  A fundamental building block in nano-photonics is the ability to
directionally excite highly squeezed optical mode dynamically, particularly
with an electrical bias. Such capabilities would enable the active manipulation
of light propagation for information processing and transfer. However, when the
optical source is built-in, it remains challenging to steer the excitation
directionality in a flexible way. Here, we reveal a novel mechanism for tunable
directional excitation of highly squeezed polaritons in graphene-hexagonal
boron nitride (hBN) heterostructures. The effect relies on controlling the sign
of the group velocity of the coupled plasmon-phonon polaritons, which can be
flipped by simply tuning the chemical potential of graphene (through
electrostatic gating) in the heterostructures. Graphene-hBN heterostructure
thus present a promising platform toward nano-photonic circuits and
nano-devices with electrically reconfigurable functionalities.
