To improve absorbing properties of phase-modulated surface (PMS), the multi-active-layer PMS composed of multiple active frequency-selective surface (AFSS) layers and one background plane is theoretically studied using time-modulation theory in this paper. The optimization of PMS's switching scheme using differential evolution (DE) algorithm is also proposed for minimizing scattering echo energy at the incident frequency. We provide analytical formulation for the scattering problem and obtain the angular scattering pattern of PMS after optimization. Simulation results indicate that the optimized switching scheme is beneficial for reducing the spatial coverage of scattering echo at incident frequency. This coverage can be further confined by the increasing number of active layers in PMS. Furthermore, it is shown that floor effect appears when the number of active layers reaches a certain value, which limits the PMS structure conversely.
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