Nanooptics and photonics
Nonlinear optical properties of 
graphene and pyrolytic carbone films.
Rudenko V.I.1, Liakhovetskyi V.R.1, Dovbeshko G.I.1, Brodyn O.M.2, Romanyuk V.R.3, Juškėnas R.4 
1 Nonlinear Optics Department, Institute of Physics of the NAS of Ukraine. Prospect Nauki, 46, Kyiv-03039, Ukraine. 
E-mail: val@iop.kiev.ua
2 National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute", 37, Prospekt Peremohy, Solomyanskyi district, Kyiv, 03056, Ukraine
3 Department of polaritonic optoelectronics, V.Ye. Lashkaryov  Institute of Semiconductor Physics, 41 pr. Nauki, 03028, Kyiv, Ukraine
4 Dpt. of Characterization of Materials Structure, Center for Physical Sciences and Technology, Saulėtekio av. 3, LT-10257 Vilnius, Lithuania.

Ever since graphene was isolated in 2004 [1], it has attached significant scientific interest due to its highly unusual properties, which also has brought about a renewed interest in graphene-like materials. Graphene has investigated, both theoretically and experimentally, in number works [2,3].  However, the results of these works are somewhat contradictory, in particular concerning the magnitude and sign of the third order nonlinearity, whereas the knowledge of these properties is essential for applications.
Pyrolytic carbon (PyC) has been known for decades, although interest to its applications in nonlinear optics has not arisen until recently, when techniques have been developed of producing ultrathin, high quality PyC films. PyC is composed of randomly intertwined nano-sized graphene flakes, and therefore one expects its properties, in particular optics, to be similar to those graphene.
We report on Z-scan investigations of the nonlinear optical properties of thin PyC films and two- and three-layer graphene films. The sign and magnitude of nonlinear refraction and absorption in these materials were analyzed. We find that nonlinear refraction of graphene changes sign with changing light intensity with concomitant subtle changes in nonlinear absorption, a behavior not reported previously that suggest an interplay of several mechanisms that depends on intensity. The maximum values of the coefficient of nonlinear refractive index and nonlinear absorption coefficient for graphene films reached ~10-7 cm2/W and ~10-3 cm/W respectively.
On the contrary, PyC films exhibit positive refraction, whose magnitude is comparable to that of graphene, and strong negative absorption, i.e. bleaching.
The maximum values of the coefficient of nonlinear refractive index and the nonlinear absorption coefficient for PyC films were 2.8×10-8 cm2/W and -1.1×10-4 cm/W respectively.
Possible mechanisms of such a phenomenon are discussed. 

1.	Novoselov K.S., Gaim A.K., Morozov S.V., Jiang D., Zhang Y., Dubonos S.V., Grigorieva I.V., Firsov A.A. Electric field effect in atomically thin carbon films. // Science.-2004.- 306.-P. 666-669.
2.	Demetriou G., Bookey H., Biancalana F., Abraham E., Wang Y., Ji W., and Kar A. Nonlinear optical properties of multilayer graphene in the infrared// Optics Express-2016.-24, Iss.12 .-P. 13033-13043.
3.	Zhang H., Virally S., Bao Q., Kian Ping L., Massar S., Godbout N., and Kockaert P. Z-scan measurement of the nonlinear refractive index of graphen // Opt. Lett.-2012.-37, N 11.-P. 1856-1858.