Infrared Fingerprints of Natural 2D Talc and Plasmon-Phonon Coupling in Graphene-Talc Heterostructures

Two-dimensional (2D) materials occupy a noteworthy place in nanophotonics providing for subwavelength light confinement and optical phenomena dissimilar to those of their bulk counterparts. In the mid-infrared, graphene-based heterostructures and van der Waals crystals of hexagonal boron nitride (hBN) overwhelmingly concentrate the attention by exhibiting real-space nano-optics features from plasmons, phonon-polaritons, and hybrid plasmon phonon-polaritons. Here we present a prime study on mid-infrared nanophotonics of talc, a natural atomically flat layered material, and graphene-talc (G-talc) heterostructures using broadband synchrotron infrared nanospectroscopy. Wavelength tuning of the talc vibrational resonances, assigned to in- and out-of-plane molecular vibrations, are achieved by changing the thickness of the crystals, which configures a tunable infrared signature for the 2D talc. In G-talc nanostructures, we unveil a coupling of the graphene plasmons polaritons with surface phonons polaritons of talc, originating hybrid surface plasmon-phonon polaritons modes. In analogy to G-hBN and G-SiO2 heterostructures, the coupling in G-talc produces a large increase in the opto-vibrational activity for the out-of-plane mode as well as it induces a blue-shift for the in-plane mode. Moreover, the coupling can be modulated by an external gate voltage to the heterostructure when mounted in a transistor configuration. Therefore, our results introduce talc and G-talc heterostructures as appealing materials for nanophotonics, especially for applications involving long wavelengths and active electric tuning of opto-vibrational activity.