Optical activity requires chirality and is a paradigm for chirality. Here, we present experiments on its mechanical counterpart, acoustical activity. The notion “activity” refers the rotation of the linear polarization axis of a transversely polarized (optical or mechanical) wave. The rotation angle is proportional to the propagation distance and does not depend on the orientation of the incident linear polarization. This kind of reciprocal polarization rotation is distinct from nonreciprocal Faraday rotation, which requires broken time-inversion symmetry. In our experiments, we spatiotemporally resolve the motion of three-dimensional chiral microstructured polymer metamaterials, with nanometer precision and under time-harmonic excitation at ultrasound frequencies in the range from 20 to 180 kHz. We demonstrate polarization rotations as large as 22° per unit cell. These experiments pave the road for molding the polarization and direction of elastic waves in three dimensions by micropolar mechanical metamaterials.

Researchers routinely sense molecules by their infrared vibrational "fingerprint" absorption resonances. In addition, the dominant handedness of chiral molecules can be detected by circular dichroism (CD), the normalized difference between their optical response to incident left-and righthanded circularly polarized light. Here, we introduce a cavity composed of two parallel arrays of helicity-preserving silicon disks that allows to enhance the CD signal by more than two orders of magnitude for a given molecule concentration and given thickness of the cell containing the molecules. The underlying principle is first-order diffraction into helicity-preserving modes with large transverse momentum and long lifetimes. In sharp contrast, in a conventional Fabry-Perot cavity, each reflection flips the handedness of light, leading to large intensity enhancements inside the cavity, yet to smaller CD signals than without the cavity.

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For analyzing displacement-vector fields in mechanics, for example to characterize the properties of 3D printed mechanical metamaterials, routine high-precision position measurements are indispensable. For this purpose, nanometer-scale localization errors have been achieved by wide-field optical-image cross-correlation analysis. Here, we bring this approach to atomic-scale accuracy by combining it with well-defined 3D printed marker arrays. By using an air-lens with a numerical aperture of $$0.4$$ 0.4 and a free working distance of $$11.2\, \mathrm{mm}$$ 11.2 mm , and an $$8\times 8$$ 8 × 8 array of markers with a diameter of $$2\, \upmu\mathrm{m}$$ 2 μ m and a period of $$5\,\upmu \mathrm{ m}$$ 5 μ m , we obtain 2D localization errors as small as $$0.9\, \AA$$ 0.9 Å in $$12.5\, \mathrm{ms}$$ 12.5 ms measurement time ($$80\, \mathrm{frames}/\mathrm{s}$$ 80 frames / s ). The underlying experimental setup is simple, reliable, and inexpensive, and the marker arrays can easily be integrated onto and into complex architectures during their 3D printing process.

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202103205.

Recently, the existence of multiple Weyl points was theoretically shown for chiral woodpile photonic crystals [1]. These Weyl points carry topological charges and thus lead to the emergence of backscattering-immune gapless surface states, making the photonic crystal a topological photonic insulator. The proposed photonic crystal structure consists of chirally stacked rods made of a perfect electric conductor (PEC), forming a hexagonal lattice. The structure parameters are designed to obtain isolated Weyl points in the THz regime. Polymer structures are fabricated using three-dimensional laser lithography and subsequently coated with silver via polymer sensitization and electroless deposition. For a silver film thickness well above the skin depth of the THz radiation, a material optically comparable to a bulk PEC can be realized. Measurements of the angle resolved transmission spectra using photoconductive antennas as THz sources and detectors will be carried out soon. From these spectra one can obtain the band structure and hence show the existence of the Weyl points.

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