High Endurance Phase-Change Devices for Electrically Reconfigurable Optical Systems

摘要：光学特性可电调谐的材料对于我们日常依赖的许多现代技术的运行至关重要，例如智能手机显示器和光纤互联网。对于可调谐光学元件、高速计算和高级光存储等新兴应用，一组可以在原子水平上重新配置的材料（称为“相变材料”）特别有前途。当它们的原子以有序或无序状态排列时，这些材料的光学性质会发生戏剧性的、稳定的和可逆的变化，这可以使器件具有非常紧凑的外形、低能耗和对振动不敏感。虽然许多概念验证光学器件已经使用相变材料进行了演示，但很少能使用电信号进行控制——这是现实世界实现的先决条件。此外，这些电控相变器件表现出较差的耐用性和开关循环性，其原因尚不清楚。为了解决这一挑战，该团队建议首先使用与时间相关的光学和电学测量来研究由这些设备组成的各个层中的热和机械膨胀的作用。其次，该团队将使用高分辨率成像技术来研究不同类型原子的迁移对相变材料可逆性的作用。最后，该团队将利用这些结果构建可靠性更高的相变器件，并探索将其扩展到需要更大可调谐光学元件的应用所需的尺寸的可能性。该团队试图通过互动研讨会和实践演示相结合的方式，对来自历史上服务于少数民族的学区的中学生进行与日常生活中新材料相关的主题教育。该项目还为两名研究生提供先进器件制造和表征技术方面的培训，并在夏季接待来自代表性不足群体的本科生，以扩大stem相关领域的参与。技术摘要：相变材料，如Ge2Sb2Te5和GeTe，由于其折射率的快速，戏剧性，非易失性和可逆变化，特别有希望用于可重构光学器件。可重构智能窗口、反射显示器、超表面和用于存储和计算的光子器件的实验演示重新点燃了人们对这些材料的兴趣。对于尺寸大于光学波长的相变器件，电热开关方法最有前途，但有限的先前工作表明，与相变数据存储的高耐久性（大于1000万周期）相比，其耐久性和可循环性较差（1000个周期或更少）。该团队提出，这些器件内材料的热性能不匹配限制了耐久性，而经常观察到的光学对比度下降是由于相变层中的相分离和空洞形成。为了验证这一假设，该项目有三个目标：(1)通过适当匹配器件层内材料的热膨胀系数来提高电热相变器件的寿命；(2)通过降低器件内的热梯度和改善沉积条件来减少循环引起的光学对比度退化；(3)确定缩放对相变光学器件的影响和限制。该方法将通过互补的热力学建模、动态光电测量和先进的纳米表征技术来研究器件层的热响应，从而克服这些电热开关相变器件有限的可循环性。通过理解和解决当前电热控制光学相变膜的局限性所获得的见解有望广泛应用于可调谐光学涂层，非冯·诺伊曼计算，电光转换以及可重构光子和射频系统等领域。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Award Title:High-endurance phase-change devices for electrically reconfigurable optical systemsNon-Technical Abstract:Materials whose optical properties can be tuned electrically are essential to operations of many modern technologies that we rely on daily, for example, smartphone displays and fiber internet. For emerging applications in tunable optical components, high-speed computing, and advanced optical storage, a group of materials that can be reconfigured at the atomic level, known as “phase-change materials,” is particularly promising. When their atoms are arranged in either an ordered or disordered state, these materials exhibit a dramatic, stable, and reversible change in their optical properties, which could enable devices with very compact form-factor, low energy consumption and insensitivity to vibration. While many proof-of-concept optical devices have been demonstrated using phase-change materials, few can be controlled using electrical signals—a prerequisite for real world implementation. Additionally, these electrically controlled phase-change devices have shown poor endurance and switching cyclability, the cause of which is not well understood. The team proposes to address this challenge by first investigating the role of heat and mechanical expansion in the various layers comprising these devices using time-dependent optical and electrical measurements. Secondly, the team will use high resolution imaging techniques to study the role that migration of various types of atoms has on the reversibility of the phase-change material. Finally, the team will use these results to construct phase-change devices with improved reliability and explore the possibility of scaling them up to sizes needed for applications requiring larger tunable optical components. The team seeks to educate middle-/high-school students on topics related to novel materials in daily life from school districts with historically serving under-represented minorities, using a combination of interactive workshops and hands-on demos. This project also provides training for two graduate students in advanced device fabrication and characterization techniques, and hosts undergraduates from underrepresented groups during the summer months to broaden participation in STEM-related fields.Technical Abstract:Phase-change materials, such as Ge2Sb2Te5 and GeTe, are particularly promising for reconfigurable optical devices owing to their fast, dramatic, non-volatile, and reversible change in refractive index. Experimental demonstrations of reconfigurable smart windows, reflective displays, metasurfaces, and photonic devices for memory and computing have re-ignited interests in these materials. For phase-change devices with dimensions greater than the optical wavelength, an electro-thermal approach to switching is most promising, but limited prior work showed poor endurance and cyclability (1000 cycles or less) compared to the high endurances (greater than 10 million cycles) demonstrated for phase-change data storage. The team proposes that the endurance is limited by poorly matched thermal properties of materials within these devices, while the degrading optical contrast often observed is due to phase segregation and void formation in the phase-change layer. To validate this hypothesis, the project has three aims: (1) improve the lifetime of electro-thermal phase-change devices by properly matching the thermal expansion coefficients of the materials within the device layers; (2) reduce the cycling-induced degradation of optical contrast by reducing thermal gradients within the device and improving deposition conditions; and (3) identify the effects and limitations of scaling on phase-change optical devices. The proposed approach will overcome the limited cyclability of these electro-thermally switched phase-change devices by studying the thermal response of the device layers through complementary thermal-mechanical modelling, dynamic optoelectronic measurements, and advanced nano-characterization techniques. The insights gained by understanding and addressing the current limitations of electro-thermally controlled optical phase-change films are expected to be broadly applicable to such fields as tunable optical coatings, non-von Neumann computing, electrical-optical conversion, and reconfigurable photonic and RF systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.