Two-dimensional transition metal dichalcogenides (TMDCs) are semiconducting materials with strong in-plane covalent bonding and weak inter-plane van der Waals forces, enabling isolation of monolayers via simple methods like mechanical exfoliation. Interest in TMDCs has surged due to their notable properties, including strong light–matter interactions at room temperature, making them promising for electronics, optoelectronics, and acousto-optics [1–2]. In the latter, TMDC flakes of varying thickness act as nanocavities generating high-frequency longitudinal acoustic phonons (~10 GHz to ~1 THz), outperforming other materials used in sensors, modulators, and filters [3]. However, a comprehensive understanding of its resonant behavior and dissipation mechanisms across a broad thickness range and under different boundary conditions remains lacking.

We present ultrafast pump–probe measurements of breathing modes in ∼300 MoS₂ flakes, from <10 to ~1000 layers, under three substrate conditions: free-standing, and supported on dense or mesoporous silica (~100 nm thick). Measured frequencies span 4–300 GHz. Quality factors (Q) generally follow expected trends based on acoustic decoherence—anharmonicities and Lamb waves at low frequencies, scattering at high frequencies. However, we observe distinct resonances at specific thicknesses. Notably, resonant enhancement of oscillation amplitude and Q occurs under two conditions: (i) when the MoS₂ mode aligns with an acoustic cavity mode in the mesoporous film, and (ii) when optical and acoustic resonances coincide in MoS₂. In peak cases, Q improves by an order of magnitude over off-resonance values.

We also detect higher harmonics in flakes with fundamental frequencies below 20 GHz, sometimes matching or exceeding the fundamental in amplitude, with parity consistent with expected boundary conditions. Furthermore, we directly observe surface waves propagating radially from the optically pumped region, attributed to Lamb-type waves in unsupported structures [4].

Our findings show that coupling between MoS₂ vibrational modes and substrate resonances—both acoustic and optical—enables enhanced, tunable mechanical behavior. This reveals complex dynamics in both free-standing and supported TMDCs and suggests potential for hybrid phononic–photonic devices.

[1] P. Soubelet et al., Nanoscale 2019, 11, 10446
[2] S. Ge et al., Sci. Rep. 2014, 4, 5722
[3] A. D. Carr et al., ACS Photonics 2024, 11, 1147−1155
[4] D. M. Photiadis et al., Phys. Rev. B 2020, 101, 245304