Large Deflection Angles Research Articles

Strong lensing and nonminimally coupled electromagnetism

The lensing at large deflection angles caused by a Schwarzschild black hole for the case of a nonminimal coupling between gravitation and electromagnetism is examined. We show that photons follow an effective geometry, which displays an effective photon sphere. For the case in which the source, lens and observer are aligned, so that relativistic Einstein rings are formed, the dependence of the angular separation $\delta\theta$ between the first and second ring with the relevant coupling parameter is calculated. We argue that such a separation, which may be measured by telescopes that will be operative in the near future, may set an upper and a lower limit for the coupling parameter.

Polarization-independent highly efficient generation of Airy optical beams with dielectric metasurfaces

Airy optical beams have emerged to hold enormous theoretical and experimental research interest due to their outstanding characteristics. Conventional approaches suffer from bulky and costly systems, as well as poor phase discretization. The newly developed metasurface-based Airy beam generators have constraints of polarization dependence or limited generation efficiency. Here, we experimentally demonstrate a polarization-independent silicon dielectric metasurface for generation of high-efficiency Airy optical beams. In our implementation, rather than synchronous manipulation of the amplitude and phase by plasmonic or Huygens’ metasurfaces, we employ and impose a 3/2 phase-only manipulation to the dielectric metasurface, consisting of an array of silicon nanopillars with an optimized transmission efficiency as high as 97%. The resultant Airy optical beams possess extraordinarily large deflection angles and relatively narrow beam widths. Our validated scheme will open up a fascinating doorway to broaden the application scenarios of Airy optical beams on ultracompact photonic platforms.

Field measurement and wind tunnel study of aerodynamic characteristics of twin-box girder

To investigate the aerodynamic characteristics of a twin-box girder, the Xihoumen suspension bridge with a main span of 1650 ​m, was selected as the research object in this study. The wind field characteristics and the wind pressure distribution on the girder surface were analyzed by comparing the field measurements and wind tunnel tests for two conditions, namely orthogonal wind and skew wind. Furthermore, the aerodynamic coefficients and admittance were studied, which fully revealed the aerodynamic characteristics of the twin-box girder. The field measurement at the bridge site showed that the wind field had a large angle of attack and deflection angle in most cases. The results from the wind tunnel tests under skew wind indicated that the yaw wind-decomposition method was not applicable in the case of large wind deflection angles. It was observed that the aerodynamic admittance obtained by the field measurement in the low frequency phase was significantly higher than that measured in the wind tunnel test; however, in the high frequency phase, both yielded similar results. This implied that the buffeting load of the girder would be underestimated in the low frequency band if the aerodynamic admittance value from the test was used.

Estimating the opening time of a direct spring operated pressure relief valve in the case of multiphase flow of fixed mass fraction in the absence of piping

This paper addresses the effect of non-flashing multiphase flow on the dynamic behaviour of direct spring operated pressure relief valves (PRV). We employ DIER’s ω method to describe the mass flux through the valve for different xg gas mass fractions, which is combined with the one-degree-of-freedom mechanical model of the PRV and the vessel pressure dynamics, providing a simulation tool for predicting venting scenarios. We emphasise on the proper description of the fluid momentum forces on the valve disc. Three typical valve geometries are considered: a ’reaction-like’ geometry with large deflection angle reversing the flow, a simple disc valve with purely radial outlet and a conical geometry, for which the direction of the flow through the valve does not change significantly. It was found that by merely varying the mass fraction of the liquid and the gas, the valve opening time changes drastically, over a range of several orders of magnitude. By using a simplified mechanical analysis and assuming frozen mixture (i.e. fixed mass fraction), a formula was developed providing an order-of-magnitude estimation of the valve opening time.

Extremely large-angle beam deflection based on low-index sparse dielectric metagratings

Metasurfaces for beam engineering are usually implemented by using dense-plasmonic or high-contrast dielectric building blocks with deep-subwavelength feature size and strong field confinement. Here, we theoretically and experimentally demonstrate extremely large angle deflection with a very steep phase gradient based on low-contrast () metagratings composed of very sparse building blocks. Only two ridges are included in a 4π variation supercell, which diffracts the normal excitation to desired angles as large as 80° with a theoretical efficiency of more than 80%. Due to the limited beam radius relative to the metasurfaces, the angular distribution of the intensity is broadened with reduced peak intensity. Such metagratings are 3D-printed for 0.14 THz operation. The measured peak intensities are 57% of the theoretical ones due to material loss and fabrication errors. Repeatability, bandwidth and angular sensitivity are systematically studied, and the results pave the way towards using practical THz elements for large-gradient wavefront shaping.

Reflective Polarization Volume Lens with Small f‐Number and Large Diffraction Angle

AbstractPlanar optics based on patterned cholesteric liquid crystals (CLCs) has attracted increasing attention owing to the self‐organized helical structure and the ability to create arbitrary reflected wavefront through spatial orientation control. However, because of the subwavelength‐orientation requirement, it is challenging for liquid crystal lens to achieve a low f‐number (f/#) and large deflection angle simultaneously. Furthermore, with the increasing demand for compact size in novel optical systems, reflective lens has advantage over the transmissive one because it can fold the optical path. Here, a new off‐axis reflective polarization volume lens (PVL) with f/# = 0.825, large aperture size, simple fabrication process, thin profile, circular polarization selectivity, and large diffraction angle is proposed. In contrast to traditional vertical spiral structure, PVL is based on patterned CLCs with a slanted helical axis. In this paper, the PVL is theoretically evaluated and then three reflective PVLs at red, green, and blue wavelengths (R = 605 nm, G = 532 nm, and B = 450 nm) are fabricated. Meanwhile, a simple approach is utilized to achieve 20 mm diameter and 16.5 mm focal length. The low scattering and good image quality of reflective PVL enrich these functional devices and provide promising applications to novel foldable optical systems and waveguide‐based wearable near‐eye displays.

Ultra-Compact Microsystems-Based Confocal Endomicroscope.

Point-of-care medical diagnosis demands immediate feedback on tissue pathology. Confocal endomicroscopy can provide real-time in vivo images with histology-like features. The working channel in medical endoscopes are becoming smaller in dimension. Microsystems methods can produce tiny mechanical scanners. We demonstrate a flexible fiber instrument for in vivo imaging as an endoscope accessory. The optical path is folded on-axis to reduce length while allowing the beam to expand and achieve a numerical aperture of 0.41. A high-speed parametric resonance mirror produces large deflection angles > 13°, and is mounted on a 2 mm diameter chip designed with clamp structures for reduced space. A compact lens assembly provides diffraction-limited lateral and axial resolution of 1.5 and [Formula: see text], respectively. A working distance of [Formula: see text] and field-of-view of [Formula: see text] m are achieved. Miniature apparatus is fabricated to assemble and align the scanhead components. The optics and scanner are packaged in a distal tip with 2.4 mm diameter and 10 mm rigid length. These dimensions allow the endomicroscope to pass forward easily through the 2.8 mm diameter working channel in medical endoscopes commonly used in clinical practice. Fluorescence images are collected in vivo at 10 frames per second in the colon of genetically-engineered mice that spontaneously develop adenomas. A FITC-labeled peptide heterodimer is administered intravenously to provide specific contrast. Sub-cellular structures are visualized to distinguish pre-malignant from normal mucosa. These results demonstrate use of microsystems methods to produce an ultra-compact instrument with sufficiently small dimensions for broad use.

Prediction for the Flexural Properties of Nanowires in Lateral Manipulation

Simple beam theory is usually employed to predict the flexural properties of nanowires (NWs) in the lateral manipulation, which results in a linear F−δ curve. However, three factors, namely, changes in the load position and direction of the AFM tip, surface effects, and the large displacement of the NW, are not considered in simple beam theory. In this work, a simple geometrical model is proposed to analyze the large deflection and rotational angle deformation of NWs in the lateral manipulation. The traditional solution to the differential equation of the deflection curve turns the solution of an integral equation. Results show that contact force does not linearly increase with increasing displacement, and the F−δ curve exists a maximum amount due to changes in the load position and direction of the AFM tip. Moreover, the rotational angle and deflection have a nonlinear relationship. Comparison of the results of this work with other authors’ measurements illustrates that the use of simple beam theory in manipulation underestimates the deflection and effective modulus of NWs. Our model provides a good approach to predict F−δ curves, rotational angles, and contact forces that closely match experimental results.

Rectangular dielectric metagrating for high-efficiency diffraction with large-angle deflection

In this Letter, we propose a metagrating consisting of simple rectangular bars for nearly unity anomalous diffraction with a large deflection angle. The analysis performed by the scattering-matrix method shows that such exceptional beam steering derives from the couplings of the two lowest propagation waveguide-array-modes and their constructive interferences. The tolerance of the incident angle for a high diffraction efficiency (e.g., >90%) is within a range of 33°. We also discuss that such an advantage still exists after considering a reasonable loss and dispersion. We envision that the proposed strategy may have wide use in the field of high-performance wavefront-shaping applications.

Flat transmitted serrated-phase high-contrast-index subwavelength grating beam splitter

We proposed a method to form a flat transmitted serrated-phase (SP) high-contrast-index subwavelength grating (HCG) beam splitter (HBS) for all dielectric materials, which is to alternately arrange two kinds of grating bars with a phase difference of π. Compared to the typical linear-phase (LP) HBS, which consists of two symmetrical deflecting gratings, the SP-HBS is extensible in size, and can achieve excellent splitting ability regardless of normal incidence or small-angle oblique incidence with large deflection angles, higher diffraction efficiency, lower energy loss, and higher tolerance of fabrication accuracy. Furthermore, the incident light can be split in half at any part of the SP-HBS, and the output beams of light maintain the original shape. In this Letter, we designed an SP-HBS with a 44.8° deflection angle and a 90.28% transmissivity.

Near-infrared beam deflector with broadband and large deflection angle based on dielectric metasurface

Near-infrared beam deflector with broadband and large deflection angle based on dielectric metasurface

Study on the control method about improving piezoelectric actuated photoelectric precision tracking system

Fast steering mirrors (FSM) driven by Piezoelectric transducer (PZT) are widely used in various precision stable tracking systems. Aiming to counteract the hysteresis and non-linear interference in PZT, this work applies a radial basis function (RBF) neural network to approximate its nonlinearity. Adaptive backstepping sliding mode (ABSM) controller combinewith a sliding mode control method and backstepping control is designed. Combining the characteristics of PZT and voice coil motors (VCM), the FSM driven by VCM is designed as the power sub-system to ensure that the large-angle deflection of the FSM can match a wider field of view. The FSM driven by PZT is designed as a correction sub-system, which can adjust the system error within a small range. Finally, the power sub-system and the correction sub-system are combined into a two-level precision tracking system. The simulation results show that the maximum steady-state error of the system is about 15 μrad, and the root mean square error is about 10 μrad. Compared with the traditional PI controller, the error is reduced by about 75%, the response speed is up to 10 ms, and the output is smooth without some serious viberation.

Large-Angle, Broadband, and Multifunctional Directive Waveguide Scatterer Gratings

Conventional surface-relief gratings are inefficient at deflecting normally incident light by large angles. This limits the overall efficiency of grating-based optical instruments and also restricts their use in many applications. Here, we demonstrate a simple approach for the design of diffraction gratings that can be highly efficient for large deflection angles, while also offering additional functionality. Such gratings are composed of a unit cell comprising a vertically oriented asymmetric slot waveguide. The unit cell shows oscillating unidirectional scattering behavior that can be precisely tuned as a function of the waveguide length. This occurs due to interference between multiple modes excited in the structure by the incident light. In contrast to metasurface-based gratings with multiple resonant subelements, a periodic arrangement of such nonresonant diffracting elements allows for broadband operation and a strong tolerance for variations in angle of incidence. Full-wave simulations show that our grating designs can exhibit diffraction efficiencies ranging from 94% for a deflection angle of 47° to 80% for a deflection angle of 80°. To show the multifunctionality of our approach, we also demonstrate a flat polarization beamsplitter and a polarization insensitive grating.

Numerical investigation of tangential blowing over the rudder of a vertical tailplane

Tangential blowing over the shoulder of a deflected rudder is applied on a vertical tailplane. For a large rudder deflection angle and without blowing the flow is mostly separated on the rudder. Three configurations are investigated in a numerical study with the aim to increase the side force coefficient. The first one is the baseline without blowing, the second one has a continuous full span slot and for the third one discrete slots are used. With sufficient blowing through the continuous slot the separation on the rudder can be removed completely while the separation extent is greatly reduced when using discrete slots. This is investigated for different sideslip angles. An approximately linear increase in the side force coefficient can be found until a sudden side force breakdown occurs. In a further study, the jet blowing velocity is varied. A smaller jet velocity leads to a smaller increase in the side force coefficient. Comparing the continuous and discrete slot configurations shows that for a similar increase in the side force coefficient a much smaller mass flow rate is needed for the discrete slots. However, for jet velocities below the sonic speed the increase in the side force coefficient is limited. It can only be increased by a larger slot extent in spanwise direction but this comes at the expense of an increase in the required mass flow rate.

Analysis and modeling of a flexible rectangular cantilever plate

Flexible plate structures with large deflection and rotation are commonly used structures in engineering. How to analyze and solve the cantilever plate with large deflection and rotation is still an unsolved problem. In this paper, a general nonlinear flexible rectangular cantilever plate considering large deflection and rotation angle is modeled, solved and analyzed. Hamilton's principle is applied to obtain the nonlinear differential dynamic equations and boundary conditions by introducing a coordinate transformation between the Cartesian coordinate system and the deformed local coordinate system. Stress function relating to in-plane force resultants and shear forces is given for the first time for complex coupling equations caused by coordinate transformation. The nonlinear equations and the solving method are validated by experiments. Then, harmonic balance method is adopted to get the nonlinear frequency-response curves, which shows strong hardening spring characteristic of this system. Runge–Kutta methods are used to reveal complex nonlinear behaviors such as 5 super-harmonic resonance, bifurcations and chaos for general nonlinear flexible rectangular cantilever plate.

Multi-element ducts for ducted wind turbines: a numerical study

Abstract. Multi-element ducts are used to improve the aerodynamic performance of ducted wind turbines (DWTs). Steady-state, two-dimensional computational fluid dynamics (CFD) simulations are performed for a multi-element duct geometry consisting of a duct and a flap; the goal is to evaluate the effects on the aerodynamic performance of the radial gap length and the deflection angle of the flap. Solutions from inviscid and viscous flow calculations are compared. It is found that increasing the radial gap length results in an augmentation of the total thrust generated by the DWT, whereas a larger deflection angle has an opposite effect. Reasonable to good agreement is seen between the inviscid and viscous flow calculations, except for multi-element duct configurations characterized by large flap deflection angles. The viscous effects become stronger at large flap deflection angles, and the inviscid calculations are incapable of taking this phenomenon into account.

Structural insight into the optical and electro-optic properties of lead zirconate titanate for high-performance photonic devices

A series of Pb(ZrxTi1-x)O3 thin films with compositions across the rhombohedral-tetragonal phase boundary (PZT, 0.2 ≤ x ≤ 0.8) have been developed by magnetron sputtering and their optical and electro-optic (EO) properties have been systematically studied. Our results reveal that the optical band gap, propagation loss, refractive index and EO performance of PZT films are strongly composition dependent. The PZT films with the composition at morphotropic phase boundary (MPB, x = 0.52) demonstrate largest EO coefficient (rc = 185.3 p.m./V), low propagation loss (3.6 dB/cm), and fast response time (5.3 μs). Further studies show that the EO coefficients of the PZT52 thin films were almost invariant with the changes of testing temperatures (in 25–250 °C) and wavelengths (λ = 532–1064 nm) and was retained nearly 83% when the testing frequency was increased from 10 Hz to 104 Hz. As an application example, a beam deflector prototype with 10 μm thick PZT52 films has been successfully fabricated and demonstrates a large deflection angle of ~3.4° at 100 V driving voltage. Our study gives an insight into the EO effects of PZT film systems, making them of interest for high-speed, low-driving-voltage and power efficient photonic devices for modern information telecommunication.

Stretchable, flexible, rollable, and adherable polarization volume grating film.

Volume Bragg gratings (VBGs) have many applications, including filters, wavelength multiplexing devices, and see-through displays. As a kind of VBGs, polarization volume gratings (PVGs) based on liquid crystal polymer have the advantages of nearly 100% efficiency, large deflection angle, and high polarization selectivity. However, previous reports regarding PVGs did not address high efficiency, tunable periodicity, and flexibility. Here, we report a stretchable, flexible, and rollable PVG film with high diffraction efficiency. The control of PVG by mechanical stretching is investigated, while the Bragg reflection band shift is evaluated quantitatively. Moreover, we quantified the deflection angle change's behavior, which has promising potential for laser beam steering applications. The mechanical robustness under stretch-release cycles is also scrutinized.

Low-index second-order metagratings for large-angle anomalous reflection.

A large-angle anomalous reflector based on a low-index polylactic acid metagrating is designed around 140GHz. By breaking the limit of fixed bar-to-bar distance and directing the beam into the second diffraction order through the optimization of the 4π phase supercell, the efficiencies for 70° and 80° reflection under normal excitation both reach 0.82 with a wide bandwidth. In contrast, the efficiency is less than 0.2 in conventional designs. The success of the second-order low-index metagrating for large-angle deflection originates from the appropriate number of Bloch modes with well-engineered mode interactions and couplings. The design can be potentially fabricated by three-dimensional printing, with promising applications in designing flat lenses of high numerical aperture and extending the material system of metadevices.

Two-dimensional environment reconstruction based on the absolute local deflection angle of laser scanning data

Successive edge following (SEF) is extensively used to extract environment characteristics from two-dimensional laser scanning data given its simplicity. Conventional SEF compares the Euclidean distance between two adjacent points to either a fixed or an adaptive threshold. However, the segmentation accuracy is low and information loss occurs during processing of planes with a large deflection angle with respect to the laser sensor because the distance estimation is sensitive to this angle. Moreover, available SEF algorithms cannot suitably determine corners in the environment. To address these problems, we propose an extended SEF algorithm based on the absolute local deflection angle of laser scanning points. The proposed algorithm determines the local deflection angle between target and reference points, and an adaptive threshold is calculated based on the range precision of the laser sensor. Furthermore, corners are efficiently estimated and noise is mitigated by using three constraints. An experiment on two-dimensional environment reconstruction of an underground parking lot verifies the accuracy and capability of the proposed algorithm to reconstruct the lines and corners representing the environment.