We investigate the polarization dependence of the carrier excitation and relaxation in epitaxial multilayer graphene. Degenerate pump-probe experiments with a temporal resolution of 30 fs are performed for different rotation angles of the pump-pulse
polarization with respect to the polarization of the probe pulse. A pronounced dependence of the pump-induced transmission on this angle is found. It reflects a strong anisotropy of the pump-induced occupation of photogenerated carriers in momentum space even though the band structure is isotropic. Within 150 fs after excitation an isotropic carrier distribution is established. Our observations imply the predominant role of collinear scattering preserving the initially optically generated anisotropy in the carrier distribution. The experiments are well described by microscopic time-, momentum, and angle-resolved modelling, which allows us to unambiguously identify non-collinear carrier-phonon scattering to be the main relaxation mechanism giving rise to an isotropic distribution in the first hundred fs after optical excitation.
We present an ultrafast graphene-based detector, working in the THz range at room temperature. A logarithmic-periodic antenna is coupled to a graphene flake that is produced by exfoliation on SiO2. The detector was characterized with the free-electro
n laser FELBE for wavelengths from 8 um to 220 um. The detector rise time is 50 ps in the wavelength range from 30 um to 220 um. Autocorrelation measurements exploiting the nonlinear photocurrent response at high intensities reveal an intrinsic response time below 10 ps. This detector has a high potential for characterizing temporal overlaps, e. g. in two-color pump-probe experiments.
