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

摘要

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 degrees C. Reduction in optical nonlinearity as a consequence of annealing is attributed to the variation of sp(2)/sp(3) fraction and higher crystallite size (decreased structural disorderness) as revealed from the Raman spectroscopy. sp(3) carbon atoms support two photon absorption, leading to RSA behavior while sp(2) structure displays SA behavior. Our study suggests that annealing paves a way to manipulate sp(2)/sp(3) fraction which can alter the structural disorderness in graphene layers and hence the optical nonlinearities.