Optofluid. Microfluid. Nanofluid. 2016; 3:53–58 Research Article Open Access Nikolaos Liaros, Ioannis Orfanos, Ioannis Papadakis, and Stelios Couris* Nonlinear optical response of some Graphene oxide and Graphene fluoride derivatives DOI 10.1515/optof-2016-0009 1 Introduction Received November 8, 2016; revised December 5, 2016; accepted De- cember 12, 2016 It is about 10 years ago that Optofluidics emerged as a new Abstract: The nonlinear optical properties of two graphene scientific research area, originating from the combination derivatives, graphene oxide and graphene fluoride, are in- of two already established research fields, micro-fluidics vestigated by means of the Z-scan technique employing and optics [1]. Optofluidics aims to combine the advan- 35 ps and 4 ns, visible (532 nm) laser excitation. Both tages of each of these two research areas in the same plat- derivatives were found to exhibit significant third-order form. In fact, Optofluidics aims to combine the appealing nonlinear optical response at both excitation regimes, versatility of using fluids to perform actions arising from with the nonlinear absorption being relatively stronger their micro- or nano-fluidic properties and/or to their op- and concealing the presence of nonlinear refraction un- tical properties in order to replace classical optical com- der ns excitation, while ps excitation reveals the presence ponents made from glass, metals, or plastics. That is why of both nonlinear absorption and refraction. Both nonlin- in the first approach, Optofluidics referred to a class of ear properties are of great interest for several photonics, optical systems employing fluids as optical components. opto-fluidics, opto-electronics and nanotechnology appli- However, the use of fluids possessing nonlinear optical re- cations. sponse or fluids containing materials (e.g. dyes, nanopar- Keywords: graphene oxide, graphene fluoride, nonlinear ticles, etc.) exhibiting nonlinear optical response can of- optical response, nonlinear absorption coefficient, non- fer some more functionalities and open new horizons for linear refractive index parameter, third-order susceptibil- optofluidic devices [1, 2]. For instance, the use of stable ity χ(3) colloidal solutions of some nonlinear optical material can be of interest for some applications as e.g. optical limiters, optical deflectors, etc. Furthermore, the ease of tunability of the nonlinear optical response by simple chemical pro- cesses (as for example by oxidation or reduction, etc.) or through the concentration of the dispersed nonlinear ma- terial can provide additional versatility to realize optoflu- idic devices making use of the combination of the nonlin- ear optical response of a material and the fluidic aspects. *Corresponding Author: Stelios Couris: Department of Physics, Graphene, the youngest of the carbon allotropes, gen- University of Patras, 26504 Patras, Greece erated an enormous scientific interest almost immediately and Institute of Chemical Engineering Sciences, Foundation for after its discovery, mainly due to the several unprece- Research and Technology-Hellas (FORTH), P.O. Box 1414, 26504 Patras, Greece dented and extraordinary optical, electrical and mechan- Nikolaos Liaros: Department of Physics, University of Patras, 26504 ical properties it exhibits, which have created great ex- Patras, Greece pectations for potential applications in several areas of and Institute of Chemical Engineering Sciences, Foundation for modern technology and nanotechnology. Graphene, be- Research and Technology-Hellas (FORTH), P.O. Box 1414, 26504 ing an atom-thick, two-dimensional carbon layer consist- Patras, Greece Current address: Department of Chemistry & Biochemistry, Univer- ing of hexagonally packed carbon atoms where carbon is sity of Maryland, College Park, MD 20742, USA the principal building block, allows the formation of many Ioannis Orfanos, Ioannis Papadakis: Department of Physics, new stable and structurally fascinating graphene-based University of Patras, 26504 Patras, Greece nanomaterials and nanostructures by means of bottom- and Institute of Chemical Engineering Sciences, Foundation for up and top-down approaches [3–6]. As for the optical Research and Technology-Hellas (FORTH), P.O. Box 1414, 26504 properties of graphene, unlike conventional semiconduc- Patras, Greece © 2016 Nikolaos Liaros et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. 54 Ë Nikolaos Liaros et al. tor materials, it exhibits an almost constant absorption (of a large energy gap, which is efficiently controlled by tun- ~2.3%) and broadband optical transparency over its entire ing the degree of oxidation, i.e. the sp2/sp3 ratio. The optical spectrum, from UV to IR. Its absorption is deter- ability to tune this ratio allows the continuous tuning of mined only by the fine-structure constant and not by the GO’s band gap, suggesting an efficient way for the tai- properties of the material itself. Furthermore, graphene loring of GO’s NLO response [24, 29]. However, this de- can form easily multilayered structures, and it has been pendence is neither simple nor straightforward because shown that its absorption scales linearly with the num- of the non-stoichiometric nature of GO samples. Another ber of layers [7]. It has also been demonstrated that when way of modifying GO’s NLO response controllably is by the incident light intensity is strong enough, graphene varying the number of graphitic layers of the sample, as exhibits significant broadband saturable absorption (SA), each layer adds about 2.3% of absorption. Moreover, GO due to the Pauli blocking principle, which makes graphene is more easily dispersed in organic solvents compared to suitable for mode-locking applications, e.g. in ultrafast pristine graphene, while it exhibits very good dispersabil- lasers [8, 9], for photodetectors [10, 11], for optical modu- ity in water, forming very stable aqueous colloidal dis- lators [12], in plasmonics [13] and for other optoelectronics persions. This makes it even more attractive in view of structures and devices [14–16]. In addition, graphene has health and environmental safety issues and, in particular, been shown to exhibit important nonlinear optical (NLO) in view of opto-fluidic applications. Furthermore, the pres- responses in a very wide range of frequencies, from mi- ence of hydroxyl and/or carboxyl functionalization groups crowaves and terahertz frequencies [17] up to optical fre- expands significantly the strategies for its further chemical quencies [18, 19]. In particular, concerning the range of derivatization [30]. the optical frequencies, inter-band optical transitions can Graphene fluoride [28] consists of a graphene layer occur at all photon energies. Beyond the typical SA re- with covalently attached fluorine atoms (instead of oxy- sponse of pristine graphene and the optical limiting ef- gen atoms), being a 2-dimensional wide band gap semi- fect due to nonlinear scattering (NLS) [20], its derivatives conductor, exhibiting broadband transparency over its en- have been shown to exhibit several other important NLO tire spectrum, while the stronger the fluorination is, the effects, such as two-photon and multiphoton absorption more the sp2carbon bonds that are transformed to sp3 (TPA) [21, 22], reverse saturable absorption (RSA) [22–24], hybridization. Once more, the ability of tuning the ratio and optical limiting [25–27]. Therefore, graphene exhibits sp2/sp3 can be an efficient way for the continuous tuning excellent photophysical properties and large optical non- of GF’s band gap, thus modifying successfully its NLO re- linearities over a wide range of laser pulse lengths, which sponse [31]. For the improvement of the stability of the GF makes it a highly promising candidate for several poten- aqueous colloids, a non-covalent functionalization of GF tial applications including fast optical communications, sheets (f-GF) with perfluorooctanoate units was used. all-optical switching, optical limiting, etc. The important fact that the studied graphene deriva- In the present work, the NLO responses of two tives dispersed in water or in other organic solvents can graphene derivatives, graphene oxide (GO) and graphene form very stable colloidal suspensions is of particular in- fluoride (GF), are studied under nanosecond and picosec- terest since it denotes the possibility of further exploita- ond visible (532 nm) laser excitation and are compared, tion to integrate optofluidic devices taking advantage of with the goal of shedding more light into the underlying both the high optical nonlinearity of the graphene flakes physical mechanisms of the corresponding NLO response and the fluidity of the liquid medium. in view of their potential for photonics, optoelectronics and nanotechnology applications. The investigation of the NLO response of GO concerns the study of two kinds of 2 Experimental procedures GO, 1-3 layered GO structures, named as SLGO, and few lay- ered GO structures (e.g. up to ~10 layers), denoted as FLGO. 2.1 Preparation of the samples Concerning the GF, the present work studies the NLO re- sponse of GF and GF stabilized with perfluorooctanoate groups [28]. The two types of GO samples were prepared and charac- The former graphene derivative, GO, consists of terized according to the procedures described in detail in a graphene layer with attached O-containing groups ref. [32] and references therein and were heavily oxidized (e.g.,