Strong and tunable interlayer coupling of infrared-active phonons to excitons in van der Waals heterostructures

Understanding and manipulating the quantum interlayer exciton-phonon coupling in van der Waals heterostructures, especially for infrared active phonons with electromagnetic fields, would set a foundation for realizing exotic quantum phenomena and optoelectronic applications. Here we report experimental observations of strong mutual interactions between infrared active phonons in hexagonal boron nitride (hBN) and excitons in ${\mathrm{WS}}_{2}$. Our results underscore that the infrared active ${A}_{2\mathrm{u}}$ mode of hBN becomes Raman active with strong intensities in ${\mathrm{WS}}_{2}$/hBN heterostructures through resonant coupling to the $B$ exciton of ${\mathrm{WS}}_{2}$. Moreover, we demonstrate that the activated ${A}_{2\mathrm{u}}$ phonon of hBN can be tuned by the hBN thickness and harbors a striking anticorrelation intensity modulation, as compared with the optically silent ${B}_{1\mathrm{g}}$ mode. Our observation of the interlayer exciton-infrared active phonon interactions and their evolution with hBN thickness provide a firm basis for engineering the hyperbolic exciton-phonon polaritons, chiral phonons and fascinating nanophotonics based on van der Waals heterostructures.