Five-layer Slab Research Articles

Mode coupling at avoided crossings in slab waveguides with comparison to optical fibers: tutorial

Avoided crossings are important in many waveguides and resonators. That is particularly the case in modern-day solid-core and air-core optical fibers that often have a complex geometry. The study of mode coupling at avoided crossings often leads to a complicated analysis. In this tutorial, we aim to explain the basic features of avoided crossings in a simple slab waveguide structure so that the modes can be found analytically with simple sinusoidal and exponential forms. We first review coupled-mode theory for the guided mode in a slab waveguide, which has a higher index in the core. We study the effective index of the guided true mode for a five-layer slab waveguide including two core layers with higher indices compared to the indices in the three cladding layers. Then, we study the same structure by using the overlap between approximate modes confined in the two individual core slabs. When the two individual core slabs are not near each other, the avoided crossing using the true modes within the two-slab waveguide agrees well with the results using the overlap between the two approximate modes. We also study coupled-mode theory and avoided crossings for leaky modes in an antiresonant slab waveguide. We obtain good agreement between the results using the true leaky mode and the results using the overlap between approximate modes. We then discuss examples of avoided crossings in solid-core and air-core optical fibers. We describe the similarities and differences between the optical fibers and simple slab waveguides that we have analyzed in detail.

Power flux in five layers slab waveguide with metamaterials

Five-layer slab a waveguide combined with a metamaterial has been studied. The central layers were the metamaterial and a normal material in the core of waveguide. The dispersion relation, field formulas, power formulas and the power flux formula has been derived. The waveguide property was controlled by changing the permittivity and permeability and thicknesses of the layers. The presence of metamaterial contributes to cut-off the mode TEo and other modes were propagated to backward. The stopping light was determined using the power flux value. However, the negative values of it refer to the backward direction of propagation. Furthermore the stopped light can be achieved in a certain value of power. Our results found it is good for different applications in communication and controlling.

Revealing the effects of nitrogen on threshold current density in GaNxAsyP1−x−y/GaP/AlzGa1−zP type I QW laser structures by hydrostatic pressure

We present a comprehensive theoretical analysis of threshold current in dilute nitride direct bandgap Ga(NAsP)/GaP/AlGaP quantum wells (QW) on silicon substrates using model calculations. The pressure dependence of band structure, radiative and non-radiative recombination rates, optical confinement factor, transparency- and threshold-carrier densities are calculated in a range of 0–1GPa at room temperature. The effect of aluminium incorporation into cladding-layer Gallium Phosphide (GaP) with the employed five-layer slab waveguide model is considered and we have shown that incorporation of Aluminium (Al) into cladding-layer GaP increases the photon confinement in well layer. It is found that incorporation of Nitrogen (N) into GaAsP reduces the non-radiative Auger recombination rates which brings an improvement in threshold current. The comparison of our calculated results with that of the experimental data indicates that the Auger effect involving CHCC process can be considered as the dominant non-radiative loss mechanism at 300K.

Sensing of refractive index based on mode interference in a five-layer slab waveguide

We propose a refractive index sensor based on the mode interference in a five-layer waveguide. The propagation properties are analyzed by the coupled mode theory, and the function of the waveguide as a refractive index sensor is also explored based on simulation results. Taking into consideration a trade-off between the sensitivity and the size of the proposed sensor, we can get a very compact device of 10.8μm×1μm, and the sensing resolution of the proposed refractive index can reach 2.25×10−5RIU for index change at around 1.455.

A New Approach to Separate Haemodynamic Signals for Brain-Computer Interface Using Independent Component Analysis and Least Squares

Brain-computer interface (BCI) is one technology that allows a user to communicate with external devices through detecting brain activity. As a promising noninvasive technique, functional near-infrared spectroscopy (fNIRS) has recently earned increasing attention in BCI studies. However, in practice fNIRS measurements can suffer from significant physiological interference, for example, arising from cardiac contraction, breathing, and blood pressure fluctuations, thereby severely limiting the utility of the method. Here, we apply the multidistance fNIRS method, with short-distance and long-distance optode pairs, and we propose the combination of independent component analysis (ICA) and least squares (LS) with the fNIRS recordings to reduce the interference. The short-distance fNIRS measurement is treated as the virtual channel and the long-distance fNIRS measurement is treated as the measurement channel. Least squares is used to optimize the reconstruction value for brain activity signal. Monte Carlo simulations of photon propagation through a five-layered slab model of a human adult head were implemented to evaluate our methodology. The results demonstrate that the ICA method can separate the brain signal and interference; the further application of least squares can significantly recover haemodynamic signals contaminated by physiological interference from the fNIRS-evoked brain activity data.

RLS adaptive filtering for physiological interference reduction in NIRS brain activity measurement: a Monte Carlo study

The non-invasive measurement of cerebral functional haemodynamics using near-infrared spectroscopy (NIRS) instruments is often affected by physiological interference. The suppression of this interference is crucial for reliable recovery of brain activity measurements because it can significantly affect the signal quality. In this study, we present a recursive least-squares (RLS) algorithm for adaptive filtering to reduce the magnitude of the physiological interference component. To evaluate it, we implemented Monte Carlo simulations based on a five-layer slab model of a human adult head with a multidistance source–detector arrangement, of a short pair and a long pair, for NIRS measurement. We derived measurements by adopting different interoptode distances, which is relevant to the process of optimizing the NIRS probe configuration. Both RLS and least mean squares (LMS) algorithms were used to attempt the removal of physiological interference. The results suggest that the RLS algorithm is more capable of minimizing the effect of physiological interference due to its advantages of faster convergence and smaller mean squared error (MSE). The influence of superficial layer thickness on the performance of the RLS algorithm was also investigated. We found that the near-detector position is an important variable in minimizing the MSE and a short source–detector separation less than 9 mm is robust to superficial layer thickness variation.

Investigation of hydrogen bonds and temperature effects on the water monolayer adsorption on rutile TiO2 (110) by first-principles molecular dynamics simulations

Density Functional Theory (DFT), based on both static and Born–Oppenheimer Molecular Dynamics approaches, has been used to investigate the effect of hydrogen bonds and temperature on the water monolayer adsorption on the rutile TiO2 (110) face. It was demonstrated that the difference between some previous theoretical results and experimental data is due to too slim slab thickness model and/or too small surface area. According to the present static calculations, water monolayer adsorbs molecularly on the five-fold titanium atoms of an optimised five-layer slab thickness, due to the stabilising lateral hydrogen bonds between molecules. From the molecular dynamics simulations, two adsorption mechanisms were described as a function of temperature. Finally, it was pointed out that the dynamics of water adsorption is strongly influenced by the structural model used. When temperature increases, the monolayer dissociates gradually. However, because of the periodic boundary conditions, the 1×1 surface unit needs to be extended to at least 2×5 to get an accurate representation of the monolayer dissociation ratio. In these conditions, this ratio is around 20%, 25% and 33% at 270, 350 and 425K, respectively.

Reduction of global interference in functional multidistance near-infrared spectroscopy using empirical mode decomposition and recursive least squares: a Monte Carlo study

Functional near-infrared spectroscopy (fNIRS) is a sensitive technique that has the potential to detect haemodynamic changes during the performance of specific activation tasks. However, in real situations, fNIRS recordings are often corrupted by physiological phenomena, especially by cardiac contraction, breathing and blood pressure fluctuations, and these forms of interference can severely limit the utility of fNIRS. We present a novel fNIRS enhancement based on the multidistance fNIRS method with short-distance and long-distance optode pairs. With this method empirical mode decomposition (EMD) is applied to decompose the short-distance fNIRS measurement into a series of intrinsic mode functions (IMFs). By utilizing the weighting coefficients for the IMFs, we derive an estimate for global interference in the long-distance fNIRS measurements. We recover the evoked brain activity by minimizing least squares between the long-distance measurements and the estimated global interference. To accelerate the computation we adopt the recursive least squares (RLS) to decrease the computation complexity due to the matrix inversion. Monte Carlo simulations of photon propagation through a five-layered slab model of a human adult head were implemented to evaluate our methodology. The results demonstrate that the EMD-RLS method can effectively remove contamination from the evoked brain activity.

Monte Carlo study for physiological interference reduction in near-infrared spectroscopy based on empirical mode decomposition

Near-infrared spectroscopy (NIRS) can be used as the basis of non-invasive neuroimaging that may allow the measurement of haemodynamic changes in the human brain evoked by applied stimuli. Since this technique is very sensitive, physiological interference arising from the cardiac cycle and breathing can significantly affect the signal quality. Such interference is difficult to remove by conventional techniques because it occurs not only in the extracerebral layer but also in the brain tissue itself. Previous work on this problem employing temporal filtering, spatial filtering, and adaptive filtering have exhibited good performance for recovering brain activity data in evoked response studies. However, in this study, we present a time-frequency adaptive method for physiological interference reduction based on the combination of empirical mode decomposition (EMD) and Hilbert spectral analysis (HSA). Monte Carlo simulations based on a five-layered slab model of a human adult head were implemented to evaluate our methodology. We applied an EMD algorithm to decompose the NIRS time series derived from Monte Carlo simulations into a series of intrinsic mode functions (IMFs). In order to identify the IMFs associated with symmetric interference, the extracted components were then Hilbert transformed from which the instantaneous frequencies could be acquired. By reconstructing the NIRS signal by properly selecting IMFs, we determined that the evoked brain response is effectively filtered out with even higher signal-to-noise ratio (SNR). The results obtained demonstrated that EMD, combined with HSA, can effectively separate, identify and remove the contamination from the evoked brain response obtained with NIRS using a simple single source–detector pair.

The observation of super-long range surface plasmon polaritons modes and its application as sensory devices

In this communication, we will describe one unique phenomenon and the potential application of it. In this work, the dispersion relation of an air-silver-silicon-silver-fluid (air-Ag-Si-Ag-fluid) five-layer slab is analyzed theoretically, in which the super-long range surface plasmon polaritons (SPP) modes, whose energy penetrates deeply into the fluid, are found with their losses being extremely small and sensitive to the change of the fluid refractive index when operating near their interspace cut-off regions, where the dispersion curves are non-continuous. By applying this phenomenon in detecting the fluid refractive index change, a SPP sensor based on intensity measurement is proposed. It is a waveguide structure with an Ag-Si-Ag slab together with a flow cell filled with the detecting fluid. It is found that a large scale of linear detection (e.g., 0.08, for 1550 nm ~1.33 to 1.41) with high resolution (e.g., 7.9 × 10(-6) Refractive Index Units) can be achieved for a very short device, which is 200 μm.

Oscillating guided modes in a symmetric five-layer slab waveguide with anisotropic-dispersive left-handed material

A symmetric five-layer slab waveguide with anisotropic and dispersive left-handed material (LHM) in the core and righthanded material (RHM) in other layers is investigated. Through Maxwell’s equations and a transfer matrix method, the dispersion equations for the TE oscillating guided modes are obtained. Under consideration of two extremely anisotropic cases, some mode dispersion curves are plotted. The zero-order TE oscillating guided mode exists. Meanwhile, with the increase of mode number, their dispersion curves move to left or right, corresponding to positive or negative dielectric permittivity and magnetic permeability in the longitudinal direction. Besides, as the core thickness increases, mode dispersion properties change and three propagation properties appear: positive group velocity, negative group velocity and zero group velocity. The negative group velocity indicates the characteristics of the left-handed materials, and the zero group velocity implies that electromagnetic waves are trapped in the waveguide completely.

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A symmetric five-layer slab waveguide with a core of left-handed material and surrounded by four normal dielectrics is investigated.Under the consideration of material dispersion,the dispersion equations for the TE oscillating guided modes are obtained and mode dispersion curves are plotted.It is found that zero-order TE oscillating guided mode is exist.With the increase of mode number,mode dispersion curves move to left,and their cutoff frequencies decrease.Besides,some modes have three dispersion properties: normal dispersion,abnormal dispersion and mode double-degeneracy.

Monte Carlo study of global interference cancellation by multidistance measurement of near-infrared spectroscopy

The performance of near-infrared spectroscopy is sometimes degraded by the systemic physiological interference in the extracerebral layer. There is some systemic interference, which is highly correlated with the functional response evoked by a task execution. This kind of interference is difficult to remove by using ordinary techniques. A multidistance measurement method is one of the possible solutions for this problem. The multidistance measurement method requires estimation parameters derived from partial pathlength values of tissue layers to calculate an absorption coefficient change from a temporal absorbance change. Because partial path lengths are difficult to obtain, experimentally, we estimated them by a Monte Carlo simulation based on a five-layered slab model of a human adult head. Model parameters such as thickness and the transport scattering coefficient of each layer depend on a subject and a measurement position; thus, we assumed that these parameters obey normal distributions around standard parameter values. We determined the estimation parameters that provide a good separation performance in average for the model parameter distribution. The obtained weighting is robust to model parameter deviation and provides smaller errors on average compared to the parameters, which are determined without considering parameter distribution.

Guided modes in a symmetric five-layer left-handed waveguide

The guided modes are investigated in a symmetric five-layer slab waveguide with a core of negative refractive index material surrounded by four normal dielectrics. The graphical method is applied to determine the guided optical modes. The left-handed material waveguide supports both oscillating and surface guided modes. For oscillating guided modes the fundamental mode can be supported. Mode double-degeneracy appears and some modes are absent in the five-layer waveguide. High-order surface guided modes exist due to the oscillating electric field in the inner claddings. Energy flux for the oscillating and surface guided modes is also discussed in detail.

Coupling characteristics between five-layer slab wave-guides including left-handed materials

To obtain the coupling characteristics between slab wave-guides including left-handed material, we modify the coupled wave equations by using Maxwell’s equations. First, we obtain new-coupled wave equations and new-coupling coefficient. Second, the coupling between two identical five-layer slab wave-guides where their cores are left-handed material, but their claddings are right-handed materials is studied. The coupling coefficient for even TE mode which is more complex than that of the riglt-handed material slab wave guides, is obtained.

Propagation properties for five-layer symmetric slab waveguides including left-handed materials

In this paper, we discussed a slab wave-guide of five layers, The core is a left-handed material, but the claddings are right-handed materials. A dispersion equation of TE modes is obtained by using Maxwell’s equations, and some new dispersion characteristics are obtained based on the equation.

Analytical model of spatial four-waveguide coupler

Analytical model of spatial four-guide coupler is presented by utilizing Marcatili's principle, where the spatial four-guide coupler can be described by two perpendicular five-layer slab waveguides. The common expressions to calculate the effective indices and the mode field distributions are shown in this paper. Introducing perturbation corrections to take the effect of the distinct regions into account, more accurate results can be obtained. The effective refractive indices and coupling lengths of specific examples are investigated to verify the feasibility of the analytical model by comparing with the numerical approaches such as finite difference (FD) numerical calculation and beam propagation method (BPM).

Relaxation of the (1 1 1) surface of δ-Pu and effects on atomic adsorption: An ab initio study

The computational formalism of the full-potential all-electron linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method has been employed to study the relaxation of the δ-Pu(1 1 1) surface and the consequent effects for atomic adsorption of C, N, and O atoms on this surface. The underlying theoretical principle is the generalized gradient approximation to density functional theory (GGA-DFT) and the surface was modeled by a five-layer slab with a (2×2) surface unit cell. Upon relaxation of the slab, the interlayer separation between the surface and the subsurface layers expanded by 7.1% with respect to the bulk interlayer separation, while the separation between the subsurface and central layers expanded by 0.4%. To study adsorption on the surface, the adatoms were allowed to approach the surface at four high symmetry adsorption sites, namely, the top, bridge, hollow FCC, and hollow HCP sites, the adlayer structure corresponding to a coverage of 0.25 of a monolayer in all cases. The hollow FCC adsorption site was found to be the most stable site for C and N with chemisorption energies of 6.420 and 6.549 eV, respectively, while the hollow HCP adsorption site was found to be the most stable site for O with a chemisorption energy of 7.858 eV. The respective distances of the C, N, and O adatoms from the surface were found to be 1.22, 1.09, and 1.22 Å. The work function and net magnetic moments, respectively, increased and decreased in all cases upon chemisorption compared with the bare δ-Pu(1 1 1) surface. The electronic structure of the interactions between the adsorbates and the substrate is discussed in detail.

Theoretical study of hydrogen adsorption on FePd face-centered cubic alloy surfaces

Adsorption of hydrogen on the ordered FePd face-centered cubic (fcc) alloys was investigated using a tight binding theoretical method and a five-layer slab surface model. The calculations predict that hydrogen is bonded mainly to Pd atoms, while the metal--metal bonding is almost not affected on the fcc surfaces. The changes in the electronic structure and bonding in the (100), (110), and (111) surfaces were also analyzed. The H--surface bond is achieved at the expense of the metal--metal bond. The results are compared with previous studies on FePd face-centered tetragonal (fct) surfaces and FePd bulk structures.

Hydrogen adsorption on β-Ga 2O 3(1 0 0) surface containing oxygen vacancies

The understanding of hydrogen (H) adsorption on gallia is an important step in the design of molecular sensors and alkane dehydrogenation–aromatization catalysts. We have simulated the (1 0 0) surface of β-Ga 2O 3, which is the more frequent cleavage plane. Our study has considered oxygen vacancies in that plane. We have used the atom superposition and electron delocalization molecular orbital (ASED-MO), a semiempirical theoretical method, to understand both the electronic and bonding characteristic of H on β-Ga 2O 3 surface. As a surface model, we have considered both a cluster and a five-layer slab. We have found that H adsorption occurs on Ga sites close to oxygen vacancies. Two types of Ga have also been considered; namely, Ga(I) and Ga(II), with different coordination: Ga(I) is four-coordinated and Ga(II) six-coordinated. The Ga(I)–H bond is ∼24% stronger than Ga(II)–H while Ga(I)–O(I) bond is ∼44% stronger than Ga(II)–O(I). Also, Ga(I)–O(III) is ∼56% stronger that Ga(II)–O(III). The Ga–H and Ga–O interactions are always bonding. The Ga–Ga overlap population is null. We have assigned the 2003 and 1980 cm −1 infrared bands to the stretching frequencies of Ga(I)–H and Ga(II)–H bonds, respectively.