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

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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.