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Absorption Saturation and Two-Photon Absorption in Graphene Weiqiang Chen, Yu Wang, and Wei Ji # National University of Singapore, Singapore # Email Address: [email protected]

ABSTRACT

We have investigated the photon-energy dependence of nonlinear optical absorption in graphene in the near infrared (NIR) and visible range (1.13 – 3.1 eV). Two nonlinear processes, namely one-photon interband absorption saturation and two-photon absorption (2PA), have been unambiguously determined in high-quality, CVD-grown, multilayer graphene films with using femtosecond Z-scan technique. The absorption saturation is found to have a square dependence on the photon energy. The 2PA spectrum is measured to be close to the theoretical prediction of ω-4 dependence at NIR wavelengths. In the visible range, however, the photon-energy dependence of 2PA is dominated by the excitonic Fano resonance.

INTRODUCTION Nonlinear optical absorption in graphene is of direct relevance to both technological applications and fundamental physics involved in interaction between intense light and two-dimensional materials [1-7]. Absorption saturation, one of the nonlinear optical processes in graphene, has enabled graphene to be an effective saturable absorber for generation of ultrafast laser pulses by mode-locking techniques [5] or short laser pulses by Q-switching methods [6]. Graphene’s absorption saturation is a consequence of Pauli blocking whereby strong interband excitation results in valence-band depletion and conduction-band filling. It should be pointed out that the previous studies [1, 4-7], either experimental or theoretical, focused on the light intensity dependence of absorption saturation in graphene at photon energy of 1.55 eV (or 800-nm wavelength) and 0.8 eV (or 1550 nm). Little is known about its photonenergy dependence, except for the theoretical estimation by Vasko [8]. We previously investigated the photon-energy dependence of absorption saturation in epitaxial graphene in the range from 1.13 to 1.59 eV [2]. Recently, absorption saturation in epitaxial graphene was observed in the range from 0.03 to 0.245 eV and its quadratic photon energy dependence was established in a phenomenological way [3]. In addition, the reports in Ref. [1, 3-7] have overlooked two-photon absorption. In our previous investigation [2], we have demonstrated, both theoretically and experimentally, that two-photon absorption (2PA) is as important as one-photon interband absorption saturation in AB-stacking bilayer graphene.

Here, we present our unambiguous determination of absorption saturation and 2PA in highquality, CVD-grown, multilayer graphene films in the near-infrared (NIR) and visible light range (1.13 – 3.1 eV) using Z-scan technique with femtosecond laser pulses.

EXPERIMENT Large-area, monolayer or 5-layer graphene films were synthesized by CVD in a quartz tube using thermally annealed copper foil (25-µm thick, 99.999% purity) as substrate [9, 10]. The highly crystalline and continuous graphene films were then transferred to quartz substrates us