Switchable bi-functional metamaterial based on vanadium dioxide for broadband absorption and broadband polarization in terahertz band

A metamaterial is a kind of artificially designed sub-wavelength composite structure that has novel optical properties and can show extraordinary physical properties that natural materials do not have, such as perfect absorption [1], anomalous reflection [2], and negative refractive index [3]. Metamaterials are widely used in stealth technology [4], perfect lenses [5], antennas [6], and other fields. In essence, metamaterial is not new material, but new structure. The properties of optical metamaterials are mainly determined by their structure, especially the design of the unit element structure. Once earlier optical metamaterials were made, it was not easy to change their optical properties. With the development of metamaterial technology, metamaterials began to be combined with tunable materials to realize switchable multi-function optical devices. Among them, VO2 is an excellent tunable material [7]; it is a metal oxide with phase transition properties in which an insulator-to-metal phase transition effect can occur at 340 K [8], [9], [10], [11], [12]. At room temperature, VO2 has a monoclinic crystal structure with high resistivity and insulation properties, and when the temperature T>340 K, VO2 transforms into a cubic rutile structure with metal properties. When T≈340 K, the electrical conductivity of VO2 will increase by 4–5 orders of magnitude in a small range of temperature changes. The phase transition of VO2 is reversible. When T<340 K, the conductivity of VO2 can be entirely restored to its initial state. The phase transition of VO2 can be promoted not only by temperature control, but also by electrical excitation, pressure, chemical doping, etc [13], [14]. The application of VO2 in metamaterials lays a foundation for adjustable multi-function devices.

Since Landy et al. [15] proposed a three-layer metamaterial absorber composed of a classic open-ring resonator, dielectric layer, and cut wire in 2008, various metamaterial absorbers have been proposed, one after another. Most of the early absorbers are narrowband and cannot be tuned. In 2017, Liu et al. [16] realized switching from narrowband absorption to broadband absorption by using the phase transition characteristics of VO2. In 2018, Zhao et al. [17] realized broadband absorption switching between different frequency bands by using VO2. In recent years, using the tunable characteristics of various materials, absorbers were gradually combined with other functions, such as perfect reflection [18], electromagnetically induced transparency [19], and polarization conversion. He et al. [20] realized the switching of perfect narrowband absorption and polarization conversion using VO2. In 2020, Song et al. [21] realized switching from broadband absorption to broadband polarization conversion by VO2. How to achieve wider operating bandwidth during metamaterial function switching is still a major challenge.

In this study, a switchable bi-functional metamaterial with broadband absorption and broadband cross-polarization conversion is proposed based on the phase transition characteristics of VO2. The proposed metamaterial is a broadband absorber when VO2 is in the metal phase. High absorptivity of more than 90% can be obtained in the 3.33–5.62 THz range. The metamaterial is a broadband cross-polarization converter when VO2 is in the insulator phase. In the 2.54–4.55 THz range, more than 90% cross-polarization conversion can be achieved at the vertical incidence of x and y polarization. Currently, most metamaterial devices have only a fixed spectrum or a single function. Compared with the switchable bi-functional metamaterials previously proposed in the terahertz band, the metamaterials proposed in this study have a wider operating bandwidth in both functions. Switchable bi-functional metamaterials have more potential applications in photodetectors, thermal imaging, thermal emitters, solar cells, and other fields.

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