Direct observation of the strong thermal non-equilibrium between optical and acoustic phonons in monolayered 2D materials

Quantifying thermal non-equilibrium between different phonon branches in supported monolayered two-dimensional materials remains a significant challenge due to complex substrate interactions and optical interference in Raman measurements. Here, we report on the direct quantification of optical-acoustic phonon thermal non-equilibrium in supported monolayered WS2 materials by leveraging the distinct thermal response of optical phonons (OPs) and acoustic phonons (APs) under modulated laser excitation of different frequencies. The temperature response of different phonon branches is separated based on the measured Raman shift power coefficient (ψ) with high precision. The reduction of the laser spot radius from ∼1.7 to ∼0.4 μm increases the ψOA/ψAP ratio by over 2.5 times, revealing a pronounced enhancement of optical-acoustic phonon thermal non-equilibrium. Comparisons between transferred monolayered WS2 and hetero-bilayered WS2/WSe2 show that the heterogeneous interface strongly strengthens optical to acoustic phonon scattering. In addition, smaller laser spots intensify the hot carrier diffusion effect, reducing the associated OP–AP temperature difference by over 60%. Our method establishes a powerful experimental platform for probing phonon transport in nanoscale materials.