Excitonic Effects on the Ultrafast Nonlinear Optical Response of MoS₂ and Fluorinated Graphene/MoS₂ Heterostructure...

In the present work, the ultrafast nonlinear optical (NLO) response of some molybdenum disulfide (MoS₂), fluorinated graphene (FG), and FG/MoS₂ heterostructure thin films was studied using the Z-scan and optical Kerr effect techniques employing femtosecond laser pulses at different excitation wavelengths (i.e., 400, 570, 610, 660, 800, and 1200 nm). The experiments have shown that the NLO response of the MoS₂ and MoS₂/FG films was significantly enhanced when the films were excited with 400, 610, and 660 nm laser pulses due to resonance effects with the close-lying excitons in these nanostructures. For a better evaluation of the resonant enhancement of the NLO response, measurements were also carried out at off-resonant wavelengths, i.e., at 570, 800, and 1200 nm. The presence of excitons in the MoS₂ and MoS₂/FG films resulted in strong saturable absorption and self-defocusing, with exceptionally large values of third-order susceptibilities χ⁽³⁾ ranging from 10^-12 to 10^-13 esu. In addition, the NLO response of the MoS₂/FG heterostructure was found to be stronger than that of the individual MoS₂ and FG films, most probably attributed to interlayer carrier transfer. The determined NLO parameters of the studied nanostructures were found to be comparable to, and in some cases exceeded, those of other reported 2D materials known to exhibit a strong NLO response as well. These findings not only advance the fundamental understanding of the contributions of excitons on the NLO response/properties of transition metal dichalcogenide-based ultrathin films but also highlight the importance of excitons for tailoring their NLO response in view of various applications in advanced optoelectronics and photonic devices.