VO2 material study and its application for terahertz modulation
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Metal-insulator transition (MIT) in vanadium dioxide (VO2) can be triggered by series of stimuli, and was accompanied by dramatic optical properties change, which makes VO2 a promising material for optical modulation and switching, especially in terahertz (THz) range. This work focuses on its growth optimization, characterization and application for THz modulation. Epitaxial VO2 thin films with good MIT characteristics and ~ 95% THz modulation depth were grown on c-cut sapphire, and was characterized to be (010) oriented multi-domain hetero-epitaxial structure. Moreover, the substrate orientation (c-, r- and m-cut sapphire) influence on structural, electrical and THz properties of VO2 thin films was also systematically studied , and r-cut samples displayed the best THz modulation performance. Finally, the electrical and optical properties of VO2 thin films were demonstrated to be effectively tuned through introduction of intrinsic vacancies, simply by adjusting growth temperature. MIT in VO2 thin films triggered by electric sources was investigated based on VO2 2-terminal devices. Voltage sweeping test unveiled a critical current density for MIT; while voltage pulses measurement demonstrated fast-speed transition within sub-s. Moreover, current sweeping test displayed a 3-stage transition process, which offers the approach to gradually control the transition process. Two prototypes of 2 × 2 pixel arrays THz spatial light modulator (TSLM) were demonstrated. TSLM1 was controlled by joule heating while TSM2 directly controlled by electric current. Both two TSLMs demonstrated the ability to independently turn on and off their pixels, and displayed uniform spatial modulation. Besides, TSLM2 suffers less influence from thermal cross-talk and displays a superior modulation depth up to 99%, showing great potential for future THz signal and image processing.