Effect of phase transition of vanadium dioxide on resonance characteristics of terahertz anti-resonant fiber and its applications
Authors: Yan Zhong-Bao , Sun Shuai , Zhang Shuai , Zhang Yao , Shi Wei , Sheng Quan , Shi Chao-Du , Zhang Jun-Xiang , Zhang Gui-Zhong , Yao Jian-Quan ,   Key Laboratory of Optoelectronic Information Science and Technology, Ministry of Education, College of Precision Instrument & Optoelectronics Engineering, Tianjin University, Tianjin 300072, China
Published: June 2021

Terahertz (THz) wave is an electromagnetic wave with frequency in a range of 0.1–10 THz, which possesses excellent photonic and electronic properties. THz wave has higher penetration and lower photon energy to non-polar materials, which makes it possess great academic value in medical, non-destructive testing and other related fields. In addition, the features such as wide bandwidth and large communication capacity of THz wave allow it to be widely used in communication, radar detection and other applications. Despite its rapid development in recent years, THz technology is used still mainly in free space currently and it is difficult to control the transmission direction of THz wave over a long distance in free space. What is more, the transmission of THz waves in free space is affected usually by the dust and water vapor. For achieving the efficient transmission of THz waves, researchers have proposed a variety of THz waveguides, including plastic fiber, Bragg fiber, photonic crystal fiber and anti-resonant fiber (ARF). The ARF confines the incident beam within the air hole of fiber center by the anti-resonance effect, which has aroused great interest because of its simple structure, low transmission loss, high damage threshold, low dispersion, and high transmission bandwidth. At present, adjustable THz fiber devices based on ARF are still reported rarely. In the near-infrared band, researchers have combined ARF with vanadium dioxide (VO<sub>2</sub>) to realize the exceptional modulation effects. The VO<sub>2</sub> is a metal oxide with insulator-metal phase transition when the ambient temperature is near 68 ℃, in which its electrical conductivity, dielectric constant and other properties will change drastically. In this paper, the VO<sub>2</sub> is coated on the inner wall of the THz ARF cladding tubes, and the effect of the phase transition of VO<sub>2</sub> on the propagation characteristics of the ARF is studied. Simulation results indicate that in the THz band, the phase transition of VO<sub>2</sub> will cause the anti-resonance period of the ARF to change greatly, in which the confinement effect of the ARF cladding tubes on the incident beam is converted from anti-resonant state to resonant state. Without changing the structure of the ARF, the effective modulation on the THz wave in the core of the ARF can be achieved only by controlling the phase transition of VO<sub>2</sub>, which has a wide application prospect in the field of THz adjustable devices. In this paper, a THz optical switch and a polarization controller based on VO<sub>2</sub>-coated ARF are proposed. With the optical switch being on and off, the corresponding losses are 0.5 dB/m and 110 dB/m respectively at 120 μm. If phase transition of VO<sub>2</sub> is induced by the excitation laser, it is expected to realize a fast-optical switch. Regarding the polarization controller, the polarization state and polarization direction of the THz wave in the core of the ARF can be controlled, and the birefringence coefficient of the ARF in the polarization state is more than 1.4 × 10<sup>–4</sup>.