Nonequilibrium plasmon emission drives ultrafast carrier relaxation dynamics in photoexcited graphene

The fast decay of carrier inversion in photoexcited graphene has been attributed to optical phonon emission and Auger recombination. Plasmon emission provides another pathway that, as we show here, drives the carrier relaxation dynamics on ultrafast time scales. In studying the nonequilibrium relaxation dynamics we find that plasmon emission effectively converts inversion into hot carriers, whose energy is then extracted by optical phonon emission. This mechanism not only explains the observed femtosecond lifetime of inversion but also offers the prospect for atomically thin ultrafast plasmon emitters.

Figure 1

Schematic of carrier recombination channels in inverted graphene. (a) Nonequilibrium plasmon emission and (b) Auger recombination both convert electron/hole pairs into hot carriers. In contrast to Auger recombination, plasmon emission is an inelastic process that is enabled by the strong coupling of electron/hole pair to 2D plasmon excitations.

Figure 2

(a) Plasmon frequency dispersion ${\omega }_{\mathrm{pl}}\left(q\right)$ and (b) loss spectrum ${\gamma }_{\mathrm{pl}}\left(\omega \right)$ of intrinsic graphene for varying temperature $T_c$ scaled with chemical potential $\mu$. The dispersion crosses through regimes of interband gain (red area) and absorption (blue area). For $\mu =0.1\mathrm{eV}$ temperatures shown range from $T_c=0–3208\mathrm{K}$.

Figure 3

Relaxation dynamics of the coupled electron/hole and plasmon system. (a) Chemical potential $\mu$ (black) and carrier temperature $T_c$ (red) for ${\tau }_{\mathrm{coll}}=100\mathrm{fs}$ (solid lines) and varying collision time (shaded area). (b) Temporal evolution of nonequilibrium plasmon distributions $n_{\mathrm{pl}}\left(q\right)$ and emission/absorption edge (black dashed lines).

Figure 4

Relaxation dynamics of carriers coupled to (a) phonon only and (b) plasmons and phonons where ${\tau }_{\mathrm{coll}}=100\mathrm{fs}$. Shown are chemical potential $\mu$ (black line), carrier temperature $T_c$ (red line), and temperatures of the $\mathrm{\Gamma }\mathrm{O},\mathrm{KO}$ phonons $T_{\mathrm{\Gamma }\mathrm{O}}$ (blue solid line), and $T_{\mathrm{K}\mathrm{O}}$ (blue dashed line); the gray area in (b) indicates change with collision time. (c), (d) Effective decay times ${\tau }_i$ and amplitudes $A_i$ extracted from triexponential fit to $\mu \left(t\right)$ considering plasmons only (dashed lines) and plasmons and phonons (solid lines); gray lines indicate decay times extracted from (a) (phonons only).