Ultrafast Photoluminescence from Graphene

Since graphene has no band gap, photoluminescence is not expected from relaxed charge carriers. We have, however, observed significant light emission from graphene under excitation by ultrashort (30-fs) laser pulses. Light emission was found to occur across the visible spectral range (1.7–3.5 eV), with emitted photon energies exceeding that of the excitation laser (1.5 eV). The emission exhibits a nonlinear dependence on the laser fluence. In two-pulse correlation measurements, a dominant relaxation time of tens of femtoseconds is observed. A two-temperature model describing the electrons and their interaction with strongly coupled optical phonons can account for the experimental observations.

Figure 1

(a) Spectral fluence of light emission from graphene for excitation with 30-fs pulses of absorbed fluences of $F=0.17$ and $0.33{\mathrm{J}\mathrm{m}}^{-2}$. The spectra are compatible with the predictions for thermal emission (dashed blue lines) for $T_{em}=2760\mathrm{K}$ and 3180 K, respectively. (b) Light emission as a function of absorbed fluence $F$. The red circles display experimental values for the total radiant fluence for photons from 1.7 to 3.5 eV. The emission can be described phenomenologically by $F^{2.5}$ (dashed blue line). The magenta squares correspond to the experimental emission temperatures $T_{em}$ for different absorbed fluences. The solid green lines in (a) and (b) represent a full calculation using the two-temperature model. The predicted emission fluence has been multiplied by a factor of $\sim 0.2$ to match the scale of the experimental data.

Figure 2

Simulations using the two-temperature model of the temporal evolution of the electronic temperature $T_{el}$ (red line), the SCOPs temperature $T_{op}$ (black line), and of the resulting graphene light emission (green line, lower panel) for photon energies from 1.7 to 3.5 eV and absorbed fluence $F=0.33{\mathrm{J}\mathrm{m}}^{-2}$. For comparison, the upper panel also shows the calculated electronic temperatures for completely decoupled electrons and for full equilibrium of all degrees of freedom of the graphene (dashed red lines).

Figure 3

Total radiant fluence emitted by graphene over photon energies of 1.7–3.5 eV (red circles), 1.75–2.0 eV (blue squares) and 2.5–2.75 eV (magenta triangles) as a function of temporal separation between two excitation pulses, each with absorbed fluence $F=0.17{\mathrm{J}\mathrm{m}}^{-2}$. The data for positive and negative delays were averaged to increase the signal-to-noise ratio. The symbols are experimental data; the green lines are the predictions of the two-temperature model, multiplied by $\sim 0.2$ to match the scale of the experimental data.

Figure 4

Simulations as in Fig. 2, but with excitation by a pair of laser pulses separated in time by 150 fs. Each pulse has absorbed fluence $F=0.17{\mathrm{J}\mathrm{m}}^{-2}$.