  Charge transfer is a fundamental interface process that can be harnessed for
light detection, photovoltaics, and photosynthesis. Recently, charge transfer
was exploited in nanophotonics to alter plasmon polaritons by involving
additional non-polaritonic materials to activate the charge transfer. Yet,
direct charge transfer between polaritonic materials hasn't been demonstrated.
We report the direct charge transfer in pure polaritonic van der Waals (vdW)
heterostructures of $\alpha$-MoO$_3$/graphene. We extracted the Fermi energy of
0.6 eV for graphene by infrared nano-imaging of charge transfer hyperbolic
polaritons in the vdW heterostructure. This unusually high Fermi energy is
attributed to the charge transfer between graphene and $\alpha$-MoO$_3$.
Moreover, we have observed charge transfer hyperbolic polaritons in multiple
energy-momentum dispersion branches with a wavelength elongation of up to 150%.
With support from the DFT calculation, we find that the charge transfer between
graphene and $\alpha$-MoO$_3$, absent in mechanically assembled vdW
heterostructures, is attributed to the relatively pristine heterointerface
preserved in the epitaxially grown vdW heterostructure. The direct charge
transfer and charge transfer hyperbolic polaritons demonstrated in our work
hold great promise for developing nano-optical circuits, computational devices,
communication systems, and light and energy manipulation devices.
