  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.
