A common experimental setup to study light-matter interactions consists of semiconducting crystals placed in a metallic cavity that can support confined photons. In that case, the regime of strong coupling can be achieved provided that the semiconducting crystal supports excitons of large oscillatory strength. Here the coupling between transverse-electric cavity-photons and excitons in crystalline ${\mathrm{C}}_{60}$ films of different thicknesses is studied in the framework of the quantum-electrodynamical Bethe-Salpeter equation. The binding strength is characterized by the Rabi splitting $\mathrm{\Omega }$ of exciton-polaritons as a function of a number of crystal layers $N$ in the van der Waals heterostructures. For the considered nanocavity system a transition from the weak ($\mathrm{\Omega }=50\mathrm{meV}$) to the strong ($\mathrm{\Omega }=350\mathrm{meV}$) coupling with an increasing number of layers N is obtained. This layer-dependent effect seems to be universal, since we also estimate an enhancement of exciton-photon binding energy by a factor of $\sim 4$ in hBN multilayers. With this we show that a few nanometer thick two-dimensional heterostructures can significantly modify the zero-point fluctuation energy of cavity photons, which may have many fundamental and practical consequences within the field of light-matter interactions.

• Bethe-Salpeter equation
• Excitons
• Optoelectronics
• Photonics
• Fullerenes
• Quantum molecular crystals