Figure 1

(Color online) (a) Schematics of the experimental setup. (b) White-light microscopy image of a thin graphene sample. (c) cw Raman spectra taken at positions (1)–(4). (d) Nonlinear PL image of the same sample as excited by 1.5 eV femtosecond pulses with signal integrated from 1.66 to 3.11 eV. Inset: PL signal along the line cut.Reuse & Permissions

Figure 2

(Color online) (a) PL image of a graphene monolayer alongside a Au film. Inset: white-light microscopy image of the same area. (b) PL spectra at various excitation frequencies (marked by arrows) from a few-layer graphene sample on glass substrate (sharp cuts are due to interference filters). The $G$ and $2D$ Stokes Raman modes upon 2.20 eV excitation are marked by “${}^{\ast }$”. The Au TPPL spectrum is shown for comparison (gray). (c) Integrated PL signal vs excitation density at various excitation frequencies (curves are vertically offset for clarity). Solid lines are fits using Eq. (2). (d) PL signal as a function of the delay between two excitation pulses. Red curve (the envelope) is a Gaussian fit. Inset: the SH autocorrelation trace from $\alpha$-quartz. The excitation frequency is 1.5 eV and the PL signal is integrated from 1.66 to 3.11 eV in (a) and (d).Reuse & Permissions

Figure 3

(Color online) (a) Schematics of multiphoton transitions in graphene near the Dirac point. (b) Schematics of elastic scattering between two carriers with initial momentum ${\mathbf{k}}_1$, ${\mathbf{k}}_2$, and exchanged momentum $\mathbf{Q}$. Upper (lower) panel is the view in $\mathbf{k}$ $\left(E\text{−}\mathbf{k}\right)$ plane, red dots (circles) refer to hot electrons (holes), respectively. The broken blue line is the trajectory of final states allowed by energy and momentum conservation. (c) Schematics of the same process depicted in (b) but in a system of parabolic bands and an energy gap $E_g$. (d) PL spectra excited at 1.5 eV for different excitation fluence. Solid lines are fits using Eq. (3). Spectra are corrected for the detection system response. Inset: fitted values of $Q_s$ vs fluence.Reuse & Permissions