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Retroreflectors that can accurately redirect the incident waves in free space back along their original channels provide unprecedented opportunities for light manipulation. However, to the best of our knowledge, they suffer from either the bulky size, narrow angular bandwidths, or time-consuming postprocessing, which essentially limits their further applications. Here, a scheme for designing ultrathin all-angle real-time retroreflectors based on hyperbolic plasmonic metasurfaces is proposed and experimentally demonstrated. The physical mechanism underlying the scheme is the orthogonality between the traveling waves in free space and the canalized spoof surface plasmon on the hyperbolic plasmonic metasurfaces, which guarantee their high-efficiency and all-angle mutual conversion. In this case, the strong confinement characteristic that benefited from the enhanced light-matter interaction enables us to route and retroreflect the canalized spoof surface plasmon with extremely thin structures. As proof of the scheme, a retroreflector prototype with a thickness approximately equal to the central wavelength is designed and fabricated. Further experimental investigation obtains a half-power field of view up to 53° and a maximum efficiency of 83.2%. This scheme can find promising applications in target detection, remote sensing, and diverse on-chip light control devices.

Terahertz (THz) vortex beams carrying orbital angular momentum (OAM) with high purity and tunable topological charge (TC) will undoubtedly bring extraordinary capacities for advanced imaging or communication systems. We propose a convenient and efficient method to generate a tunable and broadband vortex beam that is excited by superradiant Smith Purcell radiation (SSPR) on a helical grating. This scheme fully explores the advantages of natural broadband evanescent wave carried by the electron beam and the Bloch’s theorem regulated helical periodic systems. An explicit relation is established between the index of the spatial harmonic wave on the grating and the topological charge of the vortex beam. The electron energy in the SSPR can be customized to ignite the specific spatial harmonics and manipulate the OAM beam, accordingly. The separated radiation region also promises the high purity of the OAM spectrum. The harmonic excitation reduces the communality between the wavelength and the device size and alleviates the difficulty of device fabrication in the THz band. The proposed vortex scheme can not only be handily scaled to microwave and mid‑infrared regions, but also bring possibilities to applications based on compact tunable vortex beam sources.

A three-port plasmonic array is proposed to achieve a dual-field superfocusing using the forward-propagation method based on spatial Fourier transform. The algorithm of fast Fourier transform is used to facilitate the design process. Both the electric and magnetic fields can be superfocused on the focal plane of z = 0.1 λ ( λ is the free-space wavelength) based on an effect of radiationless electromagnetic interference. The full widths of the half-maximum of both two fields are 0.060 λ and 0.072 λ , respectively. Besides, the metrics of the dual-field superfocusing are 1.67 and 1.39. The proposed theory can simplify the design process of superfocusing, and the plasmonic array with brief three ports can sufficiently facilitate the construction of the desired source. This letter can be beneficial for the development of beam steering, sophisticated holography, super-resolution imaging, and other applications of near- or far-field devices.

Recent advances in spoof surface plasmon (SSP) have offered enormous opportunities for engineering electromagnetic waves at the subwavelength scale. However, the fundamental limitation of the extremely finite space channels on exciting SSP challenges their practical applications. Here, to the best of our knowledge, a new method that can realize high-efficiency infinite-channel SSP excitation based on hyperbolic plasmonic metasurfaces is proposed. Free-space propagating waves impinging from arbitrary directions can be coupled to SSP owing to the theoretical perfect wave vector match between the modulated incident waves and the hyperbolic plasmonic metasurfaces. A proof-of-method metacoupler working at 8 GHz is designed and fabricated with the novel functionality numerically and experimentally verified by the consistent guide wavelength with theory. Further experimental investigation reveals a total coupling efficiency exceeding 50% for the impinging waves with incidence angles ranging from −44° to 44°. The proposed methodology provides a new route to design multichannel antennas and detectors.

A 1 bit single-polarization broadband space-time-coding digital metasurface element with a zero cross polarization is proposed. This element consists of a modified patch, an electronic switch, a dc-bias network, a ground plane, and two-layer dielectrics. The upper patch with edge loading and centered square hole is fully optimized to achieve broadband and stable phase difference between two states. By electronically controlling the diode, the element achieves a 1 bit phase shift with a relative bandwidth of 20% within 180° ± 30° from 7.71 to 9.43 GHz. Moreover, a negligible cross polarization is obtained due to the uniaxial symmetry of the structure, and the amplitude loss is less than 0.8 dB for two states. The experimental results measured in a rectangular waveguide are well matched with the simulations, further demonstrating validity of the proposed element.

Terahertz biosensing provides a suitable method to identify biomolecules due to their rich spectral fingerprint in this electromagnetic region. However, owing to the limitations of terahertz sources and detectors, the signal is weak and requires to be enhanced by particular technologies. In this paper, we propose a terahertz plasmonic biosensor based on Goos-Hanchen effect in graphene. Sample sensing can be realized by measuring the Goos-Hanchen shift of the reflected light. Numerical simulations show that the sensitivity of this biosensor can reach a high value up to $2.5\times 10^4\mu$ m/RIU. This graphene plasmonic configuration combined with Goos-Hanchen effect provides a novel high-sensitivity approach for future terahertz biosensing applications.

In this work, we propose an angle-independent beam splitter based on spoof surface plasmon. Arbitrary incident waves can be divided in half, and scatter towards two specific and independent directions. The proposed beam splitter consists of the upper periodic metal cylinder and the metal grating below. The former is placed to modulate the incident wave and the spoof surface plasmon with the specified wave vector, and the latter is used to support the excited spoof surface plasmon. To verify the proposed functionalities, we design three beam splitters with different splitter angles and calculate their field distributions under the illumination of the same Gaussian beam. The ideal numerical results agree well with the theory.

Spoof surface plasmons (SSPs) can manipulate electromagnetic waves with subwavelength prints, which offers possibilities for many applications. To generate the SSPs propagating on a two-dimensional (2D) scale, the all-dielectric or dielectric-metal metasurfaces are generally exploited. However, the power capacity of the all-dielectric or dielectric-metal metasurfaces is relatively low. In this work, a 2D groove metal array is utilized to generate the SSPs. The dimension of the grooves is changed to possess the SSPs propagating on it. Based on this principle, a 2D half-Maxwell-fisheye lens is proposed utilizing the groove metal array, on which a spot on one side of the lens can diverge to a plane wave on the exit facet in the simulation. The all-metal half-Maxwell-fisheye lens is capable to be employed for high-power application scenarios.

Based on the effect of radiationless electromagnetic interference (REI), a dual-foci superfocusing is achieved using a plasmonic array with a physically realizable extreme size of 0.195λ, where λ is the free-space wavelength). The full widths at half maximum (FWHM) of both electric and magnetic fields are 0.061λ and 0.052λ at a distance of 0.1λ.

In this paper, a displacement sensor with an electrically extremely small size and high sensitivity is proposed based on an elaborately designed metamaterial element, i.e., coupled split-ring resonators (SRRs). The sensor consists of a feeding structure with a rectangular opening loop and a sensing structure with double-layer coupled SRRs. The movable double-layer structures can be used to measure the relative displacement. The size of microwave displacement sensors can be significantly reduced due to the compact feeding and sensing structures. By adjusting the position of the split gap within the resonator, the detection directions of the displacement sensing can be further expanded accordingly (along with the x- or y-axis) without increasing its physical size. Compared with previous works, the extremely compact size of 0.05λ0 × 0.05λ0 (λ0 denotes the free-space wavelength), a high sensitivity, and a high quality factor (Q-factor) can be achieved by the proposed sensor. From the perspective of the advantages above, the proposed sensor holds promise for being applied in many high-precision industrial measurement scenarios.

Reflective metasurfaces are utilized as a bridge linking propagating waves and surface waves. However, the effective constitutive parameters (permittivity and permeability) cannot be retrieved by conventional theory without transmission coefficients. In this paper, an innovative method based on a quasi-one-port network model is proposed to robustly retrieve the effective electromagnetic parameters ( $\varepsilon _{\mathrm {eff}}$ , $\mu _{\mathrm {eff}}$ , $n_{\mathrm {eff}}$ , and $z_{\mathrm {eff}}$ ) for reflective metasurfaces with isotropic unit cells. The validity of the method is demonstrated by imping TE-polarized terahertz waves normally and obliquely (10°) on the multiple-layer unit cells. The effective boundary of the multiple-layer cells is determined by minimizing the deviation of the effective refractive indices from 0.1018 to 0.0260 using a genetic algorithm, which increases the consistency of the retrieval results. The sensitivity of the effective constitutive parameters to the accuracy of reflection coefficients and nonzero incident angle $\theta _{i}$ is also discussed. This method is beneficial for the design of reflective metasurfaces or multichannel reflectors, the coupling of spoof space plasmon polaritons, and other applications of anomalous reflection.

Inspired by the capability of structured illumination microscopy (SIM) in subwavelength imaging, many researchers devoted themselves to investigating this methodology. However, due to the free-propagating feature of the traditional structured illumination fields, the resolution can be only improved up to two-fold of the diffraction-limited microscopy. Besides, most of the previous studies, relying on incoherent illumination sources, are restricted to fluorescent samples. In this work, a subwavelength non-fluorescent imaging method is proposed based on the illumination of terahertz traveling waves and plasmonics. Excited along with a metal grating, the spoof surface plasmons (SSPs) are employed as one of the illuminating sources. When the scattering waves with the SSPs illumination are captured, the sample’s high-order spatial frequencies (SF) components are already encoded into the obtainable low-order ones. Then, a modified post-processing algorithm is exploited to shift the modulated SF components to their actual positions in the SF domain. In this manner, the fine information of samples is introduced to reconstruct the desired imaging, leading to an enhancement of the resolution up to 0.12λ0. Encouragingly, the resolution can be further enhanced by attaching extra illumination of SSPs with an elaborately selected frequency. This method holds promise for some important applications in terahertz non-fluorescent microscopy and sample detection with weak scattering.

As a bridge linking propagating waves and surface waves, reflective metasurfaces play a significant role to manipulate electromagnetic (EM) waves. Without transmission coefficients, however, the effective constitutive parameters (e eff , μ eff , n eff , z eff ) of reflective metasurfaces cannot be retrieved by conventional theory. In this work, an innovative method using single S-parameters (only S 11 ) is proposed to retrieve the effective EM parameters for reflective metasurfaces. The validity of the method is demonstrated by imping TE-polarized EM waves normally and obliquely on the single- and multiple-layer unit cells. The retrieved results match well with each other corresponding to the assumption of passive homogeneous mediums. This method is beneficial for the design of reflective metasurfaces and other applications of anomalous reflection.

A design of real-time tunable-focus reconfigurable active metasurfaces is proposed in this paper. With the element space of 16 mm (0.458λ), a 1-bit real-time reconfigurable active metasurface element is successfully designed to work at 8.595 GHz. From 7.71 GHz to 9.43 GHz, the element realized a 1-bit phase shift with a relative bandwidth of 20% within 180o ± 30o. Moreover, a negligible cross polarization is obtained and the amplitude loss is less than 0.8 dB for two states. Furthermore, the reflective metasurface is designed with an aperture size of 512mm×512mm (32×32) and aperture efficiency of 43.7% for the tunable beam focusing. By effectively designing the states of each element on the metasurface array, a single-beam focusing on θ = 0° and dual-beam focusing on θ = ±15° are achieved, respectively. Finally, the full wave simulation is performed to demonstrate to the functionalities and effects of the proposed metasurfaces, whose results are in good agreement with the theoretical design.

Manipulation of spoof surface plasmons (SSPs) has recently intrigued enormous interest due to the capability of guiding waves with subwavelength footsteps. However, most of the previous studies, manifested for a single functionality, are not suitable for multifunctional integrated devices. Herein, a bifunctional Luneburg-fisheye lens is proposed based on a two-dimension metal pillar array. Firstly, by tuning the geometric dimension of the metal pillars in the array, its ability to precisely manipulate the excited SSPs along one direction is confirmed, achieving subwavelength focusing and imaging with the resolution up to 0.14 times the wavelength. Then, separately controlling the propagation of the SSPs along the orthotropic directions is further implemented, and the bifunctional Luneburg-fisheye lens is realized. The bifunctional lens is characterized as a Luneburg one along the x-axis, whereas in the y-axis, it presents the properties of a Maxwell fisheye lens. The experimental results almost immaculately match with the simulation ones. This bifunctional lens can validly reduce the system complexity and exert flexibility in multifunctional applications, while the proposed metal pillar-based design method broadens the application range of gradient refractive-index lens in the microwaves, terahertz, and even optical ranges., 9 pages, 5 figures

Recent advances in hyperbolic metamaterials have spurred many breakthroughs in the field of manipulating light propagation. However, the unusual electromagnetic properties also put extremely high demands on its compositional materials. Limited by the finite relative permittivity of the natural materials, the effective permittivity of the constructed hyperbolic metamaterials is also confined to a narrow range. Here, based on the proposed concept of structure-induced spoof surface plasmon, we prove that arbitrary materials can be selected to construct the hyperbolic metamaterials with independent relative effective permittivity components. Besides, the theoretical achievable ranges of the relative effective permittivity components are unlimited. As proofs of the method, three novel hyperbolic metamaterials are designed with their functionalities validated numerically and experimentally by specified directional propagation. To further illustrate the superiority of the method, an all-metal low-loss hyperbolic metamaterial filled with air is proposed and demonstrated. The proposed methodology effectively reduces the design requirement for hyperbolic metamaterials and provides new ideas for the scenarios where large permittivity coverage is needed such as microwave and terahertz focus, super-resolution imaging, electromagnetic cloaking, and so on.

In this work, aiming at reflective metasurfaces, an innovative method based on a one-port network model is proposed to retrieve the constitutive effective parameters (CEPs) only using S 11 . The theory is demonstrated by calculating the CEPs for different incident angles and multiple-layer metasurfaces. The results match well with each other.

In this work, a terahertz near-field metasurface is proposed to superfocus both electric and magnetic fields. To achieve a sharp dual-field superfocusing, a sort of minuscule unit cell is designed to elaborately arrange the array accordingly. The full widths at half-maximum (FWHMs) of the electric and magnetic fields are $0.079\lambda$ and $0.052\lambda$ at a distance of $0.1\lambda$ . The proposed dual-field superfocusing holds potential benefits for multiple-target sensing, extreme subwavelength lithography, super-resolution imaging, and data storage and access.

High power and frequency stability of electromagnetic wave input are required in the DNP-NMR system. It is necessary to monitor the electromagnetic wave energy in the transmission line in real-time to judge the magnitude and stability of the energy transmitted to the NMR spectrometer. However, the energy in the transmission system is too large to be directly measured in the transmission line. A part of the energy must be extracted to facilitate the measurement. We design a directional coupler by placing a wire array in the middle of the three-port circular corrugated waveguide for real-time extraction of transmitted energy. In the frequency range of 290 – 360 GHz, 0.05% energy can be stably coupled to port 3, so it is convenient for the energy detection. The S 11 parameter of the designed directional coupler can below −20dB from 290 to 360 GHz, and even −41dB at the operating frequency of 330 GHz.

In order to exploit orbital angular momentum (OAM) transmitting data, multiple-mode OAM waves should be generated preferably by a fixed antenna. In this paper, a programmable 1-bit digital coding metasurface is presented for the generation of multi-mode converged OAM waves. The proposed reconfigurable element consists of a microstrip patch, a PIN diode, and a DC biasing circuit. Through electronically controlled the PIN diode ON and OFF, two states with a phase difference of 180°, i.e., "0" and "1" states, can be realized. By properly designing varying states of each element on the metasurface array, the OAM waves can be generated and dynamically reconfigured among l = 0 and l = 1 modes at 7.77GHz. As a consequence, it is potential in wireless communication applications.

In this paper, a sort of dual-field superfocusing is demonstrated with a plasmonic array based on metal waveguides. A theory of steering hybrid evanescent modes is used to design the plasmonic array. Both electric and magnetic fields are converged to a focus spot at a distance of 0.11. The full widths at halfmaximum (FWHMS) of the two fields are 0.04λ and 0.069λ with the superfocusing metrics of 2.5 and 1.45. Dual-field superfocusing is beneficial for super-resolution imaging, extreme subwavelength lithography, and noncontact sensing.

Electric and magnetic modes establish the basis of the localized spoof surface plasmons (LSSPs), both of which are transverse electric (TE) modes. In this paper, the concept of transverse magnetic (TM) modes of LSSPs is proposed, and the double-layer planar-circular-grating resonator is demonstrated to support the TM modes in addition to the TE modes. In numerical simulations, the TE and TM modes were excited by a plane wave and an aperture, respectively, and their resonant frequencies and electromagnetic field distributions were demonstrated to be different. The electric field of the TE mode is parallel to the grating, while the magnetic field of the TM mode is parallel to the grating. In experiments, multiple TE and TM LSSPs modes were excited by a magnetic coupler. The magnetic coupler can be rotated to distinguish the TM modes from the TE ones, which provides conclusive evidence of the existence of the TM modes. Compared with the TE LSSPs modes, the TM LSSPs modes have the advantages of high sensitivity to the distance between layers and high quality factors (Q-factors), which hold promise for providing improvements on compact applications of LSSPs. Based on the proposed concept, therefore, the classification standard of the LSSPs is expanded to make it more scientific and comprehensive.

Recently, ultrathin localized spoof surface plasmon (LSSP) resonators are found to have intrinsic defects of relatively low quality factors (Q-factors) because of unavoidable material and radiation losses. In this paper, multilayer structures of planar-circular-grating resonators and their magnetic-coupling schemes are proposed to achieve effective excitation of high-Q LSSPs modes. By adopting the multilayer structures with air between the layers, the power dissipation effected by both material and radiation losses is significantly suppressed. Experimental results show that the Q-factors could reach more than 200 and the excitation efficiencies could reach more than 90%. Numerical simulations show the distribution of the electromagnetic field and illustrate the principle of magnetic coupling. Besides, the Q-factors of resonators with different structural parameters were measured and analyzed. This study aims to provide some inspirations on planar gyro-devices and to improve the performance of existing applications, such as sensors and filters.

In this work, a near-field metasurface is designed to overcome diffraction limit and superfocus terahertz waves by elaborately arranging the proposed unit cells with super small electric sizes. The full width at half-maximum beam width reaches up to $0.067 \lambda_{0}$ (free-space wavelength) at a distance of $0.073 \lambda_{0}$ at 0.437 THz. The near-field metasurface is beneficial for subwavelength resolution imaging, photolithography, non-contact sensing, and other applications in the near field.

In this work, an alternative method is proposed to realize terahertz far-field super-resolution imaging. Due the unique dispersive properties of the spoof surface plasmon (SSP), the spatial spectrum of the targets can be extracted and conserved by the adjacent SSP structure through a frequency canning operation. With the assist of a broadband coupler, the sampled information can be sent and retrieved in the far-filed. Then, the image with subwavelength resolution is constructed by inverse Fourier Transform. Although the resolution of $0.1\lambda$ is verified, the resolution can be further enhanced by optimizing the SSP structure. This work may find applications in non-destructive testing and biomedical imaging.

Based on the combination of propagation phase with geometric phase, to the best of our knowledge, a new reflective metasurface which can control the direction of reflected waves and excite spoof surface plasmon polariton (SSPP) simultaneously is proposed and numerically demonstrated for the circular polarization incident waves. The incident waves can be converted into SSP with longitudinal wave vector $k_{x}$ = $1.2 k_{0}$ or anomalously reflected with reflection angle $\theta _{r}$ = 13.9° under the illumination of the left or the right circular polarization beam, respectively. The respective converting efficiency of the left and right circular polarization can reach up to 70% and 82% at the frequency of 0.3 THz. Our study may open up new routes for polarization-related antennas, detectors and other practical terahertz devices.

In this Letter, we propose and experimentally demonstrate a simple but efficient method to excite spoof surface plasmons (SSP) through periodic metallic cylinders at microwave frequencies. The rigorous multiple scattering theory indicates that most of the incident propagating waves can pass the cylinders and be converted into the desired harmonics. Furthermore, by tuning the incident angle, controlling the directions of the excited SSP at different frequencies is also realized. The numerical simulations achieve a bidirectional efficiency of 90% at 9.68 GHz and unidirectional efficiency of 79%–85% at 7.46–9.7 GHz, when the incident angle changes from 60° to 120°. Meanwhile, the maximum contrast ratio between the powers of SSP launched in two opposite directions can reach up to 34 dB. The experimental results under 90° and 77.5° illuminations at 9.68 and 8.56 GHz provide strong support for the coupling mechanism. This method may provide technique support in the SSP-based communication and imaging systems.

Spoof surface plasmon (SSP) meta-couplers that efficiently integrate other diversified functionalities into a single ultrathin device are highly desirable in the modern microwave and terahertz fields. However, the diversified functionalities, to the best of our knowledge, have not been applied to circular polarization meta-couplers because of the spin coupling between the orthogonal incident waves. In this paper, we propose and numerically demonstrate a terahertz spin-decoupled bifunctional meta-coupler for SSP excitation and beam steering. The designed meta-coupler is composed of a coupling metasurface and a propagating metasurface. The former aims at realizing anomalous reflection or converting the incident waves into SSP under the illumination of the left or right circular polarization waves, respectively, and the latter are used to guide out the excited SSP. The respective converting efficiency can reach 82% and 70% at 0.3THz for the right and left circular polarization incident waves. Besides, by appropriately adjusting the reflection phase distribution, many other diversified functionalities can also be integrated into the meta-coupler. Our study may open up new routes for polarization-related SSP couplers, detectors, and other practical terahertz devices.

In this paper, a linear polarization-controlled (LPC) spoof surface plasmon polaritons (SSPPs) meta-coupler with high efficiency and directionality is proposed and demonstrated in the terahertz (THz) regime. By changing the polarization (TE or TM polarization) of the incident electromagnetic (EM) waves, the corresponding SSPPs can be directionally excited towards opposite directions. The meta-coupler is composed of a coupling metasurface (CM) and two propagating metasurfaces (PMs) on both sides of CM. Under the illumination of specified polarization Gaussian beam, the unit cells of the CMs have the similar electric field distribution with that of PMs, which can realize a smooth and efficient transition between the surface waves and SSPPs. The respective excitation efficiency of the TE and TM polarization can reach up to 87% and 90% at the frequency of 0.355 THz. Besides, the PMs are designed to support the propagation of the specified TE or TM mode SSPPs. This guarantees a relatively high directionality (> 24 dB) in the frequency range of 0.325-0.375 THz. Our study may open up new routes for polarization-related antennas, detectors and other practical THz devices.

An efficiency way to enhance the excitation efficiency of spoof surface plasmons (SSP) is proposed and demonstrated in this work. Under the rigorous theoretical deduction, it is found that almost perfect transmission through transmissive metallic grating can be realized when the Fabry–Perot (FP) resonance occurs. A significantly high conversion efficiency 92% between incident waves and surface waves is achieved by transmissive metallic gratings under the situation of FP resonances. Further studies show that tuning the direction of excited SSP and compressing the thickness of coupler are also realized by tilting the incident waves and filling high-index dielectric, respectively. This work may provide technical supports for plasmonic applications.

Terahertz wireless communication has emerged as a fast-growing research area. Routing, which is designed to control the propagation of terahertz wave, is of great importance for building the terahertz all-optical communication network. Metamaterials provide a good method for manipulating the terahertz wave, which can be used to realize the wave router. To steer the input wave in different directions, for example negative refraction angle, some structures have been proposed, such as stacked structures and nanowire arrays. However, in these metamaterials, the incident wave is generally limited to transverse magnetic polarization. Here, a new terahertz routing method for transverse electric wave is put forward with using graphene magnetic metamaterials. Theoretical analysis is performed to show that this metamaterial has negative components not only in effective permittivity tensor but also in effective permeability tensor. Routing is realized by topological transitions between different dispersion regimes in this metamaterial. Our finding provides a new candidate for signal routing of terahertz wave.

Based on phased-gradient metasurfaces, an efficient method is developed to convert waveguide modes in a parallel-plate waveguide. An effective wavevector is imparted to the incident mode at each interaction with matesurfaces, resulting in the mode conversion. The numerical simulation indicates that the efficiency and purity of TE1-to- TE2 conversion can significantly reach up to 99.98% and 99.99% at 9GHz. The conversion between TM3 and TM4 is also achieved, which gives a fair verification of the polarization insensitivity. This work can provide technical guidance for compact modes control.

A gradient index (GRIN) lens is proposed and demonstrated for efficiently focusing incident waves in terahertz regime. This lens is composed of three layers of metamaterial based on annular slots, allowing a large variation of the effective refractive index in both two bands. Thus we manage to design a dual-band focusing lens which can focus the terahertz radiation to a spot size smaller than the wavelength.

A new scenario to realize superfocusing of terahertz waves based on a gradient index (GRIN) metasurface is proposed. To design the GRIN material, a robust algorithm is presented and numerically demonstrated to retrieve the constitutive effective parameters (permittivity and permeability) of the metasurface from the measurement of S parameters. In addition, a new kind of unit cell with a simple structure is designed to verify the theory of effective parameters. By computing the effective refractive index of the unit cell using the algorithm, we design a plate with a thickness of approximately 0.11λ0 (free-space wavelength) at 0.967 THz to superfocus terahertz waves in one dimension. We also revise the theory of superfocusing to make full use of the coupling effect among the cells in the plate and are able to demonstrate it. Comprehensive simulations of focusing are provided, and the full width at half-maximum beam width reaches up to 0.167λ0 at a distance of 0.2λ0. The focusing depth can be further enhanced by decreasing the cell size of the metasurface. This work is beneficial for metasurface design, super-resolution imaging, and other applications in the near field.

In this paper, we propose and numerically demonstrate a new way to realize superfocusing of terahertz waves via the spoof surface plasmons (SSP). With the assist of a modified subwavelength metallic grating, a near-field rapid oscillation can be formed, originating from the Fabry-Perot resonances due to the reflection of SSP waves at terminations. We show that the field pattern of oscillation on textured metallic surface can be engineered by adjusting groove width and grating number. This produces a desired modulation of phase and amplitude for the radiationless electromagnetic interference (REI) focusing. The effective focusing depth through the corrugated metal is evaluated by the full-width-half-maximum (FWHM) beamwidth. At the situation of third-order Fabry-Perot resonance, the FWMH reaches up to 0.069λ at a distance of 0.1λ, improving the beamwidth by more than 540% compared with a single slit. The FWHM is optimized to 0.06λ as the order of Fabry-Perot resonance becomes seven, leading to the superfocusing metric of 1.67. On the basis of this, we further show the focusing ability can be held on the ultra-thin metallic grating. Two-dimensional subwavelength focusing behavior is also numerically verified. Our study may extend the working distance of sensing and super-resolution imaging devices at terahertz frequency.