Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics. Previous metasurfaces have been limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. Here we report a polarization-insensitive, high-contrast transmissive metasurface composed of crystalline titanium dioxide pillars in the form of metalens at the wavelength of 633 nm. The focal spots are as small as 0.54 ${\lambda }_d$ , which is very close to the optical diffraction limit of 0.5 ${\lambda }_d$ . The simulation focusing efficiency is up to 88.5%. A rigorous method for metalens design, the phase realization mechanism and the trade-off between high efficiency and small spot size (or large numerical aperture) are discussed. Besides, the metalenses can work well with an imaging point source up to ±15° off axis. The proposed design is relatively systematic and can be applied to various applications such as visible imaging, ranging and sensing systems.

Optical metasurfaces exhibit unprecedented ability in light field control due to their ability to locally change the phase, amplitude, and polarization of transmitted or reflected light. We propose a multifunctional metalens with dual working modes based on bilayer geometric phase elements consisting of low-loss phase change materials (Sb₂Se₃) and amorphous silicon (a-Si). In transmission mode, by changing the crystalline state of the Sb₂Se₃ scatterer, a bifocal metalens with an arbitrary intensity ratio at the telecommunication C-band is realized, and the total focusing efficiency of the bifocal metalens is as high as 78%. Also, at the resonance wavelength of the amorphous Sb₂Se₃ scatterer, the scatterer can be regarded as a half-wave plate in reflection mode. The multifunctional metalens can reversely converge incident light into a focal point with a focusing efficiency of up to 30%. The high focusing efficiency, dynamic reconfigurability, and dual working modes of the multifunctional metalens contribute to polarization state detection, optical imaging, and optical data storage. In addition, the bilayer geometric phase elements can be easily extended to multilayer, which significantly improves the capability of manipulating the incident light field.

Vanadium dioxide (VO₂) is a temperature phase change material that has metallic properties at high temperatures and insulation properties at room temperature. In this article, a novel device has been designed based on the dielectric metasurface consisting of VO₂ and graphene array, which can achieve multiple functions by adjusting temperature and voltage. When the temperature is high (340 K), the device is in the absorption state and its absorptivity can be dynamically controlled by changing the temperature. On the other hand, the device is in the polarization state under room temperature, and the polarization of electromagnetic waves can be dynamically controlled by adjusting the voltage of graphene. This device can achieve a broadband absorber (the maximum absorptance reaches 99.415% at wavelengths ranging from 44 THz to 52 THz) and high polarization conversion efficiency (>99.89%) in the mid-infrared range, which has great advantages over other single-function devices. Our results demonstrate that this multifunctional device may have widespread applications in emitters, sensors, spatial light modulators, IR camouflages, and can be used in thermophotovoltaics and wireless communication.

In this Letter, a new, to the best of our knowledge, type of autofocusing and symmetric beam arisen from two quartic spectral phases is introduced in theory and experiment. The symmetric Pearcey Gaussian beam (SPGB), formed with a Gaussian term and two multiplying Pearcey integrals, processes a focusing intensity approximately 1.32 times stronger than the intensity of the symmetric Airy beam. Its four off-axis main lobes split into four bending trajectories symmetrically after focusing. The rectangular intensity distribution and the focal length of the SPGB can be adjusted by two kinds of distribution factors. Additionally, the vortex-guiding property of the beam is demonstrated by embedding an off-axis vortex into the SPGB, which can be applied in particle guiding.

Focus-tunable metalenses play an indispensable role in the development of integrated optical systems. In this paper, the phase change material Sb₂S₃ is used in a thermally modulated varifocal metalens based on PB-phase for the first time. Sb₂S₃ not only has a real part of refractive index shift between the amorphous and crystalline state but also has low losses in both amorphous and crystalline states in the near-infrared region. By switching Sb₂S₃ between the two states, a metalens doublet with a variable focal length is proposed. Moreover, the full width at half maximum of each focal point is close to the diffraction limit. And the focusing efficiency can be over 50% for the two focal points. Together with the advantage of precise thermal control, the proposed metalens has great potential in the application of multi-functional devices, biomedical science, communication and imaging.

It is difficult to detect defects such as shallow dents and rust on rolling surfaces by using the traditional vision-based surface inspection method (line light source and line array camera mode) which has a low sensitivity to these defects. This paper presents a method that introduces the fringe projection technique for traditional visual inspection devices to overcome the limitations of the traditional methods and uses deep-learning techniques for detecting defects such as cuts, abrasions, dents, and rust on the rolling surfaces of drum-shaped rollers. A new artificial-intelligence-based labeling method, namely, the Padua Incremental Mask Labeling Method, has been introduced for accelerating the calibration process used for defect detection, and based on a one-stage architecture, the You-Only-Look-Once-OurNet (YOLO-OurNet) deep-learning network has been designed for detecting the defects on the rolling surfaces of drum-shaped rollers. From the results of the experimental tests, the time required for detecting a defect has been found to be 0.024s, an accuracy rate of up to 99.2%, and the value of object detection evaluation index F1 of up to 0.988. Our method outperforms the related method on the domain of rolling surface defect detection.