Evolution of anisotropic-to-isotropic photoexcited carrier distribution in graphene

Femtosecond degenerate and nondegenerate pump-probe spectroscopy in graphene reveals the evolution of photoexcited carrier distribution in the energy band during relaxation: the initial occupation of photoexcited carriers centered at the excitation state is anisotropic in momentum space; this anisotropic distribution rapidly relaxes through an intermediate state argued to be fully isotropic in the energy band due to phonon-involved cascade scattering. In addition, the experiment suggests that graphene optical absorbances for in-plane and out-of-plane optical fields are identical.

Figure 1

(a) Experimental configuration. (b) Polarization orientation of the linearly polarized pump beam.

Figure 2

Pump polarization dependent $\Delta R/R$ time traces for degenerate cases. (a1), (c1), and (c2) Pump fluence dependence of ps dynamics signal. We choose the $\Delta R/R$ at a delay time of 0.83 ps in (a1) and 0.7 ps in (c1) as representative of the ps dynamics signal; the choice is to exclude the influence of the autocorrelation signal. The solid lines in (a1) and (c2) are linear fits; the dashed line in (b1) is a quadratic fit. The $\Delta R/R$ at longer decay time ($\ge 1.2$ ps) scales linearly with pump fluence. The decay times of the ps dynamics signal in (a) and (b) are $1.1\pm 0.1$ and $80\pm 8$ ps, respectively. (b) and (d) Peak $\Delta R/R$ vs angle $\theta$ for (b) 800-nm and (d) 400-nm optical excitation; solid lines are fits. The $\Delta R/R$ time scan measured with a 3.1-eV photon agrees with that probed with a 3.2-eV photon in [26] (the 3.1-eV photon-probed $\Delta R/R$ at long delay time is shown in S7 of [22]).

Figure 3

(a) and (c) Polarization independent $\Delta R/R$ time traces for nondegenerate cases. Zero delay corresponds to the maximum pump-probe signal. (c1) Zoom in view of pump polarization independence of $\Delta R/R$. (b) and (d) Pump fluence dependence of peak $\Delta R/R$ for cases of the (b) 400-nm pump and 800-nm probe and (d) 800-nm pump and 400-nm probe; solid lines are linear fits. The decay times of the ps dynamics signal in (a) and (b) are $1.2\pm 0.2$ and $115\pm 10$ ps, respectively.

Figure 4

Schematic illustration of the photoexcited carrier distribution evolution in the graphene electronic band structure. The red and white color of the conical surface indicates carrier occupation density in the conduction and valence band, respectively. (a) The anisotropic distribution of carriers generated by a linearly polarized beam and potential scattering pathway for carriers (the dotted arrows indicate secondary scattering channels for the photoexcited carriers scattered from the excitation state). (b) Isotropic carrier distribution. Upper left inset: Electronic band curvature for excitation of 400-nm and 800-nm pulses.