Chiral optical solitons in an electrically active multiferroic guiding structure.

A stack of a dielectric planar waveguide with a Kerr-type nonlinearity, sandwiched between two oxide-based helical multiferroic layers is shown to support electrically-controlled chiral solitons. These findings follow from analytical and full numerical simulations. The analytical scheme delivers explicit material parameters for the guided mode soliton and unveils how the soliton propagation characteristics are controlled by tuning the multiferroic helicity and amplitude of the injected electromagnetic wave. Silicon and CS2 are considered as the optical media in the guiding region enclosed by the multiferroic slabs. CS2 has very similar nonlinearity characteristics to silicon but in the linear regime it exhibits a smaller refractive index in the THz frequency range. The scattering simulations are performed using our developed numerical code based on the rigorous coupled wave method and the results for the dispersion curve for the guided mode agree very well with the analytical formula that we derive in this work. The results demonstrate a case of nonlinear pulse generation with field-controlled, nontrivial topological properties.

Tunable chiral photonic cavity based on multiferroic layers.

Realization of externally tunable chiral photonic sources and resonators is essential for studying and functionalizing chiral matter. Here, oxide-based stacks of helical multiferroic layers are shown to provide a suitable, electrically-controllable medium to efficiently trap and filter purely chiral photonic fields. Using analytical and rigorous coupled wave numerical methods we simulate the dispersion and scattering characteristics of electromagnetic waves in multiferroic heterostructures. The results evidence that due to scattering from the spin helix texture, only the modes with a particular transverse wavenumber form standing chiral waves in the cavity, whereas all other modes leak out from the resonator. An external static electric field enables a nonvolatile and energy-efficient control of the vector spin chirality associated with the oxide multilayers, which tunes the photonic chirality density in the resonator.

Curvature dependent onset of quantum tunneling in subnanometer gaps.

The quantum tunneling in subnanometer gap sizes in gold dimers is studied in order to account for the dependency of the onset of quantum tunneling on the dimer's radius and accordingly the gap wall's curvature, realized in experiments. Several nanodimers both nanowires and nanospheres with various radii and gap sizes are modelled and simulated based on the quantum corrected model, determining the onset of the quantum tunneling. Results show that the onset of quantum tunneling is both dependent on the gap size as well as on the dimer's radius. As larger dimers result in larger effective conductivity volumes, the influence of the quantum tunneling begins in larger gap sizes in larger dimers.

The Importance of Subcellular Structures to the Modeling of Biological Cells in the Context of Computational Bioelectromagnetics Simulations.

Numerical investigation of the interaction of electromagnetic fields with eukaryotic cells requires specifically adapted computer models. Virtual microdosimetry, used to investigate exposure, requires volumetric cell models, which are numerically challenging. For this reason, a method is presented here to determine the current and volumetric loss densities occurring in single cells and their distinct compartments in a spatially accurate manner as a first step toward multicellular models within the microstructure of tissue layers. To achieve this, 3D models of the electromagnetic exposure of generic eukaryotic cells of different shape (i.e. spherical and ellipsoidal) and internal complexity (i.e. different organelles) are performed in a virtual, finite element method-based capacitor experiment in the frequency range from 10 Hz to 100 GHz. In this context, the spectral response of the current and loss distribution within the cell compartments is investigated and any effects that occur are attributed either to the dispersive material properties of these compartments or to the geometric characteristics of the cell model investigated in each case. In these investigations, the cell is represented as an anisotropic body with an internal distributed membrane system of low conductivity that mimics the endoplasmic reticulum in a simplified manner. This will be used to determine which details of the cell interior need to be modeled, how the electric field and the current density will be distributed in this region, and where the electromagnetic energy is absorbed in the microstructure regarding electromagnetic microdosimetry. Results show that for 5 G frequencies, membranes make a significant contribution to the absorption losses. © 2023 The Authors. Bioelectromagnetics published by Wiley Periodicals LLC on behalf of Bioelectromagnetics Society.

Thickness-dependent slow light gap solitons in three-dimensional coupled photonic crystal waveguides.

The thickness-dependent multimodal nature of three-dimensional (3D) coupled photonic crystal waveguides is investigated with the aim of realizing a medium for controlled optical gap soliton formation in the slow light regime. In the linear case, spectral properties of the modes (dispersion diagrams), location of the gap regions versus the thickness of the 3D photonic crystal, and the near-field distributions at frequencies in the slow light region are analyzed using a full-wave electromagnetic solver. In the nonlinear regime (Kerr-type nonlinearity), we infer an existence of crystal-thickness-dependent temporal solitons with stable pulse envelope and use the solitonic pulses for driving quantum transitions in localized quantum systems within the photonic crystal waveguide. The results may be useful for applications in optical communications, multiplexing systems, nonlinear physics, and ultrafast spectroscopy.

Photonic Signatures of Spin-Driven Ferroelectricity in Multiferroic Dielectric Oxides.

We study the dispersion and scattering properties of electromagnetic modes coupled to a helically ordered spin lattice hosted by a dielectric oxide with a ferroelectric polarization driven by vector spin chirality. Quasianalytical approaches and full-fledged numerics evidence the formation of a chiral magnonic photonic band gap and the presence of gate-voltage dependent circular dichroism in the scattering of electromagnetic waves from the lattice. Gating couples to the emergent ferroelectric polarization and hence, to the underlying vector-spin chirality. The theory relies on solving simultaneously Maxwell's equations coupled to the driven localized spins taking into account their spatial topology and spatial anisotropic interactions. The developed approach is applicable to various settings involving noncollinear spins and multiferroic systems with potential applications in noncollinear magnetophotonics.

Three-Dimensional Magnetic Induction Tomography: Practical Implementation for Imaging throughout the Depth of a Low Conductive and Voluminous Body.

Magnetic induction tomography (MIT) is a contactless, low-energy method used to visualize the conductivity distribution inside a body under examination. A particularly demanding task is the three-dimensional (3D) imaging of voluminous bodies in the biomedical impedance regime. While successful MIT simulations have been reported for this regime, practical demonstration over the entire depth of weakly conductive bodies is technically difficult and has not yet been reported, particularly in terms of more realistic requirements. Poor sensitivity in the central regions critically affects the measurements. However, a recently simulated MIT scanner with a sinusoidal excitation field topology promises improved sensitivity (>20 dB) from the interior. On this basis, a large and fast 3D MIT scanner was practically realized in this study. Close agreement between theoretical forward calculations and experimental measurements underline the technical performance of the sensor system, and the previously only simulated progress is hereby confirmed. This allows 3D reconstructions from practical measurements to be presented over the entire depth of a voluminous body phantom with tissue-like conductivity and dimensions similar to a human torso. This feasibility demonstration takes MIT a step further toward the quick 3D mapping of a low conductive and voluminous object, for example, for rapid, harmless and contactless thorax or lung diagnostics.

Functional all-optical logic gates for true time-domain signal processing in nonlinear photonic crystal waveguides.

We present a conceptual study on the realization of functional and easily scalable all-optical NOT, AND and NAND logic gates using bandgap solitons in coupled photonic crystal waveguides. The underlying structure consists of a planar air-hole type photonic crystal with a hexagonal lattice of air holes in crystalline silicon (c-Si) as the nonlinear background material. The remaining logical operations can be performed using combinations of these three logic gates. A unique feature of the proposed working scheme is that it operates in the true time-domain, enabling temporal solitons to maintain a stable pulse envelope during each logical operation. Hence, multiple concatenated all-optical logic gates can be easily realized, paving the way to multiple-input all-optical logic gates for ultrafast full-optical digital signal processing. In the suggested setup, there is no need to amplify the output signal after each operation, which can be directly used as a new input signal for another logical operation. The feasibility and efficiency of the proposed logic gates as well as their scalability is demonstrated using our original rigorous theoretical formalism together with full-wave computational electromagnetics.

Modeling disorder in two-dimensional colloidal crystals based on electron microscope measurements.

Self-assembled two-dimensional colloidal crystals (CCs) are critical components in many optical and optoelectronic devices. Such structures usually exhibit various types of disorder, which sometimes can be beneficial for the desired applications. However, disorder poses challenges to the modeling of two-dimensional structures. In this work, two-dimensional CCs employed in optoelectronic devices, especially dye-sensitized solar cells, are investigated. scanning electron microscope (SEM) images were used to quantify the disorder in the studied structures. As a basis for simulations, disordered model patterns were generated with properties extracted from the SEM images of prepared samples. Optical modeling was performed with a finite-difference time-domain simulator. The simulated transmission data are consistent with the experimentally measured spectra.

Three-dimensional Magnetic Induction Tomography: Improved Performance for the Center Regions inside a Low Conductive and Voluminous Body.

Magnetic induction tomography (MIT) is a contactless technique that is used to image the distribution of passive electromagnetic properties inside a voluminous body. However, the central area sensitivity (CAS) of this method is critically weak and blurred for a low conductive volume. This article analyzes this challenging issue, which inhibits even faint imaging of the central interior region of a body, and it suggests a remedy. The problem is expounded via two-dimensional (2D) and three-dimensional (3D) eddy current simulations with different transmitter geometries. On this basis, it is shown that a spatially undulating exciter coil can significantly improve the CAS by >20 dB. Consequently, the central region inside a low conductive voluminous object becomes clearly detectable above the noise floor, a fact which is also confirmed by practical measurements. The improved sensitivity map of the new arrangement is compared with maps of more typical circular MIT geometries. In conclusion, 3D MIT reconstructions are presented, and for the same incidence of noise, their performance is much better with the suggested improvement than that with a circular setup.

Theory of soliton propagation in nonlinear photonic crystal waveguides.

Propagation of the temporal soliton in Kerr-type photonic crystal waveguide is investigated theoretically and numerically. An expression describing the evolution of the envelope of the soliton based on the full-wave modal analysis, taking into account all space-harmonics, is rigorously obtained. The nonlinear coefficient is derived, for the first time, based on a modification of the refractive indices for each space-harmonic due to the Kerr-type nonlinearity. For illustrating the general formulation and results, we performed extensive computational electromagnetics simulations for the propagation of gap solitons in an experimentally feasible photonic crystal waveguides, endorsing the correctness and usefulness of the proposed formalism.

Analysis of Scattering by Plasmonic Gratings of Circular Nanorods Using Lattice Sums Technique.

A self-contained formulation for analyzing electromagnetic scattering by a significant class of planar gratings composed of plasmonic nanorods, which were infinite length along their axes, is presented. The procedure for the lattice sums technique was implemented in a cylindrical harmonic expansion method based on the generalized reflection matrix approach for full-wave scattering analysis of plasmonic gratings. The method provided a high computational efficiency and can be considered as one of the best-suited numerical tools for the optimization of plasmonic sensors and plasmonic guiding devices both having a planar geometry. Although the proposed formalism can be applied to analyze a wide class of plasmonic gratings, three configurations were studied in the manuscript. Firstly, a multilayered grating of silver nanocylinders formed analogously to photonic crystals was considered. In the region far from the resonances of a single plasmonic nanocylinder, the structure showed similar properties compared to conventional photonic crystals. When one or a few nanorods were periodically removed from the original crystal, thus forming a crystal with defects, a new band was formed in the spectral responses because of the resonant tunneling through the defect layers. The rigorous formulation of plasmonic gratings with defects was proposed for the first time. Finally, a plasmonic planar grating of metal-coated dielectric nanorods coupled to the dielectric slab was investigated from the viewpoint of design of a refractive index sensor. Dual-absorption bands attributable to the excitation of the localized surface plasmons were studied, and the near field distributions were given in both absorption bands associated with the resonances on the upper and inner surfaces of a single metal-coated nanocylinder. Resonance in the second absorption band was sensitive to the refractive index of the background medium and could be useful for the design of refractive index sensors. Also analyzed was a phase-matching condition between the evanescent space-harmonics of the plasmonic grating and the guided modes inside the slab, leading to a strong coupling.

Realization of true all-optical AND logic gate based on nonlinear coupled air-hole type photonic crystal waveguides.

In this manuscript we propose an easily scalable true all-optical AND logic gate for pulsed signal operation based on band-gap transmission within nonlinear realistic air-hole type coupled photonic crystal waveguides (C-PCW). We call it "true" all-optical AND logic gate, because all AND gate topologies operate with temporal solitons that maintain a stable pulse envelope during the optical signal processing along the different C-PCW modules yielding ultrafast full-optical digital signal processing. We directly use the registered (output) signal pulse as new input signal between multiple concatenated nonlinear C-PCW modules (i.e. AND gates) to setup a multiple-input true all-optical AND logic gate. Extensive full-wave computational electromagnetic analysis proves the correctness of our theoretical studies and the proposed operation principle of the multiple-input AND logic gate is vividly demonstrated for realistic C-PCWs.

Numerical investigation of nematic liquid crystals in the THz band based on EIT sensor.

This paper introduces the concept of electromagnetically induced transparency (EIT) into the permittivity extraction of an anisotropic material-nematic liquid crystal (NLC). A novel two-step strategy is presented to extract the complex permittivity of the NLC at the THz band, which evaluates the relative permittivity tensor from the resonant frequencies and then determines the loss tangent from the quality factor Q of the EIT sensor. The proposed method features high accuracy due to the sharp resonance of the EIT sensor and also high robustness to the thickness of the NLC layer because only amplitude rather than phase information of the transmission coefficients is required. The NLC filled EIT sensor shows a sensitivity of 56.8 µm/RIU (the resonance wavelength shift over the refractive index change unit (RIU)) and Figure of Merit (FoM) of 6.92. The uncertainty of the proposed technique in the relative permittivity and loss tangent is 3% and 8.2%, respectively.

Particle Size-Dependent Onset of the Tunneling Regime in Ideal Dimers of Gold Nanospheres.

We report on the nanoparticle-size-dependent onset of quantum tunneling of electrons across the subnanometer gaps in three different sizes (30, 50, and 80 nm) of highly uniform gold nanosphere (AuNS) dimers. For precision plasmonics, the gap distance is systematically controlled at the level of single C-C bonds via a series of alkanedithiol linkers (C2-C16). Parallax-corrected high-resolution transmission electron microscope (HRTEM) imaging and subsequent tomographic reconstruction are employed to resolve the nm to subnm interparticle gap distances in AuNS dimers. Single-particle scattering experiments on three different sizes of AuNS dimers reveal that for the larger dimers the onset of quantum tunneling regime occurs at larger gap distances: 0.96 ± 0.04 nm (C6) for 80 nm, 0.83 ± 0.03 nm (C5) for 50 nm, and 0.72 ± 0.02 nm (C4) for 30 nm dimers. 2D nonlocal and quantum-corrected model (QCM) calculations qualitatively explain the physical origin for this experimental observation: the lower curvature of the larger particles leads to a higher tunneling current due to a larger effective conductivity volume in the gap. Our results have possible implications in scenarios where precise geometrical control over plasmonic properties is crucial such as in hybrid (molecule-metal) and/or quantum plasmonic devices. More importantly, this study constitutes the closest experimental results to the theory for a 3D sphere dimer system and offers a reference data set for comparison with theory/simulations.

Examination of the Liquid Volume Inside Metal Tanks Using Noncontact EMATs From Outside.

This article discusses ultrasound echo examination of the liquid volume inside a metal enclosure using noncontact electromagnetic acoustic transducers (EMATs) from outside the tank. Because only longitudinal sound waves exist in liquids, a novel and powerful EMAT design (kA currents in small coils) with a specifically enhanced transduction of longitudinal ultrasound is presented. Different wall materials (aluminum, steel, and stainless steel) and working frequencies are considered. A particular challenge, in addition to the already weak EMAT signals across an air gap, is the highly reflective interface between the metal wall and liquid because the acoustic impedances of the two media differ considerably. With thicker metal walls (here, 3mm), only either low frequencies < 100 kHz or frequencies close to the corresponding thickness resonance of the wall (here, about 1 MHz) are more suitable. The higher frequencies are preferred, as they show advantages due to a directed beam profile, shorter wavelengths in the liquid, and an overall better pulse fidelity. The simultaneous operation of several and closely neighboring EMATs demonstrates the feasibility of more demanding detection tasks, ultimately leading toward a 3-D localization inside the liquid using a noncontact EMAT array with eight independent elements.

Formation of Fe-Ni Nanoparticle Strands in Macroscopic Polymer Composites: Experiment and Simulation.

Magnetic-field-induced strand formation of ferromagnetic Fe-Ni nanoparticles in a PMMA-matrix is correlated with the intrinsic material parameters, such as magnetization, particle size, composition, and extrinsic parameters, including magnetic field strength and viscosity. Since various factors can influence strand formation, understanding the composite fabrication process that maintains the strand lengths of Fe-Ni in the generated structures is a fundamental step in predicting the resulting structures. Hence, the critical dimensions of the strands (length, width, spacing, and aspect ratio) are investigated in the experiments and simulated via different intrinsic and extrinsic parameters. Optimal parameters were found by optical microscopy measurements and finite-element simulations using COMSOL for strand formation of Fe50Ni50 nanoparticles. The anisotropic behavior of the aligned strands was successfully characterized through magnetometry measurements. Compared to the unaligned samples, the magnetically aligned strands exhibit enhanced conductivity, increasing the current by a factor of 1000.

Optical properties of a nanomatch-like plasmonic structure.

The optical properties of a match-like plasmonic nanostructure are numerically investigated using full-wave finite-difference time-domain analysis in conjunction with dispersive material models. This work is mainly motivated by the developed technique enabling reproducible fabrication of nanomatch structures as well as the growing applications that utilize the localized field enhancement in plasmonic nanostructures. Our research revealed that due to the pronounced field enhancement and larger resonance tunabilities, some nanomatch topologies show potentials for various applications in the field of, e.g., sensing as well as a novel scheme for highly reproducible tips in scanning near field optical microscopy, among others. Despite the additional degrees of freedom that are offered by the composite nature of the proposed nanomatch topology, the paper also reflects on a fundamental complication intrinsic to the material interfaces especially in the nanoscale: stoichiometric mixing. We conclude that the specificity in material modeling will become a significant issue in future research on functionalized composite nanostructures.

Optical forces on metallic nanoparticles induced by a photonic nanojet.

We investigate the optical forces acting on a metallic nanoparticle when the nanoparticle is introduced within a photonic nanojet (PNJ). Optical forces at resonance and off-resonance conditions of the microcylinder or nanoparticle are investigated. Under proper polarization conditions, the whispering gallery mode can be excited in the microcylinder, even at off resonance provided that scattering from the nanoparticle is strong enough. The optical forces are enhanced at resonance either of the single microcylinder or of the nanoparticle with respect to the forces under off-resonant illuminations. We found that the optical forces acting on the nanoparticle depend strongly on the dielectric permittivity of the nanoparticle, as well as on the intensity and the beam width of the PNJ. Hence, metallic sub-wavelength nanoparticle can be efficiently trapped by PNJs. Furthermore, the PNJ's attractive force can be simply changed to a repulsive force by varying the polarization of the incident beam. The changed sign of the force is related to the particle's polarizability and the excitation of localized surface plasmons in the nanoparticle.

Probing the SERS brightness of individual Au nanoparticles, hollow Au/Ag nanoshells, Au nanostars and Au core/Au satellite particles: single-particle experiments and computer simulations.

Different classes of plasmonic nanoparticles functionalized with the non-resonant Raman reporter molecule 4-MBA are tested for their SERS signal brightness at the single-particle level: gold nanoparticles, hollow gold/silver nanoshells, gold nanostars, and gold core/gold satellite particles. Correlative SERS/SEM experiments on a set of particles from each class enable the unambiguous identification of single particles by electron microscopy as well as the characterization of both their elastic (LSPR) and inelastic (SERS) scattering spectra. Experimental observations are compared with predictions from FEM computer simulations based on 3D models derived from representative TEM/SEM images. Single gold nanostars and single gold core/gold satellite particles exhibit a detectable SERS signal under the given experimental conditions, while single gold nanoparticles and single hollow gold/silver nanoshells are not detectable.