Terahertz Spoof Research Articles

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.

Regenerated amplification of terahertz spoof surface plasmon radiation

Regenerated amplification induced by a Fabry–Perot (F–P) cavity is introduced to enhance the interaction efficiency of the free-electron-driven spoof surface plasmon. A direct-current electron beam flies above the meta-grating surface and seeds noise-level plasmonic waves. This weak signal experiences multiple back-and-forth regenerated amplifications in the F–P cavity loaded grating system, and the system outputs a pulsed radiation when the signal leaves the cavity. When compared with the condition without the F–P cavity, the equivalent beam-wave interaction length is effectively extended, and the interaction efficiency is improved by orders of magnitude. The proof-of-principle scheme is verified in both backward-wave and forward-wave modes using the particle-in-cell simulation. This scheme is promising for developing high-efficient on-chip terahertz free-electron radiation sources.

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.

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.

Study of terahertz spoof surface plasmons on subwavelength gratings with dielectric substance in grooves

Terahertz (THz) plasmonic devises based on periodical corrugated structures are promising for sensing applications in biology and medicine but have not been developed so far because spoof surface plasmons (SSPs) on such structures were studied insufficiently in the THz spectral range. In the paper, the propagation of THz SSPs along one-dimensional subwavelength rectangular plane gratings with a dielectric substance in the grooves was studied and optimal parameters of gratings (period, aspect ratio and groove depth) for sensing of dielectric were found. First grating samples were made and tested using the THz radiation of the Novosibirsk free electron laser.

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.

Observation of flat band for terahertz spoof plasmons in a metallic kagomé lattice

We study the dispersion relation of a metamaterial composed of metallic disks and bars arranged to have kagome symmetry and find that a plasmonic flat band is formed by the topological nature of the kagome lattice. To confirm the flat-band formation, we fabricate the metamaterial and make transmission measurements in the terahertz regime. Two bands formed by transmission minima that depend on the polarization of the incident terahertz beams are observed. One of the bands corresponds to the flat band, as confirmed by the fact that the resonant frequency is almost independent of the incident angle.

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.