Showing papers on "Moiré pattern published in 2021"

Resonance modes in moiré photonic patterns for twistoptics

Abstract: Twistronics has been studied for manipulating electronic properties through a twist angle in the formed moire superlattices of two dimensional layer materials. In this paper, we study twistoptics for manipulating optical properties in twisted moire photonic patterns without physical rotations. We describe a theoretic approach for the formation of single-layer twisted photonic pattern in square and triangular lattices through an interference of two sets of laser beams arranged in two cone geometries. The moire period and the size of unit super-cell of moire patterns are related to the twist angle that is calculated from the wavevector ratio of laser beams. The bright and dark regions in moire photonic pattern in triangular lattices are reversible. We simulate E-field intensities and their cavity quality factors for resonance modes in moire photonic pattern in square lattices. Due to the bandgap dislocation between the bright and dark regions, the resonance modes with very high quality-factors appears near bandgap edges for the moire photonic pattern with a twist angle of 9.5 degrees. At the low frequency range, the resonance modes can be explained as Mie resonances. The cavity quality factor decreases for resonance modes when the twist angle is increased to 22.6 degrees.

A Binocular Vision-Based 3D Sampling Moiré Method for Complex Shape Measurement

Abstract: As a promising method for moire processing, sampling moire has attracted significant interest for binocular vision-based 3D measurement, which is widely used in many fields of science and engineering. However, one key problem of its 3D shape measurement is that the visual angle difference between the left and right cameras causes inconsistency of the fringe image carrier fields, resulting in the phase mismatch of sampling moire. In this paper, we developed a phase correction method to solve this problem. After epipolar rectification and carrier phase introduction and correction, the absolute phase of the fringe images was obtained. A more universal 3D sampling moire measurement can be achieved based on the phase match and binocular vision model. Our numerical simulation and experiment showed the high robustness and anti-noise ability of this new 3D sampling moire method for high-precision 3D shape measurement. As an application, cantilever beams are fabricated by directed energy deposition (DED) using different process parameters, and their 3D deformation caused by residual stresses is measured, showing great potential for residual stress analyses during additive manufacturing.

Improved computer-generated moiré profilometry with flat image calibration.

Abstract: An improved computer-generated moire profilometry (CGMP) with flat image calibration is proposed. In CGMP, the purification of the AC component plays a decisive role. While a composite grating modulated with both the sinusoidal grating and its background light substitutes for the sinusoidal grating itself, the sinusoidal deformed pattern and flat image can be demodulated from the captured pattern. It is found that the sinusoidal deformed pattern and flat image may deviate, which is caused by ambient light. So flat image calibration is conducted to obtain a purer AC component that can effectively suppress the influence of ambient light and ensure the measurement accuracy, even if spectrum aliasing exists. Experimental results show the feasibility and validity of the proposed method.

One-shot dual-projection topography enhanced by phase-shifting logical moiré.

Abstract: In conventional dual-projection moire topography, the additive-type moire pattern has an amplitude modulated form, and therefore poor visibility, unable to be improved through low-pass filtering. To overcome this difficulty, this paper presents an enhanced dual-projection moire topography by means of logical moire. In its implementation, a couple of projectors project two fringe patterns of different colors onto the object simultaneously and then a color camera captures the deformed fringe pattern. Instead of directly processing the additive-type moire fringes, we separate the deformed fringes of two chromatic channels, binarize them, and then calculate their exclusive OR (XOR) moire pattern. By differentiating one of the two deformed fringe patterns, a second XOR moire pattern having exactly a π/2-rad phase shift can also be generated, so that the phases are simply calculated by using an arctangent function after low-pass filtering to the XOR moire patterns. By doing so, we can measure the three-dimensional shape of an object in an efficient way requiring a single-shot implementation. The validity of this method has been demonstrated through simulation and experimental results.

Generation of gradient photonic moiré lattice fields

Abstract: We designed and generated gradient photonic moire lattice fields comprising three varying periodic moire wavefields. Because of the common twisted angles between periodic triangular and hexagonal moire wavefields, gradient patterns can be easily obtained through coherent superposition of hexagonal-triangular-hexagonal photonic moire lattice fields. In addition, two specific twisted angles of Δα|C=3 and Δα|C=5 are proposed, which not only guarantee the periodicity of moire fields but also provide an additional degree of freedom to control the structural arrangement of the gradient photonic moire lattice fields. Further study reveals the non-diffracting character of the gradient photonic moire lattice field generated using the holographic method. This study proposes an easy way to generate and control the structures of gradient moire lattice fields that can be used to fabricate photonic lattices in optical storage media for light modulation.

Measurement of focal length and radius of curvature for spherical lenses and mirrors by using digital-grating moiré effect

Abstract: This paper proposes the use of digital-grating moire effect for measuring the focal lengths and radius of curvatures of biconvex and biconcave spherical simple lenses and spherical mirrors. Based on Fresnel diffraction, the equation for the electric field of propagated light passing through the test samples was derived. Through digital image post processing, the recorded intensity on an observation screen was superimposed on a digital grating to generate a moire pattern. On substituting the slant angle of the moire pattern into the derived equation, the focal lengths and radius of curvatures could be determined. The experimental results successfully demonstrated the feasibility of the proposed method; the percent errors for focal length and radius of curvature measurement were less than 0.5%. The measurement uncertainty was analyzed and the correctness of the derived equation was confirmed through simulation. Because of the use of digital image post processing, the proposed method has advantages such as a simple set up, easy operation, high stability, high accuracy, and low cost. Thus, the method has considerable potential in relevant application.

Design, Modeling and Validation of a Flexible Strain Sensor Based on Moire Patterns and Image Processing

Abstract: This paper reports a new flexible strain sensor using the moire pattern as the transducing signal, which can be used to map the strain/stress on a planar surface or a curved surface. Different from our previously reported flexible strain sensor based on the nanopore thin film, which uses the reflected optical signal as the transducing signal. This type of strain sensor has the following unique features: one grating (G1) is fabricated on a flexible substrate, the other grating (G2), which has a slightly different pitch, is simply stored in a computer. Images of G1 under different strains/stresses are taken using a smartphone camera. Then moire patterns are formed by superimposing the images of the two gratings (G1 and G2) in a computer. Finally, the strain/stress is determined by feature extraction of the moire patterns using image processing.

Double moiré localized plasmon structured illumination microscopy

Abstract: Structured illumination microscopy (SIM) is a well-established fluorescence imaging technique, which can increase spatial resolution by up to a factor of two. This article reports on a new way to extend the capabilities of structured illumination microscopy, by combining ideas from the fields of illumination engineering and nanophotonics. In this technique, plasmonic arrays of hexagonal symmetry are illuminated by two obliquely incident beams originating from a single laser. The resulting interference between the light grating and plasmonic grating creates a wide range of spatial frequencies above the microscope passband, while still preserving the spatial frequencies of regular SIM. To systematically investigate this technique and to contrast it with regular SIM and localized plasmon SIM, we implement a rigorous simulation procedure, which simulates the near-field illumination of the plasmonic grating and uses it in the subsequent forward imaging model. The inverse problem, of obtaining a super-resolution (SR) image from multiple low-resolution images, is solved using a numerical reconstruction algorithm while the obtained resolution is quantitatively assessed. The results point at the possibility of resolution enhancements beyond regular SIM, which rapidly vanishes with the height above the grating. In an initial experimental realization, the existence of the expected spatial frequencies is shown and the performance of compatible reconstruction approaches is compared. Finally, we discuss the obstacles of experimental implementations that would need to be overcome for artifact-free SR imaging.

Wide-view and accurate deformation measurement at microscales by phase extraction of scanning moiré pattern with a spatial phase-shifting technique.

Abstract: A scanning-based second-order moire method is proposed for high-accuracy deformation measurement in a large field of view (FOV) by analyzing the phase distribution of a single-shot scanning moire fringe image using a spatial phase-shifting technique. In this method, the grating pitch can be as small as around one pixel in the scanning moire image to ensure a wide FOV, while high-precision phase measurement is achievable. The strain measurement accuracy has been verified from simulations at different grating pitches, applied strains, and noise levels. The simulation results show that the closer the grating pitch is to the scanning pitch, the smaller the strain measurement error, and the recommended pitch ratio is 0.9∼1.1. Furthermore, the feasibility of this method has been verified from a tensile experiment on an aluminum specimen under a laser scanning microscope with scanning moire images recorded. The microscale strains of aluminum measured at different tensile loads agree well with the strain gauge results. As an integration of the scanning and sampling moire methods, this method has the advantages of a large FOV, high accuracy, strong noise immunity, and visualization of magnified deformation. Compared with the traditional phase-shifting scanning moire method, this method only needs to record a single scanning moire image and is suitable for dynamic deformation analysis.

Color-encoded single-shot computer-generated Moiré profilometry.

Abstract: A color-encoded single-shot computer-generated Moire profilometry (CSCGMP) is proposed. Two sinusoidal gratings with a π phase difference are encoded in red and blue channels respectively to combine a composite color grating. While this composite color grating is projected onto the measured object, the corresponding color deformed pattern can be captured. So two deformed patterns with a π phase difference are separated from its red and blue components respectively. After normalization and subtraction, the AC component of both separated deformed patterns can be extracted. If this AC component respectively multiplied by the two AC components of fringe patterns of reference plane with a π/2 phase difference prepared and saved on the computer in advance, two computer-generated Moire fringes just respectively standing for sine and cosine of phase which is modulated by the height of the object relative to the reference plane are figured out. So the 3D shape of the measured object can be reconstructed with normal computer-generated Moire profilometry. Both simulation and experimental results show the feasibility and validity of the proposed method. It has potential in real-time 3D measurement due to its single-shot feature.

STEM multiplication nano-moiré method with large field of view and high sensitivity.

Abstract: Strain is one of the important factors that determine the photoelectric and mechanical properties of semiconductor materials and devices.In this paper, the scanning transmission electron microscopy (STEM) multiplication nano-moire method is proposed to increase the measurement range and sensitivity for strain field. The formation principle, condition, and measurement range of positive and negative multiplication moire fringes (PMMFs and NMMFs) are analysed in detail here.PMMF generally refers to the multiplication of field of view, NMMF generally refers to the multiplication of displacement measurement sensitivity. Based on the principle of multiplication nano-moire, Theoretical formulas of the fringe spacing and strain field are derived. Compared with geometric phase analysis of deformation measurements based on high-resolution atom images, both the range of field of view and the sensitivity of displacement measurements of the multiplication moire method are significantly improved. Most importantly, the area of field of view of the PMMF method is increased by about two orders of magnitude, which is close to micrometre-scale with strain measurement sensitivity of 2×10-5. In addition, In order to improve the quality of moire fringe and the accuracy of strain measurement, the secondary moire method is developed.The strain laws at the interface of the InP/InGaAs superlattice materials are characterized using the developed method.

Computer-generated moiré profilometry based on flat image demodulation

Abstract: A computer-generated moire profilometry based on flat image demodulation is proposed. A sinusoidal fringe pattern and its background light are modulated by two-frequency carrier fringes, respectively, to combine a composite grating. While this composite grating is projected onto the measured object, the composite deformed pattern can be captured by CCD camera. So the sinusoidal deformed pattern and its corresponding flat image can be demodulated from the composite deformed pattern. And the AC component of sinusoidal deformed pattern is calculated by the subtraction between the demodulated sinusoidal deformed pattern and the flat image. Compared with normal computer-generated moire profilometry, a more intact AC component can be obtained directly no matter how severe the frequency aliasing condition is. Meanwhile, the three-dimensional (3D) shape of object can be reconstructed only using this composite deformed pattern which shows the application in real-time measurement. Experimental results show the feasibility and validity of the proposed method.

The strong modulation limit of excitons and trions in moir\'e materials

Abstract: The optical properties of weakly-doped two-dimensional materials are dominated by strong exciton and trion absorption and luminescence features. In this article we examine the influence of moir\'e patterns in semiconductor heterobilayers on exciton and trion states in the limit of strong moir\'e modulation potentials, commenting on similarities and differences compared to the case of excitons and trions in semiconductor quantum dots. We discuss strategies for using optical properties as quantitative probes of moir\'e materials, and the prospects for exploiting moir\'e materials to design unique light emitters.

Moire profilometry through simultaneous dual fringe projection for accurate phase demodulation: a comparative study

Abstract: In optical metrology, fringe projection and moire techniques have been widely used to measure the topography of objects. We can combine the advantages of the two techniques by applying a configuration of simultaneous dual projection in the fringe projection technique, which generates a superimposed fringe pattern containing a moire pattern that is phase modulated according to the topography. In this work, we present an analytic and comparative study of three methods to demodulate the phase of the moire pattern: the spatial, spatial-temporal, and temporal methods. Those methods consist of two steps: first, the moire pattern is extracted from the superimposed fringe pattern; next, the phase of the moire pattern is demodulated. The analytical results show that the resulting phase map has double phase sensitivity compared to that of the classical fringe projection technique. Experimental and numeric results prove the feasibility of this technique.

A super-grayscale and real-time computer-generated Moiré profilometry using video grating projection.

Abstract: By using the time-division multiplexing characteristics of the projector and the integral exposure characteristics of the charge coupled device (CCD) camera, a super-grayscale and real-time computer-generated Moire profilometry based on video grating projection is proposed. The traditional digital static grating is of 256-grayscale at most. If an expected super-grayscale grating with a maximum grayscale of 766 is designed and divided into three 256-grayscale fringe patterns with balanced grayscale as far as possible, they can be synthesized into a repeated playing video grating instead of the traditional static grating. When the video grating is projected onto the measured object, as long as the exposure time is set to three times the refresh cycle of the video grating, the super-grayscale deformed patterns in the 766-grayscale can be captured with a 10-bit CCD camera, so that the deformed patterns are realistic. The digital error in computer-generated Moire profilometry is effectively reduced. In addition, this method can expand the linear range of the deformed pattern by 20% in computer Moire profilometry. Therefore, the proposed method has the perspectives of high accuracy and real-time measurement. Theoretical analysis and experimental results demonstrate the validity and capability of the proposed method.

Introduction to strain characterization methods in Transmission Electron Microscopy

Abstract: In this chapter, a set of strain characterization methods in Transmission Electron Microscopy is introduced focusing on their key principles and their main interests. First, several direct methods measuring the deformation directly from the atomic columns positions on a high resolution electron micrograph are detailed. In the following, a few indirect methods measuring the strain on geometries different from the real space unit cell are described. Finally, a novel indirect strain characterization method in Scanning Transmission Electron Microscopy (STEM) is presented. In this new technique, an interference between the scanning grid of the electron beam raster and the crystal lattices is designed to generate a set of Moire fringes forming a STEM Moire hologram. The Moire arrangement is subsequently translated into 2D deformation maps with field of views up to couple of microns. STEM Moire interferometry is still in early stages of development and an up to date status is here presented.

Augmented reality display system using modulated moiré imaging technique.

Abstract: To enhance the depth rendering ability of augmented reality (AR) display systems, a modulated moire imaging technique is used to render the true three-dimensional (3D) images for AR display systems. 3D images with continuous depth information and large depth of field are rendered and superimposed on the real scene. The proposed AR system consists of a modulated moire imaging subsystem and an optical combiner. The modulated moire imaging subsystem employs modulated point light sources, a display device, and a microlens array to generate 3D images. A defocussing equal period moire imaging structure is used, which gives a chance for the point light sources to modulate the depth position of 3D images continuously. The principles of the imaging system are deduced analytically. A custom-designed transparent off-axis spherical reflective lens is used as an optical combiner to project the 3D images into the real world. An experimental AR system that provides continuous 3D images with depth information ranging from 0.5 to 2.5 m is made to verify the feasibility of the proposed technique.

Investigation of Low Temperature Deformation Measurement Problem by the Contact Holographic Interferometers

Abstract: The materials currently used in modern structures do dramatically change their mechanical properties under the influence of low operating temperatures. One of the factors affecting the breakdown of various structural elements at low temperatures, is the placement of the stress concentrations. The existing analytical design schemes cannot always correctly take into account the material mechanical properties changes caused by the temperature decrease. In the present work we consider the method of contact holographic interferometry, which made it possible to efficiently determine surface displacements and strains at the temperature of –50 °C. The contact holographic interferometer is presented as an assembly of the object surface section and holographic plate in an immediate vicinity to the object. This technique is used to determine in-plane and out-of-plane displacements of tested object. If a hologram is recorded at metallized grating surface it restores information peculiar to some optical methods: holographic interferometry, speckle photography, holographic moire and mirror-optical method. For the interpretation of interference patterns new simple analytical equations are obtained. Contact holographic interferometers are used effectively to solve some elasto-plastic deformation measurement problems of isotropic and polycristalline materials. It is specially designed to permit the accurate measurement of the in-plane component of strain in the immediate vicinity of stress concentrators in plates.

Moiré sampling in Scanning Transmission Electron Microscopy

Abstract: The image formation in Scanning Transmission Electron Microscopy (STEM) is a sequential acquisition of scattered electrons from on a thin sample illuminated by an electron probe. The acquisition sequence is set by the scanning grid defining the successive positions of the electron beam. The scanning grid defines the sampler evaluating the thin material continuum. Sampling appears to be one major component of the STEM image formation. The concepts from sampling theory such as undersampling or oversampling become naturally of interest. In the following chapter, the particular effect of undersampling crystalline materials in STEM is detailed. The undersampling artifact, often called aliasing, is formally linked the formation of 2D Moire fringes in a STEM electron micrograph. The resulting STEM Moire hologram reveals to be an undersampled sparse bandwidth limited and periodic function from which the crystalline lattices can be recovered using the Moire sampling recovery theorem. As an example, the recovery method is applied on a STEM Moire hologram recorded on an unstrained silicon material disclosing its crystalline lattices arrangement over a large field of view.

Scanning Transmission Electron Microscopy Moiré sampling Geometrical Phase Analysis (STEM Moiré GPA)

Abstract: One interest of Scanning Transmission Electron Microscopy (STEM) Moire interferometry is to increase the field of view of the electron micrograph mapping the structural properties of the material analyzed. The field of view extension is based on undersampling the features present in the sample and can be done without loss of information following the Moire sampling recovery theorem. The Moire sampling recovery theorem indeed enables the missing information between the undersampled Moire fringes to be completed reconstructing an oversampled version of the STEM Moire hologram. As STEM Moire interferometry carries all the structural properties of the sample, dedicated characterization methods can be furthermore developed. In the following chapter, the characterization of strain in STEM Moire interferometry is explored. As the phase from the Geometrical Phase Analysis (GPA) method is not distorted by the sampling process, the approach using the GPA method directly on the STEM Moire hologram is favored. From the theoretical description, a novel strain characterization technique, called STEM Moire GPA (SMG), is first presented. The application of SMG and its practical aspects are subsequently detailed on a simple case of study. SMG reveals to be an easy and efficient method mapping the 2D strain field over large field of views up to several microns and is complementary to both Dark Field Electron Holography and Nano Beam Precession Electron Diffraction techniques.