Homogeneous in-plane biaxial tensile strain strengthens the out-of-plane van der Waals (vdW) interaction in ${\mathrm{MoS}}_2$ 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 ${\nu }_{\mathrm{eff}}\approx 0.19\text{–}0.24$. Strikingly, this geometric contraction drives the system into a highly repulsive regime of the intermolecular potential, corresponding to a Grüneisen parameter of $\gamma \approx 14\text{–}20$. This value surpasses even the “giant” one reported for phosphorene, establishing these vdW BLs as highly tunable nonlinear mechanical platforms that can be addressed at the THz regime without external pressure knobs.

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• Optomechanics
• 2-dimensional systems
• Raman spectroscopy