Planar Slab Research Articles

Tabletop Tunable Chiral Photonic Emitter.

The increasing interest in chiral light stems from its spiral trajectory along the propagation direction, facilitating the interaction between different polarization states of light and matter. Despite tremendous achievements in chiral light-related research, the generation and control of chiral pulses have presented enduring challenges, especially at the terahertz and ultraviolet spectral ranges, due to the lack of suitable optical elements for effective pulse manipulation. Conventionally, chiral light can be obtained from intricate optical systems, by an external magnetic field, or by metamaterials, which necessitate sophisticated optical configurations. Here, leveraging the high harmonic generation process, we propose a versatile tunable chiral emitter, composed of only two planar Weyl semimetals slabs, addressing the challenges in both spectral ranges. Our results open the way to a compact tunable chiral emitter platform in both terahertz and ultra-violet frequency ranges. This advancement holds the potential to serve as the cornerstone for integrated chiral photonics.

Uncovering Maximum Chirality in Resonant Nanostructures.

Chiral nanostructures allow engineering of chiroptical responses; however, their design usually relies on empirical approaches and extensive numerical simulations. It remains unclear if a general strategy exists to enhance and maximize the intrinsic chirality of subwavelength photonic structures. Here, we suggest a microscopic theory and uncover the origin of strong chiral responses of resonant nanostructures. We reveal that the reactive helicity density is critically important for achieving maximum chirality at resonances. We demonstrate our general concept on the examples of planar photonic crystal slabs and metasurfaces, where out-of-plane mirror symmetry is broken by a bilayer design. Our findings provide a general recipe for designing photonic structures with maximum chirality, paving the way toward many applications, including chiral sensing, chiral emitters and detectors, and chiral quantum optics.

Counterion Release from Macroion Assemblies of Planar Geometry.

Macroions such as nanoparticles, polyelectrolytes, ionic gels, and amphiphiles can form condensed, often self-assembled, phases that are embedded in a solvent region. The condensed phase contains not only the partially or fully immobile charges of their macroions but also corresponding counterions that are mobile and thus free to migrate out of their confinement into the solvent region where they benefit from high translational entropy. Based on the nonlinear Poisson-Boltzmann model for monovalent ions, we quantify the corresponding fraction of released counterions for a planar slab geometry of the macroion phase. Slab thickness, extension of the solvent phase, fixed background charge density provided by the macroions, and dielectric constants inside slab and solvent combine into three dimensionless parameters that the fraction of released counterions depends on. We calculate that fraction and analyze the limits of a thin macroion phase, a large solvent phase, and linearized theory, where simple analytic results become available. Of particular interest is the presence of a single-planar interface that separates a bulk macroion phase from an extended solvent region. We calculate the apparent surface charge density that emerges due to the released counterions. Our model yields a comprehensive description of counterion partitioning between a planar macroion phase and a solvent region on the level of mean-field electrostatics in the absence of added salt ions.

The structural and electronic properties of (001) surface of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) with first-principles calculations.

1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is a typical insensitive energetic material. It can be used in explosive formulations, such as PBX-9502 and LX-17-0. TATB is an intriguing and unusual explosive for another reason: it crystallizes into a wide array of planar hydrogen bonds, forming a graphite-like layered structure. Therefore, TATB is one of the important research objects, and its surface structure needs to be deeply understood. In this research work, the electronic and energetic properties of TATB (001) surface are explored. In this paper, the structural, electronic, energetic properties and impact sensitivity of TATB (001) surface structure at 0 and -3 GPa along with x-axis were calculated in this study using the first-principles calculations. The calculations in this paper are performed in the CASTEP code, which is based on the density functional theory with the first-principles calculation method using the plan-wave pseudopotential approach. The exchange-correlation interaction was adopted by the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional. The DFT-D method with the Grimme correction accurately models van der Waals interactions. To model the surface structures of TATB, the planar slab method was employed. We constructed TATB (001) periodic slabs including three layers with a 15-Å vacuum layer.

Phase separation of a magnetic fluid: Asymptotic states and nonequilibrium kinetics.

We study self-assembly in a colloidal suspension of magnetic particles by performing comprehensive molecular dynamics simulations of the Stockmayer (SM) model, which comprises spherical particles decorated by a magnetic moment. The SM potential incorporates dipole-dipole interactions along with the usual Lennard-Jones interaction and exhibits a gas-liquid phase coexistence observed experimentally in magnetic fluids. When this system is quenched from the high-temperature homogeneous phase to the coexistence region, the nonequilibrium evolution to the condensed phase proceeds with the development of spatial as well as magnetic order. We observe density-dependent coarsening mechanisms-a diffusive growth law ℓ(t)∼t^{1/3} in the nucleation regime and hydrodynamics-driven inertial growth law ℓ(t)∼t^{2/3} in the spinodal regimes. [ℓ(t) is the average size of the condensate at time t after the quench.] While the spatial growth is governed by the expected conserved order parameter dynamics, the growth of magnetic order in the spinodal regime exhibits unexpected nonconserved dynamics. The asymptotic morphologies have density-dependent shapes which typically include the isotropic sphere and spherical bubble morphologies in the nucleation region, and the anisotropic cylinder, planar slab, cylindrical bubble morphologies in the spinodal region. The structures are robust and nonvolatile, and exhibit characteristic magnetic properties. For example, the oppositely magnetized hemispheres in the spherical morphology impart the characteristics of a Janus particle to it. The observed structures have versatile applications in catalysis, drug delivery systems, memory devices, and magnetic photonic crystals, to name a few.

Surface Instability in a Nematic Elastomer.

Liquid crystal elastomers (LCEs) are soft phase-changing solids that exhibit large reversible contractions upon heating, Goldstone-like soft modes, and resultant microstructural instabilities. We heat a planar LCE slab to isotropic, clamp the lower surface, then cool back to nematic. Clamping prevents macroscopic elongation, producing compression and microstructure. We see that the free surface destabilizes, adopting topography with amplitude and wavelength similar to thickness. To understand the instability, we numerically compute the microstructural relaxation of a "nonideal" LCE energy. Linear stability reveals a Biot-like scale-free instability, but with oblique wave vector. However, simulation and experiment show that, unlike classic elastic creasing, instability culminates in a crosshatch without cusps or hysteresis, and is constructed entirely from low-stress soft modes.

Validity of One-Dimensional Radiative Transfer in Participating Media

For radiative transfer through an absorbing, scattering medium with planar geometry, the validity of the one-dimensional assumption is examined. Using the generalized zonal with Monte Carlo method as a solver, exact numerical solutions are generated to identify conditions under which the one-dimensional assumption is valid. Specifically, for the predictions of transmissivity and reflectivity for a collimated or diffuse energy beam through a planar slab, numerical data are presented to show that a medium aspect ratio (ratio of the width to the thickness) of five or more is a necessary but not sufficient condition for the data to agree with the one-dimensional result. The implication of this condition on the utilization of the one-dimensional radiative transfer assumption in some practical engineering problems such as pipe flow and thermal insulation is discussed.

Advantages and Disadvantages of Computational Dosimetry Strategies in the Low mmW Range: Comparison Between Multilayer Slab and Anthropomorphic Models

Wireless technologies spanning at the higher bands of microwave range (i.e., Ka-band) are gaining increasing importance in everyday life. Most recent wireless power transfer (WPT) applications work in the microwave and millimeter wave (mmW) frequencies, overlapping with the new frequency bands of the fifth-generation (5G) mobile technology. The spread of such novel electromagnetic (EM) sources raised the need to investigate their possible effects on population’s health. Numerical dosimetry is fundamental to assess the exposure of the human body. In the range 24–28 GHz, i.e., the low-band spectrum of the mmW, and bridge between microwaves and mmW, there is poor consensus on the best strategy to model the human body. This article proposes a comparison between the two numerical methodologies typically adopted for this frequency range: the use of multilayer planar slabs and realistic anthropomorphic numerical models. The aim is to highlight the advantages and limits of each method, by comparing EM exposure results obtained on the Virtual Population (ViP) model Duke with several multilayer planar slabs. The differences between the two methods are non-negligible, suggesting the need of further studies and the necessity of improving both modeling approaches, depending on the frequency of work and the investigated application.

Grating-incoupled waveguide-enhanced Raman sensor.

We report a waveguide-enhanced Raman spectroscopy (WERS) platform with alignment-tolerant under-chip grating input coupling. The demonstration is based on a 100-nm thick planar (slab) tantalum pentoxide (Ta2O5) waveguide and the use of benzyl alcohol (BnOH) and its deuterated form (d7- BnOH) as reference analytes. The use of grating couplers simplifies the WERS system by providing improved translational alignment tolerance, important for disposable chips, as well as contributing to improved Raman conversion efficiency. The use of non-volatile, non-toxic BnOH and d7-BnOH as chemical analytes results in easily observable shifts in the Raman vibration lines between the two forms, making them good candidates for calibrating Raman systems. The design and fabrication of the waveguide and grating couplers are described, and a discussion of further potential improvements in performance is presented.

Spectral singularities and threshold gain of a slab laser under illumination of a focused Gaussian beam

Spectral singularities of an infinite planar slab containing homogeneous optically active material at the focal plane of a thin lens are investigated. The field distribution of the beam behind the lens is Gaussian that we approximate by a plane wave concentrated near the optical axis at the focal plane of the lens, such that its phase and amplitude are distance-dependent. The consequences of this configuration are explored by determining the threshold gain of the active medium and tuning the resonance frequencies related to spectral singularities. We find that the spectral singularities and the threshold gain, besides that, vary with distance from the center of the Gaussian beam; also they change with the relative aperture of the focusing lens. Numerical studies show that for the central rays of the beam compared to the side rays, the resonances related to the spectral singularities occur at lower wavelengths (higher frequencies) with the highest threshold gain. Also, for the ray at a fixed distance from the center of the beam, increasing the relative aperture of the illuminating system leads to the same results. Numerical results confirm the theoretical findings.

Transient Heat Conduction in a Planar Slab With Convection and Radiation Effects

Abstract This article presents new semi-analytical solutions for transient heat conduction in a slab, which exchanges heat at the surface with its surroundings via convection, radiation, or simultaneous convection–radiation mechanisms. The concept of thermal penetration depth together with the integral method of heat balances form the basis of the formulation. Explicit expressions are derived that eliminate the need for numerical integration of the ordinary differential equations (ODEs) obtained from space integration of the heat equation. Verification of the temperature relations with a convection boundary condition is performed using an exact solution. In the case of radiation and combined convection–radiation boundary conditions, the accuracy of the explicit solutions is assessed against a numerical model. In all three cases, the new relations predict the surface temperature with high precision. For the radiation boundary condition, the computed midpoint temperature is also in excellent agreement with the numerical solution. In the case of convection heat exchange at the surface, the temperature at internal locations has close agreement with the exact solution for Biot numbers up to Bi = 1. The model prediction deviates from the exact solution in the interior positions for higher values of Bi with the highest deviation occurring at the center. For example, at Bi = 3, the highest relative error is 7.5%. Likewise, for the case of combined convection–radiation boundary conditions, the predicted midpoint temperature is found to closely follow the numerical solution at early stages of the process only. The predictability of the explicit solution of the convection-only boundary condition is more accurate than other approximate solutions at a Fourier number less than 0.2. Additional relations are presented for determination of the total heat transfer as a function of the Fourier and Biot numbers.

Reflection and transmission properties of a graphene-dielectric-thin resistive layer structure in the THz range

We studied two-dimensional planar dielectric slab, sandwiched by graphene and thin resistive layer from two sides. Problem geometry is illuminated by a H-polarized electromagnetic plane wave from upper side. It is expected to observe the reflection and transmission performance of such a composite slab geometry depending on the electrical and geometrical parameters. We used the local reflection and transmission coefficients method to determine the overall performance. It is seen that the proper selection of the electrical resistivity of the thin resistive layer reduces the reflection from lower boundary of slab and the electrical thickness becomes less important for high THz range. Then, the geometry turns to be an air-dielectric interface. This is a novel finding and completely different from the pure dielectric slab without coatings which has frequency dependent characteristics. Also higher reflections are observed due to the higher conductivity of graphene in the low THz range. Furthermore, a sample finite plate is constructed in a same manner and it is modeled by using CST software. Presented method using equivalent 2D profile model and CST results are compared and very good consistency is observed. In both cases, the reflection can be controlled with the chemical potential at low THz range and the selection of the relative permittivity of the dielectric material determines the reflectance level at higher THz scale. We demonstrate these statements in the numerical results section for various problem parameters and angle of incidence.

Manufacturing of Er[formula omitted]-doped planar waveguides on silica-on-silicon using femtosecond laser-induced plasma

We report fabrication and characterisation of the erbium-doped planar waveguide on a silica-on-silicon (SOS) wafer-offering low loss and strong light confinement suitable for engineering optical waveguide amplifier for the C-band (1530–1565 nm) of the optical fibre communication. Here we describe an ultrafast laser plasma doping (ULPD) technique that is carried out using the plasma induced by a femtosecond laser (wavelength 800 nm) with a repetition rate of 10 kHz and a pulse duration of 45 fs. The ULPD method presented here had been applied successfully for rare earth materials doping on SOS substrates previously using a fs-laser with a pulse duration of ∼100 fs and at a repetition rate of 1 kHz. The fabricated planar optical waveguide layer onto the SOS substrate has been analysed for thickness, refractive index, optical propagation loss, photoluminescence intensity, and photoluminescence lifetime. We report a low propagation loss of <0.4 dB/cm in the C-Band, a long lifetime of 13.21 ms at 1532 nm, and the largest lifetime-density product 6.344 ×10 19 s cm −3. The low loss planar slab waveguide and a high lifetime-density product promise the further possibility of fabricating strip-loaded waveguides on the SOS platform. The proposed active waveguide fabrication methodology is potentially useful for manufacturing planar integrated optical waveguide amplifiers and lasers compatible with silicon-based photonic integrated circuits.

Amplitude Non-Uniformity of Millimeter-Wave Vortex Beams Generated by Transmissive Structures

We investigate the origin of non-uniformity in an orbital angular momentum (OAM) beam generated by a transmissive structure in the millimeter-wave frequency range. This work includes the study of amplitude non-uniformity including modal composition analysis using the angular spectrum method (ASM) for OAM beams generated by spiral phase plates (SPPs) and flat-surface metamaterial structures (MSs). The dependency of the axial dimension, over which the OAM beam transforms from a Gaussian beam, on the amplitude non-uniformity of both the structures has been rigorously analyzed. For experimental verification, we designed a perforated planar dielectric slab to convert the Gaussian beam into an OAM beam of order 2 with improved amplitude uniformity. Within the manufacturing limitations, the design of the transmissive MS was fabricated and tested to obtain optimum amplitude uniformity in the generation of OAM beams and to compare with simulation results.

Broadband circularly polarized thermal radiation from magnetic Weyl semimetals

We numerically demonstrate that a planar slab made of magnetic Weyl semimetal (a class of topological materials) can emit high-purity circularly polarized (CP) thermal radiation over a broad mid- and long-wave infrared wavelength range for a significant portion of its emission solid angle. This effect fundamentally arises from the strong infrared gyrotropy or nonreciprocity of these materials, which primarily depends on the momentum separation between Weyl nodes in the band structure. We clarify the dependence of this effect on the underlying physical parameters and highlight that the spectral bandwidth of CP thermal emission increases with increasing momentum separation between the Weyl nodes. We also demonstrate, using the recently developed thermal discrete dipole approximation (TDDA) computational method, that finite-size bodies of magnetic Weyl semimetals can emit spectrally broadband CP thermal light, albeit over smaller portion of the emission solid angle compared to the planar slabs. Our work identifies unique fundamental and technological prospects of magnetic Weyl semimetals for engineering thermal radiation and designing efficient CP light sources.

Slab cholesteric waveguide with randomly fluctuating propagation constant

In this work, we present a stochastic theoretical analysis of an electromagnetic wave propagating inside a planar slab waveguide filled with a cholesteric material. In this system, the propagation constant of the wave is assumed to be a randomly fluctuating parameter. By means of the van Kampen systematic expansion method, Maxwell’s electromagnetic equations with random coefficients, and appropriate boundary conditions, are obtained. Numerical examples of the effects of the randomness on the propagating constant, on the ratio between magnetic and electric modes, on the field profiles, and on the energy flow are given. The results show that the stochastic waveguide can support multiple Transverse Magnetic (TM) or Transverse Electric (TE) modes with the same cutoff frequency, and furthermore, they support a distribution of electric and magnetic fields with phase shift. Our results are consistent with those of an isotropic waveguide.

Droplet condensation in the lattice gas with density functional theory.

A density functional for the lattice gas with next-neighbor attractions (Ising model) from fundamental measure theory is applied to the problem of droplet states in three-dimensional, finite systems. The density functional is constructed via an auxiliary model with hard lattice gas particles and lattice polymers to incorporate the attractions. Similar to previous simulation studies, the sequence of droplets changing to cylinders and to planar slabs is found upon increasing the average density ρ[over ¯] in the system. Owing to the discreteness of the lattice, additional effects in the state curve for the chemical potential μ(ρ[over ¯]) are seen upon lowering the temperature away from the critical temperature [oscillations in μ(ρ[over ¯]) in the slab portion and spiky undulations in μ(ρ[over ¯]) in the cylinder portion as well as an undulatory behavior of the radius of the surface of tension R_{s} in the droplet region]. This behavior in the cylinder and droplet region is related to washed-out layering transitions at the surface of liquid cylinders and droplets. The analysis of the large-radius behavior of the surface tension γ(R_{s}) gave a dominant contribution ∝1/R_{s}^{2}, although the consistency of γ(R_{s}) with the asymptotic behavior of the radius-dependent Tolman length seems to suggest a weak logarithmic contribution ∝lnR_{s}/R_{s}^{2} in γ(R_{s}). The coefficient of this logarithmic term is smaller than a universal value derived with field-theoretic methods.

Slab cholesteric waveguide with randomly fluctuating director

A stochastic theoretical analysis of electromagnetic wave propagation inside a planar slab waveguide filled with a cholesteric material, whose azimuthal angular component on its director n is randomly fluctuating is presented in this article. By means of van Kampen systematic expansion method, Maxwell's electromagnetic equations with random dielectric tensor and appropriate boundary conditions are obtained. As numerical examples, the effects of the randomness on the propagating constant, on the ratio between magnetic and electric modes, on the field profiles, and on the energy flow are presented and discussed. The results in this paper are consistent with those corresponding to the deterministic conventional waveguide.

Trimetallic magnetite-Ti-Au nanoparticle formation: A theoretical approach

Geometric, electronic and magnetic structure of planar slabs consisting of magnetite Fe3O4, titanium and gold layers are investigated by DFT-GGA calculations. It is assumed that these slabs can be used to simulate the upper layers of magnetite nanoparticles covered with an intermediate layer of titanium and a gold layer on the surface. Specific energies and spreading parameters (wettability) of the magnetite-gold, magnetite-titanium and titanium-gold interfaces are calculated. The specific energy and spreading parameter of the magnetite-gold interface is found to be negative, while these values of the magnetite-titanium (for thin Ti layer) and magnetite-titan-gold interfaces are significantly positive. This allows us to hope that the intermediate thin layer of titanium at the boundary between the surface of the magnetite nanoparticle and the gold layer stabilizes this three-layer structure and allows obtaining magnetite nanoparticles covered with continuous gold coating.

A massive ancient river-damming landslide triggered by buckling failure in the upper Jinsha River, SE Tibetan Plateau

Large-scale landsliding is an extremely important geological process in shaping landscapes in the Tibetan Plateau. In this research, an ancient river-damming landslide with an estimated debris volume of 4.9 × 107 m3, located in the upper Jinsha River, SE Tibetan Plateau, was studied. The landslide once formed a dam over 60 m high and blocked the river. Lacustrine sediments, composed of silty clay with particle sizes of 0.002–0.25 mm, are intermittently distributed along both banks, extending about 6.5 km upstream. The OSL dating indicates that the lacustrine sediments have an age of 2.6 ± 0.2 ka. Detailed field investigation and theoretical analysis was performed to investigate the characteristics, potential cause and mechanism of the landslide. The results suggest that the landslide was most likely triggered by buckling of planar rock slabs under gravity. It may start as a translational sliding along the weak interlayer composed of mica schist at the upper part of the slope and then formed buckles by curving amphibolite rock beds near the slope toe. The hillslope has still been affected by gravitational deformations, with geomorphology characterized by tension cracks, buckle folds, and small landslide scars distributed on the slope surface, suggesting that the evolution of the river valley caused by buckling deformation has not achieved equilibrium.