Pluggable multitask diffractive neural networks based on cascaded metasurfaces

Optical neural networks have significant advantages in terms of power consumption,parallelism,and high computing speed,which has intrigued extensive attention in both academic and engineering communities.It has been considered as one of the powerful tools in promoting the fields of imaging processing and object recognition.However,the existing optical system architecture cannot be reconstructed to the realization of multi-functional artificial intelligence systems simultaneously.To push the development of this issue,we propose the pluggable diffractive neural networks(P-DNN),a general paradigm resorting to the cascaded metasurfaces,which can be applied to recognize various tasks by switching internal plug-ins.As the proof-of-principle,the recognition functions of six types of handwritten digits and six types of fashions are numerical simulated and experimental demonstrated at near-infrared regimes.Encouragingly,the proposed paradigm not only improves the flexibility of the optical neural networks but paves the new route for achieving high-speed,low-power and versatile artificial intelligence systems.

双反射飞行时间质量分析器高分辨光电子成像谱仪研制

本文报道了一台结合电喷雾离子源和双反射飞行时间质谱的速度成像光电子能谱仪的研制过程,其可用于开展溶液中负离子的光电子能谱研究.在该装置中,电喷雾离子源产生的负离子由四极杆组成的离子引导系统传输至冷离子阱,并对负离子进行富集和冷却,用于后端的质量选择和光电子能谱探测.该装置的关键技术之一是将双反射质量分析器与Wiley-McLaren离子加速聚焦设计进行了串联:这样可以有效地提高质谱分辨,从而准确地选择目标离子,其质量分辨率(M/△M)可达2000以上.该装置的高分辨光电子速度成像系统采用多极电子透镜设计,对于动能为0.59eV的光电子,其能谱峰的半高宽约为5.8meV,相对分辨率为~0.98%本文对该装置的工作原理,结构设计和性能测试情况进行了详细阐述。

Time-sequential color code division multiplexing holographic display with metasurface

Color metasurface holograms are powerful and versatile platforms for modulating the amplitude,phase,polarization,and other properties of light at multiple operating wavelengths.However,the current color metasurface holography can only realize static manipulation.In this study,we propose and demonstrate a multiplexing metasurface technique combined with multiwavelength code-division multiplexing(CDM)to realize dynamic manipulation.Multicolor code references are utilized to record information within a single metasurface and increase the information capacity and security for anticracks.A total of 48 monochrome images consisting of pure color characters and multilevel color video frames were reconstructed in dual polarization channels of the birefringent metasurface to exhibit high information density,and a video was displayed via sequential illumination of the corresponding code patterns to verify the ability of dynamic manipulation.Our approach demonstrates significant application potential in optical data storage,optical encryption,multiwavelengthversatile diffractive optical elements,and stimulated emission depletion microscopy.

Generation of super-resolved optical needle and multifocal array using graphene oxide metalenses

Ultrathin flat metalenses have emerged as promising alternatives to conventional diffractive lenses,offering new possibilities for myriads of miniaturization and interfacial applications.Graphene-based materials can achieve both phase and amplitude modulations simultaneously at a single position due to the modification of the complex refractive index and thickness by laser conversion from graphene oxide into graphene like materials.In this work,we develop graphene oxide metalenses to precisely control phase and amplitude modulations and to achieve a holistic and systematic lens design based on a graphene-based material system.We experimentally validate our strategies via demonstrations of two graphene oxide metalenses:one with an ultra-long(~16λ)optical needle,and the other with axial multifocal spots,at the wavelength of 632.8 nm with a 200 nm thin film.Our proposed graphene oxide metalenses unfold unprecedented opportunities for accurately designing graphene-based ultrathin integratable devices for broad applications.

Iterative freeform lens design for prescribed irradiance on curved target

Current freeform illumination optical designs are mostly focused on producing prescribed irradiance distributions on planar targets.Here,we aim to design freeform optics that could generate a desired illumination on a curved target from a point source,which is still a challenge.We reduce the difficulties that arise from the curved target by involving its varying z-coordinates in the iterative wavefront tailoring(IWT)procedure.The new IWT-based method is developed under the stereographic coordinate system with a special mesh transformation of the source domain,which is suitable for light sources with light emissions in semi space such as LED sources.The first example demonstrates that a rectangular flat-top illumination can be generated on an undulating surface by a spherical-freeform lens for a Lambertian source.The second example shows that our method is also applicable for producing a non-uniform irradiance distribution in a circular region of the undulating surface.

Analysis of teenagers'preferences and concerns regarding HMDs in education

Background Virtual reality(VR)has become a powerful and promising tool for education,and numerous studies have investigated the application and effectiveness of VR education.However,few studies have focused on the expectations and concerns of teenagers regarding head-mounted displays(HMDs),which are used for this purpose.Methods In this paper,we aim to explore the current problems and necessary advancements required in VR education based on a survey of 163 senior high school students who experience VR educational content for 1h.The usability and comfort of the HMD system,the physical and psychological effects on the students,and their preferences and concerns are investigated.Results The results show that HMDs increase students'interest,concentration,and enthusiasm for learning.However,isolated virtual environments make students feel nervous and afraid.The immersive environment also makes them worry about VR addiction and confusing the physical world with the virtual one.Conclusions VR has great potential in the field of education,but the issue of safety needs to be considered in the future.

Coded aperture compressive imaging array applied for surveillance systems

This paper proposes an application of compressive imaging systems to the problem of wide-area video surveillance systems. A parallel coded aperture compressive imaging system and a corresponding motion target detection algorithm in video using compressive image data are developed. Coded masks with random Gaussian, Toeplitz and random binary are utilized to simulate the compressive image respectively. For compressive images, a mixture of the Gaussian distribution is applied to the compressed image field to model the background. A simple threshold test in compressive sampling image is used to declare motion objects. Foreground image retrieval from underdetermined measurement using the total variance optimization algorithm is explored. The signal-to-noise ratio （SNR） is employed to evaluate the image quality recovered from the compressive sampling signals, and receiver operation characteristic （ROC） curves are used to quantify the performance of the motion detection algorithm. Experimental results demonstrate that the low dimensional compressed imaging representation is sufficient to determine spatial motion targets. Compared with the random Gaussian and Toeplitz mask, motion detection algorithms using the random binary phase mask can yield better detection results. However using the random Gaussian and Toeplitz phase mask can achieve high resolution reconstructed images.

Navigation in virtual and real environment using brain computer interface:a progress report

A brain-computer interface (BCI) facilitates bypassing the peripheral nervous system and directly communicating with surrounding devices. Navigation technology using BCI has developed-from exploring the prototype paradigm in the virtual environment (VE) to accurately completing the locomotion intention of the operator in the form of a powered wheelchair or mobile robot in a real environment. This paper summarizes BCI navigation applications that have been used in both real and VEs in the past 20 years. Horizontal comparisons were conducted between various paradigms applied to BCI and their unique signal-processing methods. Owing to the shift in the control mode from synchronous to asynchronous, the development trend of navigation applications in the VE was also reviewed. The contrast between high level commands and low-level commands is introduced as the main line to review the two major applications of BCI navigation in real environments: mobile robots and unmanned aerial vehicles (UAVs). Finally, applications of BCI navigation to scenarios outside the laboratory;research challenges, including human factors in navigation application interaction design;and the feasibility of hybrid BCI for BCI navigation are discussed in detail.

Radiationless optical modes in metasurfaces:recent progress and applications

Non-radiative optical modes attracted enormous attention in optics due to strong light confinement and giant Q-factor at its spectral position.The destructive interference of multipoles leads to zero net-radiation and strong field trapping.Such radiationless states disappear in the far-field,localize enhanced near-field and can be excited in nanostructures.On the other hand,the optical modes turn out to be completely confined due to no losses at discrete point in the radiation continuum,such states result in infinite Q-factor and lifetime.The radiationless states provide a suitable platform for enhanced light matter interaction,lasing,and boost nonlinear processes at the state regime.These modes are widely investigated in different material configurations for various applications in both linear and nonlinear metasurfaces which are briefly discussed in this review.

FreeformNet:fast and automatic generation of multiple-solution freeform imaging systems enabled by deep learning

Using freeform optical surfaces in lens design can lead to much higher system specifications and performance while significantly reducing volume and weight.However,because of the complexity of freeform surfaces,freeform optical design using traditional methods requires extensive human effort and sufficient design experience,while other design methods have limitations in design efficiency,simplicity,and versatility.Deep learning can solve these issues by summarizing design knowledge and applying it to design tasks with different system and structure parameters.We propose a deep-learning framework for designing freeform imaging systems.We generate the data set automatically using a combined sequential and random system evolution method.We combine supervised learning and unsupervised learning to train the network so that it has good generalization ability for a wide range of system and structure parameter values.The generated network FreeformNet enables fast generation(less than 0.003 s per system)of multiple-solution systems after we input the design requirements,including the system and structure parameters.We can filter and sort solutions based on a given criterion and use them as good starting points for quick final optimization(several seconds for systems with small or moderate field-of-view in general).The proposed framework presents a revolutionary approach to the lens design of freeform or generalized imaging systems,thus significantly reducing the time and effort expended on optical design.

Multichannel vectorial holographic display and encryption

Since its invention,holography has emerged as a powerful tool to fully reconstruct the wavefronts of light including all the fundamental properties(amplitude,phase,polarization,wave vector,and frequency).For exploring the full capability for information storage/display and enhancing the encryption security of metasurface holograms,smart multiplexing techniques together with suitable metasurface designs are highly demanded.Here,we integrate multiple polarization manipulation channels for various spatial phase profiles into a single birefringent vectorial hologram by completely avoiding unwanted cross-talk.Multiple independent target phase profiles with quantified phase relations that can process significantly different information in different polarization states are realized within a single metasurface.For our metasurface holograms,we demonstrate high fidelity,large efficiency,broadband operation,and a total of twelve polarization channels.Such multichannel polarization multiplexing can be used for dynamic vectorial holographic display and can provide triple protection for optical security.The concept is appealing for applications of arbitrary spin to angular momentum conversion and various phase modulation/beam shaping elements.

Review of computer-generated hologram algorithms for color dynamic holographic three-dimensional display

Holographic three-dimensional display is an important display technique because it can provide all depth information of a real or virtual scene without any special eyewear.In recent years,with the development of computer and optoelectronic technology,computer-generated holograms have attracted extensive attention and developed as the most promising method to realize holographic display.However,some bottlenecks still restrict the development of computer-generated holograms,such as heavy computation burden,low image quality,and the complicated system of color holographic display.To overcome these problems,numerous algorithms have been investigated with the aim of color dynamic holographic three-dimensional display.In this review,we will explain the essence of various computer-generated hologram algorithms and provide some insights for future research.

Graphene-enabled electrically controlled terahertz meta-lens

Metasurfaces have become a new photonic structure for providing potential applications to develop integrated devices with small thickness, because they can introduce an abrupt phase change by arrays of scatterers. To be applied more widely, active metasurface devices are highly desired. Here, a tunable terahertz meta-lens whose focal length is able to be electrically tuned by ～4.45λ is demonstrated experimentally. The lens consists of a metallic metasurface and a monolayer graphene. Due to the dependence of the abrupt phase change of the metasurface on the graphene chemical potential, which can be modulated using an applied gate voltage, the focal length is changed from 10.46 to 12.24 mm when the gate voltage increases from 0 to 2.0 V. Experimental results are in good agreement with the theoretical hypothesis. This type of electrically controlled meta-lens could widen the application of terahertz technology.

Optical design of a compact and high-transmittance compressive sensing imaging system enabled by freeform optics

Using compressive sensing for imaging has many applications, and it is an important branch of computational imaging.In this Letter, freeform surfaces are introduced in the hardware optical system design of a compressive sensing imager. The system works under the medium wave infrared band and realizes a 16× compression with a field-of-view of 7.5°× 6°. Good imaging performance is achieved in both the entire system and the freeform objective optics. Compared with the system using all spherical lenses, the volume of the freeform system is about 1/3 smaller, and the total transmittance is about 56%higher, which shows the benefits of using freeform surfaces for compressive sensing and computational imaging.

Iterative freeform lens design for optical field control

It is still very challenging to determine a freeform lens for converting a given input beam into a prescribed output beam where not only the irradiance distribution but also the phase distribution hardly can be expressed analytically.Difficulties arise because the ray mapping from the input beam to the output beam is not only intertwined with the required double freeform surfaces but also intertwined with the output phase distribution,whose gradient represents the directions of the output rays.Direct determination of such a problem is very difficult.Here,we develop a special iterative wavefront tailoring(IWT)method to tackle this problem.In a certain iteration,the current calculation data of the double freeform surfaces and the output phase gradient are used to update the coefficients of a Monge–Ampère equation describing an intermediate wavefront next to the entrance freeform surface.The solution to the wavefront equation could lead to an improved ray mapping to be used to update the corresponding phase gradient data and reconstruct the double freeform surfaces.In a demonstrative example that deviates much from the paraxial or small-angle approximation,the new IWT method can generate a freeform lens that performs much better than that designed by a conventional ray mapping method for producing two irradiance distributions in the forms of numerals"1"and"2"on two successive targets,respectively.

Full-Stokes polarization transformations and time sequence metasurface holographic display

With the development of micro/nano fabrication technology, metasurface holography has emerged as a revolutionary technology for the manipulation of light with excellent performance. However, for applications of full-Stokes polarization encryption and time sequence holographic display, multiplexing strategies of metasurfaces with large bandwidths and simple operations still need to be developed. As one of the most popular schemes of multiplexing, polarization multiplexed metasurfaces have shown flexible recording abilities for both free-space beam and surface waves. Here, by using a dielectric metasurface equipped with double phase holograms, we have achieved flexible polarization multiplexed transformations from one full-Stokes space to another. The vectorial hologram is optimized by a hybrid genetic algorithm and digitalized with subwavelength modulated units. Based on a quantitative map and remarkable information capacity, time sequence holographic display and complex optical encryption are experimentally demonstrated by changing input/output polarization channels in real time. We believe our method will facilitate applications in smart compact devices of dynamic display, dynamic optical manipulation,optical encryption, anticounterfeiting, etc.

Highly luminescent and stable lead-free cesium copper halide perovskite powders for UV-pumped phosphor-converted light-emitting diodes

Lead halide perovskites have drawn extensive attention over recent decades owing to their outstanding photo-electric performances.However,their toxicity and instability are big issues that need to be solved for further commercialization.Herein,we adopt a facile dry ball milling method to synthesize lead-free Cs3Cu2X5(X=I,CI)perovskites with photoluminescence(PL)quantum yield up to 60%.The optical features including broad emission spectrum,large Stokes shift,and long PL lifetime can be attributed to self-trapped exciton recombination.The as-synthesized blue emissive Cs3Cu2I5 and green emissive Cs3Cu2Cl5 lead-free perovskite powders have good thermal stability and photostability.Furthermore,UV-pumped phosphor-converted light-emitting diodes were obtained by using Cs3Cu2I5 and Cs3Cu2Cl5 as phosphors.

Retinal projection head-mounted display

Retinal projection displays （RPDs） are an important development direction for head-mounted dis- plays （HMDs）. This paper reviews the literature on optical engineering aspects based on the data on advanced technology in RPD design and development. The review includes the principles and applications of four theories, e. g., the Maxwellian view and its modified modality and the monocular and binocular depth cues of stereoscopic objects in the physiology of the human visual system. To support the Maxwellian view and achieve retinal projec- tion systems with depth cues, results of previous design works were summarized using different methods and their advantages and disadvantages are analyzed. With an extremely long focal depth, a prototype of a full-color stereoscopic see-through RPD system was discussed. Finally, a brief outlook of the future development trends and applications of the RPDs was presented

Freeform OST-HMD system with large exit pupil diameter and vision correction capability

Compactness and light weight,large exit pupil diameter and distance,small distortion for virtual image,and see-through light paths are pivotal factors to achieve a better,wearable experience of optical see-through headmounted displays(OST-HMDs).In addition,light efficiency of the virtual image light path is an important factor for heat dissipation in HMD devices.This paper presents a new type of OST-HMD optical system that includes three wedge-shaped freeform prisms and two symmetric lenses.Based on a 0.71 in.microdisplay,an OST-HMD prototype with a diagonal field of view(FOV)of 45.3°,an F-number(F/#)of 1.8,an exit pupil size of 12 mm×8 mm,and an eye relief of 18 mm is demonstrated.The maximum value of distortion of the final system is 0.6%and 0.4%for virtual image and see-through light path,respectively.The overall dimension of the optical system per eye is no larger than 30 mm(width)×40 mm(height)×14 mm(thickness),and the weight of the optical module including lenses,holder,and microdisplay is 12.8 g.The light efficiency of the virtual image light path is up to 50%higher than those of other OST-HMD optical solutions.

Correlated triple hybrid amplitude and phase holographic encryption based on a metasurface

Metasurface holography is becoming a universal platform that has made a considerable impact on nanophotonics and information optics,due to its advantage of large capacity and multiple functionalities.Here,we propose a correlated triple amplitude and phase holographic encryption based on an all-dielectric metasurface.We develop an optimized holographic algorithm to obtain quantitatively correlated triple holograms,which can encrypt information in multiple wavelength and polarization channels.We apply the“static”and“dynamic”pixels in our design,respectively.Two kinds of isotropic square nanofins are selected,one functioning as a transmitter and the other functioning as a blocker counterintuitively at both working wavelengths,while another anisotropic rectangle nanofin can transmit or block light in co-polarization selectively,mimicking“dynamic”amplitude switches.Meanwhile,such“dynamic”nanofins can simultaneously function as a phase modulator in cross-polarization only at the transmission wavelength.That is,through smart design,different dielectric meta-atoms functioning as spectral filters as well as phase contributors can compositely achieve triple hybrid amplitude and phase holograms.Such strategy promises to be applied in compact large-capacity information storage,colorful holographic displays,optical encryption,multifunctional imaging devices,and so on.