Active Media With Strong Micro-Wave Responses Of Nonlinearity And Gyroelectricity

In this dissertation, assisted by Maxwell-Garnett effective medium theory and the construction approach with active elements, research on active metamaterials with strong microwave responses of nonlinearity and gyroelectricity were conducted.At the microwave frequencies, natural media usually exhibit extremely weak nonlinear effects. Compared with diverse spatial nonlinear optical applications, nonlinear applications at microwave frequencies are usually limited to microwave circuits. Based on the effective medium theory, we designed and realized an active metamaterial with strong spatial nonlinear electromagnetic (EM) responses. Under the illumination of a micro-Watt spatial microwave, strong nonlinear EM responses, like spatial harmonic generation and spectrum modulation, were clearly observed. After that, the effective nonlinear constitutive parameters of the active metamaterial were successfully retrieved from measured data. Based on above research, we proposed and experimentally demonstrated the imaging of microwave sources. Spatial resolutions exceeding the Abbe diffraction limit were achieved.In Maxwell’s theory, EM waves propagating in most natural media satisfy the reciprocal theory. When energy conservation is satisfied, this propagation exhibits a local time-reversal symmetry. Few naturally occurring gyrotropic media, such as magnetized plasma and ferrite, behave relatively weak non-reciprocal, gyroelectric or gyromagnetic phenomena only when external bias magnetic fields are applied. In this dissertation, by innovatively introducing unidirectional devices commonly used in active microwave circuits, we proposed a construction method of the active metamaterial with strong gyroelectric effects in the absence of an external magnetic bias. By translating the property of a unidirectional active device into the non-reciprocal response of effective media, we realized a gyroelectric metamaterial that only depends on a DC-voltage bias. Effective constitutive parameters were retrieved by using an optimization algorithm. In further experiments, strong non-reciprocal Faraday rotations with large deflected polarization angles were observed, breaking the local time-reversal symmetry.The obtained active metamaterials fundamentally avoided the inherent performance limits of the corresponding naturally occurring media at microwave frequencies, showing a wide range of potential applications in the future.