CAREER: Understanding and Exploiting Non-linear Behavior of Phase-Change Materials for Millimeter-Wave Applications

未来的无线通信系统预期支持显著更高的数据速率。然而，更高的数据速率通常通过使用更高的频率来实现。电磁波谱中唯一适合这一目的的一个区域称为毫米波段。这个波段指的是几毫米量级的波长。具体地，毫米波段被定义为30 GHz至300 GHz之间的频率。可重构性和自适应性是未来灵敏毫米波系统用于传感、成像和无线通信的重要特征。然而，当射频系统被重新配置时，它们变得有损，从而抵消了通过重新配置实现的任何增益。换句话说，尽管增加了功能，但损耗（低效率）是任何射频可重构系统的致命弱点，通常很少讨论。本项目旨在解决上述基本限制。拟议的研究促进了对相变材料及其在毫米波领域的应用的基础研究，特别是被动成像传感器。拟议中的研究可以打开毫米波及更高波段的大门。所提出的毫米波传感器的应用包括医学成像、导航、遥感和机器人等。除研究外，该项目的教育计划还包括：1）在俄亥俄州州立大学开发新课程，2）本科和K-12暑期课程，3）参加俄亥俄州中部服务不足学生的外展计划。该项目的更广泛影响包括扩大代表性不足的群体和本科生研究的参与。相变材料是毫米波重构的有吸引力的选择，因为它们提供了实现低损耗微系统的途径。相变材料的独特特征是物理（即电或光学）性质（例如介电常数或折射率）随温度、应变和电流的非线性或突变。诸如二氧化钒的金属氧化物属于表现出可逆金属-绝缘体转变的相变材料的子组。这些材料为实现低损耗射频微系统提供了一条途径。因此，本研究的主要目的是：1）了解和分析薄膜沉积条件与包括损耗在内的毫米波段相变材料的电性能（复介电常数）之间的相关性。这种独特性质的成功证明，取决于了解薄膜生长条件及其对晶体结构的影响; 2）研究和开发新的应变诱导激发（激活）技术，并分析其对器件性能的影响; 3）探索新的器件结构，特别是使用所选择的相变材料，例如二氧化钒或其他候选物，以减少或消除损耗，同时实现独特的功能。一类新的被动成像阵列（毫米波相机）预计将在该波段表现出比最先进的传感器显着更高的响应度。除了基础研究之外，拟议的工作雄心勃勃，但可能具有变革性，因为它挑战了设计传感器的传统智慧和基于半导体的毫米波探测器的主导地位。目前，没有可接受的解决方案可用于在室温下工作的毫米波成像系统。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Future wireless communication systems are expected to support significantly higher data rates. Higher data rates, though, are generally achieved by using higher frequencies. One area of the electromagnetic spectrum that uniquely fits this purpose is called the millimeter-wave band. This band refers to wavelengths in the order of a few millimeters. Specifically, the millimeter-wave band is defined as frequencies between 30 GHz to 300 GHz. Reconfigurability and adaptability is a vital feature of future agile millimeter-wave systems for sensing, imaging, and wireless communications. However, when radio-frequency systems are made reconfigurable, they become lossy neutralizing any gain achieved by reconfiguration. In other words, despite the added functionality, losses (low efficiencies) are the Achilles heel of any radio-frequency reconfigurable system often less discussed. This project intends to address aforementioned fundamental limitation. The proposed research fosters fundamental studies on phase-change materials and their applications in the millimeter-wave domain, specifically, passive imaging sensors. The proposed research can open doors in millimeter-wave and beyond. Applications of the proposed millimeter-wave sensors include medical imaging, navigation, remote sensing, and robotics among a few. In addition to research, the education plan of this project includes: 1) develop new courses at the Ohio State University, 2) undergraduate and K-12 summer program, and 3) participation in outreach program for underserved students from Central Ohio. Broader impacts of this project include broadening participation of underrepresented groups and undergraduate research. Phase-change materials are attractive choices for millimeter-wave reconfiguration as they provide a path to achieve low-loss microsystems. Unique feature of phase-change material is non-linear or abrupt change in physical (i.e. electrical or optical) properties such as permittivity or refractive index with temperature, strain, and current. Metal oxides such as vanadium dioxide belong to a sub-group of phase-change materials that exhibit reversible metal-insulator transition. These materials provide a path for realization of low-loss radio-frequency microsystems. As a result, the main objectives of the proposed research are 1) to understand and analyze the correlation between film deposition conditions and the electrical properties (complex permittivity) of phase-change materials in the millimeter-wave band including losses. Successful demonstration of such unique properties, hinges upon understanding film growth conditions and their impact on crystal structure; 2) study and exploit new strain-induced excitation (activation) techniques on suspended millimeter-wave structures and analyze their impact on device performance; 3) explore novel device architecture, especially, using selected phase-change materials such as vanadium dioxide or other candidates, to reduce or eliminate losses while achieving unique functionalities. A new class of passive imaging arrays (millimeter-wave camera) is expected to exhibit significantly higher responsivity in this band than the state-of-the-art sensors. In addition to the fundamental studies, the proposed work is ambitious but potentially transformative as it challenges the conventional wisdom in designing sensors and dominance of semiconductor-based millimeter-wave detectors. Currently, no acceptable solution is available for millimeter-wave imaging systems operating at the room temperature.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.