Ultrafast Plasmon Thermalization in Epitaxial Graphene Probed by Time-Resolved THz Spectroscopy

The control of carrier transport by electrical, chemical, or optical Fermi level tuning is central to graphene electronics. Here, an optical pump-terahertz (THz) probe spectroscopy-is applied to investigate ultrafast sheet conductivity dynamics in various epitaxially grown graphene layers representing a large variety of carbon allotropes, including H-2 intercalated films. The graphene layers display a prominent plasmonic response connected with induced THz transparency spectra on ultrashort timescale. It is generally believed that the plasmonic excitations appear due to wrinkles, and substrate terraces that bring about natural confinement potentials. It is shown that these potentials act within micrometer-sized domains with essentially isotropic character. The measured ultrafast dynamics are entirely controlled by the quasi-Fermi level of laser-excited carriers through their temperature. The photocarriers undergo a disorder-enabled super-collision cooling process with an initial picosecond transfer of the optically deposited heat to the lattice followed by a sub-nanosecond relaxation governed by the lattice cooling. The transient spectra is described by a two-temperature Drude-Lorentz model revealing the ultrafast evolution of the carrier temperature and chemical potential and providing crucial material parameters such as Fermi energy, carrier mobility, carrier confinement length, and disorder mean free path.