Phonon polaritons in van der Waals polar heterostructures for broadband strong light-matter interactions

Phonon polaritons in polar crystals have recently gained significant attention due to their remarkable confinement and enhancement of electromagnetic fields, low group velocities, and low losses. However, these unique properties, resulting from the coupling between photons and lattice vibrations, exhibit limited spectral responses that may hinder their practical applications. Here, we propose and experimentally demonstrate that polar van der Waals heterostructures can integrate their polar constituents to enable broadband phonon polariton responses. A polar heterostructure is created by simply transferring thin flakes of two polar van der Waals materials, hexagonal boron nitride (hBN) and α-phase molybdenum trioxide (α-MoO₃), onto a polar quartz substrate. Direct infrared nanoimaging experiments show that this integrated heterostructure supports phonon polaritons in a broadband infrared spectral range from 800 to 1700 cm^-1. Further, numerical calculations predict vibrational strong coupling for a few molecule monolayers with multiple molecular absorption modes and phonon polaritons in the heterostructure. Our findings suggest that broadband phonon polariton responses in van der Waals integrated heterostructures have the potential to pave the way for the development of broadband and integrated infrared devices of molecular sensing, signal processing, and energy control.