Abstract

We propose a structure of a far-field nanofocusing metalens with focal shifting that is actively tuned at visible wavelengths. Surface plasmon polaritons (SPPs) can be excited by the metal-insulator-metal (MIM) subwavelength structure at visible wavelengths. The coherent interference of SPPs emitted by subwavelength nanostructures can form a nanoscale focus. When the SPPs are excited and pass through several concentric ring gratings with specific aspect ratios, the extraordinary optical transmission phenomenon occurs. Two metal concentric ring gratings achieve double diffraction, scattering light to the far field. An anisotropic or isotropic electrically adjustable refractive index material, such as liquid-crystal or optical phase change material, is filled in a dielectric layer between two metal layers, and the effective refractive index is modulated by electronically controlled active tuning. The focal shift is achieved by changing the effective refractive index of the intermediate dielectric. In addition, different incident wavelengths correspond to different effective refractive indices to achieve time-division-multiplexing multi-wavelength achromatic focusing. The finite-difference time-domain method was used to simulate the effect of substrate effective refractive index variation on achromatic superfocusing. The results show that the super-resolution focal spot (${\rm{FWHM}} = {0.158}{\lambda _0}$) with long focal length (${\rm{FL}} = {5.177}{\lambda _0}$) and large depth of field (${\rm{DOF}} = {3.412}{\lambda _0}$) can be achieved by optimizing the design parameters. The visible plasma metalens has potential applications in high-density optical storage and optical microscopic imaging, especially in three-dimensional display for light field and integral imaging.

© 2022 Optica Publishing Group

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