Spatiotemporal Metasurface to Control Electromagnetic Wave Scattering

Metasurfaces, the artificially planar electromagnetic structures, have exhibited strong potential for arbitrary electromagnetic wave-scattering control; however, most of the existing low-scattering metasurfaces are based on spatial phase redistributions to generate diffusive backscattering, leaving time dimension unexploited. The time-modulated mechanism provides an attractive alternative to realize backscattering control but still lacks experimental demonstration for realization of radar cross-section reduction in microwave frequencies. Here, we propose a broadband spatiotemporal metasurface approach for arbitrary scattering control and provide experimental evidence of reshaping scattering patterns in the spatial and spectral domains simultaneously. The metasurface is composed of broadband high-efficiency meta-atoms incorporating electric p-i-n diodes, which ensures independent and dynamic phase-response modulation over time under external voltage control in an octave working band from 3.4 to 7.7 GHz. The measured results demonstrate that the metasurface could redirect incidence to various spatial directions and harmonic frequencies, achieving broadband spatial and spectral control of the backward scattering. The proposed metasurface may offer promising applications in radar detection and camouflage.

Figure 1

Conceptual illustration of the spatiotemporal metasurface for scattering control operating in three mechanisms, which are (a) space modulation, (b) time modulation, and (c) spatiotemporal modulation. Schematics of the corresponding scattering patterns are also illustrated in u-v plane (u = sin θ cos φ, v = sin θ sin φ), where the color blocks represent the power of spectra. f_c is the carrier frequency, f₀ is the manipulation frequency, k is the order of harmonics. The space or time modulation methods for reflective power reduction mainly rely on the conversion of incident waves in a single spatial domain or spectral domain. While spatiotemporal metasurface could reshape the scattering patterns through effective EM wave manipulation in both the spatial and spectral domains, realizing precise scattering control and significant power-density reductions of the reflected signals. (d) Schematics of the spatiotemporal metasurface. E_i and E_r are the incident and reflected signals.

Figure 2

Theoretical confirmation of scattering control via time-coding sequences. (a) Scattering far-field patterns of the time-modulated metasurface. (b) Scattering far-field patterns of the space-time-modulated metasurface. The backscattered powers at broadside are controlled from 0 to −15 dB with the step of −5 dB approximately by preloading proper coding sequences. Corresponding coding sequences pertaining to (c) time modulation, and (d) spatiotemporal modulation, not including the sequences yielding 0 dB.

Figure 3

Theoretical scattering far-field patterns via different coding sequences with the consideration of reflection amplitudes. (a) The coding sequence for scattering control with space modulation. The blue and yellow blocks represent the normalized phases of loaded p-i-n diodes in the on and off states. (b) Scattering far-field patterns of the space-modulated metasurface. (c) Scattering far-field patterns of the time-modulated metasurface. (d) Scattering far-field patterns of the space-time-modulated metasurface.

Figure 4

(a) Schematics of the proposed broadband meta-atoms. The black and red blocks represent the p-i-n diodes and inductor, respectively. (b) Top structure of the proposed meta-atom. (c) Simulated reflection-phase characteristics of the meta-atom. The gray solid line shows the phase difference between the two coding elements. 180° ± 37° is obtained from 3.4–7.7 GHz. (d) Simulated reflection amplitude of the meta-atom. High reflectivity is maintained at the whole working band.

Figure 5

Schematic of the measurement setup for spectrum and backscattered pattern. The transmitting and receiving antennas are placed at the arc rail with equal distance and connected to the microwave signal generator and spectrum analyzer, respectively. The insets are the fabricated prototype and the FPGA-based controllable circuit board.

Figure 6

Measured spectral distributions of the 1-bit coding metasurface at 4 GHz, where three modulation methods are considered. (a)–(c) Spectra of the metasurface with different space-modulation sequences. Only the carrier frequencies can be observed. (d)–(f) Spectra of the metasurface with different time-modulation sequences, which are square waves with modulation frequencies of 100 and 500 kHz, and a quasirandom coding sequence with modulation frequencies of 12.5 kHz. (g)–(i) Spectra of the metasurface with different spatiotemporal modulation sequences. Randomly distributed spectra are observed.

Figure 7

Measured scattering patterns of the proposed spatiotemporal metasurface with different modulation mechanisms at 4–7 GHz. (a) Scattering-field distribution based on space-modulated signals. (b) Scattering-field distribution based on time-modulated signals. Unequal time steps are encoded into the metasurface with a modulation period of 0.1 ms. (c) Scattering-field distribution based on space-time-modulated signals.