High-order Surface Research Articles

Three-dimensional OCT based guinea pig eye model: relating morphology and optics.

Custom Spectral Optical Coherence Tomography (SOCT) provided with automatic quantification and distortion correction algorithms was used to measure the 3-D morphology in guinea pig eyes (n = 8, 30 days; n = 5, 40 days). Animals were measured awake in vivo under cyclopegia. Measurements showed low intraocular variability (<4% in corneal and anterior lens radii and <8% in the posterior lens radii, <1% interocular distances). The repeatability of the surface elevation was less than 2 µm. Surface astigmatism was the individual dominant term in all surfaces. Higher-order RMS surface elevation was largest in the posterior lens. Individual surface elevation Zernike terms correlated significantly across corneal and anterior lens surfaces. Higher-order-aberrations (except spherical aberration) were comparable with those predicted by OCT-based eye models.

RANS-VOF modelling of the Wavestar point absorber

Numerical modelling has become commonplace in the development of offshore structures, however for dynamic systems such as Wave Energy Convertors (WECs) conventional numerical tools may not be able to capture the full range of behaviours required for engineering analysis. This is particularly problematic in extreme seas where the nonlinearities and coupled nature of these systems become important. In this work a fully nonlinear, coupled tool for simulation of WECs is produced and compare with physical measurements. Regular wave interactions with both a fixed and freely-pitching, 1:10 scale model of the Wavestar are reproduced numerically. The numerical model is shown to be capable of predicting the pressure on the float's surface and the fully coupled motion of the device. However, the results indicate that the higher-order free surface behaviour in the vicinity of the device is not being captured correctly. Finally, the numerical model is shown to cope with an extreme regular wave including large amplitude motion, full submersion and high levels of free surface distortion. The results presented suggest that the quality of the numerical reproduction does not decrease with wave steepness; however the execution time of simulations increases significantly with increased float oscillation.

Angled cavity photonic crystal lasers with asymmetrical high-order surface gratings

980 nm angled cavity photonic crystal (PC) laser diodes with asymmetrical high-order surface gratings (aHSGs) are proposed and fabricated. The one-dimensional PC structure in the epitaxy is used to expand the fundamental transverse mode and reduce the vertical divergence. An angled cavity with aHSGs is fabricated to achieve a nearly diffraction-limited beam quality and narrow spectral width. Experimentally, a continuous-wave output of 0.85 W/facet, a low divergence of 1.5 × 10.6°, and a narrow spectral width of 0.07 nm are achieved. The lateral beam quality is superior with an of 1.96.

Employing the coupled stress components and surface elasticity for nonlocal solution of wave propagation of a functionally graded piezoelectric Love nanorod model

The higher-order stress components and surface effect are considered in this article for presentation of a nonlocal solution for a functionally graded piezoelectric nanorod. Love rod model is used for longitudinal wave propagation. The mentioned analysis is used to evaluate the governing differential equations of the system and characteristics of wave propagation in assumed nanorod. The model is excited by a two-dimensional electric potential and an initial applied voltage at top layer of rod. The mechanical and electrical properties are assumed variable along the thickness direction of rod. Hamilton’s principle is used to arrive to governing differential equations of the electromechanical system. The effect of some important parameters such as applied voltage and gradation of material properties is studied on the wave characteristics of the rod. Furthermore, the effect of different distributions of electric potential on the phase velocity of the nanorod is evaluated.

Nanowire Y-junction formation during self-faceting on high-index GaAs substrates

In this contribution, we report on the observation of high-order and bi-dimensional surface mechanisms that allows the self-assembling of an alternating array of straight and bifurcated nanowires.

Angled-cavity lasers with photonic-crystal structure and high-order surface gratings

980 nm angled-cavity laser diodes with photonic-crystal structures and high-order surface gratings (HSGs) were first designed and fabricated. These lasers were fabricated using standard photolithography on a single-growth wafer with a photonic crystal structure. In addition, the angled-cavity lasers with asymmetric HSGs offer a simple solution for laser emission with a high power, low divergence angle, and narrow spectral width. A continuous-wave output power of 848 mW facet–1 was experimentally obtained for a 100 μm-wide and 1 mm-long device. The lowest divergence angle and narrowest spectral width exhibited by these devices were 1.5° × 10.6° and 0.07 nm, respectively.

Wide-angle resonance of a photonic crystal surface mode under a surface termination and its influence on imaging

We have developed a semi-analytical approach to the modulation transfer function (MTF) for negative-index flat lenses based on photonic crystals (PhCs). Contributions of various PhC modes to the MTF have been identified and analyzed. With a certain surface termination, a high-order PhC surface mode can be tamed to produce a broad angular resonance. As such, the isotropy of the image field can be significantly enhanced, resulting in an ideal image formation with nearly perfect outgoing circular wavefronts. Ray-optics analysis has also been utilized to assist the design of a negative-index flat lens. Finite-difference time domain simulations confirm the effectiveness of PhC lens designed by this semi-analytic approach to the MTF.

Tunable Ultrahigh Order Surface Plasmonic Resonance in Multi-Ring Plasmonic Nanocavities

Optical properties of multi-ring with spatial symmetry breaking are investigated theoretically. Tunable ultrahigh order surface plasmonic resonance is achieved, which is found to be sensitive to geometric parameters. Certain high-order surface plasmonic resonances can be either suppressed or enhanced when geometrical parameters are adjusted. Moreover, more than one quadrupolar-dipolar, octupolar-dipolar, and hexadecapolar-dipolar mode of the surface plasmonic resonance can be achieved. The asymmetry also allows the generation of strong electric field enhancement with these nanostructures that can be applied in the field of surface-enhanced spectroscopy and biosensing.

Quasi-analytical solutions of hybrid platform and the optimization of highly sensitive thin-film sensors for terahertz radiation

We present quasi-analytical solutions (QANS) of hybrid platform (HP) comprising metallic grating (MG) and stacked-dielectric layers for terahertz (THz) radiation. The QANS are validated by finite difference time domain simulation. It is found that the Wood anomalies induce the high-order spoof surface plasmon resonances in the HP. The QANS are applied to optimize new perfect absorber for THz sensing of large-area thin film with ultrahigh figure of merit reaching fifth order of magnitude for the film thickness 0.0001p (p: MG period). The first-order Wood's anomaly of the insulator layer and the Fabry-Perot in the slit's cavity account for the resonance of the perfect absorber. The QANS and the new perfect absorber may lead to highly sensitive and practical nano-film refractive index sensor for THz radiation.

Higher-order surface plasmon contributions to passive and active plasmonic interferometry

We compare the intensity modulation of passive light transmission and active fluorescence emission in planar plasmonic interferometers consisting of a nano-scale hole flanked by circular grooves etched in a silver film. Discrete fast Fourier transform applied to plasmonic interferograms - i.e., optical interferograms obtained by varying the propagation phase of surface plasmon polaritons (SPPs) - reveals higher-order interference effects that can be enhanced by optimizing in-plane SPP scattering and reflection. The experimental SPP dispersion relations agree with finite-difference frequency-domain calculations. Finally, we show that odd-order SPP contributions can be suppressed by reducing the spatial coherence of the incident beam.

Analysis of High-Order Slotted Surface Gratings by the 2-D Finite-Difference Time-Domain Method

High-order slotted surface gratings are analyzed by the finite-difference time-domain method. The power reflection, transmission and loss are calculated for these gratings. Results show good agreement with those calculated by the 2-D scattering matrix method. Simulations predict that such slotted surface gratings will yield the lowest loss with slot width equal to a quarter-wavelength ( $\lambda / 4$ ) in the slot area. The calculated field distribution shows that a wider slot width results in a larger scattering loss from the gratings. It is also found that for the gratings with slot width of $5\lambda / 4$ , the power reflection will increase by more than 45% while the loss will drop to just half of the value for the $9\lambda / 4$ slot width grating that was used in previous designs. Moreover, it can still be fabricated by standard photolithography since the slot width is around 610 nm, which means that lasers based on such gratings can be easily fabricated and will have a lower threshold current with higher slope efficiency than previous designs.

Excitation of the Tunable Longitudinal Higher-Order Multipole SPR Modes by Strong Coupling in Large-Area Metal Sub-10 nm-Gap Array Structures and Its Application

The excitation of the higher-order multipole localized surface plasmon resonance (LSPR) modes in sub-10 nm-gap plasmonic structure is desired in many plasmon enhancement applications. However, unfortunately, the higher-order multipole modes are either hard to be excited by light or make a very small contribution to electromagnetic enhancement, generally. In this paper, we demonstrate that tunable longitudinal higher-order multipole LSPR modes can be excited in large-area metal sub-10 nm-gap structures based on mature PS@Au core-halfshell ordered array plasmonic nanostructure by experiment and numerical simulation. Our results show that the longitudinal higher-order multipole SPR modes can be regarded as the like-cavity modes and can be excited effectively in strong coupling sub-10 nm-gap array structures and, furthermore, make a major contribution to the huge electromagnetic enhancement. Additionally, the longitudinal higher-order multipole SPR modes can be tuned easily by changing the longitudinal size o...

Higher-order meshing of implicit geometries—Part I: Integration and interpolation in cut elements

An accurate implicit description of geometries is enabled by the level-set method. Level-set data is given at the nodes of a higher-order background mesh and the interpolated zero-level sets imply boundaries of the domain or interfaces within. The higher-order accurate integration of elements cut by the zero-level sets is described. The proposed strategy relies on an automatic meshing of the cut elements. Firstly, the zero-level sets are identified and meshed by higher-order surface elements. Secondly, the cut elements are decomposed into conforming sub-elements on the two sides of the zero-level sets. Any quadrature rule may then be employed within the sub-elements. The approach is described in two and three dimensions without any requirements on the background meshes. Special attention is given to the consideration of corners and edges of the implicit geometries.

Single-mode cylindrical graphene plasmon waveguide

A cylindrical graphene plasmon waveguide (CGPW) which consists of two rolled graphene ribbons, a dielectric core and a dielectric interlayer is proposed. An analytical model for the single-mode condition and cutoff frequency of high-order graphene surface plasmon (GSP) modes is presented and verified by finite element method (FEM) simulations. Single-mode operation region of CGPW is identified in the frequency–radius space. By varying the separation between two graphene sheets and the Fermi level of graphene, a large tunability of the mode behavior is also demonstrated. The proposed structure may provide a new freedom to manipulate GSPs, and would lead to novel applications in optics.

Extinction and Scattering Properties of High-Order Surface Plasmon Modes in Silver Nanoparticles Probed by Combined Spatially Resolved Electron Energy Loss Spectroscopy and Cathodoluminescence

We investigated the optical properties of localized surface plasmons with different orders in individual silver nanotriangles with different sizes by electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) in the same scanning transmission electron microscope. EELS and CL spectral imaging within the same nanotriangles give information about the extinction and scattering from the nanostructures. As measured from both techniques, the first two order modes showed similar spatial distributions. However, the appearances of slightly different resonant energies were confirmed in both lower and higher order plasmon modes. For the first two lower order modes, the resonant energy in CL was blue-shifted compared to that in EELS, which can be understood in a simple damped oscillator picture. This confirms the experimental and theoretical observations recently made on dipolar plasmons in gold. It also extends it to the second-order modes. The next higher order modes exhibit an apparent inverted shift, wh...

Giant Faraday Rotation of High-Order Plasmonic Modes in Graphene-Covered Nanowires.

Plasmonic Faraday rotation in nanowires manifests itself in the rotation of the spatial intensity distribution of high-order surface plasmon polariton (SPP) modes around the nanowire axis. Here we predict theoretically the giant Faraday rotation for SPPs propagating on graphene-coated magneto-optically active nanowires. Upon the reversal of the external magnetic field pointing along the nanowire axis some high-order plasmonic modes may be rotated by up to ∼100° on the length scale of about 500 nm at mid-infrared frequencies. Tuning the carrier concentration in graphene by chemical doping or gate voltage allows for controlling SPP-properties and notably the rotation angle of high-order azimuthal modes. Our results open the door to novel plasmonic applications ranging from nanowire-based Faraday isolators to the magnetic control in quantum-optical applications.

Optimized River Stream-Flow Forecasting Model Utilizing High-Order Response Surface Method

Accurate and reliable stream-flow forecasting has a key role in water resources planning and management. Most recently, soft computing approaches have become progressively prevalent in modelling hydrological variables and most specifically stream-flows. This is due to their ability to capture the non-linearity and non-stationarity characteristics of the hydrological variables with minimum information requirements. Despite this, they present several challenges in the modelling architecture, as there is a need to establish a suitable pre-processing method for the stream-flow data and an appropriate optimization model has to be integrated in order re-adjust the weights and biases associated with the model structure. On top of that, artificial intelligent models require “trial and error” procedures in order to be properly tuned (number of hidden layers, number of neurons within the hidden layers and the type of the transfer function). However, soft computing approach experienced several problems while calibration such as over-fitting. In this research, the Response Surface Method (RSM) is improved based on high-order polynomial functions for forecasting the river stream-flow namely; High-Order Response Surface (HORS) method. Several higher orders have been examined, second, third, fourth and fifth polynomial functions in order to figure out the best fit that able to mimic the pattern of stream-flow. In order to demonstrate the effectiveness of the proposed model, monthly stream-flow time series data located in Aswan High Dam (AHD) has been examined. A detailed analysis of the overall statistical indicators revealed that the proposed method showed outstanding performance for monthly stream-flow forecasting at AHD. It could be concluded that the fifth order polynomial function outperforms the other orders of the polynomial functions especially with May model who achieved minimum MAE 0.12, NRMSE 0.07, MSE 0.03 and maximum SF and R2 (0.97, 0.99) respectively.

Static and dynamic instability of nanowire-fabricated nanoelectromechanical systems: effects of flow damping, van de Waals force, surface energy and microstructure

Surface energy and microstructure-dependent size phenomena can play significant roles in physical performance of nanoelectromechanical systems (NEMS). Herein, the static and dynamic pull-in instability of cantilever and double-clamped NEMS fabricated from conductive cylindrical nanowires with circular cross section is studied. The Gurtin–Murdoch surface elasticity in combination with the couple stress continuum theory is employed to incorporate the coupled effects of surface energy and microstructure-dependent size phenomenon. Using Green–Lagrange strain, the higher order surface stress components are incorporated into the governing equation. The effect of gas damping is considered in the model as well as structural damping. The nonlinear governing equation is solved using analytical reduced order method. The effects of various parameters on the static and dynamic pull-in parameters, phase plans, and stability threshold of the nanowire-based structures are demonstrated.

A Nonlinear Model for Incorporating the Coupled Effects of Surface Energy and Microstructure on the Electromechanical Stability of NEMS

Surface energy and size phenomenon can play significant roles in physical performance of nano-electromechanical systems. Herein, the static and dynamic pull-in behavior of nano-tweezers and nano-switch fabricated from conductive cylindrical nano-wires is studied. The Gurtin–Murdoch surface elasticity in combination with the couple stress theory is employed to incorporate the coupled effects of surface energy and microstructure dependency (size phenomenon). Using Green–Lagrange strain, the higher-order surface stress components are incorporated in the nonlinear governing equation. The effect of gas damping is considered in the model as well as structural damping. The governing equation is solved using the reduced order method. The effects of various parameters on the static and dynamic pull-in parameters, phase plans and stability threshold of the nano-structures are demonstrated.

High-order spoof localized surface plasmons supported on a complementary metallic spiral structure.

We experimentally demonstrate that multiple high-order spoof localized surface plasmons (spoof-LSPs) modes can be supported on a complementary metallic spiral structure, which were absent in the previously reported spoof-LSPs modes. Through exact numerical simulations and near-field imaging experiments, we directly observe these high-order spoof-LSPs modes at microwave frequencies. We also show that these higher-order spoof-LSPs modes exhibit larger frequency shifts caused by the local environmental refractive index change than the previously reported low-order spoof-LSPs modes. Hence the complementary MSS may find potential applications as plasmonic sensor in the microwave and terahertz frequencies.