Terahertz Spectroscopy of vanadium dioxide films grown on a-plane sapphire substrate

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2013-05

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Abstract

Vanadium dioxide (VO2) has been attracting considerable attention due its unique reversible phase transformation property which changes from an insulator (semiconductor) with monoclinic crystal symmetry to a metallic phase with a tetragonal rutile structure. The phase transition is accompanied by 3-5 orders of magnitude change in electrical conductivity. The phase transition can be driven by an external perturbation such as temperature, electric field or laser excitation. The metal-insulator phase transition is of particular interest to realize reconfigurable optical devices such as switches, filters, polarizers and spatial light modulators. High quality VO2 films have been obtained using different substrates and deposition techniques. Among the various substrates used, sapphire is one of the most common ones due to important attributes such as crystalline quality, mechanical stability, and optical transparency in a wide range of frequencies. The substrate orientation plays a fundamental role on the properties of the VO2 films which include: transition temperature, resistivity-change during the metal-insulator phase transition, thermal hysteresis loop width, and crystalline quality. Detailed investigations on the influence of the substrate type and orientation are critical to obtain high-quality VO2 films for future device applications. VO2 has been recently used for switching and modulation applications and for realization of reconfigurable metamaterial filters and polarizers at terahertz (THz) frequencies. In the insulator phase VO2 grown on sapphire substrate is almost transparent to THz radiation. In contrast, above the phase transition, VO2 behaves as a metal and therefore is almost opaque to THz radiation. This makes VO2 an ideal material for applications where high amplitude modulation is a critical requirement. In this work we performed detailed investigations on the THz transmission through VO2 films grown on a-plane sapphire substrates. The temperature-driven method was used to trigger the phase transition of the VO2 films. Measurements were carried out at different temperatures using THz time-domain spectroscopy technique. Complex refractive index and optical conductivity and birefringence were determined for the VO2 films at temperatures below, during and above the phase transition. Our results indicate that VO2 films grown on a-plane sapphire substrates can be used as an anti-reflecting coating material at THz frequencies.

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Keywords

Vanadium dioxide-a plane, Terahertz spectroscopy, Birefringence

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