Deep-Learning-Assisted Simultaneous Target Sensing and Super-Resolution Imaging.

Metasurfaces have recently experienced revolutionary progress in sensing and super-resolution imaging fields, mainly due to their manipulation of electromagnetic waves on subwavelength scales. However, on the one hand, the addition of metasurfaces can multiply the complexity of retrieving target information from detected electromagnetic fields. On the other hand, many existing studies utilize deep learning methods to provide compelling tools for electromagnetic problems but mainly concentrate on resolving one single function, limiting their versatilities. In this work, a multifunctional deep learning network is demonstrated to reconstruct diverse target information in a metasurface-target interactive system. First, a preliminary experiment verifies that the metasurface-involved scenario can tolerate the system noises. Then, the captured electric field distributions are fed into the multifunctional network, which can not only accurately sense the quantity and relative permittivity of targets but also generate super-resolution images precisely. The deep learning network, thus, paves an alternative way to recover the targets' information in metasurface-target interactive systems, accelerating the progression of target sensing and superimaging areas. Besides, another new network that allows forward electromagnetic prediction is also proposed and demonstrated. To sum up, the deep learning methodology may hold promise for inverse reconstructions or forward predictions in many electromagnetic scenarios.

Spin-decoupled excitation and wavefront shaping of structured surface waves via on-chip terahertz metasurfaces.

Surface waves (SWs) are of great importance in terahertz (THz) photonics applications due to their subwavelength properties. Hence, it is crucial to develop surface wavefront shaping techniques, which is urgent in modern information technologies. In this paper, a new scheme is proposed to realize SW excitation and spin-decoupled wavefront shaping with an ultracompact planar meta-device working in the THz range. The meta-device is composed of two parts: meta-atoms (in the center) and plasmonic metals (on the left and right sides). By carefully setting the geometry size and rotation angle of each meta-atom, the encoded spin-decoupled phase distributions for both left circularly polarized (LCP) and right circularly polarized (RCP) incident THz waves are determined. In this way, circularly polarized (CP) incident THz waves can be converted to SWs propagating along plasmonic metals with unique wavefront profiles, i.e., Bessel and focusing profiles. Full-wave simulations and THz near-field scanning experiments were performed to verify the functionalities of the meta-device, both of which are in great agreement with theoretical predictions. Our findings may provide more solutions to design THz integrated photonic devices and systems.

Ultrathin All-Angle Hyperbolic Metasurface Retroreflectors Based on Directed Routing of Canalized Plasmonics.

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 non-label subwavelength imaging with composite photonics-plasmonics structured illumination.

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.

Efficient magnetic-coupling excitation of LSSPs on high-Q multilayer planar-circular-grating resonators.

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.

Design of Double-Layer Electrically Extremely Small-Size Displacement Sensor.

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.

Terahertz subwavelength edge detection based on dispersion-induced plasmons.

Terahertz imaging has recently attracted great attention owing to the abilities of high penetration and low ionizing damages. However, the low resolution and low contrast resulting from the diffraction limit and unwanted background illumination significantly hinder the extensive usage. In this Letter, we propose and numerically demonstrate a terahertz subwavelength imaging method capable of extracting only the edges and fine features of the targets. The underlying physics is the efficient transmission of the scattering evanescent waves related to key geometric information while blocking the propagating components. By exploiting the structurally induced plasmons in a bounded metallic waveguide, the transmission channel for evanescent waves is realized by hyperbolic metamaterials through periodically stacking dielectric layers. On this basis, high-contrast edge detection with a resolution up to ${0.1}\lambda$ is demonstrated at terahertz wavelengths. The proposed terahertz imaging method may find important applications in non-destructive testing, weak scattering object detection, and high-contrast microscopy.

Bifunctional Luneburg-fish-eye lens based on the manipulation of spoof surface plasmons.

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-fish-eye lens is proposed based on a 2D metal pillar array. First, by tuning the dimension of the metal pillars in the array, its ability to precisely manipulate the SSPs along one direction is confirmed, achieving subwavelength focusing and imaging with a resolution up to 0.14λ. Then, separately controlling the propagation of the SSPs along the orthotropic directions is further implemented, and the bifunctional Luneburg-fish-eye lens is realized. It is experimentally characterized as a Luneburg lens along the x axis, whereas in the y axis, it presents the properties of a Maxwell fish-eye lens. This bifunctional lens can reduce the system complexity and exert flexibility in multifunctional applications, while the proposed metal pillar-based design method broadens the application range of the gradient refractive-index lens in microwave, terahertz, and even optical ranges.

Free-electron-driven beam-scanning terahertz radiation.

A beam-scanning terahertz (THz) radiation mechanism in a free-electron-driven grating system is proposed for THz applications. By loading a period-asynchronous rod array above the grating, the spoof surface plasmon (SSP) originally excited by the electron changes its radiation characteristics owing to the rod-induced Brillouin zone folding effect. The rod array functions as an antenna and converts the SSP into a spatial coherent THz radiation. The radiation frequency and direction can be precisely controlled by the electron energy. The field intensity of the radiation is increased approximately 20 times compared with that of the conventional Smith-Purcell radiation in the same frequency range. In addition, a microwave-band scaling prototype is fabricated and the frequency-controlled radiation is measured. Excellent agreement between the experimental and simulated results is obtained. This study paves the way for the development of on-chip THz sources for advanced communication and detection applications.

High-efficiency directional excitation of spoof surface plasmons by periodic scattering cylinders.

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.

High-efficiency terahertz spin-decoupled meta-coupler for spoof surface plasmon excitation and beam steering.

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 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 right or left 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 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.

Terahertz multichannel metasurfaces with sparse unit cells.

Reflective multichannel metasurfaces are flat reflectors that can control incident and reflected waves in a number of propagating directions simultaneously. However, they are always densely discretized with a high spatial resolution, which increases the manufacturing complexity. In this Letter, to the best of our knowledge, a new method that combines the array antenna theory with the metagratings theory is proposed. We demonstrate that the unit cells with a linear gradient phase in each period of the metasurfaces can eliminate specific space harmonics. With this method, multichannel metasurfaces can be designed with sparse unit cells, and high efficiency is maintained simultaneously. As proofs of the method, we design three different terahertz multichannel metasurfaces with no more than three unit cells per period. The simplification of structures can efficiently reduce the manufacturing complexity. This work may open up new routes in designing multichannel metasurfaces.

Superfocusing of terahertz wave through spoof surface plasmons.

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.

Experimental demonstration of an ultra-broadband subwavelength resolution probe from microwave to terahertz regime.

An ultra-broadband subwavelength resolution probe that consists of a Teflon rod and six metallic strips is developed for the near-field imaging system. The slit between two metallic strips maintains quasi-TEM modes, avoiding the problem of low coupling efficiency caused by the cutoff effect. The numerical calculations visualize the process of energy compression into a 0.047λ diameter spot with great field enhancement at the taper apex, and the probe holds subwavelength focusing behavior from 10 GHz to 0.25 THz. Although limited by the fabrication, the resolution of 0.16 and 0.25λ are still experimentally demonstrated at 14 GHz and 0.1 THz. The properties of easy fabrication and no cutoff frequency would lower the threshold of a high-resolution near-field imaging system.

Terahertz super-resolution imaging using four-wave mixing in graphene.

A perfect lens made from negative refraction (NR) materials is utilized to overcome the diffraction limit. However, these NR lenses are realized by metamaterials, which suffer from high losses, and the volume is bulky. In this Letter, we propose a terahertz NR lens by using a four-wave mixing (FWM) process in graphene. NR is demonstrated because of the phase matching along the surface of graphene. Evanescent waves that store high spatial frequency information can be converted into propagating waves in the nonlinear NR process. An image with subwavelength resolution is reconstructed at the FWM wavelength. Theoretical analysis and numerical simulations are performed to demonstrate the capability of such imaging. The lens has a subwavelength resolution of around λ/5. The lens needs low field intensity due to the strong nonlinear response of graphene in the terahertz frequency. This Letter may have applications in terahertz microscopy.

Broadband terahertz-power extracting by using electron cyclotron maser.

Terahertz applications urgently require high performance and room temperature terahertz sources. The gyrotron based on the principle of electron cyclotron maser is able to generate watt-to-megawatt level terahertz radiation, and becomes an exceptional role in the frontiers of energy, security and biomedicine. However, in normal conditions, a terahertz gyrotron could generate terahertz radiation with high efficiency on a single frequency or with low efficiency in a relatively narrow tuning band. Here a frequency tuning scheme for the terahertz gyrotron utilizing sequentially switching among several whispering-gallery modes is proposed to reach high performance with broadband, coherence and high power simultaneously. Such mode-switching gyrotron has the potential of generating broadband radiation with 100-GHz-level bandwidth. Even wider bandwidth is limited by the frequency-dependent effective electrical length of the cavity. Preliminary investigation applies a pre-bunched circuit to the single-mode wide-band tuning. Then, more broadband sweeping is produced by mode switching in great-range magnetic tuning. The effect of mode competition, as well as critical engineering techniques on frequency tuning is discussed to confirm the feasibility for the case close to reality. This multi-mode-switching scheme could make gyrotron a promising device towards bridging the so-called terahertz gap.

Tunable Terahertz Deep Subwavelength Imaging Based on a Graphene Monolayer.

The resolution of conventional terahertz (THz) imaging techniques is limited to about half wavelength, which is not fine enough for applications of biomedical sensing and nondestructive testing. To improve the resolution, a new superlens, constructed by a monolayer graphene sheet combining with a grating voltage gate, are proposed in this paper to achieve deep super-resolution imaging in the THz frequency range. The main idea is based on the Fabry-Perot resonance of graphene edge plasmon waves. By shaping the voltage gate into a radial pattern, magnified images of subwavelength targets can be obtained. With this approach, the finest resolution can achieve up to λ/150. Besides, the superlens can be conveniently tuned to work in a large frequency band ranging from 4.3 THz to 9 THz. The proposal could find potential applications in THz near-field imaging systems.

Near-Field and Far-Field Directional Conversion of Spoof Surface Plasmon Polaritons.

A compact metallic meta-structure is proposed to realize directional conversion between spoof surface plasmon polaritons (SSPPs) and propagating waves at millimeter wave and THz frequencies. The structure is constructed by embedding two slits or multi-slits array into a subwavelength metallic reflection grating. When the back-side of the structure is illuminated by an oblique beam with a fixed incident angle, the propagating wave will be unidirectionally converted into SSPPs with a considerable efficiency. Both the simulations and experiments demonstrate that the excitation ratio of the SSPPs between the two possible propagating directions (left and right) reaches up to about 340. Furthermore, assisted by the structure, near-field SSPPs can be also converted into far-field narrow beams with particular directions. Through frequency sweeping, wide-angle beam scanning is verified by theory and experiments. The work paves a new way for SSPPs launching and also provides fresh ideas for super-resolution imaging in the longer wavelength range.

Terahertz far-field superresolution imaging through spoof surface plasmons illumination.

The applications of terahertz (THz) imaging are always restricted by the low resolution. We introduce here a new way to realize far-field superresolution imaging at THz frequency. Assisted by a new spoof surface plasmons (SSP) probe illumination, the images of two subwavelength separated slits can be reconstructed by a single shot. Although only 0.06λ resolution is numerically demonstrated at 0.3 THz, the resolution potentially can be further enhanced by scaling down the size of the SSP probe. Deep subwavelength focusing is also achieved by the SSP probe. Our work may open a new avenue for SSP-based superresolution at longer wavelengths.

Long-distance super-resolution imaging assisted by enhanced spatial Fourier transform.

A new gradient-index (GRIN) lens that can realize enhanced spatial Fourier transform (FT) over optically long distances is demonstrated. By using an anisotropic GRIN metamaterial with hyperbolic dispersion, evanescent wave in free space can be transformed into propagating wave in the metamaterial and then focused outside due to negative-refraction. Both the results based on the ray tracing and the finite element simulation show that the spatial frequency bandwidth of the spatial FT can be extended to 2.7k(0) (k(0) is the wave vector in free space). Furthermore, assisted by the enhanced spatial FT, a new long-distance (in the optical far-field region) super-resolution imaging scheme is also proposed and the super resolved capability of λ/5 (λ is the wavelength in free space) is verified. The work may provide technical support for designing new-type high-speed microscopes with long working distances.