Polaritons in two-dimensional
(2D) materials have shown their unique
capabilities to concentrate light into deep subwavelength scales.
Precise control of the excitation and propagation of 2D polaritons
has remained a central challenge for future on-chip nanophotonic devices
and circuits. To solve this issue, we exploit Cherenkov radiation,
a classic physical phenomenon that occurs when a charged particle
moves at a velocity greater than the phase velocity of light in that
medium, in low-dimensional material heterostructures. Here, we report
an experimental observation of Cherenkov phonon polariton wakes emitted
by superluminal one-dimensional plasmon polaritons in a silver nanowire
and hexagonal boron nitride heterostructure using near-field infrared
nanoscopy. The observed Cherenkov radiation direction and radiation
rate exhibit large tunability through varying the excitation frequency.
Such tunable Cherenkov phonon polaritons provide opportunities for
novel deep subwavelength-scale manipulation of light and nanoscale
control of energy flow in low-dimensional material heterostructures.