Search Results (143)

Whistler-mode waves, electromagnetic emissions with frequencies between the lower hybrid and electron cyclotron frequencies, are ubiquitous in planetary magnetotails. They are known to play a vital role in electron scattering and acceleration, originating primarily within strong magnetic field regions behind dipolarization fronts (DFs). In contrast to this established knowledge, we present a comprehensive analysis of whistler-mode waves generated locally within weak magnetic field regions ahead of DFs, utilizing high-cadence measurements from the MMS mission. By resolving the wave dispersion relations, we demonstrate that these emissions arise from cyclotron resonance with local electrons exhibiting weak perpendicular temperature anisotropy (A_e < 1.2). We further propose that this anisotropy may develop due to magnetic mirror structures forming upstream of DFs. Our findings challenge the conventional view that whistler-mode generation requires strong magnetic fields near DFs, providing new insights into understanding wave excitation mechanisms in planetary magnetotails. Full article

We propose a single-phase chiral elastic metamaterial capable of simultaneously exhibiting negative effective mass density and negative bulk modulus in the ultrasonic frequency range. The unit cell consists of a regular hexagonal frame connected to a central circular mass through six obliquely oriented, slender aluminum beams. The design avoids the manufacturing complexity of multi-phase systems by relying solely on geometric topology and chirality to induce dipolar and rotational resonances. Dispersion analysis and effective parameter retrieval confirm a double-negative frequency region from 30.9 kHz to 34 kHz. Finite element simulations further demonstrate negative refraction behavior when the metamaterial is immersed in water and subjected to 32 kHz and 32.7 kHz incident plane wave. Equifrequency curves (EFCs) analysis shows excellent agreement with simulated refraction angles, validating the material’s double-negative performance. This study provides a robust, manufacturable platform for elastic wave manipulation using a single-phase metallic metamaterial design. Full article

Especially in real-world circumstances with uneven road surfaces and impulsive shocks, nonlinear dynamic effects in vehicle systems can greatly skew biometric data utilized to track passenger and driver physiological states. By creating a thorough multibody human–seat–chassis model, this work tackles the effect of vehicle-induced vibrations on the accuracy and dependability of biometric measures. The model includes external excitation from road-induced inputs, nonlinear damping between structural linkages, and vertical and angular degrees of freedom in the head–neck system. Motion equations are derived using a second-order Lagrangian method; simulations are run using representative values of a typical car and human body segments. Results show that higher vehicle speed generates more vibrational energy input, which especially in the head and torso enhances vertical and angular accelerations. Modal studies, on the other hand, show that while resonant frequencies stay constant, speed causes a considerable rise in amplitude and frequency dispersion. At speeds ≥ 50 km/h, RMS and VDV values exceed ISO 2631 comfort standards in the body and head. The results highlight the need to include vibration-optimized suspension systems and ergonomic design approaches to safeguard sensitive body areas and preserve biometric data integrity. This study helps to increase comfort and safety in both traditional and autonomous car uses. Full article

Near-field imaging of the hybrid surface plasmon-phonon polaritons on the n-GaN semiconductor was performed using a scattering scanning near-field optical microscope at the selected frequencies of 920 cm⁻¹ and 570 cm⁻¹. The experimental measurements and numerical modeling data were in good agreement, revealing the large propagation distances on the n-GaN semiconductor and other insights which could be obtained by analyzing the dispersion characteristics of hybrid polaritons. In particular, the decay lengths of polaritons at the excitation frequency of 920 cm⁻¹ were measured to be up to 25 and 30 µm in experiment and theory, respectively. In the case of excitation at the frequency of 570 cm⁻¹, the surface plasmon-phonon polaritons’ decay distances were 25 µm and 105 µm, respectively, noting the limitations of the near-field optical microscope setups used. Dispersion characteristics of the resonant frequency and the damping rate of hybrid polaritons were numerically modeled and compared with the analytical calculations, validating the need for further experiment improvements. The launch conditions for the near-field observation of extraordinary coherence of the surface plasmon-phonon polaritons were also discussed. Full article

The transition to 5G and beyond has highlighted the need for efficient devices that operate at mm-wave frequencies, which require new structures and pose fabrication challenges. This paper proposes a novel non-linear antenna that combines the well-established substrate-integrated cavity (SIC) radiators and time-varying graphene for generating harmonic frequencies in the mm-wave spectrum. Graphene is represented as having a dispersive surface conductivity, while time modulation of the conductivity is introduced by varying the applied bias electric field. A modified FDTD algorithm is, additionally, used to simulate the time-varying graphene behaviour under different modulation schemes. The final antenna design involves an SIC resonator with a graphene-covered slot aperture for radiation. The numerical study highlights the effective generation of harmonics using the modulated graphene at the mm-wave regime. Finally, different modulation schemes are applied to enhance certain higher-order harmonics, demonstrating the potential of this non-linear antenna design for future mm-wave and THz frequency applications. Full article

Bow-tie cavity configurations have gained significant attention due to their efficacy in facilitating stable resonator operation for applications requiring short pulse operation and high repetition rate pulses, offering versatility and reliability. While there is an extensive body of literature addressing the theoretical aspects and applications of this laser configuration, there exists a gap in practical insights and systematic approaches guidance pertaining to the development and precision alignment of this laser type. The paper achieves this by compiling a range of analytical and optimization techniques for the bow-tie cavity configuration and delineating the necessary steps for the optimization required for continuous wave operation. This ultimately leads to the attainment of the pulsed regime through the Kerr Lens Mode-locking technique, offering a detailed account of the development, optimization, and performance evaluation of a Ti:Sapphire femtosecond laser cavity, using dispersion-compensating mirrors to produce a low-energy pulse of 1 nJ, a high repetition rate of 1 GHz, and a short pulse duration of 61 fs. This work can be useful for researchers and engineers seeking to embark on the development of compact and high-performance femtosecond lasers for a spectrum of applications, encompassing biomedical imaging, laser-assisted surgery, spectroscopy, and optical frequency combs. Full article

Studying the risk propagation mechanisms in agricultural systems is crucial for maintaining agricultural stability and promoting sustainable development. This research analyzes the risk effects and risk propagation mechanisms in agricultural systems using the DCC-t-Copula-CoVaR model, multi-layer network structures, and the mixed-frequency regression MIDAS model. The study finds that there is significant heterogeneity in risk spillover and absorption in agricultural systems; the risk propagation in agricultural systems is stable, and the stronger the connectivity of industry nodes, the greater the risk. Taking the seed industry as an example, its structural indicator values consistently range between 1.0 and 1.1, with fluctuations closely linked to industry development and policy adjustments. Major risks are caused by risk resonance across multiple industries, not triggered by a single industry alone; the interconnections between industries within the agricultural system can disperse risks, forming a collective risk-sharing mechanism. Understanding these dynamics is essential for developing resilient agricultural practices that support long-term sustainability, ensuring food security, and mitigating environmental impacts. By addressing risk propagation and fostering interconnected risk-sharing mechanisms, agricultural systems can better adapt to challenges such as climate change, resource scarcity, and market volatility, ultimately contributing to a more sustainable and stable global food system. Full article

This study investigated the mechanical, thermal, and morphological properties of acrylonitrile butadiene styrene (ABS)-based nanocomposites reinforced with different types and concentrations of nanofillers. The uniaxial tensile testing results indicated that Young’s modulus (E) generally decreased with increasing filler content, except at 0.500 w.% filler concentration, where a slight increase in stiffness was observed. A statistically significant interaction between sample type and filler concentration was identified (p = 0.045). Fracture toughness measurements revealed a significant reduction in impact resistance at 1.000 w.% filler concentration, with values dropping by up to 67% compared with neat acrylonitrile butadiene styrene. Dynamic mechanical vibration testing confirmed a decrease in stiffness, as evidenced by a shift of the first resonance frequency (f_R1) to lower values. Hardness measurements including indentation and Shore D hardness exhibited an increasing trend with rising filler concentration, with statistically significant differences observed at specific concentration levels (p < 0.05). Scanning electron microscopy analysis showed that nanofillers were well dispersed at lower concentrations, but agglomeration began above 0.500 w.%, resulting in void formation and a noticeable decline in mechanical properties. The results suggest that an optimal filler concentration range of 0.250–0.500 w.% offers an ideal balance between enhanced mechanical properties and material integrity. Full article

This paper introduces a novel method for measuring the dielectric permittivity of materials within the microwave and millimeter wave frequency ranges. The proposed approach, classified as a guided wave transmission system, employs a periodic transmission line structure characterized by mirror/glide symmetry. The dielectric permittivity is deduced by measuring the transmission properties of such structure when presence of the dielectric material breaks the inherent symmetry of the structure and consequently introduce a stopband in propagation characteristic. To explore the influence of symmetry breaking on propagation properties, an analytical dispersion equation, for both symmetries, is formulated using the Rigorous Coupled Wave Analysis (RCWA) combined with the matrix transverse resonance condition. Based on the analytical equation, an optimization procedure and linearized model for a sensing structure is obtained, specifically for X-band characterization of FR4 substrates. The theoretical results of the model are validated with full wave simulations and experimentally. Full article

The simulations presented here are based on the observational data of lightning electric currents associated with the eruption of the Hunga Tonga volcano in January 2022. The response of the lithosphere (Earth)–atmosphere–ionosphere–magnetosphere system to unprecedented lightning currents is theoretically investigated at low frequencies, including ultra low frequency (ULF), extremely low frequency (ELF), and very low frequency (VLF) ranges. The electric current source due to lightning near the location of the Hunga Tonga volcano eruption has a wide-band frequency spectrum determined in this paper based on a data-driven approach. The spectrum is monotonous in the VLF range but has many significant details at the lower frequencies (ULF, ELF). The decreasing amplitude tendency is maintained at frequencies exceeding 0.1 Hz. The density of effective lightning current in the ULF range reaches the value of the order of 10⁻⁷ A/m². A combined dynamic/quasi-stationary method has been developed to simulate ULF penetration through the lithosphere (Earth)–atmosphere–ionosphere–magnetosphere system. This method is suitable for the ULF range down to 10⁻⁴ Hz. The electromagnetic field is determined from the dynamics in the ionosphere and from a quasi-stationary approach in the atmosphere, considering not only the electric component but also the magnetic one. An analytical/numerical method has been developed to investigate the excitation of the global Schumann resonator and the eigenmodes of the coupled Schumann and ionospheric Alfvén resonators in the ELF range and the eigenmodes of the Earth–ionosphere waveguide in the VLF range. A complex dispersion equation for the corresponding disturbances is derived. It is shown that oscillations at the first resonance frequency in the Schumann resonator can simultaneously cause noticeable excitation of the local ionospheric Alfvén resonator, whose parameters depend on the angle between the geomagnetic field and the vertical direction. VLF propagation is possible over distances of 3000–10,000 km in the waveguide Earth–ionosphere. The results of simulations are compared with the published experimental data. Full article

The reliable generation of dissipative Kerr solitons (DKSs) enables applications in communications, metrology, optical clocks, and, more recently, artificial intelligence. We show how single DKS can be generated by Si₃N₄ dual-coupled microring resonators (DCMs). We modeled this coupled structure using the Lugiato–Lefever equation (LLE), including mode interactions in the dispersion profile. We also characterized the pump power and detuning parameter space for several mode interaction strengths and frequencies, and we found parameters for which a DKS could be deterministically obtained using a single, adiabatic frequency sweep with a constant pump power. We demonstrated deterministic single DKS generation for this path by simulating 200 times with different random noise inputs. This result paves the way for reliable, inexpensive, and deterministic single DKS generation in a simple setup. Full article

The intrinsic third-order nonlinearity in silicon has proven it to be quite useful in the field of quantum optics. Silicon is suitable for producing time-correlated photon pairs that are sources of heralded single-photon states for quantum integrated circuits. A quantum signal source in the form of single photons is an inherent requirement for the principles of quantum key distribution technology for secure communications. Here, we present numerical simulations of a silicon ring with a 6 μ m radius side-coupled with a bus waveguide as the source for the generation of single photons. The photon pairs are generated by exploring the process of degenerate spontaneous four-wave mixing (SFWM). The free spectral range (FSR) of the ring is quite large, simplifying the extraction of the signal/idler pairs. The phase-matching condition is considered by studying relevant parameters like the dispersion and nonlinearity. We optimize the ring for a high quality factor by varying the gap between the bus and the ring waveguide. This is the smallest ring studied for photon pair generation with a quality factor in the order of 10 5 . The width of the waveguides is chosen such that the phase-matching condition is satisfied, allowing for the propagation of fundamental modes only. The bus waveguide is pumped at one of the ring resonances with the minimum dispersion (1543.5 nm in our case) to satisfy the principle of energy conservation. The photon pair generation rate achieved is comparable to the state of the art. The photon pair sources exploiting nonlinear frequency conversion/generation processes is a promising alternative to atom-like single-photon emitters in the field of integrated photonics. Such miniaturized structures will benefit future on-chip architectures where multiple single-photon source devices are required on the same chip. Full article

Nowadays, the Internet of Things (IOT), electronics, and neural interfaces are becoming an integral part of our life. These technologies place unprecedentedly high demands on materials in terms of their mechanical and electrical properties. There are several strategies for forming conductive layers in such composites, e.g., volume blending to achieve a percolation threshold, inkjet printing, lithography, and laser processing. The latter is a low-cost, environmentally friendly, scalable way to produce composites. In our work, we synthesized AgNW and characterized them using Ultraviolet-visible spectroscopy (UV-vis), Transmission electron microscopy (TEM), and Selective area electron diffraction (SAED). We found that our AgNW absorbed in the UV-vis range of 345 to 410 nm. This is due to the plasmon resonance phenomenon of AgNW. Then, we applied the dispersion of AgNW on the surface of the polymer substrate, dried them and we got the films of AgNW.. We irradiated these films with a 432 nm laser. As a result of the treatment, we observed two processes. The first one was the sintering and partial melting of nanowires under the influence of laser radiation, as a consequence of which, the sheet resistance dropped more than twice. The second was the melting of the polymer at the interface and the subsequent integration of AgNW into the substrate. This allowed us to improve the adhesion from 0–1 B to 5 B, and to obtain a composite capable of bending, with radius of 0.5 mm. We also evaluated the shielding efficiency of the obtained composites. The shielding efficiency for 500–600 nm thick porous film samples were 40 dB, and for 3.1–4.1 µm porous films the shielding efficiency was about 85–90 dB in a frequency range of 0.01–40 GHz. The data obtained by us are the basis for producing flexible electronic components based on AgNW/PET composite for various applications using laser processing methods. Full article

In this study, the resonant characteristics of the off-center-driven Chladni plates were systematically investigated for the square and equilateral triangle shapes. Experimental results reveal that the number of the resonant modes is considerably increased for the plates under the off-center-driving in comparison to the on-center-driving. The Green’s functions derived from the nonhomogeneous Helmholtz equation are exploited to numerically analyze the information entropy distribution and the resonant nodal-line patterns. The experimental resonant modes are clearly confirmed to be in good agreement with the maximum entropy states in the Green’s functions. Furthermore, the information entropy distribution of the Green’s functions can be used to reveal that more eigenmodes can be triggered in the plate under the off-center-driving than the on-center-driving. By using the multiplication of the resonant modes in the off-center-driving, the dispersion relation between the experimental frequency and the theoretical wave number can be deduced with more accuracy. It is found that the deduced dispersion relations agree quite well with the Kirchhoff–Love plate theory. Full article