Beams carrying spin angular momentum (SAM) and orbital angular momentum (OAM) have created many application opportunities in optical communication, micromanipulation, quantum optics, and other fields. Beam arrays carrying multiple different angular momentum (AM) have attracted widespread attention due to their application prospects in high-dimensional information storage and high-security information encryption. Traditional optical devices have complex structures and large systems and are not conducive to integration. We proposed a method of using a quarter-wave plate (QWP) metasurface to regulate photon AM to generate a vortex beam array with two kinds of circularly polarized light at the wavelength 1550 nm. Different from the traditional method of using a half-wave plate to generate a vortex beam array, the use of transmission phase and geometric phase through QWP metasurface can not only realize the flexible modulation of SAM and OAM but also play a role in the optimization of partial circularly polarized light phase by geometric phase, which facilitates the generation of high-quality vortex beam arrays. Compared with traditional QWP metasurfaces that rely solely on the transmission phase, the introduction of a geometric phase can increase the number of channels without losing the quality of vortex beams. The simulation results verify the feasibility and advantages of the theory. Finally, a manufacturing tolerance analysis was conducted on the designed metasurface, verifying the feasibility of using the designed metasurface to regulate AM. We first attempt to generate circularly polarized vortex beam arrays using QWP metasurface and provide an effective idea for the regulation of optical AM.
Metasurfaces have the characteristics of ultra-thin volume, light weight, and planar structure, which can also effectively control the polarization state of electromagnetic waves. We propose single-layer all-dielectric quarter-wave plate (QWP) and half-wave plate (HWP) metasurfaces that work in the visible light region in transmission mode. The designed QWP can convert y-linearly polarized (YLP) light into left-handed circularly polarized light, whereas the HWP can convert YLP light into x-linearly polarized light. Moreover, the designed wave plates have a certain wide band working ability. When the incident angle is <30 deg for the QWP and <28 deg for the HWP, the polarization conversion performance shows strong robustness. The effects of fabrication tolerance, the height of elliptic-shaped pillar, and lattice constant on the polarization conversion performance are analyzed. In addition, the physical mechanism of polarization conversion is revealed by simulating the distribution of magnetic field and electric field inside unit structures. The results indicate that nanostructures of different sizes will excite different magnetic dipole resonance modes so as to realize the function of different polarization state conversions. It is believed that all-dielectric wave plate metasurfaces will become a good alternative for controlling the polarization state of electromagnetic waves.
Using multi-beam interferometry, a photonic structure with a graded intensity distribution is designed and obtained experimentally. Based on the structure, we choose different exposure thresholds and background materials to design a graded photonic crystal (GPC) lens with a graded air hole diameter. The transmission and focusing characteristics of the GPC lens with bismuth silicate crystal (BSO) and polymethyl methacrylate (PMMA) as background materials are investigated in the visible-light band. Comparatively speaking, the graded photonic crystal lens of BSO with a large refractive index had better performance. In addition, the focal intensity does not decrease significantly with increasing the angle of incidence. The focus point intensity remains above 93% of parallel incidence for both lenses.
A dielectric elliptic cylinder is chosen as a nanostructure unit to design unusual deflecting metasurface and multifunctional metalens. Based on Pancharatnam–Berry (PB) phase principle, the abrupt phase change is investigated by adjusting the rotation angle of the nanostructure unit. The relationship between abrupt phase change and rotation angle is optimized. Using this relationship, we have designed different metasurface elements working in the visible-light range. The designed metasurfaces can arbitrarily control the phase of the light wave, and also convert the incident left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light into each other. One of the designed structures is a deflecting metasurface, which can deflect incident LCP and RCP light by the same angle while the bending directions are different. By cascading multiple gradient phase metasurfaces, an innovative type of multiplex beam splitter is constructed. The other is a metalens that can converge the incident LCP light and diverge the incident RCP light. In addition, two types of bifocal metalenses are dexterously designed using alternately arranged nanostructures, which can arbitrarily control two focal lengths of each metalens. This is promising for developing nano-integrated elements and interconnection.
Two-dimensional two-period gradual Photonic Crystals (PhCs) structure arrays were designed by double-cone interference method (3+3), and the effects of different polarization combinations of inner and outer cone beams on the double-cone interference were studied. By adding a beam of light in the third dimension to the double-cone interference, umbrella-like double-cone interference and inverted umbrella-like double-cone interference are constructed, and the formed three-dimensional PhCs is observed to study the periodicity of the photonic structure. This structure is very important for optical coupling and light integration.
Compared with metal nano-antennas metalens, the dielectric metalens has better optical characteristics in the optical band, has smaller ohmic loss, and is more advantageous. In addition, the problem of dispersion of metalens in the wide-band range has been a hot issue that experts have been scrambling to study in recent years. In view of the excellent transmission characteristics of silicon in the near-infrared region, the optical properties of rectangular silicon nano-antennas in the near-infrared region are re-examined and simulated. And using simulation software to model and analyze it, a broadband achromatic dielectric metalens in the near-infrared band based on height variation to adjust the phase is designed. The paper demonstrates the broadband achromatic focusing characteristics of its working in transmission mode, its focusing efficiency can be as high as nearly 69%, and the focal length remains basically unchanged when λ is from 950 to 1100nm. It is a new sub-wavelength non-planar structure based on height variation, offering important opportunities for applications in circular polarized optics, fiber sensors, laser scanning fluorescence microscopy, optical signal transmission, and integrated on-chip devices in the forthcoming future.
A D-shaped fiber surface plasmon resonance (SPR) sensor based on a Hi-Bi photonic crystal fiber (PCF) is investigated with finite element method. Through changing the size of the air holes beside the fiber core, it is found that the Hi-Bi structure can enhance the plasmon resonance at mid-infrared wavelength. The effect of the two fiber core side holes as well as the different gold film thickness on plasmon resonance is numerically investigated. A high and sharp loss peak is achieved, which indicates that the sensor should have high accuracy. The sensitivity of this D-shaped PCF sensor is obtained to be 8920nm/RIU in the range of 1.37 to 1.39 and the power sensitivity is 154 dB/(cm·RIU) in the range of 1.33 to 1.36. Particularly, near 1.36, with the detection limit of 0.1 nm the resolving power of the sensor is lower than 10-4 RIU with a figure of merit of 28.6.
Three-dimensional measurement based on structured light has been widely used in many fields. Since center locations are used for calculating 3D coordinates, it is important for measurement accuracy. However, affected by the occlusion, shape or color of the measurement object, the angle between object and measurement system and so on, the gray distribution of stripe is degenerated from symmetric to asymmetric. Stripe center locating accuracy is decreased by asymmetric, and some measurement data with big error which decreasing the measurement accuracy seriously appears. In order to recognize those large error data, a new method is proposed by evaluating the quality of stripe gray distribution. The asymmetrical degree of stripe gray distribution is evaluated by the skewness coefficient of stripe gray distribution. The skewness coefficient is defined by the third-order central moment. Then the relationship between the skewness coefficient of stripe gray distribution and the stripe center locating error is analyzed and established by statistical methods. Based on the relationship, the threshold of skewness coefficient is set according to the requirement of measurement accuracy. The asymmetry of gray distribution is estimated by calculating the coefficients. According to the skewness of stripe gray distribution and threshold large error data with low reliability are identified. Higher measuring accuracy is achieved by rejecting the identified data. The validity and reliability of the method have been proved by experiments.
We propose a multi-fiber holographic interferometry for fabricate double-periodic Graded Photonic Crystal (GPC) structures over large areas. This experimental system consists of an ultraviolet laser, Single-Mode Optical Fiber Splitters (SMOFS), fiber holder and focus lens. This method simplifies the system configuration by leaving out the spatial light modulator. Holographic GPC structures have been designed by using multi-fiber interference in this paper. The interference pattern is controlled by the number and position of the beams. The simulation results show that the gradient trend in the interference graph can be accurately adjusted through the above method. Experiment results were obtained and recorded for multi-fiber interfering system. The simulated interference patterns are verified experimentally through a microscopic camera. A good consistence has been observed between the theoretical and experimental results. This proposed method is possible to develop a new technique for fabrication of GPC lens or lens array.
This work proposes to measure the topography of microstructure surfaces using a self-mixing interference (SMI) configuration. The theoretical measurement model is built using beam-expanded plane wave method and considering SMI effect. The interference patterns for different objects are obtained based on the presented model. In addition, an algorithm for reconstructing the three-dimensional surface is implemented and applied onto an object with spherical surface. The presented work shows the potential application for topography measurement using a compact SMI configuration.
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