Planar cameras with high performance and wide field of view (FOV) are critical in various fields, requiring highly compact and integrated technology. Existing wide FOV metalenses show great potential for ultrathin optical components, but there is a set of tricky challenges, such as chromatic aberrations correction, central bright speckle removal, and image quality improvement of wide FOV. We design a neural meta-camera by introducing a knowledge-fused data-driven paradigm equipped with transformer-based network. Such a paradigm enables the network to sequentially assimilate the physical prior and experimental data of the metalens, and thus can effectively mitigate the aforementioned challenges. An ultra-wide FOV meta-camera, integrating an off-axis monochromatic aberration-corrected metalens with a neural CMOS image sensor without any relay lenses, is employed to demonstrate the availability. High-quality reconstructed results of color images and real scene images at different distances validate that the proposed meta-camera can achieve an ultra-wide FOV (>100 deg) and full-color images with the correction of chromatic aberration, distortion, and central bright speckle, and the contrast increase up to 13.5 times. Notably, coupled with its compact size (< 0.13 cm3), portability, and full-color imaging capacity, the neural meta-camera emerges as a compelling alternative for applications, such as micro-navigation, micro-endoscopes, and various on-chip devices.
In virtual reality (VR) and augmented reality (AR) display, the vergence-accommodation conflict (VAC) is a significant issue. Thus, true-3D display technologies has been proposed to solve the VAC problem. Integral imaging (II) display, one of the most critical true-3D display technologies, has received increasing research recently. Significantly, anachromatic metalens array has realized a broadband metalens-array-based II (meta-II). However, the past micro-scale metalens arrays were incompatible with commercial micro-displays. Additionally, the elemental image array(EIA)rendering is slow. These device and algorithm problems prevent meta-II from being used for practical video-rate near-eye displays (NEDs). This research demonstrates a II-based NED combining a commercial micro-display and a metalens array. We make efforts in the hardware and software to solve the bottlenecks of video-rate metalens array II-based NED. The large-area nanoimprint technology fabricates the metalens array, and a novel real-time rendering algorithm is proposed to generate the EIA. We also build a see-through prototype based on our meta-II NED, demonstrating the effect of depth of field in AR, and the 3D parallax effect on the real mode. This work verifies the feasibility of nanoimprint technology for mass preparation of metalens samples, explores the potential of video-rate meta-II displays, which we can be applied in the fields of VR/AR and 3D display.
In this presentation, we briefly review the development of optical metalenses from the single metalens to the metalens array and to the metalens systems, especially focusing on the progress of silicon nitride metalenses. We then show the optical properties of silicon nitride films with different refractive index in our lab. With such silicon nitride films, we introduce our researches on the broadband achromatic metalens array, and the microscope meta-objectives with cascade metalenses, showing the visible imaging applications on noncoherent 3D integral imaging and high-resolution biological imaging.
In recent years, Meta-lens has become a new type of optical device, showing excellent performance and novel applications. The nanoantennas of meta-lens can be used to control the phase, amplitude, and polarization at well. The phase part is the most important part of the function of the meta-lens. However, so far, the phase distribution of meta-lenses has not been directly measured, which further hinders the quantitative evaluation of their performance. We have developed an interferometric imaging phase measurement system for meta-lens and meta-devices. This system can measure the phase distribution by shooting the interference pattern. The phase distribution of meta-lenses can be measured to quantitatively characterize the imaging performance. Our meta-lens phase measurement system can help for designers to optimize the design, for manufacturers to identify defects, thereby improving the manufacturing process. This work will pave the way for meta-lens in industrial applications.
Chromatic dispersion represents the wavelength-dependent behavior of optical devices and limits their operation bandwidth. Due to the material dispersion restriction of refractive elements, dispersion engineering remains a challenge to imaging technology and optical communication. Recently, metalens offers an attractive approach to engineer the dispersion by introducing the additional degree of freedom with only a single layer of nanostructures. Here, we propose a method to design the dual-wavelength metalenses with controllable dispersion characteristic in transmission mode in the visible region. Three kinds of polarization-independent metalenses are demonstrated, including those with zero dispersion, positive axial dispersion, and negative axial dispersion. All the metalenses show high resolution with nearly diffraction-limited focusing. Our findings may provide an alternative way to design dual-wavelength functional devices in the fields of optical information processing, imaging technologies and complex fluorescence techniques.
Fermionic Dirac cones have attracted tremendous attention in the electronic systems, such as topological insulator and graphene. As the classical analogs, photonic Dirac dispersions at the center of momentum space reveal a unique feature other than fermionic systems, i.e. zero-refractive-index behavior. In principle, such all-dielectric metamaterial is easily capable of scaling into optical wavelength, but it is seldom to address and promote to functional device with large area in silicon nanophotonics. Here, we show a prototype of large-area concave metalens consisting of silicon nanopillars array on silicon platform. The device was etched from n-type (100) single crystalline Si substrate by a top-down method. In theoretical prediction, such metalens can be modeled as a two-dimensional photonic crystal with conical bands at near-infrared wavelength. In this way, light focusing effect in the large-area metalens was observed directly through the out-of-plane scattering from the irregular substrate. The focal spot, which was very close to the curvature center of the metalens surface, indicated a little phase change of near-zero refractive index silicon photonic crystal. The effective refractive index retrieved from optical microscope images was quantitatively consistent with those from effective medium theory. The device performs as a near-aberration-free metalens near Dirac wavelength due to zero refractive index. Furthermore, it reveals a potential application for spectral detection based on wavelength-dependent effective index. The proposed strategy provides a feasible way for silicon-based application of zero-refractive-index photonic crystals.
Metamaterials offer unprecedented opportunity to engineer fundamental band dispersions which enable novel optoelectronic functionalities and devices. Precise control of photonic degrees of freedom can always succeed to manipulate the flow of light. For example, photonic net spin flows such as one-way transports and spin-directional locking have been realized at the boundary of topologically-protected photonic metacrystals. But this is not the only way to achieve net spin flow in solid state systems. Valley degree of freedom may provide a new route to modulate the spin flow in bulk crystals without the assist of boundary. Here, we show the molding of spin flow of light in valley photonic crystals. The coupled valley and spin physics is illustrated analytically. The associated photonic valley Hall effect and unidirectional net spin flow are well demonstrated inside the bulk crystals, instead of the assist of topologically non-triviality. We also show the independent control of valley and topology, resulting in a topologically protected flat edge state. Valley photonic crystals may open up a new route towards the discovery of fundamentally novel states of light and possible revolutionary applications.
Recently, plasmon laser has been attracted great attention at length scales below diffraction limit. It has been demonstrated not only in single nanocavity systems, but also been observed in periodic nanoplasmonic structures. We propose a kind of lasing scheme in a nano-grating with three slits and three metal strips (one fat metal strip and two thin metal strips) in each supercell. There exists a bright mode and a dark mode in the nano-grating, due to the inter-couple among the cavity modes in the slits. The most interesting issue is that such two modes can be independently controlled by tuning the widths of the fat and thin metal stripes. It enables the flexibility to choose the gain medium and the corresponding cavity surrounding in nanoscale. Based on such guideline, we investigate a lasing system consisting of nano-grating and Rhodamine dye molecules by using self-consistent finite element method. We show the lasing dynamic process with both the matched and mismatched nano-grating. As a result, when it well matches, the dark mode will provide higher feedback and amplification than those of the mismatched system. Consequently, when the same optical pump power is applied, the matched case will have shorter lasing onset time and higher output power than the mismatched structure. More calculations can conclude that the perfect-matched nano-grating system will have minimum threshold and maximum lasing slope efficiencies. Our findings may provide a new way on plasmon laser with low threshold and high efficiency.
We propose a new scheme of lasing in a nano-grating with Fano resonance by the independent control of dark and bright modes. The controllability of both dark and bright modes can be achieved by fine-tuning the widths of two kinds of metallic stripes in the nano-grating. It enables the match between resonant frequencies of the nano-grating structure and the absorption and emission frequencies of gain medium. For example, changing the width of the thin strip is to hit the emission peak of the gain medium, while the fat strip corresponds to the absorption peak of the gain medium. We work out an optimal example of silver grating immersing Rhodamine dye, verifying the lasing dynamic process with minimum threshold and maximum output power. It may provide a new way on low-threshold and high-efficient plasmon laser.
Hardware architecture of parallel computation is proposed for generating Fraunhofer computer-generated holograms (CGHs). A pipeline-based integrated circuit architecture is realized by employing the modified Fraunhofer analytical formulism, which is large scale and enables all components to be concurrently operated. The architecture of the CGH contains five modules to calculate initial parameters of amplitude, amplitude compensation, phases, and phase compensation, respectively. The precalculator of amplitude is fully adopted considering the “reusable design” concept. Each complex operation type (such as square arithmetic) is reused only once by means of a multichannel selector. The implemented hardware calculates an 800×600 pixels hologram in parallel using 39,319 logic elements, 21,074 registers, and 12,651 memory bits in an Altera field-programmable gate array environment with stable operation at 50 MHz. Experimental results demonstrate that the quality of the images reconstructed from the hardware-generated hologram can be comparable to that of a software implementation. Moreover, the calculation speed is approximately 100 times faster than that of a personal computer with an Intel i5-3230M 2.6 GHz CPU for a triangular object.
Unavoidable speckle noise on reconstructed image in laser-based holographic system has been a serious problem in holographic display, due to both temporal and spatial coherence of laser. Employing partially coherent light into optics experiment is an effective way to reduce the speckle noise and thus enhance the signal-to-noise ratio. We will show you the analysis on the coherence of input light, which will affect the quality of the reconstructed image by computer generated hologram (CGH). LED light source with band-pass filters and spatial filters is used to achieve different levels of coherence. The experimental results show high agreement with our analysis. We will also show you how to eliminate the unexpected fringes by employing fast Fourier transform, which can overcome the drawback in our previous proposal with a full analytical algorithm for encoding the CGH of polygonal model. In conclusion, we propose the LED-based holographic imaging system, in order to improve the quality of the holographic imaging.
We present a set of analytical formula on describing the diffraction field of the three dimensional (3D) triangular-meshbased
model. The advantage of the proposed method is that it can avoid using the numerical algorithm -- Fast Fourier
Transform, which leads to a depth-of-field limitation by the Whittaker-Shannon sampling theorem. We employ the
proposed method to generate the hologram of 3D texture model derived from the real scene or 3D design software. In
order to further increase the computation speed, we have rendered a real scene by employing the GPU platform. Our
homemade GPU algorithm performs hundreds of times faster than those of CPU. As we developed a general phase
adjustment technique for polygon-based algorithm, the holographic reconstructed scenes possess high performance.
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