Holographic three-dimensional telepresence using large-area photorefractive polymer

Non-convex optimization for inverse problem solving in computer-generated holography

Computer-generated holography is a promising technique that modulates user-defined wavefronts with digital holograms. Computing appropriate holograms with faithful reconstructions is not only a problem closely related to the fundamental basis of holography but also a long-standing challenge for researchers in general fields of optics. Finding the exact solution of a desired hologram to reconstruct an accurate target object constitutes an ill-posed inverse problem. The general practice of single-diffraction computation for synthesizing holograms can only provide an approximate answer, which is subject to limitations in numerical implementation. Various non-convex optimization algorithms are thus designed to seek an optimal solution by introducing different constraints, frameworks, and initializations. Herein, we overview the optimization algorithms applied to computer-generated holography, incorporating principles of hologram synthesis based on alternative projections and gradient descent methods. This is aimed to provide an underlying basis for optimized hologram generation, as well as insights into the cutting-edge developments of this rapidly evolving field for potential applications in virtual reality, augmented reality, head-up display, data encryption, laser fabrication, and metasurface design.

Generating Multi-Depth 3D Holograms Using a Fully Convolutional Neural Network

Efficiently generating 3D holograms is one of the most challenging research topics in the field of holography. This work introduces a method for generating multi-depth phase-only holograms using a fully convolutional neural network (FCN). The method primarily involves a forward-backward-diffraction framework to compute multi-depth diffraction fields, along with a layer-by-layer replacement method (L2RM) to handle occlusion relationships. The diffraction fields computed by the former are fed into the carefully designed FCN, which leverages its powerful non-linear fitting capability to generate multi-depth holograms of 3D scenes. The latter can smooth the boundaries of different layers in scene reconstruction by complementing information of occluded objects, thus enhancing the reconstruction quality of holograms. The proposed method can generate a multi-depth 3D hologram with a PSNR of 31.8 dB in just 90 ms for a resolution of 2160 × 3840 on the NVIDIA Tesla A100 40G tensor core GPU. Additionally, numerical and experimental results indicate that the generated holograms accurately reconstruct clear 3D scenes with correct occlusion relationships and provide excellent depth focusing.

Chiral Kirigami for Bend-Tolerant Reconfigurable Hologram with Continuously Variable Chirality Measures

Despite the commonality of static holograms, the holography with multiple information layers and reconfigurable grey-scale images at communication frequencies remain a confluence of scientific challenges. One well-known difficulty is the simultaneous modulation of phase and amplitude of electromagnetic wavefronts with a high modulation depth. A less appreciated challenge is scrambling of the information and images with hologram bending. Here, this work shows that chirality-guided pixelation of plasmonic kirigami sheets enables tunable multiplexed holography at terahertz (THz) frequencies. The convex and concave structures with slanted Au strips exhibit gradual variations in geometries facilitating modulation of light ellipticity reaching 40 deg. Real-time switching of 3D images of the letter "M" and the Mona Lisa demonstrates the possibility of complex grey-scale information content and importance of continuously variable mirror asymmetry. Microscale chirality measures of each pixel experiences little change with bending while retaining controllable reconfigurability upon stretching, which translates to remarkable resilience of chiral holograms to bending. Simplicity of their design with local chirality measures opens the door to information technologies with fault-tolerant THz encryption, wearable holographic devices, and new communication technologies.

UV-enhanced photorefractive response rate in a thin-film lithium niobate microdisk

The photorefractive (PR) effect plays a critical role in emerging photonic technologies, including dynamic volume holography and on-chip all-optical functionalities. Nevertheless, its slow response rate has posed a significant obstacle to its practical application. Here, we experimentally demonstrate the enhancement of the PR response rate in a high-Q thin-film lithium niobate (TFLN) microdisk under UV light irradiation. At an irradiation intensity of 30 mW/cm2, the PR effect achieves a high response bandwidth of approximately 256 kHz. By employing this UV-assisted PR effect, we have achieved rapid laser-cavity locking and self-stabilization, where perturbations are automatically compensated. This technique paves the way toward real-time dynamic holography, editable photonic devices on a lithium niobate platform, and high-speed all-optical information processing.

Full-colour 3D holographic augmented-reality displays with metasurface waveguides

Emerging spatial computing systems seamlessly superimpose digital information on the physical environment observed by a user, enabling transformative experiences across various domains, such as entertainment, education, communication and training1-3. However, the widespread adoption of augmented-reality (AR) displays has been limited due to the bulky projection optics of their light engines and their inability to accurately portray three-dimensional (3D) depth cues for virtual content, among other factors4,5. Here we introduce a holographic AR system that overcomes these challenges using a unique combination of inverse-designed full-colour metasurface gratings, a compact dispersion-compensating waveguide geometry and artificial-intelligence-driven holography algorithms. These elements are co-designed to eliminate the need for bulky collimation optics between the spatial light modulator and the waveguide and to present vibrant, full-colour, 3D AR content in a compact device form factor. To deliver unprecedented visual quality with our prototype, we develop an innovative image formation model that combines a physically accurate waveguide model with learned components that are automatically calibrated using camera feedback. Our unique co-design of a nanophotonic metasurface waveguide and artificial-intelligence-driven holographic algorithms represents a significant advancement in creating visually compelling 3D AR experiences in a compact wearable device.

Neural étendue expander for ultra-wide-angle high-fidelity holographic display

Holographic displays can generate light fields by dynamically modulating the wavefront of a coherent beam of light using a spatial light modulator, promising rich virtual and augmented reality applications. However, the limited spatial resolution of existing dynamic spatial light modulators imposes a tight bound on the diffraction angle. As a result, modern holographic displays possess low étendue, which is the product of the display area and the maximum solid angle of diffracted light. The low étendue forces a sacrifice of either the field-of-view (FOV) or the display size. In this work, we lift this limitation by presenting neural étendue expanders. This new breed of optical elements, which is learned from a natural image dataset, enables higher diffraction angles for ultra-wide FOV while maintaining both a compact form factor and the fidelity of displayed contents to human viewers. With neural étendue expanders, we experimentally achieve 64 × étendue expansion of natural images in full color, expanding the FOV by an order of magnitude horizontally and vertically, with high-fidelity reconstruction quality (measured in PSNR) over 29 dB on retinal-resolution images.

Vision transformer empowered physics-driven deep learning for omnidirectional three-dimensional holography

The inter-plane crosstalk and limited axial resolution are two key points that hinder the performance of three-dimensional (3D) holograms. The state-of-the-art methods rely on increasing the orthogonality of the cross-sections of a 3D object at different depths to lower the impact of inter-plane crosstalk. Such strategy either produces unidirectional 3D hologram or induces speckle noise. Recently, learning-based methods provide a new way to solve this problem. However, most related works rely on convolution neural networks and the reconstructed 3D holograms have limited axial resolution and display quality. In this work, we propose a vision transformer (ViT) empowered physics-driven deep neural network which can realize the generation of omnidirectional 3D holograms. Owing to the global attention mechanism of ViT, our 3D CGH has small inter-plane crosstalk and high axial resolution. We believe our work not only promotes high-quality 3D holographic display, but also opens a new avenue for complex inverse design in photonics.

Modeling and optimizing through plenoptic function for the dual lenticular lens-based directional autostereoscopic display system

We propose an autostereoscopic display system that ensures full resolution for multiple users by directional backlight and eye tracking technology. The steerable beam formed by directional backlight can be regarded as the result of sparsely sampling the light field in space. Therefore, we intuitively propose an optimization algorithm based on the characterization for the state of the steerable beams, which is computed in matrix form using the plenoptic function. This optimization algorithm aims to optimize the exit pupil quality and ultimately enhancing the viewing experience of stereoscopic display. Numerical simulations are conducted and the improvement in exit pupil quality achieved by the optimization scheme is verified. Furthermore, a prototype of the stereoscopic display that employs dual-lenticular lens sheets for the directional backlight has been constructed using the optimized optical parameters. It provides 9 independent exit pupils at the optimal viewing distance of 400 mm, with an exit pupil resolution of 1/30. The field of view is ±16.7°, the viewing distance range is 380 mm to 440 mm. At the optimal viewing distance 400 mm, the average crosstalk of the system is 3%, and the dynamic brightness uniformity across the entire viewing plane reaches 85%. The brightness uniformity of the display at each exit pupil is higher than 88%.

Stacking architecture for collimated backlight using cylindrical lens sheet with linear light sources or edge-lit/direct-lit BLU

Highly collimated and directional backlights are essential for realizing advanced display technologies such as autostereoscopic 3D displays. Previously reported collimated backlights, either edge-lit or direct-lit, in general still suffer unsatisfactory form factors, directivity, uniformity, or crosstalk etc. In this work, we report a simple stacking architecture for the highly collimated and uniform backlights, by combining linear light source arrays and carefully designed cylindrical lens arrays. Experiments were conducted to validate the design and simulation, using the conventional edge-lit backlight or the direct-lit mini-LED (mLED) arrays as light sources, the NiFe (stainless steel) barrier sheets, and cylindrical lens arrays fabricated by molding. Highly collimated backlights with small angular divergence of ±1.45°∼±2.61°, decent uniformity of 93-96%, and minimal larger-angle sidelobes in emission patterns were achieved with controlled divergence of the light source and optimization of lens designs. The architecture reported here provides a convenient way to convert available backlight sources into a highly collimated backlight, and the use of optically reflective barrier also helps recycle light energy and enhance the luminance. The results of this work are believed to provide a facile approach for display technologies requiring highly collimated backlights.

An ultrahigh-fidelity 3D holographic display using scattering to homogenize the angular spectrum

A three-dimensional (3D) holographic display (3DHD) can preserve all the volumetric information about an object. However, the poor fidelity of 3DHD constrains its applications. Here, we present an ultrahigh-fidelity 3D holographic display that uses scattering for homogenization of angular spectrum. A scattering medium randomizes the incident photons and homogenizes the angular spectrum distribution. The redistributed field is recorded by a photopolymer film with numerous modulation modes and a half-wavelength scale pixel size. We have experimentally improved the contrast of a focal spot to 6 × 106 and tightened its spatial resolution to 0.5 micrometers, respectively ~300 and 4.4 times better than digital approaches. By exploiting the spatial multiplexing ability of the photopolymer and the transmission channel selection capability of the scattering medium, we have realized a dynamic holographic display of 3D spirals consisting of 20 foci across 1 millimeter × 1 millimeter × 26 millimeters with uniform intensity.

Non-interferometric stand-alone single-shot holographic camera using reciprocal diffractive imaging

An ideal holographic camera measures the amplitude and phase of the light field so that the focus can be numerically adjusted after the acquisition, and depth information about an imaged object can be deduced. The performance of holographic cameras based on reference-assisted holography is significantly limited owing to their vulnerability to vibration and complex optical configurations. Non-interferometric holographic cameras can resolve these issues. However, existing methods require constraints on an object or measurement of multiple-intensity images. In this paper, we present a holographic image sensor that reconstructs the complex amplitude of scattered light from a single-intensity image using reciprocal diffractive imaging. We experimentally demonstrate holographic imaging of three-dimensional diffusive objects and suggest its potential applications by imaging a variety of samples under both static and dynamic conditions.

Exciting-frequency-adaptive amplitude/phase hybrid holographic inscription in plasmonic polymers

Plasmonic holography is generally regarded as an effective technology for 3D display that meets the requirements of the human visual system. However, low readout stability and large cross talk in the frequency field during a plasmonic photo-dissolution reaction set a huge obstacle for application of color holography. Herein, we propose a new, to the best of our knowledge, route toward producing exciting frequency sensitive holographic-inscription based on plasmonic nano-Ag adaptive growth. Donor-molecule-doped plasmonic polymers on polyethylene terephthalate substrates exhibit wide spectral response range, accurate optical frequency sensing, and bending durability. The resonant plasmonic particles act as optical antennas and transfer energy to surrounding organic matrices for nanocluster production and non-resonant particle growth. The surface relief hologram is also highly dependent on the excitation frequency, so we successfully obtain a controllable cross-periodic structure with amplitude/phase mixed information, as well as color holographic display. This work provides a bright way to high-density storage, information steganography, and virtual/augmented reality.

Laguerre Gaussian mode holography and its application in optical encryption

Holography provides an approach to reconstructing both intensity and phase information, and has many applications for microscopic imaging, optical security, and data storage. Recently, the azimuthal Laguerre-Gaussian (LG) mode index, orbital angular momentum (OAM), has been implemented in holography technologies as an independent degree of freedom for high-security encryption. The radial index (RI) of LG mode, however, has not been implemented as an information carrier in holography. Here we propose and demonstrate the RI holography by using strong RI selectivity in the spatial-frequency domain. Furthermore, the LG holography is realized theoretically and experimentally with the (RI, OAM) spanning from (1, -15) to (7, 15), which leads to a 26bit LG-multiplexing hologram for high-security optical encryption. Based on LG holography, a high-capacity holographic information system can be constructed. In our experiments, a LG-multiplexing holography with a span of 217 independent LG channels has been realized, which is inaccessible at present for the OAM holography.

Mechanically Tunable Flexible Photonic Device for Strain Sensing Applications

Flexible photonic devices based on soft polymers enable real-time sensing of environmental conditions in various industrial applications. A myriad of fabrication techniques have been established for producing optical devices, including photo and electron-beam lithography, nano/femtosecond laser writing, and surface imprinting or embossing. However, among these techniques, surface imprinting/embossing is simple, scalable, convenient to implement, can produce nanoscale resolutions, and is cost-effective. Herein, we utilize the surface imprinting method to replicate rigid micro/nanostructures onto a commonly available PDMS substrate, enabling the transfer of rigid nanostructures into flexible forms for sensing at a nanometric scale. The sensing nanopatterned sheets were mechanically extended, and the extension was remotely monitored via optical methods. Monochromatic light (450, 532, and 650 nm) was transmitted through the imprinted sensor under various force/stress levels. The optical response was recorded on an image screen and correlated with the strain created by the applied stress levels. The optical response was obtained in diffraction pattern form from the flexible grating-based sensor and in an optical-diffusion field form from the diffuser-based sensor. The calculated Young's modulus in response to the applied stress, measured through the novel optical method, was found in a reasonable range compared to the reported range of PDMS (360-870 kPa) in the literature.

Ultra-small low-threshold mid-infrared plasmonic nanowire lasers based on n-doped GaN

An ultra-small mid-infrared plasmonic nanowire laser based on n-doped GaN metallic material is proposed and studied by the finite-difference time-domain method. In comparison with the noble metals, nGaN is found to possess superior permittivity characteristics in the mid-infrared range, beneficial for generating low-loss surface plasmon polaritons and achieving strong subwavelength optical confinement. The results show that at a wavelength of 4.2 µm, the penetration depth into the dielectric is substantially decreased from 1384 to 163 nm by replacing Au with nGaN, and the cutoff diameter of nGaN-based laser is as small as 265 nm, only 65% that of the Au-based one. To suppress the relatively large propagation loss induced by nGaN, an nGaN/Au-based laser structure is designed, whose threshold gain has been reduced by nearly half. This work may pave the way for the development of miniaturized low-consumption mid-infrared lasers.

High-Dimensional Entanglement-Enabled Holography

As an important imaging technique, holography has been realized with different physical dimensions of light, including polarization, wavelength, and time. Recently, quantum holography has been demonstrated by utilizing polarization entangled state with the advantages of high robustness and enhanced spatial resolution, comparing with classical holography. However, the polarization is only a two-dimensional degree of freedom, which greatly limits the capacity of quantum holography. Here, we propose a method to realize high-dimensional quantum holography by using high-dimensional orbital angular momentum (OAM) entanglement. A high-capacity OAM-encoded quantum holographic system can be obtained by multiplexing a wide range of OAM-dependent holographic images. Proof-of-principle experiments with four- and six-dimensional OAM entangled states have been implemented and verify the feasibility of our idea. Our experimental results also demonstrate that the high-dimensional quantum holography shows a high robustness to classical noise. What is more, the level of security of the holographic imaging encryption system can be greatly improved in our high-dimensional quantum holography.

A Proposal for an Ultrasound/Sound Holographic Microscope Using Entangled Mobile Phone Inductors

In this study we propose a model for building a holographic ultrasound microscope. In this model two mobile phones are first connected by waves and techniques like the WhatsApp waves. If the mobile phones are close to each other, their inductors and speakers become entangled, they exchange electromagnetic and sound waves, and they vibrate many times with each other. Objects placed between two mobile phones change the sound waves and electromagnetic waves and appear as holographic images within the inductors and also on the plastic of the speakers. To see these images, a hologram machine is built from a room of plastic, one or two magnets, iron particles, and sound producers. Holographic waves change the magnetic field within the hologram machine and move the plastic and iron particles. These objects take the shape of waves and produce holographic images. To see microbes, one can send a weak current to a container of microbes and then connect it to an amplifier. The weak current takes the shape of the microbes and is amplified by one strong amplifier. Then this current goes to the mobile phone and sound card and, after passing some stages, is sent to the second mobile phone. In the second mobile phone, the sound wave is amplified by speakers and transmitted to the hologram machine. Consequently, particles within this machine move and produce big holographic images of the microbes.

Dynamic complex opto-magnetic holography

Despite recent significant progress in real-time, large-area computer-generated holography, its memory requirements and computational loads will be hard to tackle for several decades to come with the current paradigm based on a priori calculations and bit-plane writing to a spatial light modulator. Here we experimentally demonstrate a holistic approach to serial computation and repeatable writing of computer-generated dynamic holograms without Fourier transform, using minimal amounts of computer memory. We use the ultrafast opto-magnetic recording of holographic patterns in a ferrimagnetic film with femtosecond laser pulses, driven by the on-the-fly hardware computation of a single holographic point. The intensity-threshold nature of the magnetic medium allows sub-diffraction-limited, point-by-point toggling of arbitrarily localized magnetic spots on the sample, according to the proposed circular detour-phase encoding, providing complex modulation and symmetrical suppression of upper diffractive orders and conjugated terms in holographically reconstructed 3-D images.

Holography recreates both the amplitude and wave front of a three dimensional object, meaning that the observer perceives the image in the nearly same way as they would the true object. Creating such holographic images is challenging computationally, and requires extremely fast display update. Here, the authors combine a fast memoryless computation algorithm with the ultra-rapid writing based on all-optical switching of a ferrimagnetic film.

Phenanthraquinone-Doped Polymethyl Methacrylate Photopolymer for Holographic Recording

Phenanthraquinone-doped polymethyl methacrylate (PQ/PMMA) photopolymers are considered to be the most promising holographic storage media due to their unique properties, such as high stability, a simple preparation process, low price, and volumetric shrinkage. This paper reviews the development process of PQ/PMMA photopolymers from inception to the present, summarizes the process, and looks at the development potential of PQ/PMMA in practical applications.

Inverse-cavity structure for low-threshold miniature lasers

Creating micro and nano lasers, high threshold gain is an inherent problem that have critically restricted their great technological potentials. Here, we propose an inverse-cavity laser structure where its threshold gain in the shortest-cavity regime is order-of-magnitude lower than the conventional cavity configurations. In the proposed structure, a resonant feedback mechanism efficiently transfers external optical gain to the cavity mode at a higher rate for a shorter cavity, hence resulting in the threshold gain reducing with decreasing cavity length in stark contrast to the conventional cavity structures. We provide a fundamental theory and rigorous numerical analyses confirming the feasibility of the proposed structure. Remarkably, the threshold gain reduces down by a factor ~ 10⁻³ for a vertical-cavity surface-emitting laser structure and ~ 0.17 for a lattice-plasmonic nanocavity structure. Therefore, the proposed approach may produce extremely efficient miniature lasers desirable for variety of applications potentially beyond the present limitations.

Tunable liquid crystal grating based holographic 3D display system with wide viewing angle and large size

As one of the most ideal display approaches, holographic 3-dimensional (3D) display has always been a research hotspot since the holographic images reproduced in such system are very similar to what humans see the actual environment. However, current holographic 3D displays suffer from critical bottlenecks of narrow viewing angle and small size. Here, we propose a tunable liquid crystal grating-based holographic 3D display system with wide viewing angle and large size. Our tunable liquid crystal grating, providing an adjustable period and the secondary diffraction of the reconstructed image, enables to simultaneously implement two different hologram generation methods in achieving wide viewing angle and enlarged size, respectively. By using the secondary diffraction mechanism of the tunable liquid crystal grating, the proposed system breaks through the limitations of narrow viewing angle and small size of holographic 3D display. The proposed system shows a viewing angle of 57.4°, which is nearly 7 times of the conventional case with a single spatial light modulator, and the size of the reconstructed image is enlarged by about 4.2. The proposed system will have wide applications in medical diagnosis, advertising, education and entertainment and other fields.

Ultrasimple and Ultrafast Method of Optical Modulation by Perovskite Quantum Dot Attachment to a Graphene Surface

Optical modulation is the process of modifying the structure and elemental composition of materials so that the main optical parameters, including amplitude, frequency, and phase, are changed. Currently, much research attention has been directed toward ultrafast dynamics, but the process of modulation is often complex. To simplify the optical modulation process and improve the optical properties of perovskites for semiconductor quantum dot (QD) lasers, the process and physical mechanism underlying graphene QD ultrafast modulation of the optical properties of perovskite CsPbBr₃ QDs were investigated. The typical cubic structure and square shape of CsPbBr₃ QDs were characterized by transmission electron microscopy and X-ray diffraction, respectively. A luminescent peak centered near 540 nm and Stokes shift of 21.34 nm of CsPbBr₃ QDs without graphene QDs were measured by absorption and photoluminescence spectroscopy. A maximum modulation shift of 133 nm and a modulation depth of 900% were achieved in CsPbBr₃ with graphene. The results indicated that graphene QDs had the best modulation effect on perovskites when the drop volume was 0.05 mL. The process of ultrafast optical modulation via graphene QDs occurring within 1 ps was confirmed by the transient absorption spectrum. The modulation mechanism of graphene to perovskites is presented for guidance. This paper can be used as a reference for the optical modulation of perovskite materials.

Photorefractive Response Enhancement in Poly(triarylamine)-Based Polymer Composites by a Second Electron Trap Chromophore

Photorefractive (PR) performances are affected by the components of the photoconductor, sensitizer, nonlinear optical dye, and plasticizer. A photoconductor with high hole mobility promises a faster response time, whereas it induces higher photoconductivity, which leads to easy dielectric breakdown. Adding a second electron trap is effective in controlling photoconductivity. In this study, the role of a second electron trap 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) was investigated in a PR composite consisting of a photoconductor of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] with a high hole mobility, a nonlinear optical chromophore of piperidinodicyanostyrene, a plasticizer of (2,4,6-trimethylphenyl)diphenylamine, and a sensitizer of [6,6]-phenyl C₆₁ butyric acid-methyl ester. The minimum time response with the maximum optical diffraction efficiency and sensitivity was measured at a 1 wt % content of TmPyPB. These results were consistent with the number of charge carriers trapped per unit volume and per unit time N _c (cm^-3 s^-1), which is defined as the ratio between the initial trap density T _i (cm^-3) and response time τ (s), at a 1 wt % content of TmPyPB. A faster response time of 149 μs, optical diffraction of 24.1% (external diffraction of 4.8%), and a sensitivity of 2746 cm² J^-1 were measured at 50 V μm^-1 for the sample with 1 wt % TmPyPB. High loading of 5 wt % TmPyPB led to a large decrease in photoconductivity and effectively suppressed the dielectric breakdown under a stronger electric field, whereas a slower response time with lower diffraction efficiency was observed for optical diffraction.

Metasurface-empowered spectral and spatial light modulation for disruptive holographic displays

The holographic display, one of the most realistic ways to reconstruct optical images in three-dimensional (3D) space, has gained a lot of attention as a next-generation display platform for providing deeper immersive experiences to users. So far, diffractive optical elements (DOEs) and spatial light modulators (SLMs) have been used to generate holographic images by modulating electromagnetic waves at each pixel. However, such architectures suffer from limitations in terms of having a resolution of only a few microns and the bulkiness of the entire optical system. In this review, we describe novel metasurfaces-based nanophotonic platforms that have shown exceptional control of electromagnetic waves at the subwavelength scale as promising candidates to overcome existing restrictions, while realizing flat optical devices. After introducing the fundamentals of metasurfaces in terms of spatial and spectral wavefront modulation, we present a variety of multiplexing approaches for high-capacity and full-color metaholograms exploiting the multiple properties of light as an information carrier. We then review tunable metaholograms using active materials modulated by several external stimuli. Afterward, we discuss the integration of metasurfaces with other optical elements required for future 3D display platforms in augmented/virtual reality (AR/VR) displays such as lenses, beam splitters, diffusers, and eye-tracking sensors. Finally, we address the challenges of conventional nanofabrication methods and introduce scalable preparation techniques that can be applied to metasurface-based nanophotonic technologies towards commercially and ergonomically viable future holographic displays.

Recent Advances in Planar Optics-Based Glasses-Free 3D Displays

Glasses-free three-dimensional (3D) displays are one of the technologies that will redefine human-computer interfaces. However, many geometric optics-based 3D displays suffer from a limited field of view (FOV), severe resolution degradation, and visual fatigue. Recently, planar optical elements (e.g., diffraction gratings, diffractive lenses and metasurfaces) have shown superior light manipulating capability in terms of light intensity, phase, and polarization. As a result, planar optics hold great promise to tackle the critical challenges for glasses-free 3D displays, especially for portable electronics and transparent display applications. In this review, the limitations of geometric optics-based glasses-free 3D displays are analyzed. The promising solutions offered by planar optics for glasses-free 3D displays are introduced in detail. As a specific application and an appealing feature, augmented reality (AR) 3D displays enabled by planar optics are comprehensively discussed. Fabrication technologies are important challenges that hinder the development of 3D displays. Therefore, multiple micro/nanofabrication methods used in 3D displays are highlighted. Finally, the current status, future direction and potential applications for glasses-free 3D displays and glasses-free AR 3D displays are summarized.

Realization of inversely designed metagrating for highly efficient large angle beam deflection

Directional emission source is one of the key components for multiple-view three-dimensional display. It is hard to achieve high efficiency and large deflection angle direction sources via geometric optics due to the weak confinement of light. The metasurface especially metagrating provides a promising method to control light effectively. However, the conventional forward design methods for metasurface are inherently limited by insufficient control of Bloch modes, which causes a significant efficiency drop at a large deflection angle. Here, we obtained high efficiency large deflection angle metagratings by realizing the constructive interferences among the propagation Bloch modes and enhancing the outcoupling effect at the desired diffraction order. The grating structures that support the coupling of Bloch modes were designed by an inverse design method for different incident wavelengths, and the total phase response of a supercell can be tailored. For a red (620 nm) incident light, the theoretical deflection efficiency of a silicon metagrating can be higher than 80% from 30° to 80°. The experimental deflection efficiency can achieve 86.43% for a 75° deflection metagrating. The matched simulation and experimental results strongly support the reliability of developed algorithm. Our inverse design approach could be extended to the green (530 nm) and blue (460 nm) incident light with titanium dioxide metagratings, with theoretical deflection efficiency of over 80% in a large deflection angle range of 30° to 80°. Considering the multiple visible wavelength deflection capability, the developed algorithm can be potentially applied for full color three-dimensional display, and other functional metagrating devices based on different dielectric materials.

Diffractive deep neural network adjoint assist or (DNA)2: a fast and efficient nonlinear diffractive neural network implementation

The recent advent of diffractive deep neural networks or D²NNs has opened new avenues for the design and optimization of multi-functional optical materials; despite the effectiveness of the D²NN approach, there is a need for making these networks as well as the design algorithms more general and computationally efficient. The work demonstrated in this paper brings significant improvements to both these areas by introducing an algorithm that performs inverse design on fully nonlinear diffractive deep neural network - assisted by an adjoint sensitivity analysis which we term (DNA)². As implied by the name, the procedure optimizes the parameters associated with the diffractive elements including both linear and nonlinear amplitude and phase contributions as well as the spacing between planes via adjoint sensitivity analysis. The computation of all gradients can be obtained in a single GPU compatible step. We demonstrate the capability of this approach by designing several types of three layered D²NN to classify 8800 handwritten digits taken from the MNIST database. In all cases, the D²NN was able to achieve a minimum 94.64% classification accuracy with 192 minutes or less of training.

Investigation of Autostereoscopic Displays Based on Various Display Technologies

The autostereoscopic display is a promising way towards three-dimensional-display technology since it allows humans to perceive stereoscopic images with naked eyes. However, it faces great challenges from low resolution, narrow viewing angle, ghost images, eye strain, and fatigue. Nowadays, the prevalent liquid crystal display (LCD), the organic light-emitting diode (OLED), and the emerging micro light-emitting diode (Micro-LED) offer more powerful tools to tackle these challenges. First, we comprehensively review various implementations of autostereoscopic displays. Second, based on LCD, OLED, and Micro-LED, their pros and cons for the implementation of autostereoscopic displays are compared. Lastly, several novel implementations of autostereoscopic displays with Micro-LED are proposed: a Micro-LED light-stripe backlight with an LCD, a high-resolution Micro-LED display with a micro-lens array or a high-speed scanning barrier/deflector, and a transparent floating display. This work could be a guidance for Micro-LED applications on autostereoscopic displays.

Application of a Novel Nd:YAG/PPMgLN Laser Module Speckle-Suppressed by Multi-Mode Fibers in an Exhibition Environment

Laser exhibition technology has been widely used in the virtual environment of exhibitions and shows, as well as in the physical conference and exhibition centers. However, the speckle issue due to the high coherence of laser sources has caused harmful impacts on image quality, which is one of the obstacles to exhibition effects. In this paper, we design a compact Nd:YAG/PPMgLN laser module at 561.5 nm and use two different types of big-core multi-mode fibers to lower the spatial coherence. According to our experiment, the speckle contrasts relating to these two types reduce to 7.9% and 4.1%, respectively. The results of this paper contribute to improving the application effects of key optical components in the exhibitions. Only in this way can we provide technical supports and service guarantee for the development of the exhibition activities, and an immersive interactive experience for the audiences.

Nanolasers with Feedback as Low-Coherence Illumination Sources for Speckle-Free Imaging: A Numerical Analysis of the Superthermal Emission Regime

Lasers distinguish themselves for the high coherence and high brightness of their radiation, features which have been exploited both in fundamental research and a broad range of technologies. However, emerging applications in the field of imaging, which can benefit from brightness, directionality and efficiency, are impaired by the speckle noise superimposed onto the picture by the interference of coherent scattered fields. We contribute a novel approach to the longstanding efforts in speckle noise reduction by exploiting a new emission regime typical of nanolasers, where low-coherence laser pulses are spontaneously emitted below the laser threshold. Exploring the dynamic properties of this kind of emission in the presence of optical reinjection we show, through the numerical analysis of a fully stochastic approach, that it is possible to tailor some of the properties of the emitted radiation, in addition to exploiting this naturally existing regime. This investigation, therefore, proposes semiconductor nanolasers as potential attractive, miniaturized and versatile future sources of low-coherence radiation for imaging.

Edge Raman enhancement at layered PbI2platelets induced by laser waveguide effect

As a two-dimensional (2D) layered semiconductor, lead iodide (PbI₂) has been widely used in optoelectronics owing to its unique crystal structure and distinctive optical and electrical properties. A comprehensive understanding of its optical performance is essential for further application and progress. Here, we synthesized regularly shaped PbI₂platelets using the chemical vapor deposition method. Raman scattering spectroscopy of PbI₂platelets was predominantly enhanced when the laser radiated at the edge according to Raman mapping spectroscopy. Combining the outcome of polarized Raman scattering spectroscopy and finite-difference time domain simulation analysis, the Raman enhancement was proven to be the consequence of the enhancement effects inherent to the high refractive index contrast waveguide, which is naturally formed in well-defined PbI₂platelets. Because of the enlarged excited area determined by the increased propagation length of the laser in the PbI₂platelet formed waveguide, the total Raman enhancements are acquired rather than a localized point enhancement. Finally, the Raman enhancement factor is directly related to the thickness of the PbI₂platelet, which further confirms the waveguide-enhanced edge Raman. Our investigation of the optical properties of PbI₂platelets offers reference for potential 2D layered-related optoelectronic applications.

Review of Organic Photorefractive Materials and Their Use for Updateable 3D Display

Photorefractive materials are capable of reversibly changing their index of refraction upon illumination. That property allows them to dynamically record holograms, which is a key function for developing an updateable holographic 3D display. The transition from inorganic photorefractive crystals to organic polymers meant that large display screens could be made. However, one essential figure of merit that needed to be worked out first was the sensitivity of the material that enables to record bright images in a short amount of time. In this review article, we describe how polymer engineering was able to overcome the problem of the material sensitivity. We highlight the importance of understanding the energy levels of the different species in order to optimize the efficiency and recording speed. We then discuss different photorefractive compounds and the reason for their particular figures of merit. Finally, we consider the technical choices taken to obtain an updateable 3D display using photorefractive polymer. By leveraging the unique properties of this holographic recording material, full color holograms were demonstrated, as well as refreshing rate of 100 hogels/second.

Strain -multiplexing optical-tuning based on single-pulsed holographic nanostructures

Holographic flexible and rigid nanostructures in the visible to near infrared range play vital roles in various applications, including displays, data storage, imaging, and security. However, personalized use of holography is limited due to the time-consuming, costly and complex nanofabrication procedures. Personalized holography can be improved/extended through rapid, efficient and low-cost techniques on rigid, flexible and edible materials. Here, we utilize a single-pulsed nanosecond (ns) laser ablation in Denisyuk reflection mode to record one/two-dimension (1/2D) nanostructures on various substrates, including rigid glass coated with soft polymers, gelatin, and conductive (gold) and non-conductive materials (ink) as holographic multilayer metastructures (HMMs). The tunability of optical properties was investigated by illuminating monochromatic and broadband light sources on flexible-template nanostructures. The surface morphologies of the nano-structures were changed by applying mechanical force, which in turn tuned the far-field optical response depending upon the amount of applied force, acting as an optical shape and force sensor. Nanostructures engineered on an edible material (gelatin) as well as on soft polymers (polymeric diffusers) were also demonstrated for suitable application in food industries and optoelectronics.

Quasi-phase-matching-division multiplexing holography in a three-dimensional nonlinear photonic crystal

Nonlinear holography has recently emerged as a novel tool to reconstruct the encoded information at a new wavelength, which has important applications in optical display and optical encryption. However, this scheme still struggles with low conversion efficiency and ineffective multiplexing. In this work, we demonstrate a quasi-phase-matching (QPM) -division multiplexing holography in a three-dimensional (3D) nonlinear photonic crystal (NPC). 3D NPC works as a nonlinear hologram, in which multiple images are distributed into different Ewald spheres in reciprocal space. The reciprocal vectors locating in a given Ewald sphere are capable of fulfilling the complete QPM conditions for the high-efficiency reconstruction of the target image at the second-harmonic (SH) wave. One can easily switch the reconstructed SH images by changing the QPM condition. The multiplexing capacity is scalable with the period number of 3D NPC. Our work provides a promising strategy to achieve highly efficient nonlinear multiplexing holography for high-security and high-density storage of optical information.

Multicolor Holographic Display of 3D Scenes Using Referenceless Phase Holography (RELPH)

In this paper, we present a multicolor display via referenceless phase holography (RELPH). RELPH permits the display of full optical wave fields (amplitude and phase) using two liquid crystal phase-only spatial light modulators in a Michelson-interferometer-based arrangement. Complex wave fields corresponding to arbitrary real or artificial 3D scenes are decomposed into two mutually coherent wave fields of constant amplitude whose phase distributions are modulated onto the wave fields reflected by the respective light modulators. Here, we present the realization of that concept in two different ways: firstly, via temporal multiplexing using a single setup, switching between wavelengths for temporal integration of the respective wavefields; secondly, using spatial multiplexing of different wavelengths with multiple Michelson-based setups; and finally, we present an approach to magnify the 3D scenes displayed by light modulators with limited space–bandwidth product for a comfortable viewing experience.

Slim-panel holographic video display

Since its discovery almost 70 years ago, the hologram has been considered to reproduce the most realistic three dimensional images without visual side effects. Holographic video has been extensively researched for commercialization, since Benton et al. at MIT Media Lab developed the first holographic video systems in 1990. However, commercially available holographic video displays have not been introduced yet for several reasons: narrow viewing angle, bulky optics and heavy computing power. Here we present an interactive slim-panel holographic video display using a steering-backlight unit and a holographic video processor to solve the above issues. The steering-backlight unit enables to expand the viewing angle by 30 times and its diffractive waveguide architecture makes a slim display form-factor. The holographic video processor computes high quality holograms in real-time on a single-chip. We suggest that the slim-panel holographic display can provide realistic three-dimensional video in office and household environments.

Holographic Photopolymer Material with High Dynamic Range (Δn) via Thiol-Ene Click Chemistry

A high-performance holographic recording medium was developed based on a unique combination of photoinitiated thiol-ene click chemistry and functional, linear polymers used as binders. Allyl reactive sites were incorporated along the backbone of the linear polymer binder to enable facile film casting and to facilitate cross-linking by photopolymerization of the thiol-ene monomers that also serve as the writing monomers in this distinctive approach to holographic materials. The allyl content and the ratio of the linear polymer to the writing monomers were varied to maximize and control the refractive index contrast. A blade-coating-based film preparation method was developed to form films from the mixture of linear polymer and the thiol-ene monomers. This approach results in a holographic material with a peak to mean index contrast (Δn) that reaches 0.04. The refractive index contrast was stable for at least two weeks. Haze in holograms with a high writing monomer loading was significantly reduced when a higher allyl content was incorporated into the binder, resulting in the lowest haze around 0.2%. Finally, the media exhibit high resolution as demonstrated by the ability to record reflection holograms with 140 nm pitch and diffraction efficiency in excess of 90%.

View-flipping effect reduction and reconstruction visualization enhancement for EPISM based holographic stereogram with optimized hogel size

To reduce the view-flipping effect and enhance the viewing resolution, the modulation characteristics of the hogel based holographic stereogram is constructed and validated. The performance of the view-flipping effect is analyzed, and the results indicate that decreasing the size of hogel is beneficial to the reduction of the view flipping, however, which will result in significant diffraction effect which can degrade the reconstruction quality. Furthermore, a diffraction-limited imaging model of the hogel based holographic stereogram is established, where both the limited aperture of the hogel and the defocused aberration of the object point are introduced, and the effective resolvable size of the reconstructed image point is simulated. The theory shows that there is an optimal hogel size existed for the certain depth of scene. Both the numerical and optical experiments are implemented, and the results are well agreed with the theoretical prediction, which demonstrates that the view-flipping reduction and reconstruction visualization enhancement for EPISM based holographic stereogram can be achieved when the proper size of hogel is utilized.

Three-dimensional vectorial holography based on machine learning inverse design

The three-dimensional (3D) vectorial nature of electromagnetic waves of light has not only played a fundamental role in science but also driven disruptive applications in optical display, microscopy, and manipulation. However, conventional optical holography can address only the amplitude and phase information of an optical beam, leaving the 3D vectorial feature of light completely inaccessible. We demonstrate 3D vectorial holography where an arbitrary 3D vectorial field distribution on a wavefront can be precisely reconstructed using the machine learning inverse design based on multilayer perceptron artificial neural networks. This 3D vectorial holography allows the lensless reconstruction of a 3D vectorial holographic image with an ultrawide viewing angle of 94° and a high diffraction efficiency of 78%, necessary for floating displays. The results provide an artificial intelligence-enabled holographic paradigm for harnessing the vectorial nature of light, enabling new machine learning strategies for holographic 3D vectorial fields multiplexing in display and encryption.

Electromagnetically induced holographic imaging using monolayer graphene

Graphene exhibits remarkable optical and electronic properties when interacts with electromagnetic field. These properties play a vital role in a broad range of applications, such as, optical communication, optical storage, biomedical imaging and security purposes. Based on electromagnetically induced grating (EIG), we study lensless holographic imaging via quantized energy levels of two-dimensional (2D) monolayer graphene model. We observe that by exploiting electromagnetically induced grating (EIG), holographic interference patterns via electromagnetically induced classical holographic imaging (EICHI) and, non locally, electromagnetically induced quantum holographic imaging (EIQHI) can be obtained in the infrared range (THz) of the spectrum. We notice that for EIQHI one can obtain image magnification using monolayer graphene via manipulation of certain controllable parameters. The scheme provides an experimentally viable option for the classical and quantum mechanical holographic imaging and possibilities for the design of graphene-based quantum mechanical devices which can have many applications.

Holographic Sampling Display Based on Metagratings

Glasses-free three-dimensional (3D) display is considered as a potential disruptive technology for display. The issue of visual fatigue, mainly caused by the inaccurate phase reconstruction in terms of image crosstalk, as well as vergence and accommodation conflict, is the critical obstacle that hinders the real applications of glasses-free 3D display. Here we propose a glasses-free 3D display by adopting metagratings for the pixelated phase modulation to form converged viewpoints. When the viewpoints are closely arranged, the holographic sampling 3D display can approximate a continuous light field. We demonstrate a video rate full-color 3D display prototype without visual fatigue under an LED white light illumination. The metagratings-based holographic sampling 3D display has a thin form factor and is compatible with traditional flat panel and thus has the potential to be used in portable electronics, window display, exhibition display, 3D TV, as well as tabletop display.

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Metagratings are designed pixel by pixel to form converged viewpoints in 3D display

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Holographic sampling 3D display reconstruct discrete light field

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Video rate full-color display is reconstructed with a thin form factor

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Vergence and accommodation conflict is eliminated by single eye accommodation

Centered-camera-based effective perspective images' segmentation and mosaicking method for full-parallax holographic stereogram printing

The centered-camera-based effective perspective images' segmentation and mosaicking (CCEPISM) method is proposed to improve the previous EPISM-based printing of full-parallax holographic stereograms. The idea of pixel mapping from target camera images to centered camera images is analyzed, and backward pixel mapping as well as forward pixel mapping are modeled and formulated to transform the perspective images captured by different camera strategies. The principle of the proposed CCEPISM is introduced in detail along with its specific implementation. The experimental results validate the proposed method and demonstrate that CCEPISM is an effective method for printing full-parallax holographic stereograms, and the quality of optical reconstruction is the same as that of EPISM-based holographic stereograms. The whole 3D scene is captured by a centered camera image, and the pixel waste caused by simple camera capture and the number of sampled perspective images is reduced significantly.

Performance enhancement of integral imaging based Fresnel hologram capturing by the intermediate view reconstruction

A method aiming at improving the performance of integral imaging (II) based Fresnel hologram is proposed, which is generated by using the intermediate view reconstruction (IVR). The conventional integral holograms are generally generated through Fourier transforming the elemental images (EI) of II into hogels. However, a trade-off between the angular resolution and the spatial resolution of II is inevitable within the generation of integral hologram. The IVR is introduced to enhance the angular spectrum of II-based Fresnel hologram while keeping a compact image size and being free from moving the lenslet array. Multiple elemental image array (EIA) sequences are generated with the IVR and transformed to the corresponding holograms. All the generated hologram sequences shift depending on the relative position of the virtual lens array and are added together to synthesize the Fresnel hologram with a high angular spectrum. The synthesized hologram can reconstruct the 3D image with the combined light fields of all the integral hologram sequences. Finally, both the simulation with multiple objects and experiments of real 3D object are numerically and optically conducted. The high matching results among them confirm this work a better performance over the conventional methods.

Analysis on the reconstruction error of EPISM based full-parallax holographic stereogram and its improvement with multiple reference plane

To reduce the reconstruction error of holographic stereogram fabricated by effective perspective images' segmentation and mosaicking method (EPISM), a multiple-reference-plane (MRP) approach is proposed and validated. The reconstruction error for traditional EPISM is analyzed, and the results indicate that the distortion as well as the blur will be involved for object points located far away from the reference plane. A new method by introducing multiple reference planes is proposed, which divides the 3D scene into several parts along its depth direction, and sets a reference plane for each of the object part. By resynthesizing all the effectively synthetic perspective images referred to their own reference planes of the object parts, the finally effectively synthetic perspective image exposed to one holographic elemental by only once exposure is generated. The optically experimental results demonstrate the validity of the proposed method, and the reconstruction error of full-parallax holographic stereogram printed by MRP based EPISM can be reduced evidently while the displayed depth range of 3D scene can be extended, compared to the traditional EPISM approach.

Room-temperature up-conversion random lasing from CsPbBr3 quantum dots with TiO2 nanotubes

We report successful room-temperature up-conversion random lasing by distributing CsPbBr₃ quantum dots (QDs) uniformly into TiO₂ nanotubes (NTs). In order to overcome the difficulty in purifying the QDs, TiO₂ NTs were designed to collect QDs and enhance the optical multiple scattering effect. A threshold of 9.54  mJ/cm² and narrow full width at half-maximum of 0.49 nm with a relatively high quality factor of 1089 were successfully observed. These results indicate that CsPbBr₃QDs/TiO₂ NTs can be high-performance up-conversion lasers for practical applications, especially when the phase matching required by conventional approaches cannot be fulfilled.

Atomically Thin Nonlinear Transition Metal Dichalcogenide Holograms

Nonlinear holography enables optical beam generation and holographic image reconstruction at new frequencies other than the excitation fundamental frequency, providing pathways toward unprecedented applications in optical information processing and data security. So far, plasmonic metasurfaces with the thickness of tens of nanometers have been mostly adopted for realizing nonlinear holograms with the potential of on-chip integration but suffering from low conversion efficiency and high absorption loss. Here, we report a nonlinear transition metal dichalcogenide (TMD) hologram with high conversion efficiency and atomic thickness made of only single nanopatterned tungsten disulfide (WS₂) monolayer, for producing optical vortex beams and Airy beams as well as reconstructing complex holographic images at the second harmonic (SH) frequency. Our concept of nonlinear TMD holograms paves the way toward not only the understanding of light-matter interactions at the atomic level but the integration of functional TMD-based devices with atomic thickness into the next-generation photonic circuits for optical communication, high-density optical data storage, and information security.

Static volumetric three-dimensional display based on an electric-field-controlled two-dimensional optical beam scanner

Using the quadratic electro-optic effect and the gradient of the composition ratio m [Nb/(Ta+Nb) in mol. %] in a potassium tantalate niobate crystal, we have designed an electric-field-controlled two-dimensional optical beam scanner with a wide wavelength range and fast response. This scanner is used to realize a volumetric display based on a two-frequency, two-step upconversion technique that is used to address the imaging volume. Use of appropriately designed imaging optics and custom-designed software to convert 2D renderings of volumetric images into control signals for the scanner along with appropriate infrared laser source selection allows efficient single-color image generation with a large viewing zone, without flicker and with natural depth cues. The resulting system has the potential to increase image resolution to nearly 61.5×10⁹ with high scanning frequency and to expand to display three-color imagery.