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 this limitation thanks to its peculiar conical band structure, providing a universally-resonant wavelength-independent SA at low light intensity that can be further electrically tuned be means of an externally applied gate voltage. 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 and comparing results with atomistic calculations in the framework of tight-binding and random-phase approximation. Further, we investigate the optical properties of randomly-oriented undoped graphene flakes embedded in externally pumped amplifying media. We demonstrate a novel mechanism leading to stable and tunable single-mode cavity-free lasing characterized by a well-determined and highly coherent spatial pattern. This cavity-free lasing mechanism profoundly relies on graphene highly-saturated absorption at rather modest light intensities, a remarkable property which enables self-organization of light into a well determined spatial mode profile.
Andrea Marini, Joel D. Cox, and Javier F. Garcia de Abajo, "Saturable absoprtion of graphene
(Conference Presentation)," Proc. SPIE 9919, Nanophotonic Materials XIII, 99190Q (Presented at SPIE Nanoscience + Engineering: August 31, 2016; Published: 7 November 2016); https://doi.org/10.1117/12.2237745.5159519600001.
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