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Dynamic and nonlinear metamaterials: from terahertz to magnetic resonance imaging

Electromagnetic (EM) metamaterials represent an important class of artificial materials composed of arrays of subwavelength unit-cell structures, which are also known as meta-atoms, with engineered effective optical properties, such as effective permittivity and permeability. Metamaterials provide a powerful platform to implement dynamic and nonlinear electromagnetic materials. Specifically, the effective permittivity and permeability can be tailored and reconfigured to construct metamaterial devices by modulating or actuating the constituent meta-atoms. By leveraging microelectromechanical system (MEMS) technology, active metamaterial devices, such as modulators, absorbers, and tunable waveplates, may be implemented. We developed myriad functional terahertz (THz) metamaterial devices based on MEMS actuators and optical excitation to manipulate the THz waves towards practical applications. In addition to far-field radiation, metamaterials exhibit extraordinary near-field properties yielding the capacity to tailor electric and magnetic field distributions. We studied the electron emission and nonlinear resonance response in THz metamaterials due to the electric-field enhancement effect. Furthermore, intelligent magnetic metamaterials for boosting the signal to noise ratio (SNR) of magnetic resonance imaging (MRI) have been developed. We employed the nonlinear response in metamaterials consisting sub-wavelength helical resonators and varactor-loaded split ring resonators to selectively enhance the magnetic field, thereby improving the SNR of MRI. Future practical applications of metamaterials will be explored and discussed.