substrate. The SPP unit incorporates a folded stub and a conventional V-shaped SPP groove, enhancing the equivalent capacitance and consequently reducing the cutoff frequency. The cutoff frequency of the proposed SPP unit can be tuned from 285 to 390 GHz with stretchable factors of 1 ∼ 1.2, thereby achieving a reconfigurable operating frequency. The horizontal dimension of the proposed SPP waveguide can be tuned from 6.36 mm to 7.12 mm. Moreover, the SPP waveguide can generate transmission continuous phase shifts of −30°, −60°, −90°, and −120° with stretchable factors of 1.05, 1.1, 1.15, and 1.2, respectively, in the 150–190 GHz band. Applying the characteristic mode theory, a split ring resonator (SRR) functions as the equivalent magnetic dipole, which remains unaffected by stretchable deformation. When loaded with four SRR cells, the proposed SPP waveguide generates a tunable passband with a fixed notched frequency at 193 GHz. Another stretchable SPP resonator serves as the equivalent electric dipole, operating from 284 GHz to 256 GHz in 1∼1.2 stretchable states. By loading three SPP resonators, the SPP waveguide can achieve a passband for the initial state, and a tunable stopband is introduced under 1.1 and 1.2 stretchable states. The proposed stretchable method provides a promising solution for planar terahertz components and systems with reconfigurable functions.

Terahertz plasmonic functional devices enabled by multimode interference

Terahertz integrated devices have attracted great attention due to their potential applications in communication systems. Although some functional devices based on terahertz spoof surface plasmon polariton (SPP) waveguides have been studied, multimode waveguides and multimode interference mechanism have rarely been explored. This work investigates several key functional plasmonic devices based on multi-SPP mode interference in the terahertz regime. The feasibility of multimode transmission on terahertz plasmonic waveguides and control of the propagation modes by varying the waveguide parameters are first investigated. Then, the interference mechanism between two or more propagation modes and the self-imaging principle are applied to plasmonic waveguides. Finally, a series of terahertz plasmonic functional devices such as a coupler, wavelength diplexers, and cascaded devices are proposed and simulated by exploring different interference lengths, frequencies, and cascades via the self-imaging principle. These devices have good performance in transmission response, crosstalk, bandwidth, and footprint, and are expected to play an important role in the development of terahertz on-chip communication systems.

Ultracompact terahertz plasmonic mode division multiplexer.

In this Letter, an ultracompact terahertz (THz) mode division multiplexer based on THz spoof surface plasmon polaritons (SPPs) is proposed. Compared with traditional optical multiplexing devices, the proposed mode multiplexer can be designed with a reduced footprint by exploiting more degrees of freedom in the parameters of the unit cell, namely a rectangular metallic pillar. The ultracompact mode division multiplexer can simultaneously support the propagation of four mode channels: the TM0, TM1, TM2, and TM3 modes. Then, we numerically evaluate the performance of a cascaded plasmonic mode division circuit composed of a mode multiplexer and demultiplexer. The cross talk and excess loss of the whole circuit are lower than -15 dB and 3.7 dB, respectively, for all four mode channels at a center frequency of 0.65 THz. The footprint of the whole device is about 27 × 2.3 mm and the length of each coupling region is about 2.7 mm. For the first time, to the best of our knowledge, a mode division multiplexer based on THz spoof SPPs is reported, which will form core devices for future THz on-chip multimode communication systems.

Investigation of the Transitions for Coplanar Waveguide to Terahertz Spoof Surface Plasmon Polariton Waveguides

In this paper, we present the investigation and design of high-performance transitions from coplanar waveguide (CPW) to the single-conductor, corrugated waveguides, namely terahertz spoof Surface Plasmon Polariton Waveguides (SSPP WGs). In contrast with the previous studies in the literature on the CPW to SSPP WG transitions, we propose a novel methodology to examine the relationship between the guided wavenumber and momentum matching, which is based on analytical calculations, together with the effects of the flaring grounds on the transition performance. This novel methodology allows us to design a low-loss transition circuit for any given CPW and SSPP dimensions, which is demonstrated for the first time in the literature. We verify the proposed methodology by the design, fabrication, and measurement of several transition circuits. The measurement results show that the insertion losses of the proposed transition circuits can be as low as -1.2, -1, and -2.4 dB at 0.25, 0.275, and 0.3 THz, respectively, which brings a significant improvement of 0.6 to 2.3 dB in 0.25-0.3 THz band, compared to the previously reported studies in the literature for the terahertz band.

Optimization of Terahertz Spoof Surface Plasmon Polariton Waveguides for Maximum °/dB Performance

In this paper, we present an optimization study for the terahertz Spoof Surface Plasmon Polariton waveguides (THz-SSPP WGs) for the first time in the literature. The optimization study employs a selection method for the optimum dimensions for the THz-SSPP WGs, which targets maximizing the insertion phase and minimizing the insertion loss levels, and hence, finding the optimum set of dimensions that result in the best °/dB performance. The method first utilizes the analytical examination of the dispersion diagram to estimate a target range of dimensions for minimum insertion loss, which is then scrutinized by numerous simulations of the THz-SSPP WGs within the selected dimension range. In order to verify the optimization method, a set of THz-SSPP WGs are designed using the proposed method, fabricated, and measured in the 0.22-0.32 THz band. The measurements result in a record-low insertion loss per unit length of 0.75 dB/mm with the optimized THz-SSPP WGs. Additionally, the utilization of the optimized THz-SSPP WGs to compose a terahertz phase shifter enables a record-high figure of merit (FoM) of 102.8 °/dB, which is the most fundamental performance measure of the terahertz phase shifters. It should be indicated that both insertion loss per unit length and FoM levels make THz-SSPP WGs superior to the state-of-the-art planar terahertz waveguides. The presented method provides the most concrete evidence for the potential of the planar SSPP WGs for the terahertz band.

Spoof Surface Plasmon Polariton Delay Lines for Terahertz Phase Shifters

The terahertz gap attracted interest of researchers with its captivating properties such as its potential for ultrawideband communication and high-resolution imaging applications, which require robust and efficient terahertz components and circuits. Terahertz spoof surface plasmon polaritons are perfect alternatives to implement terahertz integrated circuits. We present the first-time demonstration of the terahertz spoof surface plasmon polariton delay lines for terahertz phase shifters, which present an unprecedented insertion phase versus loss performance. The electrical lengths of the proposed delay lines are tuned just by changing the corrugation depths, which allows the designer to keep the physical length of the delay lines constant. In order to verify the proposed idea, four different delay lines having the same length of 1052.5 μm are designed, where the electrical lengths of the delay lines are selected to represent the four different phase states of a 2-bit phase shifter. The measurement results of the fabricated terahertz delay lines show almost perfect phase accuracy and very good agreement with the simulations, having an ultra-low average phase error of 0.6%, and average insertion and return losses of −2.3 and −18 dB, respectively. These measurements result in, to the best of our knowledge, a record-high figure-of-merit of 90°/dB compared to all other kinds of planar waveguides at 0.3 THz. The proposed delay lines emerge as a very high-performance and promising candidate for all the waveguide-based components that are crucial for the terahertz integrated circuits.

Routing of strongly confined terahertz spoof surface plasmon polaritons on metasurfaces along straight and curved pathways with subwavelength width.

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.

Terahertz Spoof Surface Plasmon Polariton Waveguides: A Comprehensive Model with Experimental Verification

Spoof surface plasmon polariton waveguides are perfect candidates to enable novel, miniaturized terahertz integrated systems, which will expedite the next-generation ultra-wideband communications, high-resolution imaging and spectroscopy applications. In this paper, we introduce, for the first time, a model for the effective dielectric constant, which is the most fundamental design parameter, of the terahertz spoof surface plasmon polariton waveguides. To verify the proposed model, we design, fabricate and measure several waveguides with different physical parameters for 0.25 to 0.3 THz band. The measurement results show very good agreement with the simulations, having an average and a maximum error of 2.6% and 8.8%, respectively, achieving 10-to-30 times better accuracy than the previous approaches presented in the literature. To the best of our knowledge, this is the first-time investigation of the effective dielectric constant of the terahertz spoof surface plasmon polariton waveguides, enabling accurate design of any passive component for the terahertz band.

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.

Ultra-sensitive fluid fill height sensing based on spoof surface plasmon polaritons

We analyzed fluid fill height sensing supported by terahertz spoof surface plasmon polaritons (SPPs). The existence of spoof SPPs was proved experimentally and was discussed from perspective of dispersion and field distribution. For the application, we designed a fluid fill height sensor based on spoof SPPs, which is much more sensitive than previous work based on parallel plate waveguide (PPWG). For example, resonant shift of spoof SPPs is almost four times larger than the shift of PPWG at the height of 250 μm. This fluid fill height sensor calibrated the fluid volume in grooves, which contributes to improving the accuracy of high refractive index sensing. The experiment is in good agreement with simulation. Such proposed fluid height sensor has another advantage of being suitable for the fluid with refractive index higher than prism, which further benefits spoof SPPs refractive index sensing, such as blood detection in future.

Quasi-three-dimensional post-array for propagation and focusing of a terahertz spoof surface plasmon polariton

This paper presents a quasi-three-dimensional post-array designed to propagate a terahertz spoof surface plasmon polariton (terahertz spoof SPP) with confinement. A transmission line making use of a terahertz spoof SPP is a promising device in the terahertz wave band, and there are many previous reports of two-dimensional structures. Three-dimensional structures provide sophisticated designs for transmission lines propagating a terahertz spoof SPP. Eigenmode analysis is used to derive a dispersion diagram for one post with boundary conditions extracted from the full model. The propagation frequency of the terahertz spoof SPP increases with lower heights or smaller diameters, and that remains virtually unchanged for post-spacing and thickness of a substrate. The analysis of the full model confirms the confinement of a terahertz spoof SPP vertically on the post-array. The magnitude of the electric field is strong around the top and bottom and weak at approximately one-third height. The terahertz spoof SPP is confined in the space around the post-array as well as a substrate, while it is confined only on substrates in conventional two-dimensional structures. The designed post-array can control the three-dimensional focusing of a terahertz spoof SPP in an arbitrary volume of space.

Dynamic Terahertz Spoof Surface Plasmon–Polariton Switch Based on Resonance and Absorption

In this brief, a terahertz (THz) switch consisting of perfect conductor metamaterials is demonstrated both theoretically and numerically. Specifically, we build a THz logic block based on waveguide-cavity-waveguide, thus providing a strong electromagnetic field accumulation inside a small cavity. Furthermore, this brief shows that the subwavelength metallic cavity can confine light for a long time within a very small area. Therefore, an arbitrarily designed cavity with the high quality factor Q and the small effective area Aeff can be easily utilized for an efficient THz switch. These promising results can be used to provide new switch types and open up new vistas in the area of THz subwavelength optics.

Active Terahertz Spoof Surface Plasmon Polariton Switch Comprising the Perfect Conductor Metamaterial

The feasibility of realizing an active terahertz (THz) switch by utilizing artificially corrugated perfect conductor metamaterials is reported in this paper by demonstrating that the strongly localized THz spoof surface plasmon polariton (SSPP) modes can be easily controlled by changing the refractive index of dielectrics inside a longitudinal metallic structure with a periodic array of grooves. Specifically, this paper shows that incorporation of electrooptical material such as a nematic liquid crystal into the plasmonic gap leads to a highly compact and efficient THz switch that is activated by a low-voltage control-signal. The optimal design of the SSPP switch enabled by this novel method shows: 1) a strong subwavelength SSPP localization; 2) relatively high extinction ratio; and 3) small damping attenuation. Furthermore, the design flexibility associated with simple micrometer-scale architecture provides a promising method toward controlling or steering the subwavelength THz signal in the future SSPP-based compact digital circuit.