Polarization characteristics and transverse spin of Mie scattering.

Complicated polarization states in the near field of Mie scattering have aroused wide interest due to their broad potential applications. In this work, we investigated polarization properties, including polarization dimension, degree of nonregularity, and transverse electric-field spin, of scattering of a partially polarized plane wave by a dielectric nanosphere based on the rigorous Mie scattering theory. It is shown that with the decrease of the correlation coefficient, the polarization dimension and degree of nonregularity generally increase. In the limit of unpolarized incident light, a nearly-perfect nonregular polarization state (PN = 0.928) appears in the near field and the spin is transverse to the radial direction everywhere. The rich structure contained by the partially polarized scattered light offers an approach to manipulating the interaction between light and nanoparticles, which may lead to novel designs of nanoantenna, optical trap and sensing.

Thin film characterization by learning-assisted multi-angle polarized microscopy.

Thin film characterization is a necessary step in the semiconductor industry and nanodevice fabrication. In this work, we report a learning-assisted method to conduct the measurement based on a multi-angle polarized microscopy. By illuminating the film with a tightly focused vectorial beam with space-polarization nonseparability, the angle-dependent reflection coefficients are encoded into the reflected intensity distribution. The measurement is then transformed into an optimization problem aiming at minimizing the discrepancy between measured and simulated image features. The proposed approach is validated by numerical simulation and experimental measurements. As the method can be easily implemented with a conventional microscope, it provides a low cost solution to measure film parameters with a high spatial resolution and time efficiency.

Lateral shifts of linearly- and radially-polarized Bessel beams scattered by a nanosphere.

We report the investigation on the lateral shifts that linearly-polarized (LP) and radially-polarized (RP) Bessel beams experience during the Mie scattering by a nanosphere. A numerical procedure based on the angular spectrum theory is developed to solve the scattered electromagnetic field and subsequent lateral shifts with a high computational efficiency, which can be easily applied to an arbitrary shaped polarized beam. The influences of different factors, including conical angle, nanosphere radius and position, on the lateral shifts are systematically investigated. The results demonstrate that for on-axis scattering, a LP Bessel beam can be regarded as a plane wave with the same polarization state but an equivalent longer wavelength, while a RP Bessel beam can be regarded as a plane wave with a polarization state along the propagation direction exhibiting independence on the conical angle. The findings help deepen our understandings of lateral shifts in light scattering of vectorial non-diffractive beams.

Machine-learning-based computationally efficient particle size distribution retrieval from bulk optical properties.

Retrieval of particle size distribution from bulk optical properties based on evolutionary algorithms is usually computationally expensive. In this paper, we report an efficient numerical approach to solving the inverse scattering problem by accelerating the calculation of bulk optical properties based on machine learning. With the assumption of spherical particles, the forward scattering by particles is first solved by Mie scattering theory and then approximated by machine learning. The particle swarm optimization algorithm is finally employed to optimize the particle size distribution parameters by minimizing the deviation between the target and simulated bulk optical properties. The accuracies of machine learning and particle swarm optimization are separately investigated. Meanwhile, both monomodal and bimodal size distributions are tested, considering the influences of random noise. Results show that machine learning is capable of accurately predicting the scattering efficiency for a specific size distribution in approximately 0.5 µs on a standalone computer. Therefore, the proposed method has the potential to serve as a powerful tool in real-time particle size measurement due to its advantages of simplicity and high efficiency.

Geometrical optics approximation for plane-wave scattering by a rectangular groove on a surface.

Rigorous solution of plane-wave scattering by a groove based on electromagnetic theory will be time-consuming if the groove width is much larger than the illumination wavelength. To accelerate the computation, an approach based on geometrical optics approximation is developed here. The incident beam is split into several parts during reflection and refraction. Contribution of every part is superposed to obtain the electric field at the interface between the groove and air, with which diffraction theory is utilized to calculate the far-field scattered light. Results demonstrate that the approach is capable of accurately calculating plane-wave scattering by rectangular grooves with large widths in a time-efficient manner, which can be beneficial for further inverse scattering problems.

Scattering of an Airy light sheet by a chiral sphere.

Scattering of a 1D Airy beam light sheet by a chiral sphere is numerically studied within Mie theory and the plane-wave spectrum method. To testify to the validity of our code and method, the results of scattering intensity of a chiral sphere by an Airy beam light sheet reducing to a homogeneous isotropic sphere are compared with those in existing literature, which shows that these results are in good agreement. Influences of different parameters on differential scattering cross sections in the far field are investigated in detail, including the chiral parameters, sphere radius, and beam position. It is found in the scattering intensity of an Airy beam by a chiral sphere that the chiral sphere, which is compared with a homogeneous isotropic sphere, can decrease the scattering intensity in a region, and the scattering intensity distribution is sensitive to the x0 position of the Airy beam.

Defocus-based three-dimensional particle location with extended depth of field via color coding.

A color-coded lens system is designed and built for three-dimensional (3D) particle location. As it has wavelength-dependent focal length, it can obtain a series of multifocus images in a single snapshot by separating the red, green, and blue channels of the image captured by a color camera. A light source consisting of red, green, and blue LEDs is customized to provide narrowband illumination. In this way, the depth of field can be extended. A three-pinhole aperture is introduced into the system to produce defocus-based triangular patterns that help to encode the particle depth. The accurate 3D particle position can finally be obtained by fusing the information from the three color channels. Experimental results demonstrate that the system is well suited for applications where particles may be irregular or extended depth of field is needed.

Propagation of a Bessel-Gaussian beam in a gradient-index medium.

Based on the ABCD matrix method and Collins diffraction integral formula, analytical expression for Bessel-Gaussian beam propagation in a gradient-index medium is derived. The propagation trajectory, intensity, and phase distributions of the zeroth-order, second-order, and superposition cases are numerically investigated. The effect of beam waist radius w0 on the properties of beam propagation in a gradient-index medium is discussed in detail. The result shows that the beam is focused at z/L=N/2 (N=0,1,2,…) and propagates periodically in the medium. Evolution of the vortical structure of the superposed Bessel-Gaussian beam is investigated, showing that the superposed beam forms new singularities, and the rotation of the beam occurs mainly near the singularities.

Propagation of on-axis and off-axis Bessel beams in a gradient-index medium.

Bessel beams have been increasingly used for their advantages of non-diffraction and long focal depth. In this paper, we studied the propagation of on-axis and off-axis Bessel beams in a gradient-index medium. By expressing a Bessel beam in integral form, the analytical expression of an on-axis, decentered, and tilted Bessel beam through a paraxial optical system is derived with the ABCD matrix method and Collins diffraction integral formula. Main lobe size and trajectory of the zeroth- and second-order Bessel beam are obtained, demonstrating that the Bessel beam is focused by the gradient-index medium and its main lobe trajectory is exactly the same as the corresponding geometrical ray for both the decentered and tilted Bessel beam. Effects of beam apodization are finally studied by the Fourier beam propagation method, showing that the side lobes of the Bessel beam vanish when the beam is focused inside the medium as only part of the beam enters the lens.

Vision system with high dynamic range for optical surface defect inspection.

We report a machine vision system with high dynamic range designed for optical surface defect inspection. The system consists of two motorized linear stages and one motorized rotation stage for automatically scanning the surface. As the intensity of long scratches and digs differ a lot under dark-field illumination, gains of red, green, and blue channels are set to be different values to extend the dynamic range of an ordinary colored detector in a single snapshot, which greatly improves the efficiency compared with multiexposure-based approaches. Image stitching is then employed to get a high-resolution image of the entire optical surface for further processing to quantitatively evaluate surface quality based on the standards of MIL-PRF-13830B and ISO 10110-7:2008. The system can be widely applied in the optical industry, as it provides a low-cost solution for optical surface quality checks.

Scattering of one-dimensional Airy beam light sheet with finite energy by a sphere.

A one-dimensional Airy beam light sheet has the advantage of nondiffraction and self-reconstruction after encountering particles. The angular scattering of an Airy beam by a homogeneous sphere is numerically studied in this paper using Mie theory and a plane-wave spectrum method. The effects of sphere radius, refractive index, beam width, and sphere position on the angular distribution of scattering light are simulated. Correlation coefficients are employed to investigate whether Airy beam scattering can be approximated to be plane-wave scattering for computational efficiency. The results can be applied in light scattering measurement that uses an Airy beam aiming at reducing multiple scattering.

Effects of astigmatism and coma on rotating point spread function.

Rotating point spread function (PSF) provides a novel approach to detect three-dimensional position. As the optical aberration of a system can hardly be removed completely, effects of aberration on the PSF were studied both analytically and numerically. Results show that the intensity pattern is robust to both astigmatism and coma, while astigmatism affects the rotating angle of the PSF, and coma affects the lateral position of the PSF.

Development of an artificial compound eye system for three-dimensional object detection.

A compound eye has the advantages of a large field of view, high sensitivity, and compact structure, showing that it can be applicable for 3D object detection. In this work, an artificial compound eye system is developed for 3D object detection, consisting of a layer of lenslets and a prism-like beam-steering lens. A calibration method is developed for this system, with which the correspondences between incident light rays and the relevant image points can be obtained precisely using an active calibration pattern at multiple positions. Theoretically, calibration patterns at two positions are sufficient for system calibration, although more positions will increase the accuracy of the result. 3D positions of point objects are calculated to evaluate the system, which are obtained by the intersection of multiple incident light rays in the least-squares sense. Experimental results show that the system can detect an object with angular accuracy of better than 1 mrad, demonstrating the feasibility of the proposed compound eye system. With a 2D scanning device, the system can be extended for general object detection in 3D space.

Logarithmic axicon characterized by scanning optical probe system.

A scanning optical probe system is proposed to measure a logarithmic axicon (LA) with subwavelength resolution. Multiple plane intensity profiles measured by a fiber probe are interpreted by solving an optimization problem to get the phase retardation function (PRF) of the LA. Experimental results show that this approach can accurately obtain the PRF with which the optical path difference of the generated quasi-nondiffracting beam in the propagation is calculated.

Fabrication and characterization of aspherical lens manipulated by electrostatic field.

An aspherical lens is fabricated from an ultraviolet (UV) curable polymer and is characterized by measuring its focal spot. Electrostatic force is employed to manipulate the shape of the liquid polymer lens. Experiment results show that a liquid lens in a strong electrostatic field can be distorted from initial spherical shape to parabolic to even near cone shape. With in situ measurement of the surface profile and focal spot, an aspherical liquid lens with good performance can be cured to a solid aspherical lens by UV light. A probe scanning microscope is employed to accurately measure the focal spot of the aspherical lens, and the modulation transfer function (MTF) of the aspherical lens is calculated to characterize it. A focal spot of 1.825 microm diameter, an MTF of 800 line pairs/mm cutoff spatial frequency, and a Strehl ratio of 0.742 are achieved. These demonstrate the feasibility of fabricating an aspherical lens with small aberrations by using this method.