Liquid Crystal Microlens Array Research Articles

Drop-on-Demand Pyro-Electrohydrodynamic Printing of Nematic Liquid Crystal Microlenses.

Inkjet printing of liquid crystal (LC) microlens arrays is particularly appealing for the development of switchable 2D/3D organic light-emitting diode (OLED) displays, as the printing process ensures that the lenses can be deposited directly and on-demand onto the pixelated OLED layer without the need for additional steps, thus simplifying fabrication complexity. Even if different fabrication technologies have been employed and good results in LC direct printing have already been achieved, all the systems used require costly equipment and heated nozzles to reduce the LC solution's viscosity. Here, we present the direct printing of a nematic LC (NLC) lens by a Drop-on-Demand (DoD) inkjet printing by a pyro-electrohydrodynamic effect for the first time. The method works at ambient temperature and avoids dispensing nozzles, thus offering a noncontact manipulation approach of liquid with high resolution and good repeatability on different kinds of substrates. NLC microlenses are printed on different substrates and fully characterized. Polarization properties are evaluated for various samples, i.e., NLC lenses on unaligned and indium-tin oxide (ITO) aligned. Moreover, an in-depth characterization of the NLC lenses is reported by polarized optical microscopy and by analyzing the birefringence in digital holographic microscopy.

LC-based lightfield camera prototype for rapidly creating target images optimized by finely adjusting several key coefficients and a LC-guided refocusing-rendering.

A lightfield camera prototype is constructed by directly coupling a liquid-crystal (LC) microlens array with an arrayed photosensitive sensor for performing a LC-guided refocusing-rendering imaging attached by computing disparity map and extracting featured contours of targets. The proposed camera prototype presents a capability of efficiently selecting the imaging clarity value of the electronic targets interested. Two coefficients of the calibration coefficient k and the rendering coefficient C are defined for quantitively adjusting LC-guided refocusing-rendering operations about the images acquired. A parameter Dp is also introduced for exactly expressing the local disparity of the electronic patterns selected. A parallel computing architecture based on common GPU through the OpenCL platform is adopted for improving the real-time performance of the imaging algorithms proposed, which can effectively be used to extract the pixel-leveled disparity and the featured target contours. In the proposed lightfield imaging strategy, the focusing plane can be easily selected and/or further adjusted by loading and/or varying the signal voltage applied over the LC microlenses for realizing a rapid or even intelligent autofocusing. The research lays a solid foundation for continuously developing or upgrading current lightfield imaging approaches.

Laser beam homogenization based on a multifocal liquid crystal microlens array.

A novel, to the best of our knowledge, tunable multifocal liquid crystal microlens array (TMLCMA) was fabricated with a triple-electrode structure consisting of a large-hole, a small-hole array, and planar electrodes. The electro-optical performances of the TMLCMA are characterized, demonstrating the monofocal convex, multifocal convex, and multifocal concave functions when the TMLCMA is manipulated with various driving schemes. Furthermore, the homogenization of a laser beam is realized using the fabricated TMLCMA. The multifocal convex and multifocal concave functions of the TMLCMA successfully suppress the lattice phenomenon caused by the monofocal microlens array, homogenize the Gaussian beam to a flattop intensity distribution, and broaden the beam size.

Construction of cholesteric liquid crystal microlens array at air/CLC/air and water/CLC/air interfaces

This study addressed the limitations of cholesteric liquid crystal (CLC) based microlens arrays, which required submersion of the CLC in water for their formation. We successfully constructed microlens arrays at both the air/CLC/air and water/CLC/air interfaces by doping E7, a nematic host, with a strong chiral dopant, (S)− 1-Phenylethane-1, 2-diyl bis(4-(trans-4-pentyl cyclohexyl) benzoate) (S1011) at different concentrations (0.3, 0.5, and 1 wt%). By spreading the mixture on transmission electron microscopy square-geometry grids, we observed that the lenses thickened at the water/CLC/air interface and thinned at the air/CLC/air interface with increasing dopant concentrations. A non-inverted image on the same side as the object was analyzed, created from the inverted virtual image of the object on the opposite side of the lens. The non-inverted image's focal length ranged from 6–6.24 mm for air/CLC/air and 4.8–3.6 mm for water/CLC/air, corresponding at 0.3, 0.5, and 1 wt% of S1011. Furthermore, the effect of interfacial tension on focal length was also observed by replacing water with sodium dodecyl sulfate (SDS) solution, and it was found that the focal length increased by 60 µm with increasing SDS from 0.1 to 1 mM. These findings suggest that the S1011 doped E7 microlens array can be easily constructed and potentially used in optical technologies.

Snapshot Multispectral Light‐Field Imaging Based on Nematic Liquid‐Crystal Microlens Array via Deep Learning

In this study, a multispectral light‐field imaging system is developed using nematic liquid‐crystal microlens arrays (LC‐MLAs). The system solves the spectral under‐sampling and complex system issues in existing light‐field and spectral cameras. The LC‐MLA is the core optical element of the snapshot imaging system. No dispersion or customized scanning machinery is required to obtain multispectral light fields. Deep learning technology is utilized to reconstruct spectral information from raw data. In the experiments, it is demonstrated that the proposed method efficiently reconstructs multispectral data with 10 nm accuracy in the visible wavelength range of 400–700 nm. The system offers high precision, simple equipment, and relatively low cost without additional dispersion and filtering devices.

Achromatic and Resolution Enhancement Light Field Deep Neural Network for ZnO Nematic Liquid Crystal Microlens Array

Nematic liquid‐crystal microlens arrays (LC‐MLAs) often exhibit chromatic aberration and low resolution, severely compromising their optical imaging quality. This study proposes an achromatic and resolution enhancement light field (ARELF) deep neural network (DNN) to address these issues. The training set is constructed by incorporating LC‐MLA characteristics’ degradation, retrofitting the vimeo90k dataset. The network's hidden layer is trained to learn about chromatic aberration and low resolution of LC‐MLA while extracting imaging features and fusing the information of complementary features of a light field under varying voltages. The loss function includes both chromatic aberration and overall resolution. The light field images of ZnO LC‐MLA under seven consecutive voltages are used as input to test the proposed DNN model. After experimental verification, the proposed model effectively eliminates chromatic aberration while enhancing the spatial and temporal resolution of LC‐MLA. This novel network can be utilized to optimize the design process of LC‐MLA and significantly improve its imaging performance.

Cylindrical liquid crystal micro-lens array based on interdigital electrodes

ABSTRACT Based on the traditional structure of in plane switching liquid crystal display, a cylindrical liquid crystal micro-lens array driven by interdigital electrodes is proposed, and its performance is studied. Firstly, the director distribution and relative phase difference distribution of liquid crystal molecules were calculated, which shows that this structure can be used to fabricate cylindrical liquid crystal micro-lens. Then, by comparing lenses of difference sizes, it is found that narrow electrodes should be selected for lens production, and the centre of the electrode gap should be selected as the central axis of a single liquid crystal micro-lens. Finally, a certain electrode structure is selected to explore its focal length, focusing quality and zoom time. The results show that the focal length of the liquid crystal micro-lens decreases gradually with the increase of the driving voltage. When the driving voltage is between 3 Vrms and 5.5 Vrms, the liquid crystal micro-lens has good focusing quality, and its corresponding focal length changes from 1.63 mm to 20.28 mm. Based on this structure, when the liquid crystal lens is driven by the above two voltages alternately, the time required for liquid crystal molecules to reach stability is 2.35 s and 3.87 s respectively.

Flexible liquid crystal micro-lens arrays for curved integral imaging 2D/3D convertible display

ABSTRACT Glasses-free three-dimensional (3D) displays are being regarded as the key to the future of display that will redefine the display industry. Inspired by the curved screen, liquid crystal micro-lens arrays (LC MLAs) on flexible PET substrate with a radius of curvature of 5 cm was achieved. For demonstration, an integral imaging-based 2D/3D convertible display system is proposed by using the flexible LC MLAs (FLC MLAs) to switch its operation mode between 2D and 3D modes. When the FLC MLAs with an applied voltage are in the system, the prototype renders a matched 3D image and works in 3D mode. When the FLC MLAs do not have an applied voltage, the FLC MLAs are equivalent to glass, and the display system works in 2D mode. In addition, we also demonstrated that the FLC MLAs have a wider viewing angle than the flat integral imaging 3D display system. The depth of field ranges from 6.78 to 210.12 mm under low operating voltages of 3.4 to 3.1 Vrms, respectively. With its low voltage, thin structure, and adjustable focal length form factors, the developed display system can be integrated with off-the-shelf purchased flat panels, making it a promising option for portable electronics.

High-Accuracy Depth Estimation of Nematic Liquid Crystal Microlens Arrays Based on Convex Optimization Theory

We have proposed an iterative feedback method based on a nematic liquid crystal microlens array (LC-MLA) light-field system, which can obtain highly accurate depth maps. A preliminary reconstruction of the all-in-focus map of the scene is produced to obtain a high-precision depth map. A natural image sparsity prior and a chromatic aberration correction are used to ensure high-quality deblurring in the presence of inaccurate depth estimation. Subsequently, a high-quality all-in-focus image is used as a constraint to obtain a high-accuracy depth estimation result. After the experiment, it can be demonstrated that the proposed method can provide good estimation results under texture-free conditions.

Photoelectric hybrid neural network based on ZnO nematic liquid crystal microlens array for hyperspectral imaging.

The miniaturized imaging spectrometers face bottlenecks in reconstructing the high-resolution spectral image. In this study, we have proposed an optoelectronic hybrid neural network based on zinc oxide (ZnO) nematic liquid crystal (LC) microlens array (MLA). This architecture optimizes the parameters of the neural network by constructing the TV-L1-L2 objective function and using mean square error as a loss function, giving full play to the advantages of ZnO LC MLA. It adopts the ZnO LC-MLA as optical convolution to reduce the volume of the network. Experimental results show that the proposed architecture has reconstructed a 1536 × 1536 pixels resolution enhancement hyperspectral image in the wavelength range of [400 nm, 700 nm] in a relatively short time, and the spectral accuracy of reconstruction has reached just 1 nm.

Direct Imaging with Liquid Crystal Microlens Array

Direct Imaging with Liquid Crystal Microlens Array

High-resolution light field imaging based on liquid crytal microlens arrays with ZnO microstructure orientation

We proposed a liquid crystal (LC) microlens array (MLA) based on zinc oxide (ZnO) microstructure orientation and used it in a light field imaging system. The system uses convex optimization theory to jointly optimize the multi-voltage light field information and obtain a high-resolution imaging method. The core element in this system is an LCMLA, which induces the horizontal orientation of LC molecules by ZnO microstructure to effectively overcome the physical defects caused by orientation, such as mechanical friction. Combined with the proposed joint optimization high-resolution algorithm based on convex optimization theory, high-resolution imaging can be achieved using the redundant characteristics of light field images. The experiments show that the proposed method's PSNR of the reconstructed image is about 32 dB compared with the conventional method. The designed imaging system is flexible, simple, and suitable for high-resolution reconstruction.

Low-voltage driving high-resistance liquid crystal micro-lens with electrically tunable depth of field for the light field imaging system

Light field imaging (LFI) based on Liquid crystal microlens array (LC MLAs) are emerging as a significant area for 3D imaging technology in the field of upcoming Internet of things and artificial intelligence era. However, in scenes of LFI through conventional MLAs, such as biological imaging and medicine imaging, the quality of imaging reconstruction will be severely reduced due to the limited depth of field. Here, we are proposed a low-voltage driving LC MLAs with electrically tunable depth of field (DOF) for the LFI system. An aluminum-doped zinc oxide (AZO) film was deposited on the top of the hole-patterned driven-electrode arrays and used as a high resistance (Hi-R) layer, a uniform gradient electric field was obtained across the sandwiched LC cell. Experimental results confirm that the proposed LC MLAs possess high-quality interference rings and tunable focal length at a lower working voltage. In addition, the focal lengths are tunable from 3.93 to 2.62 mm and the DOF are adjustable from 15.60 to 1.23 mm. The experiments demonstrated that the LFI system based on the proposed structure can clearly capture 3D information of the insets with enlarged depths by changing the working voltage and driving frequency, which indicates that the tunable DOF LC MLAs have a potential application prospects for the biological and medical imaging.

20.2: 2D/3D Mixed Display Based on Polarization Division Multiplexing

We propose a two‐dimensional (2D)/ three‐dimensional (3D) mixed display system based on polarization division multiplexing. The polarization division multiplexing device is composed of a polarization switching layer, a polarization‐dependent liquid crystal micro‐lens array (LCMLA) and a scattering polarizer. According to the polarization state of incident light switched by the polarization switching layer, the combined action of polarization‐dependent LCMLA and scattering polarizer can form both point light source array for 3D display and scattered light source for 2D display. The 2D/3D mixed images can be reconstructed through the precise control of the polarization direction of each pixel. The advantage of the proposed system is that a point light source array and a surface light source are formed only by adjusting the state of the polarization switching layer. What's more, our proposed system has low driving voltage and relatively compact system volume.

Electrically addressed focal stack plenoptic camera based on a liquid-crystal microlens array for all-in-focus imaging.

Focal stack cameras are capable of capturing a stack of images focused at different spatial distance, which can be further integrated to present a depth of field (DoF) effect beyond the range restriction of conventional camera's optics. To date, all of the proposed focal stack cameras are essentially 2D imaging architecture to shape 2D focal stacks with several selected focal lengths corresponding to limited objective distance range. In this paper, a new type of electrically addressed focal stack plenoptic camera (EAFSPC) based on a functional liquid-crystal microlens array for all-in-focus imaging is proposed. As a 3D focal stack camera, a sequence of raw light-field images can be rapidly manipulated through rapidly shaping a 3D focal stack. The electrically addressed focal stack strategy relies on the electric tuning of the focal length of the liquid-crystal microlens array by efficiently selecting or adjusting or jumping the signal voltage applied over the microlenses. An algorithm based on the Laplacian operator is utilized to composite the electrically addressed focal stack leading to raw light-field images with an extended DoF and then the all-in-focus refocused images. The proposed strategy does not require any macroscopic movement of the optical apparatus, so as to thoroughly avoid the registration of different image sequence. Experiments demonstrate that the DoF of the refocused images can be significantly extended into the entire tomography depth of the EAFSPC, which means a significant step for an all-in-focus imaging based on the electrically controlled 3D focal stack. Moreover, the proposed approach also establishes a high correlation between the voltage signal and the depth of in-focus plane, so as to construct a technical basis for a new type of 3D light-field imaging with an obvious intelligent feature.

37‐4: Student Paper: Depth‐Enhanced 2D/3D Switchable Display Based on Integral Imaging

The development of two‐dimensional (2D)/three‐dimensional (3D) switchable displays in the display market is limited due to the narrow depth of 3D display mode. In this paper, we propose a depth‐enhanced 2D/3D switchable display based on integral imaging that stacks multiple polarization switching layers and multiple polarization‐dependent liquid crystal micro‐lens arrays (LCMLAs) together. According to the change of the incident polarization state by the polarization switching layer, the polarization‐dependent LCMLA can realize the focusing effect and transmission effect respectively. Because different layers of polarization‐dependent LCMLA have different gaps (g) and the combination of different layers of polarization‐dependent LCMLA obtains different focal lengths (f), hence, multiple central depth planes can be generated to improve the depth of field (DOF) of the 3D mode. The 2D display mode is realized when all the layers of the polarization‐dependent LCMLA are in transmission effect. We experimentally developed a prototype of the proposed 2D/3D switchable display with two polarization switching layers and two polarization‐dependent LCMLAs. The results show that the DOF of the prototype is easily increased by more than two times. At the same time, the 2D image is also clearly presented in 2D mode without any distortion.

A Light Field Display Realization with a Nematic Liquid Crystal Microlens Array and a Polymer Dispersed Liquid Crystal Film

This study demonstrates a light field display system using a nematic liquid crystal (LC) microlens array (MLA) and a polymer dispersed liquid crystal (PDLC) film. LC-MLA without polarization effects presented high-resolution intermediate 3D images by adopting a depolarization algorithm. The adopted PDLC film modulated the reconstructed 3D images to deliver full-parallax images efficiently with a wide FOV. The experimental result shows that the peak signal to noise ratio (PSNR) value of photograph accurate display results improves compared to the pure LC-MLA method. The proposed method is an essential step toward high-quality light field display.

Liquid crystal microlens array with positive and negative focal lengths based on a patterned electrode.

A liquid crystal microlens array (LCMLA) with positive and negative focal lengths based on a ring-array patterned electrodes is demonstrated. By carefully designing patterned electrodes with a circular electrode array area and outer ring electrode region area, the switching of the positive and negative lens effect can be easily achieved in a single cell. A positive lens effect appeared when the voltage was applied to the outer ring electrode region and the top substrate. The focal length changed from infinity to 1 mm as the voltage varied from 0 to 3Vrms. A negative lens effect occurred when the voltage was applied to the circular electrode array and the top substrate. The focal length varied from infinity to -1mm when the voltage changed from 0 to 2Vrms. The imaging properties of the LCMLA at different voltages are evaluated. Our LCMLA, with simple structure, low driving voltage, and good stability, has potential applications in optical communication, imaging processing, and displays.

Depth-enhanced 2D/3D switchable integral imaging display by using n-layer focusing control units

ABSTRACT Depth of field (DOF) is one of the important parameters of three-dimensional (3D) displays. A two-dimensional (2D)/3D switchable display that can display both high-resolution 2D images and vivid 3D images with large DOF is an ideal purpose. In this paper, we propose a depth-enhanced 2D/3D switchable integral imaging display by using n-layer focusing control units (FCUs). Each layer of FCU is composed of a polarization switch and a polarization-dependent liquid crystal micro-lens array (LCMLA). According to the polarization state of incident light switched by the polarization switch, the polarization-dependent LCMLA can realize focusing effect or transmission effect, respectively. Because each layer of FCU has two optional optical effects and has different gap from the display panel, the combination of n-layer FCUs could generate 2 n -1 central depth planes, hence 2 n -1 sub-DOFs can be obtained for depth-enhanced 3D display. The 2D display mode can be easily realized when all layers of FCUs are transmission effect. We experimentally set up a prototype of the proposed 2D/3D switchable display with two-layer FCUs. The experimental results indicated that the proposed prototype can effectively increase the DOF of 3D display and realize 2D/3D switchable display as well.

All-In-Focus Polarimetric Imaging Based on an Integrated Plenoptic Camera with a Key Electrically Tunable LC Device.

In this paper, a prototyped plenoptic camera based on a key electrically tunable liquid-crystal (LC) device for all-in-focus polarimetric imaging is proposed. By using computer numerical control machining and 3D printing, the proposed imaging architecture can be integrated into a hand-held prototyped plenoptic camera so as to greatly improve the applicability for outdoor imaging measurements. Compared with previous square-period liquid-crystal microlens arrays (LCMLA), the utilized hexagonal-period LCMLA has remarkably increased the light utilization rate by ~15%. Experiments demonstrate that the proposed imaging approach can simultaneously realize both the plenoptic and polarimetric imaging without any macroscopic moving parts. With the depth-based rendering method, both the all-in-focus images and the all-in-focus degree of linear polarization (DoLP) images can be obtained efficiently. Due to the large depth-of-field advantage of plenoptic cameras, the proposed camera enables polarimetric imaging in a larger depth range than conventional 2D polarimetric cameras. Currently, the raw light field images with three polarization states including I0 and I60 and I120 can be captured by the proposed imaging architecture, with a switching time of several tens of milliseconds. Some local patterns which are selected as interested target features can be effectively suppressed or obviously enhanced by switching the polarization state mentioned. According to experiments, the visibility in scattering medium can also be apparently improved. It can be expected that the proposed polarimetric imaging approach will exhibit an excellent development potential.