Predicting Optical Switching of Phase-change Materials for Nanophotonic Applications

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Metasurfaces (MSs) provide comprehensive control over light fields by manipulating the amplitude, phase and polarization of light. This is achieved by exciting resonances of individual metallic or dielectric nanoresonators ("meta-atoms") placed on a surface. Optical instruments based on MSs can therefore be much thinner than conventional optical elements (e.g., glass lenses). By incorporating active elements such as chalcogenide phase-change materials (PCMs), MSs can still be adjusted after fabrication. PCMs can be switched non-volatile between amorphous and crystalline phases, which have drastically different optical properties, especially in the infrared spectral range. Recently, we have shown that it is possible to tune MSs at the meta-atom scale with focused visible light. By changing the PCM phase at the meta-atom level, it is possible to reconfigure - or "program" - MSs after fabrication and dramatically increase their usability and versatility. Due to the general non- stationary nature of the switching process and the complexity of the meta-atom design, the deposited energy in the PCM is inhomogeneous and the resulting phase transition is spatially non- uniform. The drastic changes in material properties during the phase transition further lead to locally nonuniform time-dependent changes in absorbed energy and thermal conduction. Simple theoretical approximations that show good predictions, e.g., for switching complete MS, are not sufficient to describe phase transitions on the meta-atom length scale. The overall goal of this project is to optimize the optical switching of MSs based on PCMs. This is quite different from the switching of PCMs previously studied in optical data storage, where only the lateral contrast change was important. In contrast, it is now necessary to precisely control the 3D crystallization within a PCM-MS, where even the smallest changes in crystallization can lead to a drastic change in optical behavior. To this end, we will develop a complex, self-consistently coupled multiphysics simulation model. Experimental and theoretical work will be closely linked. Verification of the simulations by experimental investigations will allow the necessary understanding to be developed and this knowledge to be translated into a comprehensive simulation model. The implementation of this understanding for optical switching will enable a dramatic increase in the lifetime and switching stability of novel optical devices such as ultrathin adjustable lenses. The same concepts can then be applied to other PCM-incorporating systems, such as tunable waveguides in integrated photonics.