Active Metasurfaces and Their Applications
Metasurfaces are sub-wavelength scale (lambda<1/4), two dimensional (2D) periodic structures being widely studied due to their unique ability to manipulate electromagnetic waves in microwave and optical frequencies in fields of antennas, absorbers, surface waveguides, cloaking, modulators, polarizers, lenses and imaging, etc.
Despite the unique properties of the metasurfaces, they have limitations including bandwidth, reciprocity or linear response especially in microwave frequencies. By applying active electronics to the metasufaces could potentially add additional dimensions to reconfigure or modulate the property of the surface resulting in non-linear property, switchability, frequency and power dependency, non-reciprocity as well as wider bandwidth, etc. In this thesis, we studied the benefits of loading active components on metasurfaces specifically in areas of absorbers and transmitters, which overcomes some limitations from conventional approaches.
The thesis first introduces the metasurfaces and their unique properties as well as the applications. Specifically, high impedance surfaces (HIS) are widely adopted in low profile antenna applications, surface wave suppression and artificial magnetic conductors, etc due to existence of the forbidden band gap. Then the thesis studied the electric field enhancement of the high impedance surfaces in both microwave and near infrared frequencies. The bandstop properties of the HIS has great advantages in absorbers. By applying transistors on the HIS, the absorbers could be switched and tuned, which are discussed in the 3rd chapter of the thesis. On the other hand, the thesis also introduced a direct DC to radio frequency (RF) radiation by applying avalanche switches to the metasurface. A non-reciprocal surface wave metasurface absorber is introduced with time and spatial modulation technique applied.