Ultrafast Laser-Induced Photothermoelectric Currents in Graphene Junctions in the Mid-Infrared

We investigate the carrier dynamics in graphene junctions under mid-infrared excitation using ultrafast pump-probe photocurrent spectroscopy. We utilize dual split gate devices to demonstrate that the photothermoelectric effect can dominate the photocurrent response of graphene also for a mid-infrared femtosecond excitation. Graphene retains its broadband photocurrent response in this spectral region, but the photocurrent relaxation time increases from ∼2 ps below 8-9 μm up to 3 ps at longer mid-infrared wavelengths. The absence of a pronounced phonon bottleneck in the decay dynamics at room temperature suggests an efficient interplay of electron-electron and electron-phonon scattering even for photon energies below the optical phonon energy in graphene. The observed wavelength dependence of the photocurrent relaxation times is consistent with energy-dependent theoretical relaxation times as derived from a microscopic transport theory of graphene that includes electron-phonon coupling within a Holstein-Peierls Hamiltonian.