Challenges in Design of Power-amplifying Active Metasurfaces

The basic methodology in designing a transmissive or reflective power-amplifying metasurface is explained. Ensuring stability is recognized as the greatest challenge in the design. Whenever standard two-port amplifiers are used for power amplification, the stability is ensured by minimizing the input-output coupling, which inevitably leads to polarization selection, polarization conversion and nonreciprocal response of reflective metasurfaces. If these effects are not desirable, reflection amplifiers may be used. Reflection amplifiers based on negative-impedance converters employ a positive and a negative feedback loop to generate negative resistance required for power amplification, which makes them even more prone to instability. Their stability depends directly on the impedance of the radiating element. A reliable approach to stability analysis of reflection amplifiers based on negative-impedance converters is not yet available.