Incoherent partial superposition modeling for single-shot angle-resolved ellipsometry measurement of thin films on transparent substrates.

Ellipsometric measurement of transparent samples suffers from substrate backside reflection challenges, including incoherent and partial superposition issues. The recently developed angle-resolved ellipsometry (ARE) can naturally eliminate the backside reflections of substrates with a micro-spot equivalent thickness or thicker; however, for thinner substrates, ARE working with general incoherent backside reflection models shows significant inaccuracy or measurement failure. In this paper, an incoherent partial superposition (IPS) model is proposed to characterize the optical superposition effect between the frontside and uncertain backside reflections from an unknown substrate. IPS introduces a cosine-like correction of the backside reflection, corresponding to the overlapping-area change of backside and frontside reflections along with incident angles. Benefiting from ARE's wide-angle spectral imaging capability, IPS achieves single-shot measurement of thin film thicknesses on transparent substrates of unknown thickness. An ARE system was built and calibrated regarding the linear relationship between the cosine-corrected angular frequencies and substrate thicknesses. Then, commercial ITO films on glasses of different thicknesses ranging from 200 to 1000 µm were measured. Experimental results show that IPS-ARE results in a root-mean-square accuracy error of ∼1 nm in film thickness measurement and provides a ∼77% error reduction from general incoherent backside reflection models.

Honeycomb effect elimination in differential phase fiber-bundle-based endoscopy.

Fiber-bundle-based endoscopy, with its ultrathin probe and micrometer-level resolution, has become a widely adopted imaging modality for in vivo imaging. However, the fiber bundles introduce a significant honeycomb effect, primarily due to the multi-core structure and crosstalk of adjacent fiber cores, which superposes the honeycomb pattern image on the original image. To tackle this issue, we propose an iterative-free spatial pixel shifting (SPS) algorithm, designed to suppress the honeycomb effect and enhance real-time imaging performance. The process involves the creation of three additional sub-images by shifting the original image by one pixel at 0, 45, and 90 degree angles. These four sub-images are then used to compute differential maps in the x and y directions. By performing spiral integration on these differential maps, we reconstruct a honeycomb-free image with improved details. Our simulations and experimental results, conducted on a self-built fiber bundle-based endoscopy system, demonstrate the effectiveness of the SPS algorithm. SPS significantly improves the image quality of reflective objects and unlabeled transparent scattered objects, laying a solid foundation for biomedical endoscopic applications.

Error compensation for near optical coaxial phase measuring deflectometry with refraction error model.

Phase measuring deflectometry (PMD) is a key measurement technology for specular surfaces form measurement. Compared with conventional PMD techniques, the near optical coaxial PMD (NCPMD) can achieve compact configuration, light weight and reducing measurement error caused by shadows of the surface structures through utilizing a plate beamsplitter. However, the introduction of the plate beamsplitter will affect the measurement accuracy of the NCPMD system. The refraction of the plate beamsplitter needs to be considered. In this work, a virtual system of NCPMD was established, and an error model of the NCPMD system by considering the refraction influence of the plate beamsplitter was presented to analyze the shape reconstruction error caused by the plate beamsplitter. Moreover, the calibration method of the beamsplitter and the ray tracing algorithm to achieve error compensation of the beamsplitter were proposed. The proposed error compensation method can effectively improve the measurement accuracy of NCPMD system which has been confirmed by surface measurement experiments.

Adaptive coded phase mask design and high-quality image reconstruction for interference-less coded aperture correlation holography.

The interference-less coded aperture correlation holography is a non-scanning, motionless, and incoherent technique for imaging three-dimensional objects without two-wave interference. Nevertheless, a challenge lies in that the coded phase mask encodes the system noise, while traditional reconstruction algorithms often introduce unwanted surplus background components during reconstruction. A deep learning-based method is proposed to mitigate system noise and background components simultaneously. Specifically, this method involves two sub-networks: a coded phase mask design sub-network and an image reconstruction sub-network. The former leverages the object's frequency distribution to generate an adaptive coded phase mask that encodes the object wave-front precisely without being affected by the superfluous system noise. The latter establishes a mapping between the autocorrelations of the hologram and the object, effectively suppresses the background components by embedding a prior physical knowledge and improves the neural network's adaptability and interpretability. Experimental results demonstrate the effectiveness of the proposed method in suppressing system noise and background components, thereby significantly improving the signal-to-noise ratio of the reconstructed images.

Holistic calibration method of deflectometry by holonomic framework priors.

Phase measuring deflectometry is a powerful measurement tool of optical surfaces, but the measuring accuracy relies on the quality of system calibration. Calibration errors arise from the oversimplified imaging models, error accumulation and amplification, and the bias in numerical optimization. A holistic calibration method is proposed to shorten the error propagation chain. The descriptive prowess of the imaging system is enhanced by calculating each incident ray independently and compensating the systematic errors resulting from the form error of the calibration mirror. Finally, a holonomic framework prior is defined to guarantee the calibration reliability by utilizing the physical constraints of the measurement system. Experimental results demonstrate that the proposed method improves measurement accuracy by at least 38% compared to traditional approaches.

High performance sensor based on phase difference induced quasi-BIC and Fermi energy.

We propose a dielectric corrugated structure surrounded by two monolayer graphene and find that the structure supports bound states in the continuum (BIC). By introducing a phase difference between the upper and lower surface of dielectric grating, the symmetry of the structure is broken, and the BIC turns into quasi-BIC. In addition, we find that the Fermi energy of graphene strongly affect the spectral line. By controlling phase difference and Fermi energy of graphene, the ultrahigh Q-factor can be achieved. Finally, introducing a sensing medium at the incident side, the high performance sensor is realized.

Flexible Thermoelectric Films Based on Bi2Te3 Nanowires and Boron Nitride Nanotube Networks with Carbon Doping.

Energy recovery and reuse, industrial waste heat, and thermal energy recovery and conversion in emerging electronic devices are topics of widespread interest. Flexible composite thermoelectric (TE) films have become the key to TE conversion, and many studies and synthesis methods related to them have made great progress. However, little research has been performed on the corresponding composites of typical TE materials with low-dimensional nanotubular materials, particularly modulation of the overall TE properties using doped low-dimensional nanotubular materials. In this work, high-quality bismuth telluride (Bi2Te3) nanowires and boron nitride nanotubes (BNNTs) were prepared using electrolytic deposition and high-temperature catalytic deposition, respectively. Bi2Te3-BNNTs composite films were prepared using a solvent hot pressing method. The Bi2Te3-BNNTs composite film conductivity reached 179.6 S/cm at room temperature (300 K), the corresponding Seebeck coefficient was 171.4 µV/K, and the power factor (PF) was 52.8 nW/mK2. Carbon doping of BNNTs resulted in carbon-boron nitride nanotubes (BCNNTs), and Bi2Te3-BNNTs composite films were prepared. The Bi2Te3-BCNNTs composite films obtained a conductivity of 4629.6 S/cm, at room temperature (300 K), a corresponding Seebeck coefficient of 181.2 µV/K, and a PF of 1520.0 nW/mK2. This study has important reference value for the application of TE conversion. Moreover, the electrical conductivity decreased by no more than 10% after 400 cycles of bending tests, and the electrical conductivity showed signs of recovery after repressing thermally, which undoubtedly proves that Bi2Te3-BCNNTs composite films have good flexibility and thermal stability, and this has contributed to the application and promotion of flexible thermoelectric materials.

Polarized angle-resolved spectral reflectometry for real-time ultra-thin film measurement.

We propose a polarized, angle-resolved spectral (PARS) reflectometry for simultaneous thickness and refractive-index measurement of ultra-thin films in real time. This technology acquires a two-dimensional, angle-resolved spectrum through a dual-angle analyzer in a single shot by radially filtering the back-focal-plane image of a high-NA objective for dispersion analysis. Thus, film parameters, including thickness and refractive indices, are precisely fitted from the hyper-spectrum in angular and wavelength domains. Through a high-accuracy spectral calibration, a primary PARS system was built. Its accuracy was carefully verified by testing a set of SiO2 thin films of thicknesses within two µm grown on monocrystalline-Si substrates against a commercial spectroscopic ellipsometer. Results show that the single-shot PARS reflectometry results in a root-mean-square absolute accuracy error of ∼1 nm in film thickness measurement without knowing its refractive indices.

Research on Aerial Autonomous Docking and Landing Technology of Dual Multi-Rotor UAV.

This paper studies the cooperative control of multiple unmanned aerial vehicles (UAVs) with sensors and autonomous flight capabilities. In this paper, an architecture is proposed that takes a small quadrotor as a mission UAV and a large six-rotor as a platform UAV to provide an aerial take-off and landing platform and transport carrier for the mission UAV. The design of a tracking controller for an autonomous docking and landing trajectory system is the focus of this research. To examine the system's overall design, a dual-machine trajectory-tracking control simulation platform is created via MATLAB/Simulink. Then, an autonomous docking and landing trajectory-tracking controller based on radial basis function proportional-integral-derivative control is designed, which fulfills the trajectory-tracking control requirements of the autonomous docking and landing process by efficiently suppressing the external airflow disturbance according to the simulation results. A YOLOv3-based vision pilot system is designed to calibrate the rate of the aerial docking and landing position to eight frames per second. The feasibility of the multi-rotor aerial autonomous docking and landing technology is verified using prototype flight tests during the day and at night. It lays a technical foundation for UAV transportation, autonomous take-off, landing in the air, and collaborative networking. In addition, compared with the existing technologies, our research completes the closed loop of the technical process through modeling, algorithm design and testing, virtual simulation verification, prototype manufacturing, and flight test, which have better realizability.

High-accuracy camera calibration method based on coded concentric ring center extraction.

In the field of three-dimensional (3-D) metrology based on fringe projection profilometry (FPP), accurate camera calibration is an essential task and a primary requirement. In order to improve the accuracy of camera calibration, the calibration board or calibration target needs to be manufactured with high accuracy, and the marker points in calibration image require to be positioned with high accuracy. This paper presents an improved camera calibration method by simultaneously optimizing the camera parameters and target geometry. Specifically, a set of regularly distributed target markers with rich coded concentric ring pattern is first displayed on a liquid crystal display (LCD) screen. Then, the sub-pixel edges of all coded bands radial straight lines are automatically located at several positions of the LCD screen. Finally, the sub-pixel edge point set is mapped into parameter space to form a line set, and the intersection of the lines is defined as the center pixel coordinates of each target point to complete the camera calibration. The simulation and experimental results verify that the proposed camera calibration method is feasible and easy to operate, which can essentially eliminate the perspective transformation error to improve the accuracy of camera parameters and target geometry.

B-spline surface based 3D reconstruction method for deflectometry.

In the field of optical three-dimension (3-D) measurement, reconstruction usually is completed by the integration of a two-dimensional (2-D) gradient data set. Position and posture of camera and shape of the surface under test determine the location of gradient data which usually is on quadrilateral grids. This paper proposes a B-spline surface-based 3D reconstruction method for deflectometry, which reconstructs the surface under test with its 2-D gradient data set. The 2-D gradient data set consists of gradient data and the 2-D location of the gradient data in the camera coordinate system. The 2-D gradient data set is first transferred to the cameras' virtual image plane, so it locates on rectangular grids. Then, based on the properties of the B-spline basis function and characteristics of the camera, linear equations are derived to solve control points along the virtual image plane. The solved control points reconstruct the surface under test in the camera coordinate system. The property of the B-spline basis function determines the relationship between the depth of the surface and its derivative. The characteristic of the camera determines the relationship between the depth of the surface and the 2-D location of the gradient data. Meanwhile, the accuracy of the 2-D location can also be improved by the linear equations. Finally, simulated and actual experiments show that the proposed method is accurate and efficient at reconstructing surfaces in deflectometry.

Depth multiplexing in an orbital angular momentum holography based on random phase encoding.

The orbital angular momentum (OAM) holography has been identified as a vital approach for achieving ultrahigh-capacity in 3D displays, digital holographic microscopy, data storage and so on. However, depth has not been widely applied as a multiplexing dimension in the OAM holography mainly because of the serious coherence crosstalk between different image layers. The multi-layered depth multiplexing OAM holography is proposed and investigated. To suppress the coherence crosstalk between different image channels, random phases are used for encoding different image layers separately. An image can be reconstructed with high quality at a specific depth from an appropriate OAM mode. It is demonstrated that the depth multiplexing of up to 5 layers can be achieved. This work can increase the information capacity and enhance the application of the OAM holography.

Near optical coaxial phase measuring deflectometry for measuring structured specular surfaces.

Phase measuring deflectometry (PMD) is an important technique for the form measurement of specular surfaces. However, the existing stereo-PMD techniques have noticeable weaknesses for structured specular surfaces measurement due to the optical axis of the imaging system must have a notable intersection angle with the optical axis of the display system according to the law of reflection. This leads to the imaging sensor and the fringe display screen must be located on the opposite sides of the normal of the surface under test (SUT), which results in large system volume and measurement shadows when measuring discontinuous specular surfaces. In this paper, we propose a novel near optical coaxial PMD (NCPMD) by utilizing a plate beamsplitter. With the assistance of plate beamsplitter, the optical axis of display screen can be configured much closer to the optical axis of the imaging system which makes the system more compact and has significantly reduced volume compared with the conventional PMD configuration. Moreover, imaging sensors in the proposed configuration can perpendicularly capture the SUT, which can drastically decrease measurement shadows caused by discontinuous structures on the SUT and increases measurement efficiency. A comparison between the proposed NCPMD and the conventional PDM is studied by measuring a specular step to show the advantage of the proposed configuration in reducing measurement error caused by structure shadows. A portable NCPMD prototype with stereo imaging sensors is developed and verified through experiments. Experimental results show the portable prototype has comparable measurement accuracy with the existing PMD techniques while has obviously advanced performances for portable and embedded form measurement, such as small system volume, and light weight.

Boron Nitride Nanoribbons Grown by Chemical Vapor Deposition for VUV Applications.

The fabrication process of vacuum ultraviolet (VUV) detectors based on traditional semiconductor materials is complex and costly. The new generation of wide-bandgap semiconductor materials greatly reduce the fabrication cost of the entire VUV detector. We use the chemical vapor deposition (CVD) method to grow boron nitride nanoribbons (BNNRs) for VUV detectors. Morphological and compositional characterization of the BNNRs was tested. VUV detector based on BNNRs exhibits strong response to VUV light with wavelengths as short as 185 nm. The photo-dark current ratio (PDCR) of this detector is 272.43, the responsivity is 0.47 nA/W, and the rise time and fall time are 0.3 s and 0.6 s. The response speed is faster than the same type of BN-based VUV detectors. This paper offers more opportunities for high-performance and low-cost VUV detectors made of wide-bandgap semiconductor materials in the future.

Angular multiplexation of partial helical phase modes in orbital angular momentum holography.

The orbital angular momentum (OAM) holography has been identified as a vital approach for achieving ultrahigh-capacity multiplexation without a theoretical helical phase index limit. However, the encoding and decoding of an OAM hologram require a complete helical phase mode, which does not take full utilization of the angular space. In this paper, the partial OAM holography is proposed by dividing an OAM mode into several partial orbital angular momentums and encode each partial mode with a different target image. An image can only be reconstructed using an appropriate partial OAM mode within a specific illuminating angular range, henceforth holographic multiplexation of images can be realized. This method can significantly increase the holographic information capacity and find widespread applications.

Asymmetry robust centroid localization in confocal microscopy.

We present a centroid algorithm with asymmetry-robust error compensation for the peak position localization of asymmetrical axial response signals in confocal microscopy. Compared with the state-of-the-art algorithms, which are usually developed for symmetrical signals, our asymmetry robust centroid algorithm is found to have much smaller localization bias and higher precision for an asymmetrical confocal signal in numerical simulations and experiments.

Locally adaptive thresholding centroid localization in confocal microscopy.

We introduce an iteration-free approach, based on a centroid algorithm with a locally adaptive threshold, for nanometer-level peak position localization of the axial response signal in confocal microscopy. This approach has localization accuracies that are near theoretical limits, especially when there is a small number of sampling points within the discrete signal. The algorithm is also orders of magnitude faster compared to fitting schemes based on maximum likelihood estimation. Simulations and experiments demonstrate the localization performance of the approach.

Two-dimensional spectral signal model for chromatic confocal microscopy.

In chromatic confocal microscopy, the signal characteristics influence the accuracy of the signal processing, which in turn determines measurement performance. Thus, a full understanding of the spectral characteristics is critical to enhance the measurement performance. Existing spectral models only describe the signal intensity-wavelength characteristics, without taking the displacement-wavelength relation into consideration. These models require prior knowledge of the optical design, which reduces the effectiveness in the optical design process. In this paper, we develop a two-dimensional spectral signal model to describe the signal intensity-wavelength-displacement characteristics in chromatic confocal microscopy without prior knowledge of the optical design layout. With this model, the influence of the dimensional characteristics of the confocal setup and the displacement-wavelength characteristics and monochromatic aberrations of the hyperchromatic objective are investigated. Experimental results are presented to illustrate the effectiveness of our signal model. Using our model, further evaluation of the spectral signal can be used to enhance the measurement performance of chromatic confocal microscopy.

Advances in optical metrology and instrumentation: introduction.

Optical measurement and characterization are two of the pillars of metrology. The ability to measure precisely with high dynamic range and accuracy betters our understanding of nature and the universe. In this feature issue, we present a collection of articles that delves into the fundamental techniques used to advance the field.

Analysis of the synchronous phase-shifting method in a white-light spectral interferometer.

For white-light spectral interferometry, the phase information is usually retrieved via the Fourier transform method and the temporal phase-shifting method. In comparison, the synchronous phase-shifting method can be used to synchronously acquire interferometric signals with good accuracy and reduced noise. Therefore, it has potential for online measurement and is suitable for application in precision industries and for ultrahigh-speed measurement. In this work, a white-light spectral interferometer for synchronous phase shifting based on polarization interference was built, and the two-step phase-shifting algorithm was used to retrieve phase information. A variety of spectral interferometric signals were simulated based on the mathematical model of the two-step phase-shifting algorithm to illustrate the effects of differences in intensity and envelope shape, random noise, and phase-shift error on measurement of the absolute distance. Measurements of the absolute distance were conducted, and they indicated that the system had high accuracy.