Nano-imaging of Plasmon polaritons of graphene on SrTiO₃ using terahertz scanning near-field optical microscope

The electrical and optoelectronic properties of graphene are significantly influenced by the underlying substrate. Replacing the conventional SiO₂/Si substrate with a high-dielectric, atomically smooth ferroelectric substrate such as SrTiO₃ dramatically changes the carrier densities dynamics in graphene. To explore the unique properties of surface plasmon polaritons (SPPs) in graphene on a dielectric substrate, we experimentally apply two advanced imaging techniques: nano-photocurrent mapping in magneto-scanning near-field optical microscopy (m-SNOM) and terahertz time-domain scanning near-field optical microscopy (THz-SNOM) with spacetime metrology. Utilizing THz-SNOM with fine spatial and temporal resolution, we investigate the charge density inhomogeneities and group velocities of SPPs, revealing key insights into the plasmonic behavior of 2D plasmons with a SrTiO₃ substrate.