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Metasurfaces have emerged as a promising technology for the manipulation ofelectromagnetic waves within a thin layer. In planar ultrathin metasurfaces,there exist rigorous narrowband design methods, based on the equivalent surfaceimpedance of patterned metallic layers on dielectric substrates. In this work,we derive a limit on bandwidth achievable in these metasurfaces, based onconstraints that their meta-atoms should be passive, causal and lossless, andthat they should obey the time-bandwidth product rules of a single resonancestructure. The results show that in addition to elementary design parametersinvolving variation of the surface impedance, the bandwidth is criticallylimited by the dielectric substrate thickness and permittivity. We then proposea synthesis method for broadband ultrathin metasurfaces, based on an LCresonance fit of the required surface impedance, and experimentally verify abroadband dispersive structure at millimeter-wave frequencies. This results ina bandwidth enhancement of over 90%, relative to a reference metasurfacecreated with the narrowband design process.

Bandwidth Limit and Synthesis Approach for Single Resonance Ultrathin Metasurfaces