Robust stability characterizations of active metamaterials with non-Foster loads

Active metamaterials incorporating with non-Foster elements have been considered as one of the means of overcoming inherent limitations of the passive counterparts, thus achieving broadband or gain metamaterials. However, realistic active metamaterials, especially non-Foster loaded medium, would face the challenge of the possibility of instability. Moreover,they normally appear to be time-variant and in unsteady states, which leads to the necessity of a stability method to cope with the stability issue considering the system model uncertainty. In this paper, we propose an immittance-based stability method to design a non-Foster loaded metamaterial ensuring robust stability. First, the principle of this stability method is introduced after comparing different stability criteria. Based on the equivalent system model, the stability characterization is used to give the design specifications to achieve an active metamaterial with robust stability. Finally, it is applied to the practical design of active metamaterial with non-Foster loaded loop arrays. By introducing the disturbance into the nonFoster circuit（NFC）, the worst-case model uncertainty is considered during the design, and the reliability of our proposed method is verified. This method can also be applied to other realistic design of active metamaterials.

Active elastic metamaterials for subwavelength wave propagation control

Recent research activities in elastic metamaterials demonstrate a significant potential for subwavelength wave propagation control owing to their interior locally resonant mechanism. The growing technological developments in electro/magnetomechanical couplings of smart materials have introduced a controlling degree of freedom for passive elastic metamaterials. Active elastic metamaterials could allow for a fine control of material physical behavior and thereby induce new functional properties that cannot be produced by passive approaches. In this paper, two types of active elastic metamaterials with shunted piezoelectric materials and electrorheological elastomers are proposed. Theoretical analyses and numerical validations of the active elastic metamaterials with detailed microstructures are pre- sented for designing adaptive applications in band gap structures and extraordinary waveguides. The active elastic metamaterial could provide a new design methodology for adaptive wave filters, high signal-to-noise sensors, and structural health monitoring applications.

Tailoring electromagnetic responses in terahertz superconducting metamaterials

Superconducting terahertz metamaterials have attracted significant interest due to low loss, efficient resonance switching and large-range frequency tunability. The super conductivity in the metamaterials dramatically reduces ohmic loss and absorption to levels suitable for novel devices over a broad range of electromagnetic spectrum. Most metamaterials utilize subwavelength-scale split-ring resonators as unit building blocks, which are proved to support fundamental inductive-capacitive reso- nance, to achieve unique resonance performance. We presented a review of terahertz superconducting metama- terials and their implementation in multifunctional devices. We began with the recent development of superconducting metamaterials and their potential applications in control- ling and manipulating terahertz waves. Then we explored the tuning behaviors of resonance properties in several typical, actively controllable metamaterials through integrating active components. Finally, the ultrafast dynamic nonlinear response to high intensity terahertz field in the superconducting metamaterials was presented.