Tunable polarization-insensitive multifocal metalens based on an inverse design framework.

Multifocal metalenses are effective elements for longitudinal light field modulation and have important applications in long-focal depth imaging and three-dimensional display. However, the forward design method is subject to destructive interference generated by phase discontinuity, and cannot achieve high-efficiency, tunable multifocal metalenses. Therefore, we propose an efficient and tunable inverse design framework based on the adjoint method and gradient strategy, transforming light field modulation into mathematical optimization of nonlinear constraints. As proof, a trifocal metalens based on the inverse design framework is proposed with a focusing efficiency of 41%, and the focal length deviation is less than 1 µm. Then, trifocal metalenses operating in the visible range with focusing efficiencies of more than 30% are designed to demonstrate the multi-wavelength optimization capability of the framework. Besides, we verified the tunable ability of the inverse design framework and achieved trifocal metalenses with a relative light intensity tuning range of 0.3-1 and a focal length interval tuning range of 20-60 µm, respectively. The inverse design framework avoids complex physical reasoning and prior knowledge in the design process and promotes the development of multifunctional photonic devices.