Abstract
Saturable absorption (SA) is an inherent property of photonic materials that manifests itself as an absorption quenching at high light intensities and is a key element for passive mode-locking (PML) in laser cavities, where continuous waves break into a train of ultrashort optical pulses. Currently, state-of-the-art semiconductor-based SA mirrors are routinely employed for PML lasers. However, these mirrors operate in a narrow spectral range, are poorly tunable, and require advanced fabrication techniques. Graphene overcomes these limitations thanks to its peculiar conical band structure, providing a universally-resonant wavelength-independent SA at low light intensity [1] that can be further electrically tuned be means of an externally applied gate voltage, thus enabling PML ultrafast laser operation [2]. Here, we calculate intraband and interband contributions to SA of extended graphene by solving non-perturbatively the single-particle Dirac equation for massless Dirac fermions in the presence of an external electromagnetic field. Further, we investigate the dependence of the intensity-saturated graphene conductivity on doping, temperature, and optical frequency. We find a remarkably low intensity-threshold for SA, which is consistent with reported experimental findings. In addition, our calculations indicate a strong quenching of absorption depth ensuing from doping through an externally applied voltage, and a weak dependence on electron temperature. We envisage that our results will be relevant for the engineering of graphene-based PML fibre lasers and single-mode random lasers [3].
© 2017 IEEE
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