The realization of invisibility cloaks has long been a challenging goal in electromagnetics. Here, we propose and experimentally demonstrate a novel dual-layer diamondoid invisibility cloak capable of concealing objects under transmissive plane waves. The cloak integrates an outer layer of transparent metasurfaces and an inner layer of reflective metasurfaces, exploiting polarization conversion and multiple reflections to restore the wavefront and suppress scattering. Both full-wave simulations and microwave experiments validate the design, showing that the cloak effectively reconstructs the transmitted and reflected waves, independent of the shape or number of objects within the cloaked region. Furthermore, by applying the mirror-image principle, the design is readily extended to carpet cloaks, offering near-perfect reflection restoration for objects on a ground plane. This work introduces a versatile, scalable, and experimentally feasible approach for transmissive and carpet cloaks, opening new avenues for advanced electromagnetic wave manipulation at microwave, terahertz, and optical frequencies.
This study proposes a composite ventilated noise reduction structure based on Helmholtz resonators to meet the low-frequency, broadband noise reduction requirements of equipment pipelines under ventilation constraints. The structure uses a combination of air-filled cavities to achieve low-frequency broadband noise reduction. First, a tube muffling and noise reduction model was adopted for the theoretical analysis of the structure. Based on this, a ventilation noise reduction unit was designed via spatial folding technology, and the effects of structural parameters on sound absorption performance as well as the coupling effects of multi-unit parallel metamaterials were systematically analyzed. Simulations verify that the designed structure not only achieves a noise reduction level above 10 dB in the 250–2500 Hz frequency range but also has a structural thickness of only one-tenth of the wavelength and a ventilation rate of 13%. The correctness of the simulation results was validated by standing wave tube experiments. The proposed structure offers a potential solution for architectural sound absorption and aircraft cabin noise reduction under ventilation conditions.
HoFeO 3 rare earth ferrite exhibits distinctive crystallographic properties; however, its regulatory mechanisms and methods within the terahertz band remain complex. This study employed variable-temperature Raman spectroscopy to explore atomic interactions within the crystal structures of HoFeO 3 across various modes, spanning temperatures from 20 to 300 K. The fabrication of HoFeO 3 metamaterials was achieved using 3D direct ink writing additive manufacturing technology, accompanied by the design of a three-coordinate spatial intelligent control structure to manipulate terahertz electromagnetic waves. Variable-temperature Raman spectroscopy revealed twelve active peaks, which included the stretching vibrations of Ho and O, the torsion of FeO 6 , and the stretching vibrations of Fe-O. Notably, these phonons exhibited softening phenomena with increasing temperature. A low transmittance (transmittance <0.1) of terahertz waves at 0.82 THz was attained through the use of 3D-printed double-layer 90° metamaterials. Furthermore, when the double-layer 45° metamaterials were rotated in the E-H plane around the propagation direction as the central axis, the terahertz waves demonstrated a rotational symmetry transmission law at 0.5 THz. This research offers effective materials and construction methods for metamaterials aimed at regulating terahertz electromagnetic waves.
Developing electromagnetic metamaterials that simultaneously offer broadband response, a thin profile, and multifunctional capabilities remains a key challenge in stealth technology. Based on the Pancharatnam–Berry (PB) phase principle, this study proposes a multilayer cascaded geometric phase metasurface. By rotating the meta-atoms to introduce a controllable phase gradient, the metasurface efficiently converts incident circularly polarized waves into co-polarized reflected vortex waves, while leveraging electromagnetic coupling and resonance characteristics between the multilayer structures to achieve broadband energy dissipation. Experimental results demonstrate that this merely 2.72 mm thick multilayer cascaded structure achieves an effective absorption bandwidth of 7.9 GHz (8.1–16.0 GHz), representing a threefold increase over a single-layer design, and exhibits excellent radar cross-section (RCS) reduction performance. Near field and scattering confirm vortex wave generation with specific topological charges, revealing the intrinsic physical mechanism underlying broadband absorption and stealth through energy scattering across a wide angular domain. This study provides new insights for addressing the technical challenges of broadband and multifunctional absorption materials, laying an important foundation for the development of next-generation intelligent stealth metamaterials.
Optical resonators with high quality (Q) factors are highly desirable in meta-optic and nanophotonic fields. Recently, bound states in the continuum (BIC) have emerged as a pivotal solution to enhance the Q factors of optical resonators to theoretically infinite values, leveraging non-dissipative energy confinement, specific resonance conditions, and destructive interference. However, in all-dielectric structures, the field distribution of BIC is predominantly confined within the metasurface (MS), posing significant challenges to the precise control of mode volume. Here, we propose a metal-dielectric hybrid MS that supports two symmetry-protected BIC, both of which are topologically protected by polarization vortices. Notably, the BIC mode volume is drastically reduced via coupling with surface plasmon polaritons, forming a hybrid mode with enhanced field localization. Specifically, we achieve high-efficiency terahertz third harmonic generation (THG) by breaking the dielectric constant symmetry of the structure to excite quasi-BIC. When the incident power of the fundamental frequency wave reaches 1 kW/cm2, the THG conversion efficiency peaks at an impressive 1.5%. Our findings not only provide a pathway for designing optical components with both high Q factors and low mode volumes but also offer a feasible strategy to boost nonlinear optical effect, opening new avenues for advanced nanophotonic applications.
