Microsphere-assisted super-resolution imaging is a promising technique that can significantly enhance the resolution of conventional optical microscopes. The focus of a classical microsphere is called photonic nanojet, which is a symmetric high-intensity electromagnetic field. Recently, patchy microspheres have been reported to have superior imaging performance than pristine microspheres, and coating microspheres with metal films leads to the formation of photonic hooks, which can enhance the imaging contrast of microspheres. Understanding the influence of metal patches on the near-field focusing of patchy particles is important for the rational design of a nanostructured microlens. In this work, we theoretically and experimentally showed that the light waves can be focused and engineered using patchy particles. When coating dielectric particles with Ag films, light beams with a hook-like structure or S-shaped structure can be generated. Simulation results show that the waveguide ability of metal films and the geometric asymmetry of patchy particles cause the formation of S-shaped light beams. Compared with classical photonic hooks, S-shaped photonic hooks have a longer effective length and a smaller beam waist at far-field region. Experiments were also carried out to demonstrate the generation of classical and S-shaped photonic hooks from patchy microspheres.

Microsphere-assisted imaging is a label-free super-resolution optical microscopic technique. In 2021, Shang et al. found that by coating microspheres with Ag films, the super-resolution imaging performance of microspheres can be significantly enhanced. Here we reported the progress of using patchy particles for super-resolution imaging, and we showed that the performance of the imaging system can also be enhanced by coating microspheres with Al films. Using Al instead of Ag can significantly reduce the cost of fabrication, and facilitates the commercialization of this technique.

Photonic hooks (PHs) are non-evanescent light beams with a highly concentrated curved optical fields. Since their discovery, PHs always have one single inflection point and thus have a hook-like structure. In this work, a new type of PHs with two inflection points and S-shaped structures were reported for the first time. We theoretically studied the effects of various physical parameters on the generation of S-shaped photonic hook (S-PH). The S-PH may have potential applications in super-resolution imaging, subwavelength micromachining, particle and cell manipulation, etc.