Lifetimes of optical phonons in graphene and graphite by time-resolved incoherent anti-Stokes Raman scattering

We report lifetimes of optical phonons (OPs) in graphene and graphite measured by time-resolved anti-Stokes Raman scattering. Lifetimes in graphite and monolayer graphene are 2.4 and 1.2 ps, respectively. For graphite and graphene with more than five layers, the lifetimes decrease with increasing temperature as $\sim 1/T$, indicating the dominance of anharmonic processes in the decay of the OP population. The decrease in lifetime with decreasing number of layers suggests an additional decay channel through which excitations in graphene interact directly with lattice vibrations of the ${\text{a-SiO}}_2$ substrate.

Figure 1

(Color online) Time-resolved anti-Stokes Raman spectra for monolayer graphene normalized by the integration time, power in the probe beam, and spectral width of one CCD pixel. The lower curve is for a time delay between pump and probe of $t=-10\text{ps}$ (black line) and the upper curve is for $t=0.6\text{ps}$ (red line). The oscillations in the data at $<-1800{\text{cm}}^{-1}$ are artifacts from optical interference in the CCD detector.

Figure 2

(Color online) Normalized intensity change of anti-Stokes G band Raman scattering in graphite and monolayer graphene at room temperature as a function of delay time $t$. Symbols (open blue triangles for graphite, solid red circles for monolayer graphene) and the solid lines are the experimental data and model fits, respectively. The extracted exponential relaxation times for graphite and monolayer graphene from these fits are $T_1=2.4\pm 0.1\text{ps}$ and $T_1=1.2\pm 0.1\text{ps}$, respectively. The time correlation between the pump and probe was measured by two-photon absorption in a GaP photodiode and is shown as a black dashed-dot line; the pump-probe correlation has a FWHM of 0.7 ps.

Figure 3

(Color online) Lifetimes of OPs in graphite (solid green diamonds), thick-layer graphene (solid pink squares), 5-LG (solid brown stars), bilayer graphene (open red triangles), and monolayer graphene (solid blue circles) as a function of the temperature of the substrate. Data for the lifetime of the OP in HiPCO nanotubes (open black triangles) and arc-discharge nanotubes (open black inverted triangles) from Ref. 13 are included for comparison. Error bars are assigned from the uncertainties in the model fits.