Guided Acoustic Waves in MoS ₂ Thin Flakes

Guided acoustic waves in 2D materials are vital for energy transport, yet their behavior in transition metal dichalcogenides remains poorly understood. Here, we use spatially decoupled ultrafast optical techniques to investigate guided waves in MoS 2 flakes (90–410 nm). We observe propagating acoustic excitations with a constant velocity of (6.7 ± 0.8) km s –1, independent of thickness. Simulations and symmetry analysis reveal these vibrations deviate from classical Lamb wave models, instead acting as a superposition of decoupled longitudinal and shear modes. Their optical detectability is governed by the Poisson effect: longitudinal components modulate thickness to generate signals, while shear motion remains invisible. An intrinsic attenuation length of ∼13 μm suggests dissipation is influenced by material-specific mechanisms rather than only geometric spreading. Finally, we demonstrate remote excitation across nanometric steps, enabling acoustic generation in inaccessible regions. These findings provide a foundation for nanoscale phononic circuits and engineered energy transport in optomechanical and quantum acoustic devices.