General phase-shifting algorithm for hybrid errors suppression using variable-frequency fringes.

In measurements based on phase-shifting fringe pattern analysis, residual ripple-like artifacts often appear due to the co-influence of several error sources, e.g., phase-shifting errors, temporal intensity fluctuations and high-order fringe harmonics, when existing algorithms are adopted to retrieve phase using limited number of fringe patterns. To overcome this issue, a general phase-shifting algorithm for hybrid errors suppression by variable-frequency fringes is proposed in this paper for what we believe to be the first time. A corresponding fringe model is deduced to represent real patterns more accurately under the co-influence of these error factors. Variable-frequency fringes are introduced to provide a least and sufficient system of equations, while a least-squares iterative technique with a grouped step-by-step strategy is adopted for stable calculating a larger number of desired parameters in the constructed model. For the phase jump problem caused by non-full rank matrices at certain sampling points, a regularization combined with constraints between coefficients of high-order fringe harmonics is further proposed for identification and processing. Simulations and experimental results have shown that compared with the prior techniques, the accuracies of the proposed algorithm have been significantly enhanced at least 2.1 (simulations) and 1.5 (experiments) times respectively using bi-frequency equal three-step as an example in the study.

Large aperture and defect-free liquid crystal planar optics enabled by high-throughput pulsed polarization patterning.

The liquid crystal (LC) geometrical phase optics, which is realized by the high-resolution control of the optical axis orientation in transparent micrometer-thin polymer films, is emerging as a next generation of planar optics. It features pronounced optical properties and stimuli-responsive behaviors, which could introduce appealing and new possibilities for photonic purposes. The development of fabrication techniques producing elements with large aperture sizes and arbitrarily varying molecular orientation is of significance in terms of practical utility. Here, we propose the pulsed polarization patterning technique to create large-aperture and defect-free LC geometrical phase elements. We investigated the capability of the azo photo-alignment material responding to nanosecond laser pulses and the corresponding anchoring behaviors to LCs. The threshold was reduced to one fourth of that under the continuous wave recording. The patterning resolution was found to be enhanced to around 0.71 µm, due to the ultra-fast interaction nature of the photo-alignment material with the polarized light field. We proposed the flying exposure mode to deliver high frequency modulated polarized laser pulses (8 kHz), with the precision stage moving in a uniform velocity for light-field stitching and the servo auto-focusing in the sample normal, enabling the stable and reliable polarization patterning for large aperture sizes. We further report on representative fabrication of LC polarization gratings with an aperture of 4 inch and 99.2% average diffraction efficiency.

Full-visible achromatic imaging with a single dual-pinhole-coded diffractive photon sieve.

Conventional diffractive optical elements suffer from large chromatic aberration due to its nature of severe dispersion so that they can only work at a single wavelength with near zero bandwidth. Here, we propose and experimentally demonstrate an achromatic imaging in the full-visible wavelength range with a single dual-pinhole-coded diffractive photon sieve (PS). The pinhole pattern (i.e., distribution of the position and size of each pinhole) is generated with dual wavelength-multiplexing coding (WMC) and wavefront coding (WFC), in which WMC makes multiple wavelengths that are optimally selected within the full visible range focus coherently on a common designed focal length while WFC expands the bandwidth of the diffracted imaging at each of the selected wavelengths. Numerical simulations show that when seven wavelengths (i.e., 484.8, 515.3, 547.8, 582.4, 619.1, 658.1 and 699.5 nm) within the visible range between 470 nm to 720 nm and a cubic wavefront coding parameter α = 30π are selected, a broadband achromatic imaging can be obtained within the full range of visible wavelength. Experimental fabrication of the proposed dual-pinhole-coded PS with a focal length of 500 mm and a diameter of 50 mm are performed using the mask-free UV-lithography. The experimental imaging results agree with the numerical results. The demonstrated work provides a novel and practical way for achieving achromatic imaging in the full visible range with features of thin, light and planar.

Roll-to-plate additive manufacturing.

In this paper, we propose a roll-to-plate (R2P) projection micro-stereolithography (PSL) 3D printer, where layers of photopolymer are transferred and photopolymerized through a flexible membrane. Benefitting from the "coat-expose-peel" procedure, highly viscous material can be printed quickly with good vertical resolution. Most importantly, the multinozzle dispensing method enables the fabrication of multimaterial architectures with high throughput, low material consumption, and low cross-contamination. R2P-PSL exhibits superior features for flexible 3D printing in terms of material complexity. For this purpose, we envision infinite scenarios involving potential applications in bionics, biotechnology, microcircuit graphics, photonic devices, microfluidics and material science.

An Investigation of the Work Hardening Behavior in Interrupted Cutting Inconel 718 under Cryogenic Conditions.

The severe work hardening phenomenon generated in the machining of Inconel 718 is harmful to continue cutting processes, while being good for the component's service performance. This paper investigates the performance of cryogenic assisted machining used in the cutting processes, which can reduce the waste of fluids. The influence of dry and cryogenic machining conditions with different cutting speeds on the work hardening layer is investigated based on the interrupted cutting of Inconel 718. Cutting temperature distribution obtained from simulations under different conditions is used to discuss the potential mechanism of work hardening. Then, the depth of work hardening and degree of work hardening (DWH) are investigated to analyze the surface deformation behavior, which strengthens the machined surface during metal cutting processes. From the cutting experiments, the depth of the work hardening layer can reach more than 60 µm under the given cutting conditions. In addition, a deeper zone can be obtained by the cooling of liquid nitrogen, which may potentially enhance the wear performance of the component. The results obtained from this work can be utilized to effectively control the work hardening layer beneath the surface, which can be applied to improve the service performance.

Flexible graphical black structural color with high angular tolerance featuring subwavelength nickel cylinder array pixels.

Black structural color has attracted particular interest due to its attractive applications in various fields. Until now, however, the reported graphical black structural color (GBSC) devices are mainly realized by means of electron beam lithography or focused ion beam technology, inevitably suffering from the obstacles of high production cost and time-consuming processing. Moreover, the limited and small area of the GBSC constitutes another issue for real applications and little attention has been devoted to flexible GBSC because of the limitations of this manufacturing approach. In this paper, we experimentally demonstrate and theoretically analyze a novel flexible GBSC architecture capitalized on a pixelated embedded nickel cylindrical array using a reliable, low-cost and self-developed continuously variable spatial frequency lithography. The fabricated graphical and large-area flexible GBSC sample (4 cm × 4 cm) exhibits a measured absorbance of ∼92% over the entire visible regime from 400 nm to 700 nm. Furthermore, the desirable absorptivity is well retained at incident angles up to 60°. It is anticipated that the facile, controllable and scalable approach developed here opens new exciting perspectives for industrial production.

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.

Compact compound-eye imaging module based on the phase diffractive microlens array for biometric fingerprint capturing.

A compound-eye imaging system based on the phase diffractive microlens array as a compact observation module is proposed. As compared with the refractive microlens in common compound-eye imaging systems, the diffractive microlens is a flat imaging optics featuring high relative aperture, thin component thickness and compatibility with lithography techniques. As an application, a compact fingerprint imaging module was demonstrated using this compound-eye imaging system. The phase Fresnel microlens array with continuous trough morphology was fabricated via the self-developed gray-scale laser direct write equipment. An image reconstruction method is proposed by extracting the effective image information of each Fresnel microlens, removing the complex signal separator layer from the compound-eye imaging system. The illumination optics is further planarized through the waveguide backlighting and the waveguide functions as the touch panel for fingerprint recording. The novel compound-eye imaging device length was only restricted by the focal length of the microlens with a low limit of 4.12f. The applicability of this novel compound-eye imaging system was further demonstrated by recording the human fingerprint texture, paving ways for various applications as a compact imaging system.

Cost-effective near-perfect absorber at visible frequency based on homogenous meta-surface nickel with two-dimension cylinder array.

To date, near-perfect light absorbers at visible frequency are still severely impeded by the complicated architecture design and time-consuming costly fabrication procedures. In this work, we design and fabricate a new cost-effective near-perfect absorber at visible frequency based on homogenous meta-surface nickel (Ni) with a two-dimension cylinder array. The simulated and measured average absorption at normal incidence are beyond 94% and 92% over the entire visible wavelength band from 400 nm to 700 nm, respectively. Additionally, the absorbance property was well retained, and the absorptivity still remained beyond 70% when the incident angles vary from 0° to 60°. Our theoretically and experimentally results indicate that the broadband wide-angular absorption can be ascribed to the Rayleigh-Wood anomaly combined with slot modes induced by excited surface plasmon polaritons. Moreover, the low-cost double-beam interference lithography followed by soft nano-imprinting and electroforming technology, which are directly compatible with the cost-effective and high volume manufacturing requirements, are employed to fabricate the proposed absorber. The proposed approach is simple and inexpensive and the obtained ultrathin homogenous meta-surface nickel absorber can be rolled or folded on the surface of various optoelectronics, such as solar system and radiation thermal devices.

Scalable Fourier transform system for instantly structured illumination in lithography.

We report the development of a unique scalable Fourier transform 4-f system for instantly structured illumination in lithography. In the 4-f system, coupled with a 1-D grating and a phase retarder, the ±1st order of diffracted light from the grating serve as coherent incident sources for creating interference patterns on the image plane. By adjusting the grating and the phase retarder, the interference fringes with consecutive frequencies, as well as their orientations and phase shifts, can be generated instantly within a constant interference area. We demonstrate that by adapting this scalable Fourier transform system into lithography, the pixelated nano-fringe arrays with arbitrary frequencies and orientations can be dynamically produced in the photoresist with high variation resolution, suggesting its promising application for large-area functional materials based on space-variant nanostructures in lithography.

Ultra-broadband achromatic imaging with diffractive photon sieves.

Diffractive optical elements suffer from large chromatic aberration due to the strong wavelength-dependent nature in diffraction phenomena, and therefore, diffractive elements can work only at a single designed wavelength, which significantly limits the applications of diffractive elements in imaging. Here, we report on a demonstration of a wavefront coded broadband achromatic imaging with diffractive photon sieves. The broadband diffraction imaging is implemented with a wavefront coded pinhole pattern that generates equal focusing power for a wide range of operating wavelength in a single thin-film element without complicated auxiliary optical system. Experimental validation was performed using an UV-lithography fabricated wavefront coded photon sieves. Results show that the working bandwidth of the wavefront coded photon sieves reaches 28 nm compared with 0.32 nm of the conventional one. Further demonstration of the achromatic imaging with a bandwidth of 300 nm is also performed with a wavefront coded photon sieves integrated with a refractive element.

Efficient fabrication method of nano-grating for 3D holographic display with full parallax views.

Without any special glasses, multiview 3D displays based on the diffractive optics can present high resolution, full-parallax 3D images in an ultra-wide viewing angle. The enabling optical component, namely the phase plate, can produce arbitrarily distributed view zones by carefully designing the orientation and the period of each nano-grating pixel. However, such 3D display screen is restricted to a limited size due to the time-consuming fabricating process of nano-gratings on the phase plate. In this paper, we proposed and developed a lithography system that can fabricate the phase plate efficiently. Here we made two phase plates with full nano-grating pixel coverage at a speed of 20 mm2/mins, a 500 fold increment in the efficiency when compared to the method of E-beam lithography. One 2.5-inch phase plate generated 9-view 3D images with horizontal-parallax, while the other 6-inch phase plate produced 64-view 3D images with full-parallax. The angular divergence in horizontal axis and vertical axis was 1.5 degrees, and 1.25 degrees, respectively, slightly larger than the simulated value of 1.2 degrees by Finite Difference Time Domain (FDTD). The intensity variation was less than 10% for each viewpoint, in consistency with the simulation results. On top of each phase plate, a high-resolution binary masking pattern containing amplitude information of all viewing zone was well aligned. We achieved a resolution of 400 pixels/inch and a viewing angle of 40 degrees for 9-view 3D images with horizontal parallax. In another prototype, the resolution of each view was 160 pixels/inch and the view angle was 50 degrees for 64-view 3D images with full parallax. As demonstrated in the experiments, the homemade lithography system provided the key fabricating technology for multiview 3D holographic display.

High performance organic distributed Bragg reflector lasers fabricated by dot matrix holography.

We report distributed Bragg reflector (DBR) polymer lasers fabricated using dot matrix holography. Pairs of distributed Bragg reflector mirrors with variable mirror separations are fabricated and a novel energy transfer blend consisting of a blue-emitting conjugated polymer and a red-emitting one is spin-coated onto the patterned substrate to complete the device. Under optical pumping, the device emits sing-mode lasing around 622 nm with a bandwidth of 0.41 nm. The working threshold is as low as 13.5 µJ/cm² (~1.68 kW/cm²) and the measured slope efficiency reaches 5.2%. The distributed feedback (DFB) cavity and the DBR cavity resonate at the same lasing wavelength while the DFB laser shows a much higher threshold. We further show that flexible DBR lasers can be conveniently fabricated through the UV-imprinting technique by using the patterned silica substrate as the mold. Dot matrix holography represents a versatile approach to control the number, the size, the location and the orientation of DBR mirrors, thus providing great flexibility in designing DBR lasers.

Ultra-large multi-region photon sieves.

A novel method of designing ultra-large photon sieves in visible regime with multi-region structure is proposed and experimentally demonstrated. Design principle that is based on both phase matching and total pinhole area matching among regions is introduced. The focusing properties of the multi-region structure and the conventional monolithic structure of the same numerical aperture in terms of energy efficiency and the sidelobe suppression are compared. Two photon sieves of focal length 500 mm and diameters 50mm and 125 mm with respectively 3 and 4 regions at working wavelength 632.8 nm are fabricated using UV lithography to validate the proposed method. Good performance of the multi-region photon sieves are evaluated by imaging test. The extension of the proposed method suggests a new concept of ring-to-ring design in terms of pinhole size and density of each individual ring for photon sieves with superior suppressed sidelobes towards ultra-large dimension, high numerical aperture that can be implemented with UV lithography which is otherwise impossible with e-beam technique.

Diffraction characteristics of a submicrometer grating for a light guide plate.

The diffraction transmission characteristics of submicrometer gratings (SMGs) designed for coupling tricolor light out of a light guide plate (LGP) are discussed. Three discrete SMGs are designed for three special wavelengths: red (700 nm), green (546.1 nm), and blue (435.8 nm). The propagation direction of the output tricolor light is perpendicular to the surface of the LGP and can pass through the corresponding pixels of liquid crystal. Calculated by the rigorous coupled-wave theory, the first-order transmission efficiency as a function of grating depth is found to be approximate to a sinusoidlike curve and can be utilized to obtain uniform illumination. The theoretical maximum transmission is as much as 44%. The performance of the LGP composed of SMGs is also demonstrated by our experiments. Compared with other types of LGP, the present device has the advantage of flexible control of illumination angle and wavelengths. Additionally, lossy and costly color filters are unnecessary in the proposed configuration.