A numerical study of optical reconfiguration dynamics of phase change material metasurfaces

Tunability is a highly desirable feature for nanophotonic devices and metasurfaces that can enable a plethora of exciting applications like dynamic color filtering and displays, motionless beam scanning, and fast focal length tuning compact imagers. Among several alternatives being explored for realizing tunable nanophotonics, phase change materials have been receiving much attention. In particular, chalcogenide glasses like GeSeTe alloys possess several advantages like large refractive index contrast and rapid phase switching properties which enable non-volatile reconfigurable metasurfaces. While previous workers have reported high reflection contrast changes ensuing from laser-induced amorphous-to-crystalline phase changes, detailed studies of the reconfiguration dynamics and optimization of switching processes have not been adequately considered. In this work, we consider simple and dimerized one-dimensional gratings of GST225 and numerically study phase switching as a function of reconfiguration pulse intensity with the objective of minimizing reconfiguration threshold and maximizing the figure of merit (defined as the rate of change of reflection contrast in % to change in pulse intensity beyond the reconfiguration threshold). The numerical study employs coupled electromagnetic and thermal solvers to ascertain the temperature profile and material phase profile for a particular reconfiguration pulse (assumed to be rectangular shaped). This work hopes to provide insights into the reconfiguration dynamics of PCM gratings while scaling down the reconfiguration threshold intensity requirements which can guide experimental activity in PCM based active metasurfaces.