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.

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.

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.

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.

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.

Emerging trends and hot spots in subacute thyroiditis research from 2001 to 2022: A bibliometric analysis.

Background: Subacute thyroiditis (SAT) is the most prevalent self-limiting thyroid disease that causes pain, accounting for about 5% of all clinical thyroid disorders. Numerous clinically noteworthy results have been published in this area over the last 20 years. However, no article has comprehensively assessed the relevant literature yet. We conducted a bibliometric analysis of SAT to provide light on the dynamic nature of scientific advancement and aid researchers in gaining a global perspective while examining research core themes and hotspots. Methods: SAT-related articles and reviews from 2001 to 2022 were retrieved from the Science Citation Index-Expanded of Web of Science Core Collection (WoSCC). We analyzed current research trends and hotspots in this area using CiteSpace and Vosviewer. Results: A total of 568 studies associated with SAT research were published in 282 academic journals by 2,473 authors in 900 institutions from 61 countries/regions. The United States was a crucial link in inter-country/region collaboration and was the most frequently involved country in international cooperation. The University of Missouri System was the top organization, and Braley-Mullen H. was the most productive researcher. Thyroid published the most papers, with 36 publications. The most co-cited article was "Clinical features and outcome of subacute thyroiditis in an incidence cohort: Olmsted County, Minnesota, study" (by Fatourechi V., 2003). The clustered network and timeline view of keywords showed that the prevalence, diagnosis, and treatment of SAT were the research core themes during the past 20 years. Analysis of keyword bursts indicated that the clinical characteristic and the influence of COVID-19 on SAT appeared to be the current research hotspots. Conclusion: This bibliometric analysis conducted a thorough review of the SAT research. The clinical characteristics and the genetic background of SAT under the influence of COVID-19 are current research hotspots. However, there is still a need for further study and global collaboration. Our findings can aid researchers in understanding the current status of SAT research and immediately pinpoint new directions for further investigation.

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.