Predicting Optical Switching of Phase-change Materials for Nanophotonic Applications

超表面（MSs）通过操纵光的振幅、相位和偏振来提供对光场的全面控制。这是通过激发放置在表面上的单个金属或介电纳米谐振器（“元原子”）的共振来实现的。因此，基于MSs的光学仪器可以比传统的光学元件（如玻璃透镜）薄得多。通过加入活性元素，如硫系相变材料（PCMs）， MSs仍然可以在制造后进行调整。pcm可以在非晶相和结晶相之间切换，这两种相具有截然不同的光学特性，特别是在红外光谱范围内。最近，我们已经证明可以用聚焦的可见光在元原子尺度上调整MSs。通过在元原子水平上改变PCM相位，可以在制造后重新配置或“编程”MSs，并显着提高其可用性和多功能性。由于开关过程的非平稳性和元原子设计的复杂性，沉积在PCM中的能量是不均匀的，由此产生的相变在空间上是不均匀的。相变过程中材料性质的剧烈变化进一步导致吸收能量和热传导的局部不均匀随时间变化。简单的理论近似显示出良好的预测，例如，切换完全质谱，不足以描述元原子长度尺度上的相变。本课题的总体目标是优化基于pcm的MSs光交换。这与先前在光学数据存储中研究的pcm开关有很大不同，在光学数据存储中，只有横向对比度变化是重要的。相比之下，现在需要在PCM-MS中精确控制3D结晶，即使是最小的结晶变化也会导致光学行为的剧烈变化。为此，我们将开发一个复杂的，自一致耦合的多物理场仿真模型。实验和理论工作将紧密联系在一起。通过实验调查验证模拟将使必要的理解得以发展，并将这些知识转化为全面的模拟模型。这种对光开关的理解的实现将使超薄可调透镜等新型光学器件的寿命和开关稳定性大大增加。同样的概念也可以应用于其他结合pcm的系统，如集成光子学中的可调谐波导。

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.