Temporal Refraction in an Acoustic Phononic Lattice

In this Letter, we present the first experimental demonstration of the temporal refraction of acoustic waves in a phononic lattice. A step change in grounding stiffness results in a discontinuous change in group velocity across a so-called temporal boundary. This leads to frequency translation of incident signals, which maintain constant wavelength. We use the system to construct phononic analogs of the classical Snell and Fresnel relationships for temporal boundaries, providing evidence of temporal refraction. Last, we propose the ability to design systems to achieve tunable slow sound.