Fig. 3. Harmonic wave generation and unidirectional propagation in the time-varying metasurface.

(A) The schematic of one-way harmonic wave generation with the pump frequency $f_0$ in the time-varying metasurface under the CW driving. (B) The time-series signals for achieving the CW and CCW driving protocols. (C) The electric field response (output signal) in the frequency domain of the metasurface under pump (input signal) frequency $f_0$ and switching frequency $f_{\mathrm{s}}=450$ MHz. Clear response peaks at $f_0+n{f}_{\mathrm{s}}$ (n = 1, 2, and 3) in the output signal are observed. (D) Floquet bulk and edge state spectra in the momentum space, where the edge states corresponding to the response peaks in (C) are marked. (E) Electric field distributions ( $\mid E_z\mid$ ) of the generated harmonic waves and their unidirectional propagation under CW (top three panels) and CCW (bottom three panels) driving. The pump frequency is at $f_0=200\text{MHz},$ whereas the generated harmonic waves are at $f_0+f_s$ and $f_0+2f_s$ with the driving frequency of $f_s=450\text{MHz}$.