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Terahertz (THz) technology has received considerable interest because of its potential in a wide variety of applications such as wireless communication, spectroscopy, imaging, and sensing. In the past several decades, cost-effective and compact THz sources and detectors have been intensively developed to use the THz wave in industrial applications. For more practical THz applications, various active and passive devices such as THz filters, modulators, phase shifters, switches, and mirrors must be developed. However, the development of THz devices is very deficient compared to microwave and light wave bands because the electromagnetic properties of most natural materials are not suitable to be used in the THz frequency range. To overcome the limitations of natural materials in the THz band, research on the utilization of metamaterials, which can artificially control electrical and magnetic properties, as devices in the THz band has attracted much attention. The controllable resonances of artificially engineered metamaterials can offer opportunities to realize novel THz devices for a wide variety of THz applications.
Numerous research studies on the realization of tunable characteristics for THz metamaterials have been reported by using semiconductors, graphene, and tunable functional materials. Tunable metamaterials based on vanadium dioxide (VO2) present a promising approach to spatially manipulate the THz wave thanks to easy fabrication and high tunability. Several studies have researched tunable THz metamaterials based on the phase transition of VO2 by applying temperature, a THz field, or light. However, these methods require external devices such as a heater or a source of THz waves or light, and the external devices cause these THz tunable devices to be expensive and bulky. Thus, electrical control of the phase transition of VO2 is preferred for practical applications. The metamaterial that can be electrically controlled but has low Q-factor is limited in improving the performance when applied as a device. To improve the performance of THz devices using metamaterials, it is very important to increase the quality factor of the metamaterials.
In this paper, we proposed an asymmetric split-loop resonator with an outer square loop (ASLR-OSL) based on vanadium dioxide thin film, which can actively control the transmission properties of a terahertz (THz) wave while maintaining a high quality factor of the asymmetric split-loop resonator (ASLR). The outer square loop was combined with the ASLR to be used as a microheater capable of controlling the temperature of the VO2 thin film through a directly applied bias voltage. Therefore, the transmission characteristics of the ASLR-OSL based on VO2 thin film were successfully controlled by directly applying a bias voltage. In addition, the ASLR-OSL could well maintain a high quality factor of the ASLR. The transmittance of the ASLR-OSL based on VO2 thin film was changed from 43.16% to 2.41% in mode 1 (0.7 THz) and 30.86% to 4.23% in mode 2 (1.1 THz). Based on these results, it is possible to impose active properties on a common metamaterial having a high quality factor by adding a simple loop structure that works as a microheater.
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