  In this study, we demonstrate simplified equivalent circuit models to
effectively approximate responses of recently reported waveform-selective
metasurfaces that distinguish different electromagnetic waves even at the same
frequency depending on their waveforms or pulse widths. Compared to
conventional equivalent circuit models that represent behaviors of ordinary
metasurfaces in the "frequency" domain, the proposed models enable us to
explain how waveform-selective metasurfaces respond in the "time" domain. Our
approach well estimates not only time constants of waveform-selective
metasurfaces but also their entire time-domain responses. Particularly, this
study reports the importance of resistive components of diodes as well as the
limitation of our models with respect to the power dependence although still
the models effectively work when waveform-selective mechanisms are clearly
exhibited with a sufficiently large input power. Thus, the idea of the proposed
equivalent circuit models contributes not only to a greater understanding of
waveform-selective mechanisms but also to facilitating the design process of
such unique structures.
