Inverse-designed non-volatile phase change varifocal metalens at the edge of the visible spectrum

Abstract

Reconfigurable metalenses capable of large focal length tuning, fast response times, and high focusing efficiency while maintaining diffraction-limited operation are highly desirable for next-generation adaptive imaging systems. Phase change chalcogenides provide a promising platform for such devices by exploiting the reversible amorphous-to-crystalline transition to achieve non-volatile tuning with relatively fast switching. However, extending these approaches towards the visible spectrum is challenging because of the reduced meta-atom dimensions, stringent phase coverage requirements, intrinsic material absorption and the need to simultaneously preserve focusing efficiency and image quality across multiple material states. Here, we present a dynamically tunable metalens based on $Sb_2$$S_3$ operating at the edge of the visible spectrum. The design framework combines finite element computations with a genetic algorithm-based inverse design approach to achieve robust phase control in both amorphous and crystalline states. The resulting metalens shows near-diffraction-limited performance with a focal length tunability of $33\%$, Strehl ratios of $0.80$ and $0.77$ in the amorphous and crystalline states, respectively, and focusing efficiencies of approximately $40\%$ which are among the highest reported for metalenses designed entirely within an active phase-change chalcogenide medium.