Zahra Dehghani - Department of Physics, University of Neyshabur, Neyshabur, ۹۳۱۹۷۷۴۴۴۶, Iran.
Iman Sahraneshin Bazzi - Department of Physics, University of Neyshabur, Neyshabur, ۹۳۱۹۷۷۴۴۴۶, Iran.
Davoud Vahedi Fakhrabad - Department of Physics, University of Neyshabur, Neyshabur, ۹۳۱۹۷۷۴۴۴۶, Iran.
Fatemeh Ostovari - Department of Physics, Faculty of Science, Yazd University, Yazd, Iran.

The next generation of photonics and nano-optical devices may be based on two-dimensional(۲D) transition metal dichalcogenides (TMDs). In this research, molybdenum diselenide (MoSe۲) nanosheets, as one important member of TMDs, have been synthesized by the solvothermal method and characterized through XRD patterns, SEM, and TEM images. Nanosheets were found to have a hexagonal phase based on XRD patterns and the crystallinity percentage is ۲۴.۸ %.  The lattice constants of the hexagonal phase of MoSe۲ are calculated as a= ۳.۰۸ ͦ A, c= ۱۳.۷۲ ͦ A. The calculated average value of the crystallite size, dislocation density, and micro strain are ۲۱.۹۳۵ nm, ۲.۱۳۸   nm-۲ and ۹.۰۷۰ , respectively. A few layers of nanosheets without wrinkles were observed on TEM and SEM. Next, the synthesized nanosheets were employed to prepare thin films with three different thicknesses using the spin coating method.  By employing a continuous wave (CW) Nd : YAG laser at ۵۳۲ nm via a Z-scan approach, this study investigates how thin film thickness affects the thermal nonlinear optical (NLO) responses of MoSe۲ nanosheets. The magnitude of NLO coefficients of the prepared thin films decreased with increasing film thickness. It is observed that the prepared thin films possess saturable absorption (SA) as well as the self-focusing effect. Saturable absorbers and mode-locking devices can be developed with MoSe۲ thin films because of their improved NLO properties.

[MoSe]۲, Nonlinear optics, Saturable Absorption, Self-focusing, Thin Film