Overview: We propose to investigate dramatic changes in electronic and photonic properties of new functional devices based on the reversible metal-insulator transition (MIT) of vanadium dioxide. The electrical resistivity of VO2 changes by 4 to 5 orders. Variations in refractive index exceed 15%, giving an index contrast of greater than 3 relative to free space. This unique combination of properties motivates the study of its phase transition mechanisms, control of its properties, and development of terahertz (THz) functional components. Intellectual Merit: We will explore the fundamental physics of VO2 phase transition, particularly when driven electrically in pulsed current mode or in gate electric-field mode, and develop new functional devices for THz switching and modulation applications with the aim of reconfigurable manipulation. Through the study of phase transition mechanisms and factors to influence the transition ?speed? we will engineer the material structure to achieve abrupt ?digital? switching and controllable percolative phase transition for ?analog? modulation. The result will provide fundamental understanding of electrically controlled metal-insulator transition and promote the application of phasetransition- based materials for electronic and photonic applications. Broader Impact: Study of phase transitions will provide new opportunities to achieve innovative functional devices. These devices will exhibit unprecedented operational principles for electronic and photonic applications, in addition to fundamental scientific understanding. The proposed components will find broad applications in THz technology. The project provides excellent opportunities for interdisciplinary research education and training. Outreach efforts will attract new students, especially those from underrepresented groups, to science and engineering fields.
We carried out synthesis, material physics, and terahertz (THz) optical studies of functional vanadium dioxide (VO2) thin films, which undergoes metal-insulator transition (MIT) under thermal, optical, and electric control, towards the understanding of fundamental material properties and its utilization as active materials in tunable THz optical devices. We have studied the epitaxial growth of VO2 on sapphire substrates with different orientations and investigated the non-stoichiometric effects by growth control and intentional impurity doping. MIT evolution process under thermal, electrical and optical control were studied. We devoted special efforts to investigate the relationship between crystal anisotropy and phase transition, and the relationship of material structure with phase transition abruptness and the percolation process. Impurity doping, particularly H or W doping were studied to significantly modify the MIT characteristics, including stabilizing the metallic phase to room temperature and expanding the temperature transition window from 5o C to 35 oC with gradual property change. Using several spectroscopic tools, we studied the fundamental optical properties and shed light on the fundamental mechanisms driving its MIT. Transformative switches, modulators, tunable filters, and spatial light modulator arrays (SLM), based on thin-film or metamaterial structures, have been developed for reconfigurable THz wave manipulation by exploiting the tunable dielectric function. We demonstrated the first broad band THz modulator with modulation depth ~ 98%, and the first VO2 based primitive THz-SLM. We demonstrated several tunable THz filters based on VO2 memtamatterial structures. The use of VO2 as coating material to suppress Fabry-Perot resonances at THz frequencies was also confirmed. We have published 11 journal papers, 3 more are under review or in preparation. We have also made 14 conference presentations, and published 3 conference papers. Three PhD students and two MS students trained in this effort have graduated, and another two PhD students are still working in this project. Three additional PhD students are also involved in this project. We developed a new graduate level course PHYS5300 "Terahertz Optics" for both physics and EE students at Texas Tech University. REU students Ms. Kay Igwe, a minority female student, and Mr. Josh Wyatt were trained on device fabrication, including photolithography, plasma etching, metallization, and on electrical and optical characterization of the fabricated devices. Both presented their studies in conference meetings. Another undergraduate student Ms. Venesa Hallum carried out atomic force microscopy (AFM) measurements and contributed to a journal paper. Two minority students from Frenship High School in Lubbock, Mr. Zach Aguirre and Mr. Tyler Wiley, participated learning of the basic procedures of microfabrication. A third high school student, Mr. Fabio Arai, was involved in the development of the control software used in the THz imaging system.