Nonlinear optical properties of graphene: a comparison of liquid-phase exfoliation and chemical vapor deposition

The z-scan technique is used to measure and compare the near-infrared nonlinear optical properties of graphene films fabricated via liquid-phase exfoliation (LPE) and chemical vapor deposition (CVD). Experiments are performed with 170 fs pulses at 1030 nm and 50 fs pulses at 800 nm, revealing that both LPE and CVD graphene exhibit strong saturable absorption characterized by a β coefficient on the order of -10 4 cm/GW. Additionally, we find an effective nonlinear refractive index ( n 2 ) of -3 cm 2 /GW. Both graphene samples exhibit a markedly reduced depth of the absorption modulation when the pulse duration is extended into the ps range. Our results demonstrate that an inexpensive, fast, and scalable fabrication technique such as LPE can produce graphene-based thin layers displaying nonlinear optical properties comparable to, and potentially exceeding, those of CVD graphene. Both types of samples are promising for photonic applications such as mode locking via saturable absorption.