Nonlinear thermal and electronic optical properties of graphene in N-methylpyrrolidone at 800 nm with femtosecond laser pulses

Graphene is a useful saturable absorber in a variety of lasers working in mode-locking or Q-switch regimes. The optical performance of chemically synthesized graphene is still not completely characterized. In this study, the saturable absorption and the nonlinear refractive index of graphene flakes in N-methylpyrrolidone, in both liquid and solid phases, have been studied at 800 nm with the z-scan technique using femtosecond laser pulses. The results obtained using a Ti:sapphire laser oscillator in the mode-locking regime (6 fs, 78 MHz) or in the continuous wave shows that the optical properties of graphene have a thermal origin, while at the lower repetition rate and higher energy and intensity of a Ti:sapphire amplifier (95 fs, 1 kHz), it shows the electronic Kerr effect. Solid samples with very high optical densities, equivalent to 60 layers of graphene grown by chemical vapor deposition (CVD), can be fabricated. They show a higher saturation intensity ( I s ≈ 100 GW cm) than CVD-grown (74 MW cm) or epitaxially grown (4 GW cm) graphene and intensity-dependent changes in transmission from 25% to 43%. This change in transmission in multilayer solid samples points to a good performance as a saturable absorber in laser cavities.We gratefully acknowledge the support from projects Santander-Complutense FIS2013-41709-P, MAT2013.47898- C2-2-R, and PCIN2016-021(M-ERA-NET2/0002/2016) from MINECO. Also, ICTS Centro Nacional de Microscopía Electrónica (Universidad Complutense de Madrid, Spain) for the AFM and TEM measurements.Peer Reviewe