Outgoing Directions Research Articles

Semi-analytical Monte Carlo simulation of underwater target echoes from a small UAV-based oceanic lidar

Small unmanned airborne oceanic lidars are becoming increasingly popular for ocean surveys. They typically weigh less than 10 kg and can be mounted on commonly used commercial drones, making them easier to use than traditional large oceanic lidars, which typically weigh up to 80 kg and can only be carried on manned aircraft. Unmanned airborne oceanic lidars typically fly at lower altitudes and use higher laser emission frequencies, making them suitable for detecting underwater targets. However, most of the existing LiDAR underwater echo simulation models are designed for flat terrain and assume that the target fills the LiDAR receiving field of view, and there are few studies on the echoes of suspended targets smaller than the field of view range. Therefore, we combined ray tracing and semi-analytical Monte Carlo to model the laser echo energy of the underwater target. The study investigated the energy change of the target echo in the direction of deviation from the laser outgoing direction, and the impact of absorption and scattering coefficients on it. The difference in echo energy between different fields of view for extended and non-extended targets and the effect of water quality on it were explored. The echo waveforms of the target under different tilt angles were simulated, and it was found that the target echo pattern will be tilted with the change of tilt angle and offset position.

Metasurface‐Based Free‐Space Multi‐Port Beam Splitter with Arbitrary Power Ratio

AbstractA beam splitter (BS) is one of the most critical building blocks in optical systems. Despite various attempts of flat‐type BSs to miniaturize the conventional cube BS reported, it remains a challenge to realize an ultrathin optical BS with multi‐port output, non‐uniform splitting ratio, and steerable outgoing directions. Herein, a free‐space optical multi‐port beam splitter (MPBS) based on a polarization‐independent all‐dielectric metasurface is demonstrated. By applying an optimized phase‐pattern paradigm via a gradient‐descent‐based iterative algorithm to amorphous silicon (α‐Si) metasurfaces, a variety of MPBS samples with arbitrarily predetermined output port number (2–7), power ratio, and spatial distribution of output beams are prepared. The experimental results reveal that the fabricated MPBSs can achieve high total splitting efficiency (TSE, above 74.7%) and beam‐splitting fidelity (similarity, above 78.4%) within the bandwidth of 100 nm (1500–1600 nm). Such proposed MPBS can provide fabulous flexibility for optical integrated system and diverse applications.

Estimation of micro-Hall-Petch coefficients for prismatic slip system in Mg-4Al as a function of grain boundary parameters

Grain size strengthening, referred to as the Hall-Petch effect, is a common strategy to improve the yield strength of magnesium (Mg) alloys. Several experimental studies have reported that the Hall-Petch slope strongly depends on the texture of the alloy. This effect arises from altering grain boundaries (GBs) resistance to different slip systems to transfer across adjacent grains. The grain boundary barrier strength of certain grain boundaries to basal slip, referred to as basal micro-Hall-Petch, was investigated in the previous work. In this study, the micro-Hall-Petch coefficient values for the prismatic slip (kμprismatic) in Mg-4Al and their correlation with the grain boundary parameters were investigated. An experimental method was developed to initiate the prismatic slip band at low-stress levels. High-resolution electron backscatter diffraction (HR-EBSD) was used to measure the residual stress tensor, from which the resolved shear stress ahead of blocked prismatic slip bands was computed for seven different grain boundaries. kμprismatic values for each individual GB were calculated by coupling the stress profile information with a continuum dislocation pile-up model. The kμprismatic values vary from 0.138 MPa.m1/2 to 0.685 MPa.m1/2 which are almost three times larger than the calculated values for the basal micro-Hall-Petch. The kμprismatic values were correlated with the GB parameters, and a functional relationship depending on the two most effective angles, the angle between the traces of the slip planes on the GB plane (θ) and the angle between incoming and outgoing slip directions (κ), was proposed to estimate the Hall-Petch barrier for prismatic slip system. The work provides coefficients that can be supplied as input to crystal plasticity models to couple the effect of texture and grain size effectively.

On-Chip Metasurface for Optical Directional Rectification

On-chip two-dimensional (2D) metasurfaces create a new degree of freedom of light manipulation within a 2D surface, which is an emerging research area with fascinating possibilities for realization of very dense integration and miniaturization in photonic systems. As a longstanding challenge in optics steering, light rectification, which means to manipulate beams with multiple directional wavevectors to exit to the same outgoing direction, could hardly be made possible based on conventional optics unless with zero-index medium. However, on-chip metasurface could potentially come to rescue with deep-subwavelength features and its engineered phase signature depending on wavevectors. Here, we design and demonstrate a surface-wave rectification on-chip metasurface (SROM) for visible light, which realizes light rectification for oblique incidences within +/-20°. The SROM is formed by cascading two plasmonic on-chip nanowires of semi-elliptical and trapezoidal structures as its unit cell. Through the geometrical change induced by the semi-elliptical and trapezoidal structures and the plasmonic interaction between the two cascaded nanowires, the incident surface waves with multiple wavevector angles would exit and propagate normally along 0° around 500 nm wavelength, thus mimicking a zero-index-like phenomenon. The equal-frequency contour (EFC) of the designed SROM is also numerically retrieved to exhibit flat angular dispersion, which verifies the proof-of-concept light reification phenomenon. The on-chip miniature SROM is of merely ~ 3 μm length, and the light rectification performance remains for the visible-frequency band of 500 - 560 nm. Such new on-chip architecture only involves the mature and conventional nanofabrication techniques to manufacture without any alignment for multilayer metasurfaces. We envision that the created beam-steering freedom of light rectification opens a viable and compatible route towards new invention of photonic integrated devices.

NeuMIP

We propose NeuMIP, a neural method for representing and rendering a variety of material appearances at different scales. Classical prefiltering (mipmapping) methods work well on simple material properties such as diffuse color, but fail to generalize to normals, self-shadowing, fibers or more complex microstructures and reflectances. In this work, we generalize traditional mipmap pyramids to pyramids of neural textures, combined with a fully connected network. We also introduce neural offsets, a novel method which enables rendering materials with intricate parallax effects without any tessellation. This generalizes classical parallax mapping, but is trained without supervision by any explicit heightfield. Neural materials within our system support a 7-dimensional query, including position, incoming and outgoing direction, and the desired filter kernel size. The materials have small storage (on the order of standard mipmapping except with more texture channels), and can be integrated within common Monte-Carlo path tracing systems. We demonstrate our method on a variety of materials, resulting in complex appearance across levels of detail, with accurate parallax, self-shadowing, and other effects.

Causal description of marginally trapped surfaces in D dimensions

In this paper, we analyse the causal aspects of evolving marginally trapped surfaces in a D-dimensional spherically symmetric spacetime, sourced by perfect fluid with a cosmological constant. The norm of the normal to the marginally trapped tube is shown to be the product of lie derivatives of the expansion parameter of future outgoing null rays along the incoming and outgoing null directions. We obtain a closed form expression for this norm in terms of principal density, pressure, areal radius and cosmological constant. For the case of a homogeneous fluid distribution, we obtain a simple formula for determining the causal nature of the evolving horizons. We obtain the causal phase portraits and highlight the critical radius. We identify many solutions where the causal signature of the marginally trapped tube or marginally anti-trapped tube is always null despite having an evolving area. These solutions don't comply with the standard inner and outer horizon classification for degenerate horizons. we propose an alternate prescription for this classification of these degenerate horizons.

Identifying Central Nodes in Directed and Weighted Networks

An issue of critical interest in complex network analysis is the identification of key players or important nodes. Centrality measures quantify the notion of importance and hence provide a mechanism to rank nodes within a network. Several centrality measures have been proposed for un-weighted, un-directed networks but applying or modifying them for networks in which edges are weighted and directed is challenging. Existing centrality measures for weighted, directed networks are by and large domain-specific. Depending upon the application, these measures prefer either the incoming or the outgoing links of a node to measure its importance. In this paper, we introduce a new centrality measure, Affinity Centrality, that leverages both weighted in-degrees as well as out-degrees of a node’s local neighborhood. A tuning parameter permits the user to give preference to a node’s neighbors in either incoming or outgoing direction. To evaluate the effectiveness of the proposed measure, we use three types of real-world networks - migration, trade, and animal social networks. Experimental results on these weighted, directed networks demonstrate that our centrality measure can rank nodes in consonance to the ground truth much better than the other established measures

Plasmonic Dirac Cone in Twisted Bilayer Graphene.

We discuss plasmons of biased twisted bilayer graphene when the Fermi level lies inside the gap. The collective excitations are a network of chiral edge plasmons (CEP) entirely composed of excitations in the topological electronic edge states that appear at the AB-BA interfaces. The CEP form a hexagonal network with a unique energy scale ε_{p}=(e^{2})/(ε_{0}εt_{0}) with t_{0} the moiré lattice constant and ε the dielectric constant. From the dielectric matrix we obtain the plasmon spectra that has two main characteristics: (i)a diverging density of states at zero energy, and (ii)the presence of a plasmonic Dirac cone at ℏω∼ε_{p}/2 with sound velocity v_{D}=0.0075c, which is formed by zigzag and armchair current oscillations. A network model reveals that the antisymmetry of the plasmon bands implies that CEP scatter at the hexagon vertices maximally in the deflected chiral outgoing directions, with a current ratio of 4/9 into each of the deflected directions and 1/9 into the forward one. We show that scanning near-field microscopy should be able to observe the predicted plasmonic Dirac cone and its broken symmetry phases.

Elucidating the capability of electron backscattering for 3D nano-structure determination

Reflection electron energy loss spectroscopy (REELS) is well established for the study of homogeneous materials with flat surfaces. Here we extend the use of this technique to nano-structures consisting of silicon and silica and show that the experimentally-observed peculiar dependence of the REELS spectra on the sample orientation can be reproduced by Monte Carlo simulations using the known sample morphology. A sample with a 3D structure, resembling those found in FinFET transistors, was analyzed through electron Rutherford backscattering (ERBS, revealing the mass of the atoms near the surface) and REELS (revealing the electronic structure). ERBS/REELS spectra were taken at two incoming electron energies (5 and 40 keV) and in two experimental geometries with the component of the outgoing propagation direction along the surface being either parallel or perpendicular to the fins. The measured spectra were different for the two geometries due to attenuation effects within the fins, especially at 5 keV where the inelastic mean free path is of the order of the fin dimensions. This means that the 3D structure induces shadowing effects which suppress the elastic peaks and enhanced the inelastic signal. A Monte Carlo code was used to simulate multiple elastic and inelastic interactions of the electrons with these 3D structures and was indeed able to reproduce these experimental results, including the shadowing effects. A sub-angstrom layer of Au was evaporated on the sample and the changes induced by the Au layer were dependent on the orientation of the fins and were again reproduced by the simulation.

Wide-spatial scattering and arbitrary-angular tunability based on hybridized phase gradients from metasurface hologram algorithm

Light steering and wavefront shaping by metasurfaces typically relies on the elaborate phase arrangement from discrete plasmonic or dielectric resonators. A linear phase gradient profile could simply enable the beam bending (anomalous reflection/transmission) functionality. Comparing to the single beam steering performance, the applicable wide-scattering property is also highly in demand for more practical applications of full-spatial imaging and detection, etc. However, it is still challenging to come up with an efficient design route to manipulate the arbitrary wide-spatial scattering and complex phase gradient hybridization. Here, we propose a metasurface design route that could arbitrarily expand the visible-frequency light scattering into wide outgoing directions. An inverse-design metasurface based on hologram algorithm is created to form the complex phase gradient hybridization and realize wide-spatial scattering covering the outgoing angle range between ±60°. Such design method can also tailor to the arbitrary angular scattering functionality to outgoing angular direction at 15°–30°, 30°–45°and 45°–75°, respectively. Additionally, the proposed architecture operates at broadband visible wavelength rather than a single wavelength, and it also enjoys the robust optical scattering performance which is irrelevant to the incident angle variation. Overall, the presented design route is universal and can easily find applications for optical diffuser, light coupling/de-coupling, directional optical emitter, full-spatial imaging, light trapping and spectrum splitting, etc.

A proposal on complementary determination of the effective electro-weak mixing angles via doubly heavy-flavored hadron production at a super Z-factory

A proposal on determination of the effective electro-weak mixing angle ${\rm sin^2\theta_{eff}^f\;(f=c,b)}$ at a super Z-factory (an $e^+e^-$ collider designed with the possible highest luminosity and running around the center mass energy $\sqrt s=m_Z$) is proposed. It is to determine how the effective mixing angles depend on the flavors, especially the flavors $c$, $b$ (with the determined effective ones, the electro-weak mixing angles ${\rm sin^2\theta_W^f\;(f=c,b)}$ of Standard Model can be derived out precisely). The key point of the proposal is that at such a super Z-factory, via measuring the forward-backward ($A_{FB}$), left-right ($A_{LR}$) and combined left-right forward-backward ($A^{FB}_{LR}$) asymmetries of the produced doubly heavy-flavored hadrons (such as $B_c$, $B_c^*$ and the baryons $H_{QQ'q}$: $\Xi_{cc}$, $\Xi_{bc}$ and $\Xi_{bb}$ etc) in the relevant production processes $e^+e^-\to B_c+\cdots$ and $e^+e^-\to H_{QQ'q}+\cdots$ one may complement the final determination of the mixing angle ${\rm sin^2\theta_W^f\;(f=c,b)}$ of Standard Model. The advantage of the proposed way to determine the effective electro-weak mixing angles is that the doubly heavy flavor(s) and the out-going direction of the produced doubly-heavy hadron can be experimental determined precisely, i.e. there are no such errors caused by missing identification of the heavy flavor(s) and by determining the so-called thrust axis of the produced jets, which can not be avoided in the early determination at LEP-I and SLC.

Limits on field of view for Risley prisms.

The constraints on directions of incoming and outgoing rays of Risley prisms, caused by total internal reflection and surface tilt, are investigated for the four typical configurations. After applying a nonparaxial ray-tracing method based on the vector form Snell's law, the incident angles at prisms' surfaces are calculated and compared with the critical angle. On this basis, the direction limitations of incoming and outgoing rays are investigated. The permissible incoming directions as well as the achievable outgoing directions depend on prisms' orientation. Their ranges over which the rays can pass through the system at any prism orientation are defined as the angular aperture and angular field of view (FOV). The effects of the prisms' refractive index, opening angle, and arrangement on angular aperture/FOV are discussed. It is shown that there exists a direct trade-off between the deviation power and the angular aperture/FOV for Risley prisms, that is, high refractive index and large opening angle yield a smaller angle aperture/FOV. Large angular aperture can be achieved by employing the 2121 configuration, while it is desirable to adopt a 1212 configuration to obtain large angular FOV. The research can afford guidance for prism material, geometry choices, and configuration setting in the design of a Risley prism system for wide-angle beam steering or imaging adjustment.

Geodesic and Curvature of Piecewise Flat Finsler Surfaces

A piecewise flat Finsler metric on a triangulated surface M is a metric whose restriction to any triangle is a flat triangle in some Minkowski space with straight edges. One of the main purposes of this work is to study the properties of geodesics on a piecewise flat Finsler surface, especially when it meets a vertex. Using the edge-crossing equation, we define two classes of piecewise flat Finsler surfaces, namely, Landsberg type and Berwald type. We deduce an explicit condition for a geodesic to be extendable at a vertex, and define the curvature which measures the amount of such extensions. The dependence of the curvature on an incoming or outgoing tangent direction corresponds to the feature of flag curvature in Finsler geometry. When the piecewise flat Finsler surface is of Landsberg type, the curvature is only relevant to the vertex, and we prove a combinatoric Gauss–Bonnet formula which generalizes both the Gauss–Bonnet formulas for piecewise flat Riemannian manifolds and for smooth Landsberg surfaces.

Acoustic focusing lens with near-zero refractive index based on coiling-up space structure

An acoustic focusing lens based on a coiling-up space structure with near-zero refractive index is studied. According to the direction selection mechanism for acoustic waves in a near-zero refractive index material, we adopt the coiling-up space structure as a basic unit for arrangement, and design a geometric structure with specific incident and outgoing interfaces which is used to manipulate the outgoing direction of transmitted wave. Thus, the focusing effects for plane acoustic wave and cylindrical acoustic wave are realized. Besides, the influences of rigid scatterers inside the lens on the focusing performance are also discussed in detail. Moreover, the shape and direction of the acoustic waveform can be manipulated accurately by changing the outgoing interface of the lens with the near-zero refractive index. The results show that the lens with a single and two circular surfaces could realize the focusing effects of the plane and cylindrical acoustic waves, respectively, and the rigid scatterers inside the lens have no effects on the focusing performance. In addition, the cylindrical acoustic wave could be transformed into the plane acoustic wave through the lens with the circular incident surface and the plane exit surface, and the inclined angle of the exit surface could be used to manipulate the propagation direction of the plane wave. The simulation results between the lenses composed of the coiling-up space structure and the effective medium are in good agreement with each other. This type of lens has the advantages of single cell structure, high focusing performance, and high robustness. This work provides theoretical guidance and experimental reference for designing a novel acoustic focusing lens with the near-zero refractive index, and offers a new idea for studying the manipulation of the acoustic waveforms.

Elastic and inelastic collisions of swarms

Scattering interactions of swarms in potentials that are generated by an attraction–repulsion model are studied. In free space, swarms in this model form a well-defined steady state describing the translation of a stable formation of the particles whose shape depends on the interaction potential. Thus, the collision between a swarm and a boundary or between two swarms can be treated as (quasi)-particle scattering. Such scattering experiments result in internal excitations of the swarm or in bound states, respectively. In addition, varying a parameter linked to the relative importance of damping and potential forces drives transitions between elastic and inelastic scattering of the particles. By tracking the swarm’s center of mass, a refraction rule is derived via simulations relating the incoming and outgoing directions of a swarm hitting the wall. Iterating the map derived from the refraction law allows us to predict and understand the dynamics and bifurcations of swarms in square boxes and in channels.

Perceptually Motivated BRDF Comparison using Single Image

Surface reflectance of real-world materials is now widely represented by the bidirectional reflectance distribution function (BRDF) and also by spatially varying representations such as SVBRDF and the bidirectional texture function (BTF). The raw surface reflectance measurements are typically compressed or fitted by analytical models, that always introduce a certain loss of accuracy. For its evaluation we need a distance function between a reference surface reflectance and its approximate version. Although some of the past techniques tried to reflect the perceptual sensitivity of human vision, they have neither optimized illumination and viewing conditions nor surface shape. In this paper, we suggest a new image-based methodology for comparing different anisotropic BRDFs. We use optimization techniques to generate a novel surface which has extensive coverage of incoming and outgoing light directions, while preserving its features and frequencies that are important for material appearance judgments. A single rendered image of such a surface along with simultaneously optimized lighting and viewing directions leads to the computation of a meaningful BRDF difference, by means of standard image difference predictors. A psychophysical experiments revealed that our surface provides richer information on material properties than the standard surfaces often used in computer graphics, e.g., sphere or blob.

Switching in Heteroclinic Networks

We study the dynamics near heteroclinic networks for which all eigenvalues of the linearization at the equilibria are real. A common connection and an assumption on the geometry of its incoming and outgoing directions exclude even the weakest forms of switching (i.e. along this connection). The form of the global transition maps, and thus the type of the heteroclinic cycle, plays a crucial role in this. We look at two examples in $\mathbb{R}^5$, the House and Bowtie networks, to illustrate complex dynamics that may occur when either of these conditions is broken. For the House network, there is switching along the common connection, while for the Bowtie network we find switching along a cycle.

Surface excitation parameter for rough surfaces

In order to assess quantitatively the importance of surface excitation effect in surface electron spectroscopy measurement, surface excitation parameter (SEP) has been introduced to describe the surface excitation probability as an average number of surface excitations that electrons can undergo when they move through solid surface either in incoming or outgoing directions. Meanwhile, surface roughness is an inevitable issue in experiments particularly when the sample surface is cleaned with ion beam bombardment. Surface roughness alters not only the electron elastic peak intensity but also the surface excitation intensity. However, almost all of the popular theoretical models for determining SEP are based on ideal plane surface approximation. In order to figure out whether this approximation is efficient or not for SEP calculation and the scope of this assumption, we proposed a new way to determine the SEP for a rough surface by a Monte Carlo simulation of electron scattering process near to a realistic rough surface, which is modeled by a finite element analysis method according to AFM image. The elastic peak intensity is calculated for different electron incident and emission angles. Assuming surface excitations obey the Poisson distribution the SEPs corrected for surface roughness are then obtained by analyzing the elastic peak intensity for several materials and for different incident and emission angles. It is found that the surface roughness only plays an important role to SEP only for large interaction angle cases (larger than 60°) and large surface roughness (root-mean-square value lager than 15nm). This result is a clear evidence that the SEP database calculated based on ideal plane surface model are still efficient for realistic sample surface with common surface roughness.

Normal Mode Analysis of Three-Dimensional Propagation Over a Small-Slope Cosine Shaped Hill

Three-dimensional propagation over an infinitely long cosine shaped hill is examined using an approximate normal mode/parabolic equation hybrid model that includes mode coupling in the out-going direction. The slope of the hill is relatively shallow, but it is significant enough to produce both mode-coupling and horizontal refraction effects. In the first part of the paper, the modeling approach is described, and the solution is compared to results obtained with a finite element method to evaluate the accuracy of the solution in light of assumptions made in formulating the model. Then the calculated transmission loss is interpreted in terms of a modal decomposition of the field, and the solution from the hybrid model is compared to adiabatic and N × 2D solutions to assess the relative importance of horizontal refraction and mode-coupling effects. An analysis using a horizontal ray trace is presented to explain differences in the modal interference pattern observed between the 3D and N × 2D solutions. The detailed discussion provides a thorough explanation of the observed 3D propagation effects and demonstrates the usefulness of the approximate normal mode/parabolic equation hybrid model as a tool to understand measured transmission loss in complex environments.

Coupled mode analysis of three-dimensional propagation over a cosine shaped hill

Three-dimensional propagation over an infinitely long cosine shaped hill is examined using an approximate normal mode/parabolic equation hybrid model that includes mode coupling in the out-going direction. The slope of the hill is relatively shallow, but it is significant enough to produce both mode-coupling and horizontal refraction effects. The modeling approach is described and the solution is compared to results obtained with a finite element method to evaluate the accuracy of the solution in light of assumptions made in formulating the model. Then, the calculated transmission loss is interpreted in terms of a modal decomposition of the field, and the solution from the hybrid model is compared to adiabatic and Nx2D solutions to assess the relative importance of horizontal refraction and mode-coupling effects. An analysis using a horizontal ray trace is presented to explain differences in the modal interference pattern observed between the 3D and Nx2D solutions. The detailed discussion provides a thoro...