Underwater polarization imaging for visibility enhancement of moving targets in turbid environments.

Polarization imaging techniques have more prominent advantages for imaging in strongly scattered media. Previous de-scattering methods of polarization imaging usually require the priori information of the background region, and rarely consider the effect of non-uniformity of the optical field on image recovery, which not only reduces the processing speed of imaging but also introduces errors in image recovery, especially for moving targets in complex scattering environments. In this paper, we propose a turbid underwater moving image recovery method based on the global estimation of the intensity and the degree of polarization (DOP) of the backscattered light, combined with polarization-relation histogram processing techniques. The full spatial distribution of the intensity and the DOP of the backscattered light are obtained by using frequency domain analysis and filtering. Besides, a threshold factor is set in the frequency domain low-pass filter, which is used to adjust the execution region of the filter, which effectively reduces the error in image recovery caused by estimating the DOP of the backscattered light as a constant in traditional methods with non-uniform illumination. Meanwhile, our method requires no human-computer interaction, which effectively solves the drawbacks that the moving target is difficult to be recovered by traditional methods. Experimental studies were conducted on static and moving targets under turbid water, and satisfactory image recovery quality is achieved.

Generation and characteristics of hollow structured optical fields based on multiple off-axis vortices.

In this paper, various hollow structured optical fields are generated by skillfully adjusting the number and positions of multiple off-axis vortices loaded in a Gaussian beam. The focal-field characteristics of the generated hollow structured optical fields after passing through an ordinary lens are studied based on the scalar diffraction theory. Firstly, a variety of hollow structured optical fields are theoretically simulated by adjusting the number and positions of multiple off-axis vortices loaded in the Gaussian beam. The focal-field characteristics of the hollow structured optical fields after passing through a lens are theoretically analyzed. On this basis, the experiments are implemented in the built optical system for multi-off-axis vortex beam focusing through an ordinary lens. In the experiments, various hollow structured optical fields are detected in CCD which are consistent with the theoretical results. The manipulations of size and rotation direction of the hollow structured optical fields are realized. We believe that this study will contribute to extending the potential applications of off-axis vortex beams in fields such as optical field shaping, optical manipulation and laser processing.

Understanding of transverse spin angular momentum in tightly focused linearly polarized vortex beams.

Spin angular momentum (SAM) is widely used in spin-dependent unidirectional optical interfaces, optical manipulation, integrated optical signal processing, laser structuring and other fields, but its physical mechanism has not been fully understood so far. In this work, we investigate the three-dimensional (3D) SAM in tightly focused x-polarized first-order vortex beams from the perspectives of light field itself, phase distribution, and focusing propagation. It is shown that the distribution of three orthogonal components of SAM at the focal plane has pseudo two-fold rotational symmetry, because the cycloidal rotation of the electric field of the tightly focused vortex beam is opposite. The 3D SAM distribution in the focal region is visualized by mapping the 3D distribution of state of polarization (SoP). In addition, a principle experimental method for identifying the transverse SAM by using the direction of particle's rotation axis in optical tweezers is proposed.

Transversal energy flow of tightly focused off-axis circular polarized vortex beams.

The transversal energy flow characteristics of tightly focused circular polarized beams carrying off-axis vortices are examined in this research work. The results reveal that the symmetry of the focal fields are destroyed and energy flow is offset by the existence of off-axis vortices. Therefore, the focal field and energy flow distribution of polygons (bar-type-like, triangle-like, and square-like) can be realized by the superposition of multiple off-axis vortices with controllable positions. Furthermore, based on off-axis vortex energy flow characteristics, the force exerted on the metal particles in polygon focal fields is found to rotate the particles clockwise along the outlines of the polygon energy flow. The results will potentially provide new ideas and theoretical guidance to explore focal field and particle control methods.

Angular momentum separation in focused fractional vector beams for optical manipulation.

The generation, propagation, and applications of different types of integer vector beams have been extensively investigated. However, little attention focuses on the photophysical and photomechanical properties of the fractional vector beam (FVB). Herein, we theoretically and experimentally investigate the spin angular momentum (SAM) separation and propagation characteristics of weakly focused FVBs. It is demonstrated that such a beam carrying no SAM leads to both the transverse separation of SAM and the special intensity patterns in the focal region. Furthermore, we study the intensity, SAM, and orbital angular momentum (OAM) distributions of the tightly focused FVBs. It is shown that both three-dimensional SAM and OAM are spatially separated in the focal region of tightly focused FVBs. We investigate the optical forces, spin torques, and orbital torques on a dielectric Rayleigh particle produced by the focused FVBs. The results reveal that asymmetrical spinning and orbiting motions of optically trapped particles can be realized by manipulating FVBs.

Tightly focusing terahertz wave using gradient-type slotted grating based on spoof surface plasmons.

The d1-d2-d3-d4-d5 gradient-type spoof surface plasmons (SSP) grating was designed and found to exert an obvious effect on electric field localization. Two gradient-shaped planar ports were added to the bottom of this grating to form a gradient-type slotted SSP grating and achieve tight focusing and local electric field enhancement for a terahertz wave. The size of the focal spot was optimized to 0.01λ. The single-gradient-type slotted SSP grating was considered as a unit and arranged in one and two dimensions to generate a longitudinal focal line and square focal spots array. This did not only improve the resolution of terahertz imaging, but also simultaneously scan multiple focal spots to increase the speed of terahertz imaging. This work makes the manipulation of terahertz wave more flexible and efficient which has great potential in terahertz high-resolution near-field scanning imaging.

Ultrathin Ruddlesden-Popper Perovskite Heterojunction for Sensitive Photodetection.

Thanks to their unique optical and electric properties, 2D materials have attracted a lot of interest for optoelectronic applications. Here, the emerging 2D materials, organic-inorganic hybrid perovskites with van der Waals interlayer interaction (Ruddlesden-Popper perovskites), are synthesized and characterized. Photodetectors based on the few-layer Ruddlesden-Popper perovskite show good photoresponsivity as well as good detectivity. In order to further improve the photoresponse performance, 2D MoS2 is chosen to construct the perovskite-MoS2 heterojunction. The performance of the hybrid photodetector is largely improved with 6 and 2 orders of magnitude enhancement for photoresponsivity (104 A W-1 ) and detectivity (4 × 1010 Jones), respectively, which demonstrates the facile charge separation at the interface between perovskite and MoS2 . Furthermore, the contribution of back gate tuning is proved with a greatly reduced dark current. The results demonstrated here will open up a new field for the investigation of 2D perovskites for optoelectronic applications.

Enhanced second harmonic emission with simultaneous polarization state tuning by aluminum metal-insulator-metal cross nanostructures.

Aluminum (Al) plasmonic nanostructures have recently demonstrated remarkable optical nonlinear phenomena, such as enhanced second harmonic (SH) generation. However, the relatively weak field enhancement resulted from large optical losses associated with aluminum nanostructures in combination with the difficulties in controlling the emission polarization pose as a challenge for SH enhancement and tuning. In this paper, we show that the SH emission of aluminum nanostructures can be efficiently enhanced with the polarization properties simultaneously tunable by using metal-insulator-metal (MIM) nanostructures, constituting of Al cross nanoantenna arrays on top of Al mirrors with a SiO2 spacing layer. Specifically, femtosecond laser beam with a linear polarization parallel to one arm illuminates on the structure while the orthogonal arms were physically modified by the laser-induced photothermal reshaping technique to control the SH radiation by the plasmonic resonances. Under the resonance at the SH wavelength, we observed one order of magnitude larger emission enhancement compared to that at the off-resonant condition. Interestingly, the polarization states can be well manipulated simultaneously by controlling the resonances of the orthogonal arms. The enhanced SH conversion and tunable polarization states pave the way for the development of nonlinear optical sources and advanced functional metasurfaces.

Three-dimensional magnetization needle arrays with controllable orientation.

Based on the Richards-Wolf vectorial diffraction theory and inverse Faraday effect, we first propose a scheme to generate three-dimensional magnetization needle (MN) arrays with arbitrary orientation for each individual needle and controllable spatial position and number by reversing the electric dipole array radiation. To achieve this, each unit of the electric dipole array has two electric dipoles with orthogonal oscillation directions and quadrature phase and is located mirror-symmetric with respect to the focal plane of the high numerical aperture lens. Uniformly distributed MNs with a subwavelength lateral size of 0.44λ and a longitudinal depth of 5.36λ with four different orientations are obtained by optimized arrangement for 2N (here, N=2) units of the electric dipole array. The corresponding purity of MNs is also discussed in detail. Furthermore, two combinations of MN arrays with orthogonal orientation are emphatically exploited in the hybrid bit-patterned media recording. The results illustrate the richness of the proposed methods to locally control the particular orientation properties of the MN and find many potential applications in multichannel/multilayer magneto-optical storage, information security, and spintronics.

Theoretical investigation on asymmetrical spinning and orbiting motions of particles in a tightly focused power-exponent azimuthal-variant vector field.

We generate a new kind of azimuthal-variant vector field with a distribution of states of polarization (SoPs) described by the square of the azimuthal angle. Owing to asymmetrical SoPs distribution of this localized linearly polarized vector field, the tightly focused field exhibits a double half-moon shaped pattern with the localized elliptical polarization in the cross section of field at the focal plane. Moreover, we study the three-dimensional distributions of spin and orbital linear and angular momenta in the focal region. We numerically investigate the gradient force, radiation force, spin torque, and orbital torque on a dielectric Rayleigh particle produced by the tightly focused vector field. It is found that asymmetrical spinning and orbiting motions of trapped Rayleigh particles can be realized by the use of a tight vector field with power-exponent azimuthal-variant SoPs.

All-optical generation of magnetization with arbitrary three-dimensional orientations.

In this Letter, all-optical generation of magnetization with arbitrary three-dimensional (3D) orientations is numerically demonstrated through the inverse Faraday effect (IFE) by using a reversing calculation method. The IFE-induced magnetization with an expected 3D orientation is initially conceived by coherently configuring two orthogonally arranged electric dipoles with a phase difference of π/2 in the focal region of a to-be-determined incident light field. Based on the dipole antenna theory, this required incident light field can be deduced analytically according to the orientations of the electric dipoles. By utilizing this field as illumination and reversing the field propagation, magnetization with the expected orientation can be obtained in the focal region through the IFE. Moreover, this method showcases a high magnetization orientation purity (greater than 93%) within the focal volume defined by the full width at half maximum when the numerical aperture of the focal lens is 0.95. This result demonstrates extended flexibility of magnetization manipulations in an all-optical fashion and possesses great potential in spintronics and all-optical magnetic recording.

Creation of identical multiple focal spots with three-dimensional arbitrary shifting.

We propose a simple and flexible method to create identical multiple focal spots with three-dimensional arbitrary shifting without moving lenses or laser beams. The incident cylindrical vector (CV) beam superposed with predesigned phase and amplitude modulations is tightly focused by a single lens. The multiple focal spots with predetermined number and positions are generated and the identical intensity distribution as well as the polarization distribution for each individual focal spot is demonstrated. We also present a three-dimensional dynamic shifting with four identical focal spots along Pyramid-like trajectory by continuously regulating the phase and amplitude modulations on the incident CV beam. Furthermore, multiple focal spots with unique intensity profile can also be achieved when proper diffractive optical element (DOE) is associated in the focusing system. These engineered focal fields may find potential applications in 3D laser printing, moving multiple particles trapping and manipulations.

Generation of equilateral-polygon-like flat-top focus by tightly focusing radially polarized beams superposed with off-axis vortex arrays.

In this paper, the general formula for tightly focusing radially polarized beams (RPB) superposed with off-axis vortex arrays is derived based on Richard-Wolf vector diffraction theory. The off-axis vortex breaks the rotational symmetry of the energy flow along the axial direction and leads to the spatial redistribution of intensity within the focal plane. The dependence of the consequent focal intensity redistribution on the off-axis distance of vortices as well as the numerical aperture of the lens is theoretically studied. Based on this intriguing feature, generation of equilateral-polygon-like flat-top focus (EPFF) with a flat-top area on the level of sub-λ2 is realized. The demonstrated method provides new opportunities for focus shaping and holds great potentials in optical manipulation and laser fabrication.

Experimental study on optical image encryption with asymmetric double random phase and computer-generated hologram.

Optical image encryption, especially double-random-phase-based, is of great interest in information security. In this work, we experimentally demonstrate the security and feasibility of optical image encryption with asymmetric double random phase and computer-generated hologram (CGH) by using spatial light modulator. First of all, the encrypted image modulated by asymmetric double random phase is numerically encoded into real-value CGH. Then, the encoded real-value CGH is loaded on the spatial light modulator and optically decrypted in self-designed experimental system. Experimental decryption results are in agreement with numerical calculations under the prober/mistaken phase keys condition. This optical decryption technology opens a window of optical encryption practical application and shows great potential for digital multimedia product copyright protection and holographic false trademark.

Generation and manipulation of super-resolution spherical magnetization chains.

Based on the inverse Faraday effect, the light-induced magnetization field distributions are investigated for a 4π tight focusing configuration with azimuthally polarized beams. It is found that a superlong (16λ) magnetization chain, composed of 19 subwavelength (0.44λ) spherical spots with longitudinal magnetization field, can be achieved in the focal volume of the objective. Moreover, the magnetic force on a magnetic particle or particle trains produced by tightly focused azimuthally polarized beams are calculated and exploited for the stable trapping of magnetic particles. These unique focal field distributions may find potential applications in confocal microscopy, atom control, and magneto-optical data storage.

Propagation evolution of an off-axis high-order cylindrical vector beam.

The propagation characteristics of an off-axis high-order cylindrical vector beam (OHCVB) are studied in this paper. The analytic expressions for the electric field and intensity distribution of the OHCVB propagating in free space are presented, to our knowledge for the first time. The transverse intensity of the OHCVB, different from that of the input Gaussian beam, does not have an axially symmetric distribution, owing to a slight dislocation between the polarization singularity located in the vector field generator and the center point of the Gaussian beam. Numerical results show that the intensity distribution during propagation strongly depends on the propagation distance, dislocation displacement, and topological charge. Accompanied by beam expansion, the intensity distribution of the OHCVB tends to eventually become steady, and the dark core of the vector beam will disappear gradually during the process of propagation. Moreover, with the increase of the topological charge, more energy will be transferred from the x axis to the y axis, and the annular intensity is split into two parts along the y-axis direction. The results help us to investigate the dynamic propagation behaviors of the HCVB under the off-axis condition and also guide the calibration of the off-axis high-order cylindrical vector field in practice.

Risk Factors and Risk Model Construction of Stroke in Patients with Vertigo in Emergency Department.

Objective: We aimed to explore the risk factors of stroke in patients with vertigo in the emergency department and establish a risk prediction model for stroke patients. Methods: A total of 301 patients experiencing vertigo in our hospital from January 2020 to January 2021 were retrospectively included. Patients were divided into the stroke group (n = 56) and the nonstroke group (n = 245). The clinical characteristics of patients in both groups were collected and compared, followed by binary logistic regression that was employed to determine the risk factors that affect stroke diagnosis. The receiver operating characteristic (ROC) curve was used to clarify the effectiveness of the constructed model. Results: Patients in the stroke group were older and had higher systolic and diastolic blood pressure on admission than the nonstroke group. Meanwhile, they demonstrated a higher proportion of diabetes and atrial fibrillation and focal muscle weakness, dysarthria, dysphagia, or ataxia in neurological examinations compared to the nonstroke group (all P < 0.05). The proportion of patients in the nonstroke group who had a history of vertigo or inner ear disease was significantly higher than that in the stroke group (P < 0.05). The patient's age ≥ 60 years old (OR = 3.57), diabetes (OR = 4.57), atrial fibrillation (OR = 4.26), previous history of vertigo or inner ear disease (OR = 0.16), focal muscle weakness (OR = 4.34), and dysphagia or ataxia (OR = 4.08) were associated with a higher risk of stroke. The area under the curve for stroke was 0.87, and the sensitivity and specificity were 98.2% and 57.6%, respectively, as the sum of the assigned scores was greater than 3. Conclusions: Age ≥ 60 years old, diabetes, atrial fibrillation, previous history of vertigo or inner ear disease, focal muscle weakness, dysphagia, or ataxia were associated with a higher risk of stroke. The risk model constructed based on our findings may help to assess the risk of stroke in patients with vertigo in the emergency department.

Quantitative angle-insensitive flow measurement using relative standard deviation OCT.

Incorporating different data processing methods, optical coherence tomography (OCT) has the ability for high-resolution angiography and quantitative flow velocity measurements. However, OCT angiography cannot provide quantitative information of flow velocities, and the velocity measurement based on Doppler OCT requires the determination of Doppler angles, which is a challenge in a complex vascular network. In this study, we report on a relative standard deviation OCT (RSD-OCT) method which provides both vascular network mapping and quantitative information for flow velocities within a wide range of Doppler angles. The RSD values are angle-insensitive within a wide range of angles, and a nearly linear relationship was found between the RSD values and the flow velocities. The RSD-OCT measurement in a rat cortex shows that it can quantify the blood flow velocities as well as map the vascular network in vivo.