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Double-bended saturation of optically induced bleaching in graphene
Winnerl, S.; Winzer, T.; Mittendorff, M.; Mittenzwey, H.; Jago, R.; Schneider, H.; Helm, M.; Malic, E.; Knorr, A.;
Saturable absorption due to Pauli blocking is a fundamental optical phenomenon that can be described fully analytically for a two-level system. In solids, the related carrier dynamics is typically much more complex. Nevertheless, the fluence dependence of the induced bleaching is typically qualitatively similar to the behaviour of a two-level system. Saturable absorbers are important photonic devices for realizing short laser pulses.
We present a joint theory-experiment study, where the bleaching of graphene is studied in a wide range of fluences. In pump-probe experiments utilizing 30 fs near-infrared (wavelength 800 nm) pulses the pump-induced transmission is measured. The study reveals an unusual double-bended saturation behaviour. For fluences in the mJ/cm2 range the induced transmission saturates due to Pauli blocking. Interestingly, a qualitatively similar behaviour is found at fluences that are 1000 times smaller. In this range one would expect a linear fluence dependence of the induced transmission. Microscopic theory based on the density matrix formalism shows that the unexpected saturation at low fluences is related to intensity dependent many-particle scattering. The crucial point is the balance between in- and out-scattering of electrons from the optically excited k-space regions. The occupation of this region determines the observed transmission [1].
Full understanding of the saturation behaviour in graphene is of relevance for graphene-based saturable absorbers. Graphene is an interesting material for this purpose as it can be applied in a very broad spectral range from THz to UV [2,3]. Also the high damage threshold, which is verified in our experiments, is an attractive feature.

[1] T. Winzer et al., Nature Commun. 8 (2017) 15042
[2] V. Bianchi et al., Nature Commun. 8 (2017) 15763
[3] D. G. Purdie et al., Appl. Phys. Lett. 106 (2015) 253101
Keywords: graphene, ultrafast dynamics, saturable absorption
  • Poster
    Graphene 2018, 26.-29.06.2018, Dresden, Deutschland

Publ.-Id: 28703 - Permalink