Hybrid Si-GST Polarization-Insensitive Dynamically Tunable Bifocal Metalens Operating at 1.55-$\mu$m Wavelength

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 be reversibly switched 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 \textmu m. The device achieves a variable focal length from 70 \textmu m to 200 \textmu m, 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 ~8 ns and crystallization in ~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.