Graphene as an inhomogeneously broadened two-level saturable absorber

Abstract

We show that the inter-band optical conductivity of graphene follows a dependence on intensity that is characteristic of inhomogeneously broadened saturable absorbers, and we obtain a simple formula for the saturation intensity. We compare our results with those from more exact numerical calculations and selected sets of experimental data, and obtain good agreement for photon energies much larger than twice the chemical potential.

Momentum-Space Observation of Optically Excited Nonthermal Electrons in Graphene with Persistent Pseudospin Polarization

“…At delays (Δ t ) longer than our IRF width, i.e. delays where the pump and probe pulses no longer overlap significantly, the spectra are well described with the simulated thermal signals with temperatures consistent with previous work. , Note that it is expected that the temperature does not scale linearly with the fluence due to both the nonconstant graphene DoS and the onset of saturated absorption in this fluence regime . For all fluences, the momentum- and energy-integrated signal for all electrons above the Dirac point is fit well by a biexponential decay with τ 1 ≈ 200 fs and τ 2 = 1–3 ps depending on fluence, as shown in Figure S3 of the Supporting Information.…”

“…29,35 Note that it is expected that the temperature does not scale linearly with the fluence due to both the nonconstant graphene DoS and the onset of saturated absorption in this fluence regime. 10 For all fluences, the momentum-and energy-integrated signal for all electrons above the Dirac point is fit well by a biexponential decay with S3 of the Supporting Information. However, near Δt = 0, where the pump and probe pulses overlap, a significant population is observed above 0.8 eV that cannot be described by a Fermi−Dirac distribution.…”

“…The gapless conical band structure of graphene is the origin of many exotic optical and electronic phenomena that can be harnessed for applications in optoelectronic devices. The broadband absorption arising from the band linearity makes graphene suitable for optical sensors , or photodetectors in the near-IR to visible range and graphene is now routinely used in technologies such as saturable absorbers in passively mode-locked lasers. − The ultrafast optical response of graphene is due to carrier relaxation mediated by different scattering processes, which lead to rapid thermalization of excited electrons. Electron–electron (e-e) scattering is very efficient in graphene due to the gapless band structure and high carrier mobility, and also electron–phonon (e-ph) coupling between high-energy electrons and graphene’s optical phonons is calculated to be exceptionally large .…”