Xiaoyu Weng

Near-field focusing of the dielectric microsphere with wavelength scale radius

We focus on physically analyzing the origins of the numerical aperture (NA) and the spherical aberration of the microsphere with wavelength scale radius. We demonstrate that the microsphere naturally has negligible spherical aberration and high NA when the refractive index contrast (RIC) between the microsphere and its surrounding medium is about from 1.5 to 1.75. The reason is due to the spherical aberration compensation arising from the positive spherical aberration caused by the surface shape of the microsphere and the RIC and the negative spherical aberration caused by the focal shifts due to the wavelength scale dimension of the microsphere. We show that, only within the approximate region of 1.5 ≤ RIC ≤ 1.75 with the proper radius r of microsphere, the microsphere can generate a near-field focal spot with lateral resolution slightly beyond λ/2ns, which is also the lateral resolution limit of the dielectric microsphere. The r for each RIC can be obtained by optimizing r from 1.125λ/n o to 1.275λ/n o. Here λ, n s, and n o are the wavelength in vacuum and the refractive indices of microsphere and its surrounding medium, respectively. For the case of the near-field focusing, we also develop a simple transform formula used to calculate the new radius from the known radius of microsphere corresponding to the original illumination wavelength when the illumination wavelength is changed.

Tight focusing of a higher-order radially polarized beam transmitting through multi-zone binary phase pupil filters

When the pupil filters are used to improve the performance of the imaging system, the conversion efficiency is a critical characteristic for real applications. Here, in order to take full advantage of the subwavelength focusing property of the radially polarized higher-order Laguerre-Gaussian (LG) beam, we introduce the multi-zone binary phase pupil filters into the imaging system to deal with the problem that the focal spot is split along the z axis for the small size parameter of the incident LG beam. We provide an easy-to-perform procedure for the design of multi-zone binary phase pupil filters, where the zone numbers of π phase are uncertain when the optimizing procedure starts. Based on this optimizing procedure, we successfully find the set of optimum structures of a seventeen-belt binary phase pupil filters and generate the excellent focal spot, where the depth of focus, the focal spot transverse size, the Strehl ratio, and the sidelobe intensity are 9.53λ, 0.41λ, 41.75% and 16.35% in vacuum, respectively. Most importantly, even allowing the power loss of the incident LG beam truncated by the pupil of the imaging system, the conversion efficiency is still as high as 37.3%. Theoretical calculations show that we succeed to have sufficient conversion efficiency while utilizing the pupil filters to decrease the focal spot and extend the depth of focus.

Multifocus with small size, uniform intensity, and nearly circular symmetry.

We investigate in detail the focusing properties of the composite vector beam (CVB) composed of two orthogonally linearly polarized beams with inhomogeneous polarization modulation. By optimizing the modulation factor, a multifocus with excellent quality is obtained, where the sizes of each focus are fairly smaller than that of the focusing spot of a radially polarized beam, the uniformity in the intensity of the focal spots is as high as 1, and the distributions of each focal spot have nearly circular symmetry. In order to decrease the power loss of the incident beam, the CVB formed by an annular beam is demonstrated as the substitute for the optimized CVB formed by a Gaussian beam. This work is important for high-resolution and high-speed imaging in biology and micro-nanofabrication.

Design of cylindrical vector beams based on the rotating Glan polarizing prism

Recently the cylindrically polarized beams have been gained highly attention in the fields of particle manipulation, material processing, nanoscale imaging, etc. So the methods to create the cylindrically polarized beams become more important. Here, based on the principle of the Glan polarizing prism, we design two types of the structures of the cylindrical polarization analyzer that can convert directly a linearly or circularly polarized beam into various cylindrical vector beams. The key optical element in the cylindrical polarization analyzer is the cylindrical polarizing prism with unique structure. We demonstrate the operating principle and the feasibility of the fabrication of the cylindrical polarization analyzer in detail. Analyses show that the cylindrical polarization analyzer designed by us not only have novel structures and excellent characteristics, such as the compact and stabile structures, high extinction ratio, high polarization purity, no requirements on the mode and the wavelength of the incident light (only for the first type), not changing the intensity distribution of the incident light, and easily integrated into the optical systems, but also is easy to be fabricated, especially for the second type.

Control of the multifocal properties of composite vector beams in tightly focusing systems

The numbers of the focal spots and the dominant field (i.e., whether the axial field or the transverse fields play dominant role in the focusing field) have significant effects on various applications. In this paper, we have derived the universal imaging model of the composite vector beam (CVB) composed of two orthogonally linearly polarized beams with inhomogeneous polarization modulation, which is also suitable for various polarized beams, such as linearly, circularly, radially, azimuthally, and vortex polarized beams. Moreover, the sin&cos amplitude modulation with arbitrary orders and the pupil filters with cylindrical symmetry are also involved in this imaging model. On the basis of this imaging model, the regulars to control the focal numbers and the dominant field are drawn. For the various applications, some important conclusions and constructive advices are given.

Creation of tunable multiple 3D dark spots with cylindrical vector beam

We present a method to generate multiple three-dimensional (3D) dark focal spots along the optical axis by focusing a cylindrical vector beam. The formation of uniform 3D dark spots is determined by optimizing three parameters of the pupil filter in the wavefront of the lens and the polarized state of the incident cylindrical vector beam. By adjusting the three parameters of the pupil filter, the number and position of dark spots can be controlled willingly, and the uniformity of dark spots can be adjusted by modulating the polarized state of the incident cylindrical vector beam. A single dark spot with uniform surrounding intensity is formed and double dark spots are also obtained with the proper parameters and can be separated with an equal distance away from the geometric focus. Moreover, multiple dark spots numbering up to six are formed along the optical axis. The size of each 3D dark spot is almost the same, and the central light intensity of the dark spots is nearly zero while the light intensity surrounding around is almost uniform. This work may find valuable application in particle trapping, microscopes, optical engineering, and so on.