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Sound absorbing materials capable of working in harsh environments (e.g., fire resistance, moisture resistance, and pressure resistance) while having excellent low-frequency sound absorption properties are highly desirable for many practical applications. Recently, a new class of acoustic absorbers based on space coiling and folding metamaterials (SCFMs) has emerged. The SCFMs could be comprised of rigid plastics, ceramics, and metals and could be a good candidate for developing sound-absorbing devices capable of working in harsh environments. The SCFMs also enable flexible control of low-frequency sound with subwavelength structures, addressing the problem of poor absorption of low-frequency noise encountered by conventional acoustic materials. However, the current geometries of SCFM meta-atoms do not take full advantage of the configuration of three-dimensional space, limiting their ability for flexible control of sound absorption properties. Here, we propose an acoustic absorber based on stacked SCFMs with side openings, which could effectively absorb line-spectrum noise as well as broadband noise. The effects associated with the geometry and resonance modes of SCFMs are shown to significantly influence the sound absorption properties. To understand the sound-metamaterial interaction, an analytical model is obtained to study the acoustic response of SCFMs, which is further validated by numerical simulations. Further experiments have demonstrated the acoustic properties of our proposed metamaterial structure and confirmed the capability of stacked SCFMs for flexible control of the acoustic absorption spectrum. This work provides new possibilities for the development of novel sound absorption metamaterials desirable for many practical applications.

Spin-orbital interaction of light attracts much attention in nanophotonics opening new horizons for modern optical systems and devices. The photonic spin Hall effect or Imbert-Fedorov shift takes a special place among the variety of spin-orbital interaction phenomena. It exhibits as a polarization-dependent transverse light shift usually observed in specular scattering of light at interfaces with anisotropic materials. Nevertheless, the effect of the polarization mixing caused by anisotropy on the Imbert-Fedorov shift is commonly underestimated. In this work, we demonstrate that polarization mixing contribution cannot be ignored for a broad range of optical systems. In particular, we show the dominant influence of the mixing term over the standard one for the polarized optical beam incident at a quarter-wave plate within the paraxial approximation. Moreover, our study reveals a novel contribution with extraordinary polarization dependence not observable within the simplified approach. We believe that these results advance the understanding of photonic spin Hall effect and open new opportunities for spin-dependent optical phenomena.