Acoustic Guided Waves in MoS2 thin flakes

TL;DR

This study investigates guided acoustic waves in MoS2 thin flakes using ultrafast optical techniques, revealing unique propagation modes, their detection mechanisms, and potential for nanoscale phononic applications.

Contribution

It uncovers the nature of guided acoustic waves in MoS2, showing they deviate from classical models and are governed by optical detectability and material-specific dissipation.

Findings

Propagation velocity of 6.7 km/s independent of thickness

Vibrations are superpositions of decoupled longitudinal and shear modes

Intrinsic attenuation length of approximately 3.3 microns

Abstract

Guided acoustic waves in two-dimensional materials are a key channel for energy transport and dissipation, yet their generation and propagation in transition metal dichalcogenides remain poorly understood. Here, we employ in-situ and spatially decoupled ultra-fast optical pump-probe techniques to investigate guided waves in MoS2 flakes with thicknesses between 90 and 410 nm. We observe a propagating acoustic excitation with a constant velocity of (6.7 +/- 0.8) km/s, independent of thickness. Finite element simulations and symmetry analysis reveal that these vibrations deviate from the classical Lamb wave model and are better described as a superposition of decoupled longitudinal and shear modes. We show that their optical detectability is governed by the Poisson effect: longitudinal components modulate the flake thickness and generate a measurable signal, whereas shear motion remains largely optically invisible. An intrinsic attenuation length of approximately 3.3 microns indicates that dissipation is dominated by material-specific mechanisms rather than geometric spreading. Finally, we demonstrate remote excitation across a nanometric step, enabling acoustic generation in optically inaccessible regions. These results provide a foundation for nanoscale phononic circuits and engineered in-plane energy transport in 2D-based optomechanical and quantum acoustic devices.