Graphene plasmonics is of great interest for compact optical devices working in broad frequency domains with ultrahigh speed and very low energy consumption. However, graphene plasmons damp out quickly on most substrates mainly due to scattering loss from substrate surface phonons and impurities. Here we discover a new hybridized plasmon-phonon polariton mode in graphene/h-BN van der Waals heterostructures, which enables ultralong hybrid plasmon lifetime up to 1.6 picosecond, the longest plasmon lifetime ever demonstrated. Such remarkably long lifetime arises from the coupling of long-lifetime h-BN transverse optical phonon with graphene plasmons, which uniquely exists in monolayer heterostructures. Our findings and understanding of this unexploited hybrid mode offer a novel approach to tune the plasmon behaviours in the frequency, time and space domains. This can potentially introduce a new paradigm to generate highly-confined plasmons with ultra-long lifetime for various applications, such as deep-subwavelength metamaterials, ultra-low-loss waveguides, and ultrafast optical switches.
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