Unraveling absorptive and refractive optical nonlinearities in CVD grown graphene layers transferred onto a foreign quartz substrate

Absorptive and refractive optical nonlinearities in chemical vapor deposited graphene layers, transferred onto a foreign quartz substrate was studied by performing Z-scan experiment under continuous wave excitation. Open aperture Z-scan results exhibit two shoulder peaks along with a valley at the focal plane and is interpreted in terms of combined effect of saturation absorption (SA) and reverse saturation absorption (RSA). Self-defocusing effect and hence negative nonlinear refraction was observed from the closed aperture Z-scan experiment. Optical nonlinearity is found to decrease upon annealing of graphene layers at 250 °C. Reduction in optical nonlinearity as a consequence of annealing is attributed to the variation of sp²/sp³ fraction and higher crystallite size (decreased structural disorderness) as revealed from the Raman spectroscopy. sp³ carbon atoms support two photon absorption, leading to RSA behavior while sp² structure displays SA behavior. Our study suggests that annealing paves a way to manipulate sp²/sp³ fraction which can alter the structural disorderness in graphene layers and hence the optical nonlinearities.