Exploiting the Metal-Insulator Transition of VO 2 Thin Films for Terahertz Wave Modulation and Switching
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Exploiting the Metal-Insulator Transition of VO2 Thin Films for Terahertz Wave Modulation and Switching Md Nadim Ferdous Hoque1, Gulten Karaoglan-Bebek2, Mark Holtz3, Ayrton A. Bernussi1 and Zhaoyang Fan1 1
Department of Electrical and Computer Engineering and Nano Tech Center, Texas Tech University, Lubbock, Texas 79409, USA 2 Department of Physics and Nano Tech Center, Texas Tech University, Lubbock, Texas 79409, USA 3 Department of Physics and MSEC, Texas State University, San Marcos, Texas 78666, USA ABSTRACT VO2 is one of the very few natural materials that can be used to modulate terahertz (THz) radiations. A 100-nm thick VO2, when in its metallic phase, has a charge density of more than ~ 1015 cm-2 which will strongly reflect and absorb the THz radiation; while in its insulator state, the charge density is lowered by several orders of magnitude to be THz transparent. Therefore, exploiting the metal-insulator transition of VO2 is a potential approach to modulate or even switch THz radiation for THz optics. Here we report that VO2 epitaxial thin films on sapphire substrate exhibits 85% amplitude modulation depth in a broad bandwidth, while this value can be improved to 95% when VO2 film is coated on both sides of a substrate. We further demonstrate that with wafer bonding, 4-layered VO2 thin films exhibit a transmittance as low as -20 dB to -30 dB at their metallic state, enough for switching applications. We also report our proof-of-concept demonstration of THz spatial light modulator that exhibits amplitude modulation as large as 96%, -30 dB pixel-to-pixel crosstalk, and a broad THz bandwidth. INTRODUCTION VO2 undergoes a first order metal-insulator transition (MIT) with its conductivity change by 4-5 orders of magnitude at 68oC, accompanied by a dramatic optical property change [1,2]. Exploring the MIT of VO2 for tunable, switchable and other functional microwave, terahertz (THz), and infrared optical devices is very promising, especially in the THz radiation range [3,4]. A 100-nm thick VO2, when in its metallic phase, has a charge density of more than ~ 1015 cm-2 which will strongly reflect and absorb the THz radiation; while in its insulator state, the charge density is lowered by several orders of magnitude to be THz transparent. VO2 has been studied for modulation and filtering in various structures i.e. as thin film [5], metamaterial [6,7], nano antenna [8], photonic crystal [9] etc. However, the modulation depth and extinction ratio is still not satisfactory in those works which encouraged this work, where double sided thin films have been grown along with using wafer bonding technique to achieve higher modulation depth and better extinction ratios. Previously reported THz modulators lack enough modulation depth for better contrast and good thermal isolation at the same time[10], which is critical for specific applications such as static high contrast imaging or long wavelength astronomy. We have addressed these issues by demonstrating the device functionality in THz by fabricating a 2x2 prototype spa
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