Thermal tuning nanoprinting based on liquid crystal tunable dual-layered metasurfaces for optical information encryption.

Dynamic tuning metasurfaces represent a significant advancement in optical encryption techniques, enabling highly secure multichannel responses. This paper proposes a liquid crystal (LC) tunable dual-layered metasurface to establish a thermal-encrypted optical platform for information storage. Through the screening of unit cells and coupling of characteristics, a dynamic polarization-dependent beam-steering metasurface is vertically cascaded with an angular multiplexing nanoprinting metasurface, separated by a dielectric layer. By integrating high-birefringence LCs into dual-layered metasurfaces, the cascaded meta-system can achieve dynamic thermal-switching for pre-encoded nanoprinting images. This work provides a promising solution for developing compact dynamic meta-systems for customized optical storage and information encryption.

Technique for enhancing the accuracy of the Rayleigh-Sommerfeld convolutional diffraction through the utilization of independent spatial sampling.

The Rayleigh-Sommerfeld diffraction integral (RSD) is a rigorous solution that precisely satisfies both Maxwell's equations and Helmholtz's equations. It seamlessly integrates Huygens' principle, providing an accurate description of the coherent light propagation within the entire diffraction field. Therefore, the rapid and precise computation of the RSD is crucial for light transport simulation and optical technology applications based on it. However, the current FFT-based Rayleigh-Sommerfeld integral convolution algorithm (CRSD) exhibits poor performance in the near field, thereby limiting its applicability and impeding further development across various fields. The present study proposes, to our knowledge, a novel approach to enhance the accuracy of the Rayleigh-Sommerfeld convolution algorithm by employing independent sampling techniques in both spatial and frequency domains. The crux of this methodology involves segregating the spatial and frequency domains, followed by autonomous sampling within each domain. The proposed method significantly enhances the accuracy of RSD during the short distance while ensuring computational efficiency.

Technique for enhancing the accuracy of the Rayleigh-Sommerfeld convolutional diffraction through the utilization of independent spatial sampling: publisher's note.

This publisher's note contains a correction to Opt. Lett.49, 1385 (2024)10.1364/OL.509688.

From picture to 3D hologram: end-to-end learning of real-time 3D photorealistic hologram generation from 2D image input.

In this Letter, we demonstrate a deep-learning-based method capable of synthesizing a photorealistic 3D hologram in real-time directly from the input of a single 2D image. We design a fully automatic pipeline to create large-scale datasets by converting any collection of real-life images into pairs of 2D images and corresponding 3D holograms and train our convolutional neural network (CNN) end-to-end in a supervised way. Our method is extremely computation-efficient and memory-efficient for 3D hologram generation merely from the knowledge of on-hand 2D image content. We experimentally demonstrate speckle-free and photorealistic holographic 3D displays from a variety of scene images, opening up a way of creating real-time 3D holography from everyday pictures.

Three-dimensional computer holography enabled from a single 2D image.

To compute a high-quality computer-generated hologram (CGH) for true 3D real scenes, a huge amount of 3D data must be physically acquired and provided depending on specific devices or 3D rendering techniques. Here, we propose a computational framework for generating a CGH from a single image based on the idea of 2D-to-3D wavefront conversion. We devise a deep view synthesis neural network to synthesize light-field contents from a single image and convert the light-field data to the diffractive wavefront of the hologram using a ray-wave algorithm. The method is able to achieve extremely straightforward 3D CGH generation from hand-accessible 2D image content and outperforms existing real-world-based CGH computation, which inevitably relies on a high-cost depth camera and cumbersome 3D data rendering. We experimentally demonstrate 3D reconstructions of indoor and outdoor scenes from a single image enabled phase-only CGH.

Deep-learning-based computer-generated hologram from a stereo image pair.

We propose a deep-learning-based approach to producing computer-generated holograms (CGHs) of real-world scenes. We design an end-to-end convolutional neural network (the Stereo-to-Hologram Network, SHNet) framework that takes a stereo image pair as input and efficiently synthesizes a monochromatic 3D complex hologram as output. The network is able to rapidly and straightforwardly calculate CGHs from the directly recorded images of real-world scenes, eliminating the need for time-consuming intermediate depth recovery and diffraction-based computations. We demonstrate the 3D reconstructions with clear depth cues obtained from the SHNet-based CGHs by both numerical simulations and optical holographic virtual reality display experiments.

Foveated holographic near-eye 3D display.

We present a foveated rendering method to accelerate the amplitude-only computer-generated hologram (AO-CGH) calculation in a holographic near-eye 3D display. For a given target image, we compute a high-resolution foveal region and a low-resolution peripheral region with dramatically reduced pixel numbers. Our technique significantly improves the computation speed of the AO-CGH while maintaining the perceived image quality in the fovea. Moreover, to accommodate the eye gaze angle change, we develop an algorithm to laterally shift the foveal image with negligible extra computational cost. Our technique holds great promise in advancing the holographic 3D display in real-time use.

Development of an ultra-compact optical combiner for augmented reality using geometric phase lenses.

We present an ultra-compact optical combiner using a waveguide and geometric phase lenses (GPL) for augmented reality displays. By sandwiching the output coupler of a planar waveguide between two flat, thin GPLs, we create two optical sub-systems with different optical powers for displaying the virtual objects and transmitting the ambient light rays, respectively. We implemented our method in a scanning-based Maxwellian display and demonstrated the augmentation of an all-in-focus Maxwellian-view image with real-world objects within a 15° field of view. Our device is light (50 g) and thin (4 mm), making it well suited for wearable applications.

A flexible numerical calculation method of angular spectrum based on matrix product.

Fast Fourier transform (FFT) is the most commonly used mathematical method in numerical calculation, and the FFT-based angular spectrum method (ASM) is also used widely in diffraction calculation. However, the frequency and spatial sampling rules in FFT limit the effective propagation distance and the observation window range of ASM. A novel method for calculating the angular spectrum based on the matrix product is proposed in this Letter. This method realizes the fast calculation of discrete Fourier transform (DFT) based on the matrix product, in which the sampling matrix is orthogonally decomposed into two vectors. Instead of FFT, angular spectrum diffraction calculation is carried out based on the matrix product, which is named the matrix product ASM. The method in this Letter uses a simple mathematical transformation to achieve maximum compression of the sampling interval in the frequency domain, which significantly increases the effective propagation distance of the angular spectrum. Additionally, the size of the observation window can be enlarged to obtain a wider calculation range by changing the spatial sampling of the output plane.

Holographic multiplane near-eye display based on amplitude-only wavefront modulation.

We present a holographic multiplane near-eye display method based on Fresnel holography and amplitude-only wavefront modulation. Our method can create multiple focal images across a wide depth range while maintaining a high resolution (1080P) and refresh rate (60 Hz). To suppress the DC and conjugation signals inherent in amplitude-only wavefront modulation, we develop an optimization algorithm which completely separates primary diffracted light from DC and conjugation at a pre-defined intermediate plane. Spatial filtering at this plane leads to a dramatic increase in the image contrast. The experimental results demonstrate our approach can create continuous focus cues in complex 3D scenes.

Ultraviolet Photoluminescence of Carbon Nanospheres and its Surface Plasmon-Induced Enhancement.

Ultraviolet (UV) light can be used in versatile applications ranging from photoelectronic devices to biomedical imaging. In the development of new UV light sources, in this study, stable UV emission at ≈350 nm is unprecedentedly obtained from carbon nanospheres (CNSs). The origin of the UV fluorescence is comprehensively investigated via various characterization methods, including Raman and Fourier transform infrared analyses, with comparison to the visible emission of carbon nanodots. Based on the density functional calculations, the UV fluorescence is assigned to the carbon nanostructures bonded to bridging O atoms and dangling -OH groups. Moreover, a twofold enhancement in the UV emission is acquired for Au-carbon core-shell nanospheres (Au-CNSs). This remarkable modification of the UV emission is primarily ascribed to charge transfer between the CNSs and the Au surface.

Phase-shifting-free resolution enhancement in digital holographic microscopy under structured illumination.

In this paper, we present a phase-shifting-free method to improve the resolution of digital holographic microscopy (DHM) under the structured illumination (SI). The SI used in the system is different from the traditional SI for it is free of the visible structure due to two illumination lights with orthogonal polarization states. To separate the recorded information and also retrieve the object phase, two reference beams with different carrier frequencies and orthogonal polarization states are adopted. The principle component analysis (PCA) algorithm is introduced in the reconstruction process. It is found that the modulated frequency of SI besides the quadratic phases of the imaging system can be easily removed with help of PCA. Therefore, phase-shifting is not required both in recording and reconstruction process. The simulation is performed to validate our method, while the proposed method is applied to the resolution enhancement for amplitude-contrast and phase-contrast objects imaging in experiments. The resolution is doubled in the simulation, and it shows 78% resolution improvement in the experiments.

Generation of optical vortex array along arbitrary curvilinear arrangement.

We propose an approach for creating optical vortex array (OVA) arranged along arbitrary curvilinear path, based on the coaxial interference of two width-controllable component curves calculated by modified holographic beam shaping technique. The two component curve beams have different radial dimensions as well as phase gradients along each beam such that the number of phase singularity in the curvilinear arranged optical vortex array (CA-OVA) is freely tunable on demand. Hybrid CA-OVA that comprises of multiple OVA structures along different respective curves is also discussed and demonstrated. Furthermore, we study the conversion of CA-OVA into vector mode that comprises of polarization vortex array with varied polarization state distribution. Both simulation and experimental results prove the performance of the proposed method of generating a complex structured vortex array, which is of significance for potential applications including multiple trapping of micro-sized particles.

Bright, stable, and tunable solid-state luminescence of carbon nanodot organogels.

Despite the sustained enthusiastic interest in fluorescent carbon nanodots (FCNDs), it is still challenging to achieve bright and widely tunable solid-state luminescence. Herein, organogels embedded with FCNDs were simply synthesized via a one-pot pyrolysis method. Subsequently, the excitation of a single ultraviolet (UV) excitation line results in tunable solid-state luminescence ranging from blue to red with quantum yields (QYs) >14%. In this study, N and S elements were co-doped to regulate the aggregation of FCNDs, which consequently modulated the Stokes shift of the photoluminescence (PL) by managing the degree of photon reabsorption. Notably, without compact aggregations, the dispersions of FCNDs in the organogel matrix indeed render bright fluorescence, which results from the suppression of excessive photon reabsorption and nonradiative resonant energy transfer (NRET).

Speckle reduced lensless holographic projection from phase-only computer-generated hologram.

This paper presents a method for the implementation of speckle reduced lensless holographic projection based on phase-only computer-generated hologram (CGH). The CGH is calculated from the image by double-step Fresnel diffraction. A virtual convergence light is imposed to the image to ensure the focusing of its wavefront to the virtual plane, which is established between the image and the hologram plane. The speckle noise is reduced due to the reconstruction of the complex amplitude of the image via a lensless optical filtering system. Both simulation and optical experiments are carried out to confirm the feasibility of the proposed method. Furthermore, the size of the projected image can reach to the maximum diffraction bandwidth of the spatial light modulator (SLM) at a given distance. The method is effective for improving the image quality as well as the image size at the same time in compact lensless holographic projection system.

Shaping of optical vector beams in three dimensions.

We present a method of shaping three-dimensional (3D) vector beams with prescribed intensity distribution and controllable polarization state variation along arbitrary curves in three dimensions. By employing a non-iterative 3D beam-shaping method developed for the scalar field, we use two curved laser beams with mutually orthogonal polarization serving as base vector components with a high-intensity gradient and controllable phase variation, so that they are collinearly superposed to produce a 3D vector beam. We experimentally demonstrate the generation of 3D vector beams that have a polarization gradient (spatially continuous variant polarization state) along 3D curves, which may find applications in polarization-mediated processes, such as to drive the motion of micro-particles.

Resolution enhancement of the microscopic imaging by unknown sinusoidal structured illumination with iterative algorithm.

Microscopy by sinusoidal structured illumination is a conventional method to improve resolution, which largely depends on accurate knowledge of the illumination pattern. Two steps are included in the reconstruction process of our proposed technique. The first step solves the parameters of the structured illumination in the spatial domain. Besides the phase-shifting amounts, the period, the modulation factor, and the background intensity of the pattern are extracted from three segmented raw images by the iterative algorithm. The second step is retrieval and synthesis of the low- and high-frequency information of the object in the Fourier space with obtained data. Since the unknown object information is not involved in the pattern parameters' solving process, it is possible to figure out the problem with higher precision and less requirements. We test the performance of this method in the experiments. The resolution is improved with the designed carrier frequency of the illumination pattern.

Speckleless holographic display by complex modulation based on double-phase method.

The purpose of this study is to implement speckle reduced three-dimensional (3-D) holographic display by single phase-only spatial light modulator (SLM). The complex amplitude of hologram is transformed to pure phase value based on double-phase method. To suppress noises and higher order diffractions, we introduced a 4-f system with a filter at the frequency plane. A blazing grating is proposed to separate the complex amplitude on the frequency plane. Due to the complex modulation, the speckle noise is reduced. Both computer simulation and optical experiment have been conducted to verify the effectiveness of the method. The results indicate that this method can effectively reduce the speckle in the reconstruction in 3-D holographic display. Furthermore, the method is free of iteration which allows improving the image quality and the calculation speed at the same time.

Simple calculation of a computer-generated hologram for lensless holographic 3D projection using a nonuniform sampled wavefront recording plane.

In this paper, we present a method for calculation of a computer-generated hologram (CGH) from a 3D object. A virtual wavefront recording plane (WRP) which is close to the 3D object is established. This WRP is nonuniformly sampled according to the depth map of the 3D object. The generation of CGH only involves two nonuniform fast Fourier transform (NUFFT) and two fast Fourier transform (FFT) operations, the whole computational procedure is greatly simplified by diffraction calculation from a 2D planar image instead of 3D object voxels. Numerical simulations and optical experiments are carried out to confirm the feasibility of our proposed method. The CGH calculated with our method is capable to project zoomable 3D objects without lens.

Nonlinear dynamic phase response calibration by digital holographic microscopy.

An accurate phase characterization method by digital holographic microscopy for spatial light modulators (SLMs) is proposed. This method permits high precision measurement for individual SLM pixels. Based on this method, the nonlinear dynamic phase response of the SLM is analyzed and calibrated in two steps: the global phase calibration and the local phase calibration. After the calibrations, both the phase modulation deficiency and the sharp phase jump of the 26-step grating are optimized. The root mean square error of the phase grating is reduced from 1.6319 to 0.2132 rad. The accurate phase distribution control may find various applications concerning high-resolution and high-accuracy wavefront modulation.