Topological mode conservation and conversion in phononic crystals with temporal interfaces

TL;DR

This paper explores topologically non-trivial phononic lattices with temporal interfaces, demonstrating mode conservation and conversion, and generalizing classical wave laws to phononic systems through analytical, numerical, and experimental methods.

Contribution

It introduces the first experimental realization of topologically non-trivial phononic lattices with temporal interfaces, revealing mode conservation and conversion phenomena and extending classical wave laws to phononics.

Findings

Demonstrated single-frequency mode conservation after a temporal interface.

Observed multi-frequency mode splitting in phononic lattices.

Developed a phononic time lens and analogues of quantum phase transitions.

Abstract

A sudden change in material properties creates a temporal interface and forces a propagating wave to change its frequency while preserving its wavenumber. In contrast to monoatomic lattices with a single frequency-wavenumber pair, polyatomic lattices support multiple frequencies for each wavenumber. To date, experimental observations are limited to topologically trivial monoatomic phononic systems. Here, we utilize analytical, numerical, and experimental methods to examine topologically non-trivial phononic lattices subject to temporal interfaces. In particular, we realize phononic lattices demonstrating single-frequency shift (i.e., mode conservation) and multi-frequency splitting (i.e., mode conversion) following a temporal interface. Accordingly, we generalize temporal analogues of Snell's law and Fresnel equations. Moreover, we utilize Bloch mode overlaps to obtain a phononic time lens and a classical analogue of dynamic quantum phase transitions for phonons. Such overlap determines the probability of mode conversion or conservation after a temporal interface and, more importantly, can carry hidden topological characteristics. Our methodology paves the way for the use of temporal interfaces in probing phonon band topology and the realization of advanced acoustic devices.