Wideband tunable terahertz metamaterial absorber based on vanadium dioxide (VO2) for terahertz sensing and communication

A micro-cavity based on phase-change material is a very important strategy for the realization of tunable absorption and conversion of terahertz waves. In this work, a tunable terahertz metamaterial absorber based on the phase-change material germanium–antimony–tellurium (GST) is demonstrated. The device features a metal–insulator–metal triple-layer structure, where the dynamic switching of absorption characteristics is achieved via thermally controlled GST phase transition. In the amorphous state, the absorber exhibits a single absorption peak at 7.7 THz. Upon crystallization, the absorption switches to dual peaks at 5.1 THz and 8.3 THz, achieving near-perfect absorption in both states. Full-wave electromagnetic simulations and theoretical analysis based on a multiple-reflection interference model indicate that this performance tuning originates from the GST-phase-transition-induced change in the equivalent optical cavity length. This corresponds to a switch between two resonant modes: coupled inner–outer ring resonance and independent outer ring resonance. These results provide a foundation for developing dynamically tunable terahertz devices with promising applications in terahertz communications, imaging, and sensing.

Due to its low static power consumption, the memcapacitor has potential applications in the development of high-density neural network computing. Herein, the stable and uniform capacitive switching behaviors are demonstrated through thousands of voltage sweep cycles in the Ni/Ga2O3/ZnO metal–insulator–semiconductor heterostructure memcapacitor. Furthermore, these capacitive states remain stable at different read frequencies. Meanwhile, the multilevel capacitive states are obtained by controlling the amplitude of the applied voltage. This depletion capacitance switching mechanism can attribute to the depletion region width modulation of Ni/Ga2O3/ZnO heterojunction due to the migration of the oxygen ion or electron under the external electric field. This study provides new insights for the development of memcapacitive materials and devices.

Wide-Range Tuning of Insulator–Metal Transition via Microstructure and Defect Engineering of Sputtered VO ₂ Thin Films

Abstract Correlation-driven metal–insulator transitions and temperature-driven coherence–incoherence crossovers strongly influence the dynamical responses of correlated electron systems. Motivated by recent experimental probes of spin and charge excitations, we investigate the real-frequency local spin and charge dynamical susceptibilities of the one-band Hubbard model within dynamical mean-field theory (DMFT), employing numerical renormalization group (NRG) impurity solvers. We analyze both the interaction-driven transition at half filling and the temperature evolution in the doped regime proximate to the Mott insulator. 
To characterize low-energy relaxation processes within the local DMFT framework, we examine normalized dissipative spectra constructed from the imaginary parts of the local susceptibilities. From these spectra we extract characteristic fluctuation scales and track their evolution with interaction strength, temperature, and band filling. At half filling, the spin and charge channels exhibit qualitatively distinct low-frequency structures governed by symmetry and correlation effects. In the doped Mott regime, we find that spin and charge fluctuations reorganize on parametrically different temperature scales, signaling a separation of local coherence scales within a purely local theory.
We emphasize that our analysis pertains to on-site dynamical relaxation within DMFT rather than hydrodynamic transport. Nevertheless, the distinct temperature evolution of spin and charge fluctuation scales is qualitatively consistent with multistage crossover behavior observed in transport quantities. These results highlight how local two-particle dynamics reorganize across interaction- and temperature-driven crossovers near the Mott transition.

Unlocking the potential of the insulator–metal transition for photocatalysis

Metal-insulator-metal (MIM) plasmonic metasurfaces provide a powerful platform for enhancing light-matter interactions; however, achieving simultaneous spectral tunability, fabrication reproducibility, and ultrasensitive detection remains a challenge. Here, we present the rational design, simulation, and lithographic fabrication of three distinct MIM metasurfaces (bowtie, honeycomb, and nanotriangle) optimized for plasmon-enhanced Raman spectroscopy (PERS). Finite-difference time domain (FDTD) simulations reveal localized surface plasmon resonances with electric field enhancement factors (EF) exceeding |E|2 ∼ 1600, supported by experimental reflection spectra and the fidelity of nanofabrication. Raman sensing of molecular probes (R6G, 4-ATP, 4-CTP) demonstrates analytical enhancement factors reaching 107 and detection limits as low as ∼10-15 M. This is made possible by the designed nanogap resonances, and the broadband localized surface plasmon resonance (LSPR) overlap with the excitation and scattering bands. Our findings establish the lithographically defined MIM metasurfaces as reliable, tunable, and ultrasensitive surface-enhanced Raman spectroscopy (SERS) platforms, making them suitable for next-generation portable chemical and biological sensing systems.

Excellent gate electrostatics in field effect transistors (FETs) based on 2D transition metal dichalcogenide (2D TMD) channels can dramatically decrease static power dissipation. Energy-efficient FETs operate in enhancement mode with a small and positive threshold voltage (Vth) for n-type devices. However, most state-of-the-art FETs based on monolayer MoS2 channel operate in depletion mode with negative Vth due to doping from the underlying dielectric substrate. In this work, we identify key properties of the semiconductor/dielectric interface (MoS2 on industrially relevant high dielectric constant (k) HfO2, ZrO2 and hBN for reference) responsible for realizing enhancement-mode operation of 2D MoS2 channel FETs. We find that hBN and ZrO2 dielectric substrates provide low defect interfaces with MoS2 that enables effective modulation of the Vth using gate metals of different work functions (WFs). We use photoluminescence (PL) and synchrotron X-ray photoelectron spectroscopy (XPS) measurements to investigate doping levels in monolayer MoS2 on different dielectrics with different WF gate metals. We complement the FET and spectroscopic measurements with capacitance-voltage analysis on dielectrics with varying thicknesses, which confirms that Vth modulation in ZrO2 devices is correlated with WF of the gate metals - in contrast with HfO2 devices that exhibit signatures of Vth pinning induced by oxide/interface defect states. Finally, we demonstrate FETs using a 2D MoS2 channel and a 6 nm of ZrO2 dielectric, achieving a subthreshold swing of 87 mV dec-1 and a threshold voltage of 0.1 V. Our results offer insights into the role of dielectric/semiconductor interface in 2D MoS2 based FETs for realizing enhancement mode FETs and highlight the potential of ZrO2 as a scalable high-k dielectric.

• Continuous hydrothermal flow synthesis enables rapid formation of high-crystallinity VO 2 nanoparticles within seconds. • Reaction temperature and residence time play key roles in controlling particle size and crystallinity. • The thermochromic performance of composite films is governed by nanoparticle microstructure. • The developed flow-based approach offers a scalable route toward thermochromic smart window materials. Monoclinic vanadium dioxide (VO 2 (M)) is a promising thermochromic material for smart windows due to its reversible metal–insulator transition near 68 °C and strong near-infrared modulation. Conventional fabrication methods, such as batch hydrothermal synthesis, are limited by a high cost, long processing times, and reliance on hazardous reducing agents, thus hindering large-scale implementation. In this study, a reducing-agent-free continuous hydrothermal flow synthesis (CHFS) route was developed using a commercial T-junction reactor to rapidly and safely produce VO 2 (M) nanoparticles. The effects of the reaction temperature (340–400 °C) and residence time (0.5–10 s) on the particle formation and crystallinity were systematically investigated. VO 2 (M) nanoparticles with an average size of around 20 nm were obtained at 400 °C within 5 s, while an extended residence time promoted coarsening to 32 nm. Differential scanning calorimetry revealed phase transition enthalpies of 24–58 J g −1 , indicating a strong dependence on the reaction conditions. Composite films fabricated from optimized nanoparticles exhibited an excellent thermochromic performance, achieving a transmittance modulation (ΔT 1200 nm) of 47% with repeatable switching. These findings demonstrated a scalable, reducing-agent-free CHFS process for high-quality VO 2 (M) nanoparticles and their integration into thermochromic films, thus offering a practical route toward energy-efficient smart window applications.

ABSTRACT La 0.67 Ca 0.33 MnO 3 is a typical perovskite manganese oxide material with rich physical properties, especially in the field of colossal magnetoresistance properties. La 0.67 Ca 0.33 MnO 3 : x Fe 3 O 4 (0 ≤ x ≤ 0.005) composite ceramics were prepared by sol–gel and solid‐phase methods to study the effect of Fe 3 O 4 addition on the electrical transport and magnetoresistive properties. The resistivity progressively increased with higher dopant content, while the metal–insulator transition temperature exhibited a systematic decrease with increasing composition fraction. Temperature coefficient of resistance (TCR) and magnetoresistance (MR) are considered key parameters for enabling practical applications of such ceramics. At x = 0.002, the TCR reached 37.32%·K −1 , representing an increase of 3.51%·K −1 compared with the pristine sample. At x = 0.003, the MR reached 61.49% (1 T), corresponding to an increase of 4.28% compared with the pristine sample. This series of results shows that the addition of Fe 3 O 4 to the composite ceramic can significantly increase the TCR and MR, which in turn can facilitate the potential applications of these materials in bolometers and magnetic sensors.

Terahertz (THz) broadband absorbers with high efficiency and tunability are crucial for applications in electromagnetic shielding, sensing, stealth technology, and THz communication systems. In this work, an ultra-broadband, thermally tunable THz absorber with high fabrication tolerance is proposed based on the phase-change material vanadium dioxide (VO2). The absorber adopts a metal-dielectric-metal (MDM) configuration, consisting of a gold reflective layer, a SiO2 dielectric spacer, and a patterned VO2 top layer. When VO2 is in the metallic phase, the absorber achieves absorptance exceeding 90% over the frequency range of 3.1-10.0 THz, with a large fractional bandwidth of 105.34%. The broadband absorption mechanism is revealed through impedance matching analysis, multiple reflection interference theory, electric-field distribution analysis, and multipole decomposition. The results show that the absorption is primarily driven by electric dipole resonance, with contributions from toroidal and magnetic dipole resonances, which effectively confine electromagnetic energy and suppress reflection. Thermal modulation of the VO2 phase transition enables dynamic tunability of the absorption response, while parametric and structural-shape analyses confirm excellent fabrication tolerance. This work demonstrates that the proposed VO2-based metamaterial absorber provides a practical solution for advanced THz functional devices, combining high efficiency, broadband performance, and robust fabrication compatibility.

Flexible photodetectors represent a key enabling technology for next-generation wearable and biointegrated optoelectronic systems. Integrating plasmonic metal–insulator–metal (MIM) structures into flexible electronics is challenging due to the mechanical fragility of continuous metal layers. Here, we report a flexible organic field-effect transistor (OFET) photodetector enhanced by an embedded MIM architecture (Ag/P3HT/AuNPs). Unlike rigid periodic metallic structures, this design leverages the stochastic spatial distribution of Au nanoparticles (AuNPs) to maintain a stable magnetic resonance effect even under mechanical deformation. This unique structure property correlation resolves the conflict between optical enhancement and mechanical flexibility. Consequently, the optimized device achieves a high responsivity of 441 mA/W and a detectivity of 1.23 × 1011 Jones (under 10 μW/cm2 illumination at VG = 0 V, VDS = −40 V), representing 19-fold and 16-fold enhancements over pristine devices. Remarkably, the device retains ∼ 80% of its initial performance after 1000 bending cycles. This work provides a facile, lithography-free strategy for constructing high-performance, strain-tolerant flexible optoelectronics.

LaCoO3 (LCO) undergoes a cubic to rhombohedral structural phase transition at ∼1600 K, a metal–insulator transition at ∼550 K, and a spin-state transition at ∼100 K. The thermodynamics of spin-state phase transitions for LCO have been studied using the 2-4-6 Landau–Ginzburg (LG) free energy functional (ΔG) developed in terms of mode amplitude of octahedral tilt (primary order parameter), symmetry-adapted strains, and pure electronic free energy (depicting gradual change of high-spin population fraction). The expansion coefficients of (ΔG) are determined using experimentally reported structural parameters and by employing first principle-based calculations. The spin crossover transition is predicted to be ∼75 K. The developed LG free energy functional model can successfully explain the temperature evolution of mode amplitude of primary order parameter (QR4+), symmetry-adapted strains ea (particularly characteristic hump ∼70 K) and e4, and population of high-spin state (n). The present study depicts the primary role played by octahedral tilt in the spin-state phase transition of LCO.

Abstract The Talbot effect in periodic metallic metasurfaces offers a powerful approach for manipulating electromagnetic waves at the nanoscale. However, achieving active control of this effect remains challenging due to the intrinsic limitations of conventional materials and the static nature of fabricated nanostructures. Here, we present a multifunctional periodic metal–insulator–metal (MIM) nanograting metasurface that exploits quantum plasmonic tunneling to enable active and continuous modulation of Talbot self-imaging. By applying an external bias across the MIM junctions, the tunneling current significantly modifies the plasmonic coupling, allowing the metasurface to switch dynamically among reflective, absorptive, and reconfigurable Talbot-imaging states. The device exhibits up to 80% reflectivity in the non-tunneling state and 90% absorption in the tunneling state. Moreover, through electrical control of the spatial distribution of tunneling and non-tunneling unit cells within each super-period, key Talbot imaging parameters—including the Talbot distance, hot-spot intensity, full-width at half-maximum (FWHM), and pattern contrast ratio—can be continuously tuned. This work provides a versatile platform for actively modulating classical optical self-imaging phenomena via quantum plasmonic effects and opens new pathways toward dynamically reconfigurable photonic devices.

With the rapid development of electrified railways in high-altitude regions, section insulators in catenary systems frequently experience gap breakdown and surface flashover under low atmospheric pressure conditions, posing serious threats to safe train operation. This paper investigates the discharge mechanisms of section insulators in high-altitude environments and conducts research on discharge characteristics under extremely non-uniform electric fields, along with structural optimization. First, the physical mechanisms of gap discharge and surface flashover in section insulators are analyzed. A three-dimensional electric field simulation model of the section insulator is established, and numerical analysis is performed to reveal the electric field distribution characteristics. The results indicate that the electric field is predominantly concentrated at the junction between metal electrodes and insulators, as well as at the tip of the arcing horn. The local maximum field strength reaches 3.84 × 105 V/m, exceeding the corona inception field strength of air, which readily induces discharge. Subsequently, power frequency and lightning impulse discharge tests are conducted in both plain region and regions at an altitude of 4300 m. The results show that under high-altitude conditions, the power frequency breakdown voltage decreases by 28%, and the 50% lightning impulse breakdown voltage decreases by 42%. The discharge voltages under standard atmospheric conditions are obtained through correction. Finally, optimization schemes involving arcing horn structural modification and surface coating application are proposed. Adjusting the arcing horn angle to 55° and adding a grading ring structure with a radius of 70 mm reduces the local maximum field strength by 26%. After applying an RTV insulating coating, the field strength at the junction decreases by 35.9%, effectively enhancing the insulation performance of section insulators in high-altitude regions.

Abstract To overcome the intrinsic physical constraints of strict near-field coupling in conventional plasmonic sensors, this work proposes a metal-insulator-metal (MIM) waveguide configuration. By integrating a periodic array of silver nanoblocks within a sensing region positioned over 2 μm from the bus waveguide, the sensor achieves effective remote refractive index sensing at the micrometer scale. Finite-difference time-domain (FDTD) simulations demonstrate that the localized surface plasmon resonance (LSPR) peaks can be deterministically tuned by adjusting the dimensions of the composite periodic cavity. This versatility allows for precise adaptation to specific detection windows, such as [1.0, 1.2] or [1.2, 1.4]. After optimizing the structure of the resonator, although the sensing region is far away from the bus waveguide, it can still obtain a sensitivity of 1105 nm RIU −1 while maintaining a low signal loss. Through the numerical verification of glucose solutions with different concentrations, the device achieved a mass concentration sensitivity of 0.131 nm·l g −1 . This result shows that the structure has good application potential in the field of high-performance remote biomedical sensing.

This study proposes a metal–insulator-metal(MIM)-graphene hybrid waveguide structure by integrating graphene nanotubes into MIM ring resonators. The transmission spectrum and electric field distribution are analyzed using the finite element method (FEM), and the results show excellent agreement with multimode interference coupled mode theory (MICMT). Based on this design, the refractive index sensor achieves sensitivity reaching up to 1900nm/RIU, with a peak figure of merit (FOM) of 96.15 RIU-1. Importantly, compared to conventional MIM waveguide devices, the introduction of graphene enables dynamic tunability of the waveguide response.

ABSTRACT A metal–dielectric–metal nanocavity with a chiral nanopatterned metal electrode provides an effective platform for circularly polarized photodetectors, which allows the chiral optical response and photon‐to‐charge conversion to be optimized with minimal mutual interference. We employ a genetic algorithm to inversely design the electrode pattern to maximize the chiral response while incorporating a vacuum‐deposited small‐molecule multilayer, which enables precise alignment of the photoactive layer with the helicity‐dependent field distribution. The optimization yields a non‐intuitive metal nanopattern that achieves a high dissymmetry factor ( = 1.32) of external quantum efficiency ( = 16.7%) at a target wavelength, outperforming a representative conventional chiral nanopattern. The fabricated device with the optimized electrode achieves = 0.67 and = 8.1% near the target wavelength, and simulations of the fabricated device geometry, accounting for curvature in the multilayer stack, confirm helicity‐dependent plasmonic field distributions consistent with those of the idealized flat‐layer device used in the optimization. These results demonstrate the effectiveness of our inverse design strategy and provide a framework for the development of high‐performance thin‐film chiral optoelectronic devices.

This work presents a composite infrared camouflage coating for low Earth orbit (LEO) spacecraft, integrating multiband infrared stealth, narrowband laser suppression, and enhanced radiative heat dissipation within a unified framework. The proposed “metal-dielectric-metal (MDM) micrograting + molybdenum/silicon (Mo/Si) multilayer” structure achieves low emissivity (0.21 in MWIR, 0.27 in LWIR), high emissivity (0.56 in 5–8 µm), and strong absorption (0.91 at 10.6 µm). Interfacial risk analysis identifies the TiN/top Si interface as the primary reliability bottleneck, with deep-cold conditions as the most critical thermal phase. These results provide guidance for material selection, fabrication, and durability optimization for future spacecraft coatings.

In this design, we propose a completely new sensor structure. It features a metal-insulator-metal (MIM) waveguide and a circular four-stub resonator (CFSR). Using the finite element method, we analyzed the performance of the sensor structure. We examined the impact of different parameters and structural variations on its performance. Ultimately, we determined the optimal performance parameters for the best configuration. The modified device demonstrated a sensitivity (S) of 2940 nm/RIU and a figure of merit (FOM) of 52.5. Furthermore, this sensor design demonstrates significant potential for temperature measurement applications, with a core parameter of 1.508 nm/°C.