Experimental Comparison of Nonlinear Optical Properties Between Graphene Oxide and Reduced Graphene Oxide MORTEZA A. SHARIF , 1,4 SOMAYEH SALMANI, 2,3 SALMAN MOHAJER, 2,3 and M.H. MAJLES ARA 2,3 1.—Optics and Laser Engineering Group, Faculty of Electrical Engineering, Urmia University of Technology, Band Road, Urmia, Iran. 2.—Research Institute of Applied Sciences, Kharazmi University, Karaj, Iran. 3.—Photonics Laboratory, Kharazmi University, Mofatteh, Tehran, Iran. 4.—e-mail: m.abdolahisharif@ee.uut.ac.ir We delineate a comparative investigation between dispersed graphene oxide (GO) and reduced graphene oxide (RGO) by experimentally measuring their principal/nonlinear optical parameters. We show that the nonlinear refractive index of RGO is larger than that of GO while the nonlinear absorption coef- ficient of RGO is almost negligible. We particularly organize an experimental plan using GO and RGO inks included in a Mach–Zehnder interferometer and illuminated by a light beam with the wavelength 650 nm. We obtain that the lower threshold input power and more pronounced hysteresis loops are ob- tained for RGO in comparison to GO. We infer that the contribution of non- linearity is majorly refractive in RGO rather than absorptive. In return, GO shows a larger absorptive nonlinearity compared to RGO in consequence of the larger nonlinear absorption coefficient. Although GO seems to be appropriate for the saturable absorption-based applications, we deduce that partially re- duced GO is more preferred since it can appear as an active electrode pro- viding then an ultrathin electric double layer required for ultrashort pulse generation. Our results indicate that RGO is also suitable for the electro- optical applications like the modulation through which the Fermi energy is to be tuned with a low bias voltage. As well, we propose RGO for all-optical applications like the optical switching for which the highly nonlinear response is required. Key words: Graphene oxide, reduced graphene oxide, nonlinear Schro ¨dinger equation, nonlinear refractive index, nonlinear absorption INTRODUCTION Due to the superlative optical properties like the broad transmission range, ultrafast electric response, large optical nonlinearity and high charge carrier mobility, 1–8 graphene is distinguished as a useful two dimensional nanostructure for various applications in nanophotonics. 9–14 Graphene can be produced in different types: monolayer/few layers pristine graphene, graphene oxide (GO), reduced graphene oxide (RGO), etc. GO is a prevalent type; it has high transparency and can be cheaply prepared. It also possesses the considerable nonlinear charac- teristics particularly suitable for the application of Saturable Absorption (SA). 10,15–17 However, GO with high oxidation degree is almost an insulator. This in turn confines its usefulness for the applications through which the tunability of optical conductivity is required. 18–22 The latter can be modified by reduc- ing the GO to RGO. On this base, easy reduction methods have been proposed and developed to pre- pare RGO in a large deal, with an inexpensive cost compared to that of the pristine graphene. 23–31 This will be especially determinative if one notices that RGO has similar optical properties to the pristine graphene. 32–34 Highly reduced GO in its turn suffers (Received January 9, 2019; accepted July 9, 2019) Journal of ELECTRONIC MATERIALS https://doi.org/10.1007/s11664-019-07442-w Ó 2019 The Minerals, Metals & Materials Society