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International audience; The study of periodic artificial structures for elastic/acoustic waves, or phononic crystals, opens the possibility of engineering mechanical and acoustical effective properties. Potential applications to ultrasonics, such as imaging techniques, typically require band gaps for frequencies extending from 1 to 15 MHz. Thanks to the development of additive manufacturing, the fabrication of complex structures such as 3D phononic crystals is now possible for a reasonable cost. However, the limited resolution of classical 3D printers, a few tens of micrometers, imposes their working frequencies to remain well below 1 MHz. This frontier can be overcome using two-photon direct laser writing. In this work we design, fabricate at the microscale, and characterize a 3D phononic crystal that has a wide band gap in the MHz frequency range. The cubic perovskite structure is considered for the unit cell of the phononic crystal, since it can achieve very wide phononic band gaps. Thanks to the present development of two-photon lithography, it is possible to manufacture a structure at the microscale with high complexity. To characterize samples, a piezoelectric patch glued at the bottom of the supporting glass substrate produces longitudinal waves. Then, with the help of a laser Doppler vibrometer, the out of plane displacements are measured as a function of frequency. With a lattice constant of 300 μm we find a theoretical complete band gap extending from 0.6 MHz to 7.7 MHz. The experimental band gap is found to extend from 0.6 MHz to 7.5 MHz, resulting in an experimental band gap to mid-gap ratio of 170%.

Previously, rotons were observed in correlated quantum systems at low temperatures, including superfluid helium and Bose-Einstein condensates. Here, following a recent theoretical proposal, we report the direct experimental observation of roton-like dispersion relations in two different three-dimensional metamaterials under ambient conditions. One experiment uses transverse elastic waves in microscale metamaterials at ultrasound frequencies. The other experiment uses longitudinal air-pressure waves in macroscopic channel–based metamaterials at audible frequencies. In both experiments, we identify the roton-like minimum in the dispersion relation that is associated to a triplet of waves at a given frequency. Our work shows that designed interactions in metamaterials beyond the nearest neighbors open unprecedented experimental opportunities to tailor the lowest dispersion branch—while most previous metamaterial studies have concentrated on shaping higher dispersion branches.