High quality holographic 3D display with enhanced focus cues based on multiple directional light reconstruction.

Holographic display faces the trade-off between image quality and focus cues, resulting from the specific choice of phase distribution. In this Letter, we propose a speckle-free holographic display with enhanced focus cues by multiple directional light reconstruction. The uniform phase hologram is first generated by the gradient descent optimization algorithm. The blazed grating phase is used to steer the object light to a specific direction. Multiple sub-holograms with different blazed gratings are refreshed fast to reconstruct the images from different directions. Thus, the defocus blur is improved due to the separation of multiple spots on the defocus plane. The multi-plane reconstruction is also realized by pre-processing the depth images to eliminate image separation. The proposed method provides apparent focus cues while maintaining high image qualities, which is expected to realize comfortable holographic near-eye display in the near future.

Lens array-based holographic 3D display with an expanded field of view and eyebox.

Conventional spatial light modulator (SLM)-based holographic 3D display faces limited field of view (FOV) and eyebox, due to its limited pixel number. In this paper, a lens array is used to expand the FOV and eyebox of an SLM-based holographic display. The hologram is calculated to reconstruct a 3D sub-image array, each sub-image corresponding to a specific perspective of the 3D object. Then, the 3D sub-image array is imaged and magnified by the lens array to integrate to the original 3D image. The FOV is expanded due to the large numerical aperture of the lens, and the eyebox is expanded because the lens array generates multiple viewpoints with a large pitch. The optical experiment realizes a 17.6° FOV and 50 mm eyebox, which contains 4 × 4 viewpoints. Apparent motion parallax is observed through the viewpoint array, which is usually hard to observe in a conventional SLM-based holographic display. The proposed method provides a novel, to the best of our knowledge, way to expand the FOV and eyebox of holographic 3D display without increasing the total pixel number of the SLM.

Generation of a Focused THz Vortex Beam from a Spintronic THz Emitter with a Helical Fresnel Zone Plate.

Similar to optical vortex beams, terahertz (THz) vortex beams (TVBs) also carry orbital angular momentum (OAM). However, little research has been reported on the generation of TVBs. In this paper, based on the detour phase technique, we design a series of spintronic terahertz emitters with a helical Fresnel zone plate (STE-HFZP) to directly generate focused TVBs with topological charges (TCs) of l = ±1, ±2 and ±3, respectively. The STE-HFZP is a hybrid THz device composed of a terahertz emitter and a THz lens, and it has a high numerical aperture (NA), achieving subwavelength focal spots. Its focus properties are surveyed systemically through accurate simulations. This STE-HFZP can also generate focused TVBs with higher order TCs. More importantly, the components of the focused electric field with OAM make up the majority of the intensity and have potential applications in the field of THz communications, THz imaging and atom trapping.

Cross talk-free retinal projection display based on a holographic complementary viewpoint array.

In near-eye displays (NEDs), retinal projection display (RPD) is one kind of promising technology to alleviate the vergence-accommodation conflict (VAC) issue due to its always-in-focus feature. Viewpoint replication is widely used to enlarge the limited eyebox. However, the mismatch between viewpoint interval and eye pupil diameter will cause the inter-viewpoint cross talk when multiple viewpoints enter the pupil simultaneously. In this Letter, a holographic complementary viewpoint method is proposed to solve this cross talk problem. Instead of avoiding observing multiple viewpoint images simultaneously, it is designed that multiple complementary viewpoints jointly project the complete image on the retina without cross talk. To do this, the target image is segmented into multiple sub-images, each multiplied with a corresponding partial spherical phase to converge to a specific complementary viewpoint. A group of complementary viewpoint enter the eye pupil simultaneously, and each viewpoint project a corresponding sub-image on a specific area of the retina and splice to a complete image. All of the complementary viewpoints are duplicated to an interlaced two-dimensional array to extend the eyebox in both horizontal and vertical directions. Optical experiment verifies that the proposed method could present smooth transition between viewpoints to avoid both inter-viewpoint cross talk and blank image issues.

Reducing crosstalk of a multi-plane holographic display by the time-multiplexing stochastic gradient descent.

Multi-plane reconstruction is essential for realizing a holographic three-dimensional (3D) display. One fundamental issue in conventional multi-plane Gerchberg-Saxton (GS) algorithm is the inter-plane crosstalk, mainly caused by the neglect of other planes' interference in the process of amplitude replacement at each object plane. In this paper, we proposed the time-multiplexing stochastic gradient descent (TM-SGD) optimization algorithm to reduce the multi-plane reconstruction crosstalk. First, the global optimization feature of stochastic gradient descent (SGD) was utilized to reduce the inter-plane crosstalk. However, the crosstalk optimization effect would degrade as the number of object planes increases, due to the imbalance between input and output information. Thus, we further introduced the time-multiplexing strategy into both the iteration and reconstruction process of multi-plane SGD to increase input information. In TM-SGD, multiple sub-holograms are obtained through multi-loop iteration and then sequentially refreshed on spatial light modulator (SLM). The optimization condition between the holograms and the object planes converts from one-to-many to many-to-many, improving the optimization of inter-plane crosstalk. During the persistence of vision, multiple sub-hologram jointly reconstruct the crosstalk-free multi-plane images. Through simulation and experiment, we confirmed that TM-SGD could effectively reduce the inter-plane crosstalk and improve image quality.The proposed TM-SGD-based holographic display has wide applications in tomographic 3D visualization for biology, medical science, and engineering design, which need to reconstruct multiple independent tomographic images without inter-plane crosstalk.

Quantitative detection of high-order Poincaré sphere beams and their polarization evolution.

The high-order Poincaré sphere (PS) introduces a mapping whereby any vector beams with spatially homogeneous ellipticity are represented by a specific point on the surface of the sphere. We propose the quantitative detection of high-order PS beams by introducing three sets of nonuniform polarization bases in the high-order Stokes parameters. Overall polarization detection is realized by directly separating and measuring the respective intensity of different nonuniform polarization bases based on S-plate. The polarization evolution of the PS beams on the high-order PS and between the conventional and the high-order PS are achieved by S-plate. The results provide new insights for the generation, evolution and detection of arbitrary beams on the high-order PS.

Lensless phase-only holographic retinal projection display based on the error diffusion algorithm.

Holographic retinal projection display (RPD) can project images directly onto the retina without any lens by encoding a convergent spherical wave phase with the target images. Conventional amplitude-type holographic RPD suffers from strong zero-order light and conjugate. In this paper, a lensless phase-only holographic RPD based on error diffusion algorithm is demonstrated. It is found that direct error diffusion of the complex Fresnel hologram leads to low image quality. Thus, a post-addition phase method is proposed based on angular spectrum diffraction. The spherical wave phase is multiplied after error diffusion process, and acts as an imaging lens. In this way, the error diffusion functions better due to reduced phase difference between adjacent pixels, and a virtual image with improved quality is produced. The viewpoint is easily deflected just by changing the post-added spherical phase. A full-color holographic RPD with adjustable eyebox is demonstrated experimentally with time-multiplexing technique.

Unidirectional spintronic terahertz emitters with high efficiency.

Due to the high performance and low cost, spintronic terahertz emitters (STEs) have been a hot topic in the field of terahertz sources. However, most of the research focuses on the THz generation process and little attention has been paid to the control and modulation of the THz wave generated by the STE. In this Letter, a unidirectional spintronic terahertz emitter (USTE) integrating a common STE with a metal grating is proposed to manipulate the THz emission process. The dyadic Green's function method and finite element method are adopted to survey the characteristics of the USTE. Simulations show that the metal grating not only has a transmission larger than 97% in the optical band, but also has a higher reflectivity larger than 99% in the THz band. As a result, the USTE has a unidirectional THz emission along the direction of the pump beam with a larger than 4-fold enhancement in intensity. Moreover, the USTE has the capability of tuning the central frequency and THz wave steering by adjusting the distance and angle between the STE and the metal grating. We believe that this USTE can be used in THz wireless communications and holographic imaging, especially in the field of THz bio-sensing, which needs some resonance frequencies to sense.

Simultaneous multi-channel near-eye display: a holographic retinal projection display with large information content.

Augmented reality (AR) near-eye displays (NEDs) are emerging as the next-generation display platform. The existing AR NED only present one single video channel at a time, same as traditional media such as TVs and smartphones. In this Letter, to the best of our knowledge, we propose for the first time a multi-channel holographic retinal projection display (RPD), which can provide multi-channel image sources simultaneously, thus greatly increasing the information content. Due to the superposition capacity of a hologram, multiple images are projected to different viewpoints simultaneously through multiple spherical wave encoding, so that the viewer can switch among playing channels very fast through eye rotation. A full-color dynamic multi-channel holographic near-eye display is demonstrated in the optical experiment. The proposed method provides a good prospect that the future AR glasses can play dozens of video channels in parallel, and the user can switch among channels freely and efficiently just through a simple eye rotation.

Release of phthalate esters from a local landfill in the Tibetan Plateau: Importance of soil particle-size specific association.

High loads of phthalate esters (PAEs) in background regions can be directly attributed to the local sources, and their association with soil particles may determine the environment behaviors. However, little is known about the particle-size specific distributions of PAEs in soils from point source to the surroundings. In this study, 12 PAE congeners were measured in clay (< 2 µm), silt (2-63 µm) and sand fractions (63-250 µm) from surficial soils and soil profiles (0-200 cm) around the Lhasa landfill. The total concentrations of PAEs in bulk soils varied from 0.44 to 22.3 µg/g, with a dominance of bis(2-ethylhexyl) phthalate (DEHP). The clay-sorbed PAEs exhibited a decreasing trend with the increasing distance from landfill. This distribution pattern was well described by the Gaussian air pollution model, suggesting the airborne particles/gaseous transport of clay-sorbed PAEs. The Boltzmann equation explained the spatial variation of silt-sorbed PAEs, reflecting the atmospheric dispersion of silt-sorbed PAEs. In comparison, the sand-sorbed PAEs in surrounding soils showed downslope accumulation possibly due to the aeolian transport of sand particles. Half-life of the most abundant PAE congener DEHP was assumed based on the soil inventories from observed concentration and the Level III fugacity model simulations, and the results indicated significant longer half-life of DEHP in deeper soils (~24,000 h) than in surficial soils (5500 h). This study elucidates that the distribution and fate of soil PAEs would depend on their association with particles in the source area, and the relative stability of DEHP in deeper soils would further increase PAE inventory in soil compartment.

Conjugate wavefront encoding: an efficient eyebox extension approach for holographic Maxwellian near-eye display.

Conventional holographic display suffers from the conjugate light issue. In this Letter, we propose to efficiently extend the eyebox of holographic Maxwellian near-eye display by encoding the conjugate wavefront as the multiplication of plane wave phase with the target image. It is interesting that after being focused by the lens, the generated conjugate viewpoints also present erect virtual images with the same image quality as the signal viewpoints. Multiple plane wave encoding is used for eyebox extension, and, because of the utilization of conjugate light, the effect of eyebox extension is doubled. That is, the space bandwidth of the amplitude-type hologram is fully used. A speckless holographic image is produced in mid-air with high quality within a large depth range. The proposed display is compact and promising for the augmented reality near-eye display. Furthermore, it may inspire better solutions for the conjugate light issue of amplitude-type holography.

Lensless full-color holographic Maxwellian near-eye display with a horizontal eyebox expansion.

A lensless full-color holographic Maxwellian near-eye display using a single amplitude-type spatial light modulator is proposed in this Letter. The color holographic image is directly projected onto the retina without any eyepiece. The color crosstalk is clearly separated from the signal in the space owing to the encoded spherical wave and carrier wave. An aperture numerical filter and a real polarized filter are used at the pupil plane to accurately stop the crosstalk light. A high-quality dynamic speckless color holographic image was produced in the mid-air within a specific depth range. The horizontal eyebox expansion is achieved simply through multiple spherical wave encoding and verified through an optical experiment. The proposed display is compact and promising as the augmented reality near-eye display.

Cascaded stimulated Brillouin scattering erbium-doped fiber laser generating orbital angular momentum beams at tunable wavelengths.

We experimentally demonstrate a method to obtain central wavelength tunable orbital angular momentum beams with switchable topological charges (+1 or -1) in a stimulated Brillouin scattering erbium-doped fiber laser. Multiwavelength operation is achieved through cascaded stimulated Brillouin scattering in a single-mode fiber with a length of 6 km initiated by an external Brillouin pump. High-efficiency mode conversion between the fundamental mode and the orbital angular momentum modes is realized through a broadband two-mode long-period fiber grating. High-purity orbital angular momentum beams with up to 10 stable wavelength channels with a tuning range of 35 nm are achieved, which is the highest number of operating wavelengths and tuning range in an all-fiber laser for orbital angular momentum beam emission to the best of our knowledge. Both the operational central wavelength and number of operating wavelengths can be tuned by adjusting the primary pump power and the center wavelength of the tunable bandpass filter in conjunction with changing the Brillouin pump wavelength.

Polycyclic aromatic hydrocarbons in sedimentary cores of Tibetan Plateau: Influence of global warming on cold trapping.

Cold condensation is an important pathway for polycyclic aromatic hydrocarbons (PAHs) depositing at remote alpine lakes after long-range atmospheric transportation. However, in the context of global warming, the obvious temperature rise in the Tibetan Plateau (TP) might have an impact on the air deposition of PAHs by controlling the extent of cold condensation. To investigate the influence of rising temperatures on the atmospheric deposition of PAHs, two dated sedimentary cores from Pumoyum Co Lake (PC) and Selin Co Lake (SC) were collected, respectively and concentrations of 16 individual PAHs were measured. In both PC and SC, the total concentration of 16 PAHs presented relatively lower levels in four historical periods of "hot anomaly" including 1973-1975, 1988-1989, 1998-1999, and 2006-2007. This indicated that the hot temperatures might restrict the atmospheric deposition of PAHs. Besides, the results of the principal component analysis did discriminate those "hot anomalies". As the temperature kept increasing in TP, for low molecular weight PAHs and high molecular weight PAHs, the influence of rising temperatures on the cold condensation was different. Therefore, it was identified that the effect of global warming on the environmental fate of POPs cannot be neglected, especially in alpine regions like TP.

Hybrid holographic Maxwellian near-eye display based on spherical wave and plane wave reconstruction for augmented reality display.

The holographic Maxwellian display is a promising technique for augmented reality (AR) display because it solves the vergence-accommodation conflict while presenting a high-resolution display. However, conventional holographic Maxwellian display has the inherent trade-off between depth of field (DOF) and image quality. In this paper, two types of holographic Maxwellian displays, the spherical wave type and the plane wave type, are proposed and analyzed. The spherical wavefront and the plane wavefront are produced by a spatial light modulator (SLM) for Maxwellian display. Due to the focusing properties of different wavefronts, the two types of display have complementary DOF ranges. A hybrid approach combining the spherical wavefront and plane wavefront is proposed for a large DOF with high image quality. An optical experiment with AR display is demonstrated to verify the proposed method.

Enhanced resolution of holographic stereograms by moving or diffusing a virtual pinhole array.

In this paper, two different display modes, the "pinhole mode" and the "lens mode" of the pinhole-type integral imaging (PII) based hologram are demonstrated by proper use of random phase. The performances of resolution, fill factor and image depth, of the two display modes are analyzed. Two different methods, the moving array lenslet technique (MALT) and the high-resolution elemental image array (EIA) encoding are introduced for the spatial resolution enhancement of the two display modes, respectively. Both methods enhance the spatial resolution without increasing the total pixel number or the space-bandwidth product (SBP) of the hologram. Both simulation and optical experiments verify that the proposed methods enhance the spatial resolution of PII-based hologram at a very low cost.

Unraveling the behaviors and significances of waste biomass ashes as underlying emission sources of soil polycyclic aromatic hydrocarbons in Tibetan Plateau.

Domestic consumption of biomass fuels has been found as a leading source of polycyclic aromatic hydrocarbons (PAHs) in pristine regions. The biomass ashes would serve as both source and vector for PAHs, which may threaten residents' health. However, research focusing on the behaviors of waste biomass ashes acting as emission resources of PAHs is still lacking. In present study, occurrence and fate of PAHs released from disposal biomass ashes in the Tibetan Plateau were investigated by evaluating their patterns and levels in soils at different distances from the Lhasa landfill. Concentrations of 16 PAHs averaged at 1009 ng/g dry weight and 103 ng/g dry weight in landfill cover soils and surrounding soils, respectively. The spatial distributions of PAHs evidenced the local source of biomass ashes in landfill, while the fractionations of PAHs in soils profiles emphasized the post-deposal leaching. Since PAH compositions altered significantly during their transport and sequestration, the rectification factors were suggested to adjust the source diagnostic ratios. In study area, the emission flux of PAHs from waste biomass ashes to soils was 0.93 ton/year; extended to the Tibetan Plateau, the flux was estimated to be 13.1-32.5 ton/year. This study confirmed that waste biomass ashes would represent significant contributions of PAH emissions in this pristine region, which might remarkably exacerbate the total emissions of PAHs in Tibet by more than 25%.

Quick stabilization of capillary for rapid determination of potassium ions in the blood of epilepsy patients by capillary electrophoresis without sample pretreatment.

Mutations in the potassium channel genes may be linked to the development of epilepsy and affect the blood potassium levels. Therefore, accurate determination of potassium in the blood will be critical to diagnose the cause of epilepsy. CE is a competent technique for the fast detection of multiple ions, but complicated matrices of a blood sample may cause significant variation of migration times and the peak shape. In this work, a procedure for rapid stabilization of the capillary inner surface through preflushing of a blood sample was employed. The process takes only 40 min for a capillary and then it can be used for more than 2 weeks. No pretreatment of the blood sample or other surface modification of the capillary is needed for the analysis. The RSDs of the migration time and peak area were reduced to 1.5 and 5.1% from 12.6 and 14.5%, respectively. The proposed method has been successfully applied to the determination of the potassium contents in the blood sample of patients with epilepsy at different stages. The recoveries of potassium ions in these blood samples are in a range from 86.5 to 104.5%.

Fully controllable three-dimensional light-induced longitudinal magnetization using a single objective lens.

With features of fast and energy-efficient data writing, all-optical helicity-dependent switching (AO-HDS) has emerged as a competitive technology to satisfy the demand for the next-generation volume data storage. Unfortunately, to switch the magnetizations in different positions of the magnetic-optic film, the laser beam, the objective lens, or the magnetic recording film should be moved, limiting the advantage of AO-HDS in fast data writing. To achieve on-the-fly magnetization switching, the induced magnetization should be fully controllable. In this Letter, by focusing an azimuthally polarized vortex beam (APVB) and introducing an additional phase, a feasible strategy constructing subwavelength light-induced pure longitudinal multi-magnetization spots is proposed. In addition, the position of the multi-magnetization spots can be dynamically controlled. The distributions of the focused APVBs with different orbital angular momentum, and the induced magnetizations are surveyed. We believe that this is a practical and flexible three-dimensional magnetic recording technique with dynamic control of the recording position.

Single frontal projection autostereoscopic three-dimensional display using a liquid crystal lens array.

Frontal projection autostereoscopic three-dimensional (3D) display is a kind of excellent 3D display technique with large display size and efficient space utilization, especially suitable for the future glasses-free 3D cinema. In this paper, we propose a frontal projection autostereoscopic 3D display using a liquid crystal lens array (LCLA) and a quarter-wave retarding film. The LCLA acts as two roles, refraction and transparency, for different polarized light. The forward projected polarized light can pass through the LCLA as a transparency, and then pass through the quarter-wave retarding film. After reflecting from a polarization-preserving screen, the returned light will pass through the quarter-wave retarding film again and turn to an orthogonal polarization. This polarized light will be refracted by the LCLA and reconstruct the 3D image. The demonstrated LCLA has the merits of no driving voltage, simple fabrication, and cost-effective. Optical experiment verifies the proposed method, which is promising for its potential application in the future glasses-free 3D cinema.