Tuning Terahertz Optomechanics of MoS2 Bilayers with Homogeneous In-plane Strain

Abstract

Homogeneous in-plane biaxial tensile strain strengthens the out-of-plane van der Waals (vdW) interaction in \MoS\ bilayers (BLs) and can be used to fine-tune their terahertz (THz) oscillations. Using ultralow-frequency Raman spectroscopy on hexagonal (2H) and rhombohedral (2R) stacked BLs, we observe a hardening of the interlayer breathing modes originating from a strain-induced Poisson contraction of the vdW separation between the layers characterized by an effective out-of-plane Poisson's ratio of $\hat{I}{½}_{\mathrm{eff}}\approx 0.19\text{–}0.24$. Strikingly, this geometric contraction drives the system into a highly repulsive regime of the intermolecular potential, corresponding to a Grüneisen parameter of ${\hat{I}}^3\approx 14\text{–}20$. This value surpasses even the `giant' one reported for phosphorene, establishing these van der Waals BLs as highly tunable nonlinear mechanical platforms that can be addressed at the THz regime without external pressure knobs.