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We propose a switchable metamaterial absorber (MMA) for bandwidth tunable broadband and four-band absorption in the terahertz (THz) band based on the phase-transition characteristics of vanadium dioxide (VO2). The proposed MMA can achieve over 90% broadband absorption in the 3.06–7.61, 3.18–8.91, and 2.78–9.22 THz ranges when the conductivity of VO2 is 3 × 1 0 5 , 3 × 1 0 4 , and 2 × 1 0 4 S m − 1 , respectively, and has the advantages of polarization insensitivity and wide-angle absorption. When VO2 is in the insulator phase, the MMA can achieve four-band high absorption, and the absorptance can reach 99% at 2.23, 4.06, 5.80, and 9.82 THz. Through the joint action of the multi-layer structure, the proposed MMA solves the problem that when only VO2 is used to tune the broadband absorption, the decrease in the conductivity of VO2 by 1 order of magnitude will lead to the obvious decrease in the broadband absorption bandwidth. A switchable absorber designed by a simple tuning method without changing the structure has significant development prospects in stealth technology and selective thermal emitters. [ABSTRACT FROM AUTHOR]

A switchable bi-functional optical metamaterial for broadband absorption and broadband cross-polarization conversion is proposed in the terahertz band based on the phase transition of vanadium dioxide (VO 2). When VO 2 is in the metal phase, the designed metamaterial has the function of a broadband absorber. This absorber consists of a VO 2 top structure, a polyimide layer, and VO 2 film. Calculated results show that the absorptance of the absorber is higher than 90% in the 3.33–5.62 THz range. Because of its symmetry, the absorber is insensitive to polarization and maintains high absorptance in incident angles from 0° to 60°. When VO 2 is in the insulator phase, the designed metamaterial has the function of a broadband cross-polarization converter. This polarization converter comprises two gap gold square rings, a polyimide layer, and a gold substrate. The calculated results show that more than 90% cross-polarization conversion can be achieved at the vertical incidence of x and y polarization in the 2.54–4.55 THz range. Nearly 100% cross-polarization conversion can be achieved at 2.62, 3.29, and 4.33 THz. The designed metamaterial has a wide potential application prospect in fields including stealth technology, solar electromagnetism, and radar communication. • Have the dual functions of broadband absorption and broadband polarization. • The switching of dual functions can be realized within similar operating bandwidths. • Function switching can be realized by a small range of temperature changes. • High working efficiency. [ABSTRACT FROM AUTHOR]

• Modified vermiculite achieved a better expansibility under microwave irradiation. • A novel quantitative analysis method of microwave energy absorption was proposed. • Influence mechanism of Mg2+ modification on microwave absorption was analyzed. This work investigated the effect of microwave power on vermiculite expansibility for different exposure time and compared the changes after Mg2+ modification. The expansion ratio of the modified vermiculite sample reached up to 5.29 at 800 W with a significant increase of 23.89 % in contrast to the raw vermiculite sample. Aimed to quantify the microwave energy absorption of vermiculite, a novel method was proposed by employing water as a medium material due to its sensitivity to microwave energy. Using this method, the effect of microwave power on microwave energy absorption of vermiculite was studied. Meanwhile, the influence mechanism of Mg2+ modification on microwave energy absorption of vermiculite was analyzed by means of energy dispersive X-ray spectroscopy and thermogravimetric-differential scanning calorimeter. Mg2+ modification could significantly improve the microwave energy absorption ability of vermiculite, which was mainly ascribed to the increase of interlayer water content through Mg2+ hydration reaction. [ABSTRACT FROM AUTHOR]