Thanh Son Pham

In this paper, we present a reconfigurable digital two-dimensional metasurface consisting of an 8 × 8 matrix of specialized unit cells that can dynamically switch between two resonance modes at 11 MHz and 13.56 MHz. By carefully designing defect-based metamaterial cavities, flexible waveguides are formed while effectively minimizing leakage and transmission losses typically observed in homogeneous structures. Analyses, simulations, and experiments verify that the proposed metasurface enables diverse magneto-inductive wave propagation patterns on a single physical platform under a deep subwavelength mechanism. Furthermore, a step-defect configuration is introduced and experimentally validated, demonstrating its effectiveness in suppressing reflections and impedance mismatch while enhancing transmission efficiency when the receiver is moved to arbitrary positions on the metasurface. The unidirectional waveguide achieves transmission coefficients of S ₂₁ = 0.76 and 0.4, corresponding to source diameter-to-transmission length ratios of 1:1 and 1:8, respectively.

This work presents a systematic study on the tunability of negative magnetic permeability in bilayer metallic disk metamaterials based on plasmon hybridization. Both numerical and experimental results show that the single-layer disk structure exhibits a magnetic resonance at 14.36 GHz, while the bilayer structure splits this resonance into two hybridized modes at 14 101 and 14.623 GHz, leading to an approximately 1.6 times broader negative-permeability bandwidth. By introducing a graphene conductive coating with controllable sheet resistance, the hybridization strength can be modulated without changing the structural geometry. When the graphene sheet resistance is increased to 15 Ω sq ⁻¹ , the two hybridized resonances shift to 14 189 and 14.613 GHz; at 30 Ω sq ⁻¹ , the second hybridized mode is strongly suppressed, leaving a weakened single resonance at 14 442 GHz. The roles of dielectric and Ohmic losses are also clarified: increasing the dielectric loss tangent from 0.0027 to 0.05 nearly eliminates the resonance splitting, whereas changing conventional metals among Cu, Au, and Ag produces only minor spectral variations in the GHz regime. The approach is further extended to a disk grid configuration exhibiting a negative refractive index, where graphene-induced attenuation is found to be frequency dependent. At 14.63 GHz, the attenuation is mainly governed by absorption loss, while at 14.98 GHz it results from the combined effects of absorption and impedance mismatch. These findings provide a practical material-based strategy for tuning microwave metamaterial responses and provide new opportunities for the design of reconfigurable microwave metamaterials.

Reconfigurable photonic metasurfaces enable tunable thermal-emission engineering in the long-wave infrared (LWIR), particularly within the 8–13 μm atmospheric window. This work includes the investigation on a concentric-ring VO₂/SiO₂/Au metasurface for LWIR spectral-emissivity modulation. Full-wave simulations showed that, in the metallic phase (σ = 2 × 10⁵ S/m where σ is conductivity), the structure exhibited an absorption over 90% across the 9.3–15 μm sub-band, with two near-unity resonances near 10.2 and 13.3 μm. Control structures, gap-dependent spectra, E-field maps, and current-density Cartesian multipole decomposition supported a hybridized-ring mechanism in which both dominant resonances were predominantly electric-dipole-like ring branches whose spectral positions and field localizations were modified by inter-ring coupling. Across the conductivity sweep, the normal-incidence band-averaged 8–13 μm emissivity changed from 0.0184 to 0.8844, corresponding to a switching ratio of 48.06. The four-fold symmetry of unit cell also yielded polarization-insensitive and angularly robust LWIR absorption, while the simplified endpoint thermal-balance estimate indicated a metallic-state net cooling power of 49.3 W m⁻² at T = T_amb = 300 K, where T_amb was the ambient temperature, and an estimated equilibrium temperature drop of 4.4 K below the ambient for the metallic-state endpoint, whereas the insulating-state one suppressed this response. These results identify concentric VO₂ ring metasurfaces as promising candidates for switchable LWIR thermal-emission control.

Advancements in Wireless Power Transfer (WPT) technology have enabled a wide range of applications across both industrial and civilian sectors. WPT systems can operate in environments where conventional wired systems are difficult or impractical to implement. These environments often exhibit complex characteristics, including electrical conductivity, which significantly affects WPT performance. This paper presents a WPT system operating in submerged conductive medium with conductivities ranging from 0 to 10 S/m. Theoretical investigations of eddy current formation in conductive environments that lead to efficiency degradation were validated through simulations of electric and magnetic field distributions and experimental measurements of transmission efficiency under different operating conditions. The results obtained can provide useful insights for designing WPT systems in conductive environments, such as underwater, underground, or biomedical implant power-delivery applications.

Vật liệu biến hoá nói chung và vật liệu biến hoá từ tính (magnetic metamaterial nói riêng ngày càng được ứng dụng trong nhiều lĩnh vực. Tuy nhiên, trong một số thiết bị thế hệ mới yêu cầu vật liệu biến hoá từ tính phải kết hợp được cả tính chất điện từ độc đáo và tính chất cơ học linh hoạt. Nhằm từng bước giải quyết bài toán tối ưu cấu trúc vật liệu biến hoá từ tính cho các ứng dụng yêu cầu tính linh hoạt, sử dụng phương pháp tính toán lý thuyết chúng tôi đã xây dựng thành công chương trình tính toán khảo sát độ tự cảm của vật liệu vật liệu biến hoá từ tính cấu trúc xoắn ốc vuông ở cả trạng thái phẳng và khi uốn cong. Nghiên cứu này kết hợp cả tính toán lý thuyết và mô phỏng bằng phần mềm CST Studio Suite để nghiên cứu độ tự cảm của vật liệu biến hóa từ tính xoắn ốc vuông khi uốn. Kết quả cho thấy độ tự cảm vẫn duy trì trên 96% giá trị độ tự cảm ở trạng thái phẳng khi uốn từ 0°–180°. Các kết quả thu được là tiền đề quan trọng cho các nghiên cứu tối ưu cấu trúc vật liệu biến hoá từ tính ứng dụng trong các hệ thống truyền năng lượng không dây yêu cầu cấu trúc vật liệu linh hoạt.

This paper presents the design, fabrication, and characterization of a broadband metamaterial absorber (MMA) capable of optical switching between high- and low-absorbing states. The proposed metasurface exhibits a wide absorption bandwidth ranging from 13 to 17.3 GHz, maintaining a high absorption for wide incidence angles and for both transverse electric and transverse magnetic polarizations, demonstrating the polarization-insensitive performance. The metasurface is composed of periodically arranged unit cells integrated with photoresistors that enable tunable electrical resistance under optical illumination. When exposed to light, the photoresistors reduce their resistance, allowing the structure to achieve a high absorption. In the absence of illumination, the increased resistance suppresses the absorption response, effectively switching the metasurface to be a reflective state. This optically controllable behavior provides a simple and efficient approach for realizing dynamically reconfigurable MMAs, offering potential applications in adaptive stealth technology, electromagnetic shielding, and tunable sensing systems.

Control and localization of magnetic fields are critical issues in wireless power transfer (WPT) technology. Managing and localizing magnetic fields minimizes energy losses in regions without a load, thereby enhancing the WPT efficiency through magnetic induction or resonance. In this study, we propose the application of a digital metamaterial (DMM) capable of tuning localized field on the DMM as called the space-modulated reconfigurable region to improve the WPT efficiency. Based on the exceptional flexibility of the DMM slab, magnetic fields can be localized to various receiver coil locations. The experimental results indicate that the WPT efficiency remains relatively stable, ranging from 67% to 50%, for an Rx:Tx coil area ratio of 1:32 as the transfer distance increases from 15 cm to 20 cm. Furthermore, the proposed DMM enables the WPT system to maintain an efficiency above 36% even under a coil misalignment of up to 15 cm at 15 MHz over a transfer distance of 15 cm.