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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 has not been demonstrated. We report the direct charge transfer in pure polaritonic van der Waals (vdW) heterostructures of α-MoO3/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 α-MoO3. Moreover, we have observed charge transfer hyperbolic polaritons in multiple energy–momentum dispersion branches with a wavelength elongation of up to 150%. With the support from the density functional theory calculation, we find that the charge transfer between graphene and α-MoO3, 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. 

Two-dimensional van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe5GeTe2 with a Curie temperature T$_c = 332$ K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The experiments were performed with external magnetic fields applied along the c-axis (H$\parallel$c) and the ab-plane (H$\parallel$ab), with temperatures ranging from 300 to 10 K. We have found a sizable Landé g-factor difference between the H$\parallel$c and H$\parallel$ab cases. In both cases, the Landé g-factor values deviated from g = 2. This indicates contribution of orbital angular momentum to the magnetic moment. The FMR measurements reveal that Fe5GeTe2 has a damping constant comparable to Permalloy. With reducing temperature, the linewidth was broadened. Together with the VSM data, our measurements indicate that Fe5GeTe2 transitions from ferromagnetic to ferrimagnetic at lower temperatures. Our experiments highlight key information regarding the magnetic state and spin scattering processes in Fe5GeTe2, which promote the understanding of magnetism in Fe5GeTe2, leading to implementations of Fe5GeTe2 based room-temperature spintronic devices.