Transient charge and energy balance in graphene induced by ultrafast photoexcitation

TitleTransient charge and energy balance in graphene induced by ultrafast photoexcitation
Publication TypeJournal Article
Year of Publication2013
AuthorsZhang JH, Schmalian J, Li TQ, Wang JG
Journal TitleJournal of Physics-Condensed Matter
Date Published08
Type of ArticleArticle
ISBN Number0953-8984
Accession NumberWOS:000321944000003
Keywordsepitaxial graphene, graphite films

Ultrafast optical pump-probe spectroscopy measurements on monolayer graphene reveal significant optical nonlinearities. We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac-fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates for absorption loss in the graphene layer. To quantitatively investigate this transient, extremely dense photoexcited Dirac-fermion state, we construct a two-chemical-potential model, in addition to a time-dependent transient carrier temperature above the lattice temperature, to describe the population inverted electronic state metastable on the time scale of tens of femtoseconds generated by a strong exciting pulse. The calculated transient optical conductivity reveals a complete bleaching of absorption, which sets the saturation density during the pulse propagation. In particular, the model calculation reproduces the negative optical conductivity at lower frequencies in the states close to saturation, corroborating the observed femtosecond stimulated emission and optical gain in the wide near-infrared window.

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