All-Optical Varifocal Switching in a Polarization-Insensitive Si--GST Metalens

Abstract: Metasurfaces have become a cornerstone of flat-optics, enabling precise control over light propagation through nanoengineered materials. Dynamic and reconfigurable metalenses are key to next-generation flat-optics platforms, yet their practical realization remains limited by slow response, optical loss, and polarization sensitivity. The integration of chalcogenide phase-change materials with metasurface architectures offers a powerful platform for dynamic optical tunability, owing to materials such as Ge$_2$Sb$_2$Te$_5$ (GST) that can reversibly switch between amorphous and crystalline states with distinct refractive indices. However, the strong optical absorption of crystalline GST in the visible to near-infrared range has hindered its widespread use in reconfigurable metalenses. In this study, we design an all-dielectric polarization-insensitive metasurface based on hybrid Si--GST nanostructures to realize a dynamically tunable bifocal metalens operating at 1.55 μm. The device achieves a variable focal length from 70 μm to 200 μm, with focusing efficiencies of 30% in the amorphous state and 20% in the crystalline state, as validated through finite-difference time-domain (FDTD) simulations. Using COMSOL Multiphysics, we show that flat-top laser excitation enables uniform, reversible phase transitions within tens of nanoseconds -- amorphization in approximately 13~ns and crystallization in approximately 90~ns -- without mechanical motion or electrical bias. For next-generation metasurfaces intended for uses including beam steering, dynamic holography, optical routing, multi-depth imaging, and optical communication, this method shows great promise due to its control and stability.