Coupled Topological Interface States and Phonon Molecules in GaAs/AlAs Superlattices

TL;DR

This paper demonstrates the creation and control of coupled topological phonon states in GaAs/AlAs superlattices, enabling tunable nanophononic devices in the GHz range through experimental and theoretical methods.

Contribution

It introduces the engineering of coupled topological phonon molecules and chains in superlattices, with tunable splitting and miniband formation, validated by experiments and models.

Findings

Coupled interface states hybridize into symmetric and antisymmetric modes.

Splitting of modes can be tuned over tens of gigahertz.

Chains of up to six coupled states form narrow topological minibands.

Abstract

Topological interface states in one-dimensional superlattices provide spatially localized phonon modes protected by the topology of the underlying band structure. In GaAs/AlAs distributed Bragg reflectors (DBRs), such states can be engineered through band inversion between superlattices with opposite Zak phases within the Su-Schrieffer-Heeger (SSH) framework. Here, we demonstrate topological phonon molecules and extended chains formed by coupled nanophononic interface states. By concatenating three superlattices with alternating topology, we realize two coupled interface states that hybridize into symmetric and antisymmetric modes, whose splitting can be tuned over tens of gigahertz by varying the reflectivity of the central DBR. Extending this concept, we engineer chains of up to N=6 coupled interface states that form narrow topological minibands while remaining strongly localized at the interfaces. We experimentally observe these coupled states in molecular-beam-epitaxy-grown GaAs/AlAs heterostructures using time-domain pump-probe transient reflectivity measurements, and reproduce their behavior using transfer-matrix calculations and a simple analytical model for the mode splitting. These results establish topological interface states as a robust platform for engineering coupled phononic systems and tunable nanophononic architectures in the GHz regime.