Fig. 4. Strongly coupled acoustic cavities constructed as a vertically-stacked heterostructure.

a Spectrum for the MoS₂/h-BN/MoS₂ tri-layer; b Spectrum of the neighboring MoS₂/MoS₂ control area. The red tick in b highlights the location of the non-active second harmonic peak. Only odd modes are excited with the homogeneous pump excitation here. c FEM-predicted frequencies for the three lowest vibrational modes of the tri-layer system as function of the spacer h-BN layer thickness. Experimental data (open symbols) are included for the tri-layer (h-BN thickness = 5 L, normalized ε_ZZ strain configurations are shown on the right) and for the control sample (h-BN thickness = 0 L). d, e The normalized ε_ZZ strain profiles for vibrational modes in monolithic MoS₂ (dotted lines) are shown in comparison with strain in the tri-layer system (solid lines) at the labeled frequencies.