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.