Geometric-phase-based axicon lens for computational achromatic imaging.

Conventional optical imaging systems usually utilize several lenses within a precise assembly to eliminate chromatic aberration, which increases the difficulty of system integration. In recent years, with the rapid development of metasurfaces and liquid crystals (LCs), planar optical elements provide feasible solutions to realize flexible light manipulation and lightweight systems. However, there also exists chromatic aberration, which can be corrected but at the cost of a complex device design. Here, a geometric-phase-based axicon lens is utilized to correct chromatic aberration across a broadband wavelength with the assistance of a post-process algorithm. The axicon lens is fabricated through arranging orientations of liquid-crystal molecules with a standard photoalignment technique, and it produces an approximately invariant point spread function (PSF) at several discrete wavelengths, which is used as the prior information to extract the object in the blurred image. In the experiment, the reconstruction quality is significantly improved after the post-process algorithm. We expect our work to provide further development to reduce the dispersion with both the device design and the computational image technique.

Reflective Quasi-Continuous Metasurface with Continuous Phase Control for Light Focusing.

Benefitting from the arbitrary and flexible light modulation, metasurface has attracted extensive attention and been demonstrated in different applications. However, most reported metasurface-based devices were normally composed of discrete micro or nano structures, therefore the discretization precision limited the performance of the device, including the focusing efficiency, stray light suppression, and broadband performance. In this work, an all-metallic reflective metasurface consisting of numerous quasi-continuous nanostructures is proposed to realize high-efficiency and broadband focusing. The constructed quasi-continuous metasurface (QCMS) is then verified numerically through electromagnetic simulation, and the numerical results show a higher focusing efficiency and a better stray light suppression effect, compared to a binary-phase-based metalens. Through the same design strategy, a QCMS with the ability to overcome the diffraction limit can also be constructed, and a focal spot with the size of 0.8 times the diffraction limit can be achieved. We expect that this quasi-continuous structure could be utilized to construct other high-performance devices that require continuous phase controls, such as the beam deflector, orbital angle momentum generator, and self-accelerating beam generator.