Scalable Thin-Film Fabrication for THz Optical Switching Devices in Vanadium Dioxide

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Technical Description: Vanadium dioxide exhibits a reversible insulator-to-metal transition that can be initiated optically with femtosecond lasers, and is thus potentially ideal for high-speed electronic, photonic and memory devices. This research project addresses two challenges to the realization of this potential: First, the insulator-to-metal transition is fast (sub-picosecond), but the metal-to-insulator transition is slow (sub-nanosecond) because it requires a rearrangement of crystal structure. Second, scalable low-temperature thin-film fabrication has not been demonstrated for epitaxial vanadium-dioxide films on technologically useful substrates. The activity addresses these issues by correlating materials properties (film morphology, substrate interactions, dopant concentration, lattice match, strain and crystalline phases) to the static and dynamic optical constants of the thin-film structures and to the time-dependent electronic response and lattice vibrations that govern the speed of the transition. The relative merits of sputter, electron-beam and atomic-layer film-deposition techniques are assessed and the equilibrium optical and physical properties of the films are characterized by ellipsometry, X-ray diffraction and by high-resolution transmission-electron, Raman and X-ray photoelectron microscopy. Dynamical optical properties are measured with femtosecond time resolution by visible, infrared and THz pump-probe spectroscopy, time-dependent ellipsometry, interferometric autocorrelation and photoelectron electron microscopy.Non-technical Description. Vanadium dioxide is a remarkable material that changes from an insulator to a metal in about a trillionth of a second when illuminated by a short burst of laser light, making it ideal for applications in telecommunications, electronics and optical computing. The goal of this project is to optimize vanadium dioxide properties to make the fastest possible optical switch while developing fabrication tools to make vanadium dioxide thin-film devices on a size scale suitable for real-world applications. The project promotes the development of the future science and engineering work force by training graduate students to think and work at the interdisciplinary boundary between science and engineering. The research fosters work-force diversity by recruiting a graduate student from, and collaborating with researchers at, two universities that serve underrepresented populations. Undergraduate students involved in the research present the concept of optical switching to local high school students through Vanderbilt Students Volunteering for Science. Statewide impact on future science and engineering undergraduates is fostered by a long-standing interaction between the graduate students and the annual Tennessee Governor's School for Emerging Technologies. The research project also motivates ongoing dialogue with industrial partners who are developing technologies based on vanadium dioxide.