Slab Modes Research Articles

In situ evaluation of dynamic characteristics of prefabricated ballastless track slab using EMA and OMA techniques

Abstract This study focused on the prefabricated ballastless slab tracks first applied in urban rail transit of a city in China. By combining the experimental modal analysis (EMA) and operational modal analysis (OMA) techniques, we evaluated the dynamic characteristics of the tracks using the eigensystem realization algorithm and stochastic subspace identification algorithm to identify modal parameters. This study attempted to determine the characteristics and correlation of the two modal analysis methods to comprehensively reflect the dynamic characteristics of the track slabs. We established a detailed finite element model to analyze the influence of the construction method and structural characteristics of the track slabs on the modal parameters. The results show that the in-situ testing of the track slab modes were subject to the influence of many uncertain factors, resulting in differences between the numerical calculation-based ideal mode shapes and the actual EMA- and OMA-based counterparts.

Coupling between plasmonic and photonic crystal modes in suspended three-dimensional meta-films.

A complementary metal oxide semiconductor (CMOS) compatible fabrication method for creating three-dimensional (3D) meta-films is presented. In contrast to metasurfaces, meta-films possess structural variation throughout the thickness of the film and can possess a sub-wavelength scale structure in all three dimensions. Here we use this approach to create 2D arrays of cubic silicon nitride unit cells with plasmonic inclusions of elliptical metallic disks in horizontal and vertical orientations with lateral array-dimensions on the order of millimeters. Fourier transform infrared (FTIR) spectroscopy is used to measure the infrared transmission of meta-films with either horizontally or vertically oriented ellipses with varying eccentricity. Shape effects due to the ellipse eccentricity, as well as localized surface plasmon resonance (LSPR) effects due to the effective plasmonic wavelength are observed in the scattering response. The structures were modeled using rigorous coupled wave analysis (RCWA), finite difference time domain (Lumerical), and frequency domain finite element (COMSOL). The silicon nitride support structure possesses a complex in-plane photonic crystal slab band structure due to the periodicity of the unit cells. We show that adjustments to the physical dimensions of the ellipses can be used to control the coupling to this band structure. The horizontally oriented ellipses show narrow, distinct plasmonic resonances while the vertically oriented ellipses possess broader resonances, with lower overall transmission amplitude for a given ellipse geometry. We attribute this difference in resonance behavior to retardation effects. The ability to couple photonic slab modes with plasmonic inclusions enables a richer space of optical functionality for design of metamaterial-inspired optical components.

Revealing the underlying mechanisms behind TE extraordinary THz transmission

Transmission through seemingly opaque surfaces, so-called extraordinary transmission, provides an exciting platform for strong light–matter interaction, spectroscopy, optical trapping, and color filtering. Much of the effort has been devoted to understanding and exploiting TM extraordinary transmission, while TE anomalous extraordinary transmission has been largely omitted in the literature. This is regrettable from a practical point of view since the stronger dependence of the TE anomalous extraordinary transmission on the array’s substrate provides additional design parameters for exploitation. To provide high-performance and cost-effective applications based on TE anomalous extraordinary transmission, a complete physical insight about the underlying mechanisms of the phenomenon must be first laid down. To this end, resorting to a combined methodology including quasi-optical terahertz (THz) time-domain measurements, full-wave simulations, and method of moments analysis, subwavelength slit arrays under s-polarized illumination are studied here, filling the void in the current literature. We believe this work unequivocally reveals the leaky-wave role of the grounded-dielectric slab mode mediating in TE anomalous extraordinary transmission and provides the necessary framework to design practical high-performance THz components and systems.

Effects of hydrostatic pressure on the photonic band structure and quality factor of an L3 cavity in a photonic crystal slab

In this work, we investigate the effects of hydrostatic pressure on the photonic band structure and quality factor of the fundamental mode in an L3 cavity using the guided-mode expansion method in two-dimensional photonic crystal slabs. The two-dimensional photonic crystal is composed of air holes with circular cross-sections arranged in a hexagonal lattice. Here, the dielectric constant of the slab is a function of hydrostatic pressure and temperature. Upon increasing pressure at a given temperature, the photonic band structure exhibits a shift towards higher frequencies for the TE-like and TM-like modes of the photonic crystal slab. In the L3 cavity, the lateral displacement of the holes must also be considered as the quality factor of the fundamental mode increases with the displacement of the holes. However, there is evidence that the confinement of the L3 cavity decreases when both the hydrostatic pressure and the symmetric lateral shift of the holes increase.

Tunneling-induced broadband and tunable optical emission from plasmonic nanorod metamaterials

Abstract We demonstrate a metamaterial platform for electrically driven broadband light emission induced by electron tunneling. Both the Fabry-Perot and waveguided modes of the metamaterial slab as well the plasmonic mode of the tunneling gap are identified as contributing to shaping the emission spectrum. This opens up an opportunity to design the spectrum and polarization of the emitted light by tuning the metamaterial modes via the geometric parameters of the nanostructure throughout the visible and near-infrared spectral ranges. The efficient coupling of the tunneling-induced emission to the waveguided modes is beneficial for the development of integrated incoherent light sources, while the outcoupled emission provides a source of free-space radiation. The demonstrated incoherent nanoscale light sources may find applications in the development of integrated opto-electronic circuits, optical sensing platforms, imaging, and metrology.

Analytical approximations for the dispersion of electromagnetic modes in slabs of biaxial crystals

Anisotropic crystals have recently attracted considerable attention because of their ability to support polaritons with a variety of unique properties, such as hyperbolic dispersion, negative phase velocity, or extreme confinement. Particularly, the biaxial crystal $\alpha$-MoO$_3$ has been demonstrated to support phonon polaritons, light coupled to lattice vibrations, with in-plane anisotropic propagation and unusually long lifetime. However, the lack of theoretical studies on electromagnetic modes in biaxial crystal slabs impedes a complete interpretation of the experimental data, as well as an efficient design of nanostructures supporting such highly anisotropic polaritons. Here, we derive the dispersion relation of electromagnetic modes in biaxial slabs surrounded by semi-infinite isotropic dielectric half-spaces with arbitrary dielectric permittivities. Apart from a general dispersion relation, we provide very simple analytical expressions in typical experiments in nano-optics: the limits of short polaritonic wavelength and/or very thin slabs. The results of our study will allow for an in-depth analysis of anisotropic polaritons in novel biaxial van der Waals materials.

Experimental Investigation of Impact Behaviour of RC Slab with Different Reinforcement Ratios

Reinforced concrete (RC) slabs may be exposed to the low-velocity impact load during their service periods. In low-velocity impact scenarios, the effect of strain rates has been remarkably higher than quasi-static loading because the loading duration is very short. Thus, structural responses and failure modes will be different. The present study aims to investigate dynamic response and failure modes of simply supported two-way RC slabs exposed to low-velocity impact load. In the experimental part of this study, nine RC slabs with the dimension of 1,000 × 1,000 × 80 mm were tested. The reinforcement ratio of RC slabs and the input impact energy applied to RC slabs were experimental variables investigated. A drop-weight test setup was utilized to apply impact load to RC slabs. By varying drop-height as 1,000, 1,250 and 1,500 mm, three different impact energies have been applied to RC slabs via a hammer of which weight is 84 kg. The time histories of the accelerations, displacements and impact loads were recorded. The dynamic responses obtained by tests and the failure modes observed has been interpreted in detail. Besides, a finite element model where explicit dynamic analysis is performed has been established for verification of the experimental results. There was observed good accordance between numerical and experimental results. Consequently, it is considered that the present finite element treatment can be used for the evaluation of the dynamic responses and failure modes of RC slabs exposed to low-velocity impact load.

Investigation of backward cladding-mode coupling in Bragg gratings implemented on a Si3N4 waveguide platform

We present experimental and simulation results of sidewall Bragg gratings (BGs) implemented on a silicon nitride waveguide platform, with emphasis on the appreciable drop of transmission on the high-frequency side of the grating stopband due to the coupling into backward cladding/radiative modes. We differentiate this from the coupling into backward slab modes as observed in BGs implemented in silicon-on-insulator ridge waveguides. The dimension of the device top cladding affects the shape of the drop pattern. Different approaches for circumventing this drop pattern are discussed. A better understanding of this physical phenomenon will help guide the design of BGs on integrated photonic platforms.

Coupled microstrip-cavities under oblique incidence of semi-guided waves: a lossless integrated optical add-drop filter

We investigate optical microresonators consisting of either one or two coupled rectangular strips between upper and lower slab waveguides. The cavities are evanescently excited under oblique angles by thin-film guided, in-plane unguided waves supported by one of the slab waveguides. Beyond a specific incidence angle, losses are fully suppressed. The interaction between the guided mode of the cavity-strip and the incoming slab modes leads to resonant behavior for specific incidence angles and gaps. For a single cavity, at resonance, the input power is equally split among each of the four output ports, while for two cavities an add-drop filter can be realized that, at resonance, routes the incoming power completely to the forward drop waveguide via the cavity. For both applications, the strength of the interaction is controlled by the gaps between cavities and waveguides.

Ridge Resonance in Silicon Photonics Harnessing Bound States in the Continuum

AbstractThe theoretical analysis and experimental demonstration of a waveguide resonator are presented based on bound states in the continuum in an integrated photonic chip platform. The continuum has the form of a collimated beam of light which is confined vertically in a transverse electric (TE) mode of a silicon slab. The bound state is a discrete transverse‐magnetic (TM)‐like mode of a ridge on the silicon slab. The coupling between the slab and ridge modes results in a single sharp resonance at the wavelength where they phase match. This phenomenon is experimentally demonstrated on a silicon photonic chip using foundry‐compatible parameters and it is interfaced on‐chip to standard single‐mode silicon nanowire waveguides. The fabricated chip exhibits a single sharp resonance near 1550 nm with a line width of a few nanometers, an extinction ratio of 25 dB, and a thermal stability of 19.5 pm °C−1. It is believed that the demonstration of a resonance operating near a bound state in the continuum realized using guided wave components will form the basis of a new approach to on‐chip wavelength filtering and sensing applications.

Geometric controls on flat slab seismicity

The relationship between intraslab seismicity and the dynamics of subduction is a subject of ongoing debate. Uncertainty surrounds (1) the extent to which the stress regime associated with slab earthquakes reflects the driving/resisting forces of subduction, or more localised processes such as metamorphic or thermo-elastic volume change, and (2) the relative contribution of uniform (stretching/shortening) and flexural (bending/buckling) deformation modes in slabs. Because variations in slab curvature are very clear in subduction zones with flat slab segments, such settings allow for assessment of the relationship between slab geometry, bending and seismicity. Using a numerical model that reproduces published flat slab geometries we show how bending rates, which are dominated by the advective component, relate to downdip curvature gradients. Based on published slab geometries, we show that the patterns of seismicity in the Peruvian and Mexican flat slabs, vary systematically with slab curvature gradient. Seismicity is restricted to regions colder than about 600°C. This means that only the upper half of flat slab bending zones are expressed seismically, providing the appearance of a uniform extensional regime.

Lateral Leakage in Silicon Photonics: Theory, Applications, and Future Directions

This paper presents an overview of lateral leakage in silicon photonics due to polarization coupling between the guided mode of a ridge waveguide and unguided slab mode in the etched cladding. We explain the physical origins and provide insight into how this effect can be suppressed or harnessed. We show how lateral leakage can manifest as new resonant behaviour and explore this effect in the context of bound states in the continuum. We review a number of applications for devices based on lateral leakage and present outlook for a new generation of polarization manipulators, antennas and filters.

Magnetophonon resonance in quantum wells due to absorption and emission of confined phonon

In this paper, the effect of confined phonons described by the Huang-Zhu model, Ridley's guided mode model, and Fuchs-Kliewer slab mode model on the full-width at half-maximum (FWHM) of the optically detected magnetophonon resonance (ODMPR) peak for the inter-subband transitions (1–2) with the absorption/emission of phonon in GaAs quantum wells is investigated by using the projection operator method. The ODMPR conditions depend on the photon energy and well width are presented. Our numerical results show that the FWHM of the ODMPR peak for the inter-subband transitions with the absorption/emission of phonon increases with the growing temperature and decreases with the rise of well width for all models of phonons. Besides, the FWHM of the above ODMPR peaks for the bulk phonons has a smaller value and changes slower than it does for the confined phonons. The FWHM for the Huang-Zhu model is largest among three models of confined phonons (Huang-Zhu model, Ridley's guided mode model, and Fuchs-Kliewer slab mode model), while it for Ridley's guided mode model is smallest. This result is in good agreement with the theoretical results in Refs. [1–3]. In addition, the FWHM of the ODMPR peak for the inter-subband transitions with the emission of phonon has a smaller value than that for absorption of phonon for both the bulk and confined phonons. This result is in good agreement with the theoretical results in Ref. [4] but very different from that the theoretical results in Refs. [5,6].

Liquid crystal-based widely tunable integrated optic wavelength filters

We report widely tunable wavelength filters using liquid crystal (LC)-based long-period waveguide gratings (LPWGs). The high birefringence and electrically tunable refractive index of the LC have been used to achieve a tuning range covering most of the S, C, and L bands of an optical communication system. E7 LC has been used as a cover layer in an LPWG rib waveguide structure. The LPWG structure couples light from the fundamental mode to the slab modes of the waveguide. This results in transmission dips at resonance wavelengths. The mode constants of the coupling modes can be changed by controlling the refractive index of the LC and hence the resonance wavelengths by using the externally applied electric field. We have carried out numerical simulations to analyze the performance of E7 LC-based LPWGs on various substrates such as ion-exchanged BK7 glass and proton-exchanged LiNbO3 and silicon-on-insulator and silicon nitride substrates. The potential applications of the device are electrically tunable wavelength filters for optical communication systems.

Numerical Study on the Performance of RC Slab with Protective Aluminum Foam Cladding under Near-field Explosion

Aluminium foam material has excellent properties and a wide application prospect in the field of protective engineering. Present study is to study the protecting effect of aluminium foam plate on RC slab. First, quasi-static compression test and SHPB test were conducted to verify the material parameters of aluminium foam with different porosity (87% and 77%). Then, the dynamic response and the failure modes of RC slabs were calculated and analysed using LS-DYNA software. The results show that the increase in the thickness of aluminium foam plate changes the failure mode of RC slabs, from the overall bending failure mode to the local spall failure mode. Reasons were discussed and some useful suggestions for design were given at last.

Temperature insensitive long period waveguide gratings in rib waveguide

We propose long period waveguide gratings (LPWGs) in a rib structure in which the coupling takes place between the fundamental mode and the higher order modes (slab modes). Such an LPWG avoids the need of finite over cladding. The structure is inherently temperature insensitive as the involved modes have light confinement in the same material in contrast to the LPWGs, which use the over cladding of different material. Moreover, such a structure has an added advantage of less critical and easier fabrication process than the conventional LPWGs, in which the choice and deposition process of over cladding layer imposes several limitations. We demonstrate formation of such rib LPWGs in ion exchanged BK7 glass by laser ablation method and show their applications as wavelength filters and refractive index sensors. In particular we demonstrate temperature insensitive wavelength filter having the center wavelength of 1557.5 nm, an extinction ratio of ˜ 12 dB and 3-dB bandwidth of 3.8 nm. We also show a refractive index sensing characteristics of the structure with a sensitivity of 246 nm/RIU as an additional application.

Resilience and Robustness of Composite Steel and Precast Concrete Track Slabs Exposed to Train Derailments

Sydney Harbour Bridge (SHB) is an iconic structure constructed in 1923 in Sydney, connecting North Sydney and Sydney CBD. It is a steel-arch structure that caries 8 road lines and 2 rail tracks. Rail tracks of the bridge presently use conventional timber transoms (or sleepers) as track support structure. Replacement of aging and failing timber transoms annually cost public tax money significantly as a result of the high turnover rate of components caused by salt spray, electrolysis, humidity, aggressive dynamic condition and so on. Steel-concrete composite transoms have been found to be a feasible alternative to timber transoms, which enable systems compatibility with the structural configuration of Sydney Harbour Bridge. However, critical literature review on composite transoms reveals that their impact failures due to train derailments have not yet been considered. In fact, the derailment impacts can cause progressive failure to the bridge structure system. This paper therefore investigates the unprecedented impact damage and failure modes of composite transoms during train derailments. The derailment loads was considered in accordance with Australian Bridge Standard AS5100. The train derailment load was simulated using three dimensional non-linear finite element modelling by ABAQUS. The damage and failure behaviour of the precast concrete-steel composite transoms was then analysed. The ductility of composite slabs can be observed from the yielding of both reinforcements and steel sheets during the impact loading. The dynamic finite element analysis is found to be capable of making reasonable predictions by determining the possible failure modes of steel-concrete composite track slabs subjected to impact loads.

Nanofocusing in SOI-based hybrid plasmonic metal slot waveguides.

Through a process of efficient dielectric to metallic waveguide mode conversion, we calculate a >400-fold field intensity enhancement in a silicon photonics compatible nanofocusing device. A metallic slot waveguide sits on top of the silicon slab waveguide with nanofocusing being achieved by tapering the slot width gradually. We evaluate the conversion between the numerous photonic modes of the planar silicon waveguide slab and the most confined plasmonic mode of a 20 x 50 nm2 slot in the metallic film. With an efficiency of ~80%, this system enables remarkably effective nanofocusing, although the small amount of inter-mode coupling shows that this structure is not quite adiabatic. In order to couple photonic and plasmonic modes efficiently, in-plane focusing is required, simulated here by curved input grating couplers. The nanofocusing device shows how to efficiently bridge the photonic micro-regime and the plasmonic nano-regime whilst maintaining compatibility with the silicon photonics platform.

Experimental investigation of RC slabs under air and underwater contact explosions

The blast experimental test is an effective method to observe the failure characteristics of reinforced concrete (RC) slabs to contact explosion. However, the existing explosion experiments on RC slabs are mainly focused on air blast. Corresponding experiments of RC slabs to underwater explosion are limited. In this article, both air and underwater contact explosion experiments are carried out. Ten specimens of concrete slabs including three plain concrete slabs and seven RC slabs are tested. Five specimens are subjected to air contact explosion, and other five specimens are subjected to underwater contact explosion. The influence of the boundary condition, reinforcement, charge shapes, and charge mass on failure modes of concrete slabs is investigated. The damage characteristics and failure modes of concrete slabs subjected to air and underwater contact explosions are compared. The results indicate that underwater contact explosion can cause more damage to the concrete slab than the same amount of explosive in air. Even one electric detonator detonated in the water can cause damage to the RC slab.

Photonic waveguide to free-space Gaussian beam extreme mode converter

Integration of photonic chips with millimeter-scale atomic, micromechanical, chemical, and biological systems can advance science and enable new miniaturized hybrid devices and technology. Optical interaction via small evanescent volumes restricts performance in applications such as gas spectroscopy, and a general ability to photonically access optical fields in large free-space volumes is desired. However, conventional inverse tapers and grating couplers do not directly scale to create wide, high-quality collimated beams for low-loss diffraction-free propagation over many millimeters in free space, necessitating additional bulky collimating optics and expensive alignment. Here, we develop an extreme mode converter, which is a compact planar photonic structure that efficiently couples a 300 nm × 250 nm silicon nitride high-index single-mode waveguide to a well-collimated near surface-normal Gaussian beam with an ≈160 µm waist, which corresponds to an increase in the modal area by a factor of >105. The beam quality is thoroughly characterized, and propagation over 4 mm in free space and coupling back into a single-mode photonic waveguide with low loss via a separate identical mode converter is demonstrated. To achieve low phase error over a beam area that is >100× larger than that of a typical grating coupler, our approach separates the two-dimensional mode expansion into two sequential separately optimized stages, which create a fully expanded and well-collimated Gaussian slab mode before out-coupling it into free space. Developed at 780 nm for integration with chip-scale atomic vapor cell cavities, our design can be adapted for visible, telecommunication, or other wavelengths. The technique can be expanded to more arbitrary phase and intensity control of both large-diameter, free-space optical beams and wide photonic slab modes.