Experimental Observation of Superscattering

Superscattering, induced by degenerate resonances, breaks the fundamental single-channel limit of the scattering cross section of subwavelength structures; in principle, an arbitrarily large total cross section can be achieved via superscattering. It thus provides a unique way to strengthen the light-matter interaction at the subwavelength scale, and has many potential applications in sensing, energy harvesting, bioimaging (such as magnetic resonance imaging), communication, and optoelectronics. However, the experimental demonstration of superscattering remains an open challenge due to its vulnerability to structural imperfections and intrinsic material losses. Here we report the first experimental evidence for superscattering by demonstrating the superscattering simultaneously in two different frequency regimes through both the far-field and near-field measurements. The underlying mechanism for the observed superscattering is the degenerate resonances of confined surface waves, by utilizing a subwavelength metasurface-based multilayer structure. Our work paves the way towards practical applications based on superscattering.

Figure 1

Multifrequency superscattering of TE waves from a subwavelength structure. (a) Photograph of the fabricated subwavelength multilayer circular rod. Here the realistic 3D rod with its finite length much larger than the interested wavelength can be reasonably treated as an ideal rod in two dimensions (with an infinite length). The rod is composed by three conformal metasurfaces which are parallel to each other; see colored dashed lines in the right inset. The ultrathin metasurface, constructed by deep-subwavelength periodic copper strips (see the left inset), can be theoretically described by a lossless surface conductivity of ${\sigma }_s$ with $\mathrm{Im}({\sigma }_s)<0$. The designed metasurfaces support TE surface waves, instead of TM surface waves which require $\mathrm{Im}({\sigma }_s)>0$. (b) Analytical total scattering cross section and the contribution from individual channel, from the designed circular rod in two dimensions located in free space. The degenerate resonance of confined surface waves emerges simultaneously at 2.2 and 3.73 GHz, leading to the multifrequency superscattering.

Figure 2

Far-field measurement of the total scattering cross section of TE waves (a) from a subwavelength multilayer structure (denoted as superscatterer) and (b) from a circular perfect electric conductor (PEC) rod. The superscattering of the designed superscatterer appears simultaneously at 2.2 and 3.73 GHz. The PEC rod has the same diameter as the superscatterer shown in Fig. 1. The measured results (colored dots) are consistent with the simulated results for the real 3D structure by CST Microwave Studio (colored lines) and the analytical results for the ideal structure in two dimensions as shown in Fig. 1.

Figure 3

Far-field measurement of the scattering cross section per azimuthal angle of TE waves (a),(b) from the superscatterer and (c),(d) from the PEC rod. The two superscattering frequencies for the designed superscatterer are 2.2 and 3.73 GHz. The cross section per azimuthal angle is shown as a function of the azimuthal angle $\varphi$, where $\varphi$ denotes the angle between the $\widehat{x}$ coordinate and the wave vector $\overline{k}$ of scattered waves. The incident waves propagate along the $+\widehat{x}$ direction, i.e., $\varphi =0°$. The radial length represents the magnitude of cross section per azimuthal angle, which is in unit of $2\lambda /\pi$. The measured results (colored dots) are consistent with the analytical results (colored lines).

Figure 4

Near-field measurement of the scattered field ${|E_z^{\mathrm{sca}}|}^2$ (a),(c),(e),(g) from the superscatterer and (b),(d),(f),(h) from the PEC rod. Much pronounced scattered fields are observed in the superscatterer. The measured results in (e)–(h) are consistent with the analytical results in (a)–(d). The designed superscatterer has the superscattering phenomenon simultaneously at 2.2 and 3.73 GHz. The incident plane waves come from the left side and propagate along the $+\widehat{x}$ direction. The superscatterer and the PEC rod are highlighted by gray and black circles, respectively.