Fully fluorinated graphene (fluorographene) exhibits a weak nonlinear optical response, in contrast to its unfunctionalized counterpart, graphene. However, diethyl-amino-functionalized fluorographene is shown to exhibit a dramatic enhancement of its nonlinear optical response. Diethyl-amino-fluorographene is obtained by the reaction of fluorographene with lithium diethylamide. Partial nucleophilic substitution/reduction of the substrate results in sp 3 carbons bonded to diethylamine or fluorine groups and in distinct sp 2 domains. The diethylamine groups act as strong electron donors, the C−F groups as strong electron acceptors, and the sp 2 domains as π-conjugated bridges, thus forming a donor−π bridge−acceptor nonlinear optical chromophore scheme. As such, the diethyl-aminofluorographene displays a large enhancement of its nonlinear optical response compared to fluorographene and other fluorographene derivatives under ps, both visible and infrared laser excitation.
In this work, the nonlinear optical (NLO) response of some graphene dispersions is investigated under low (i.e., 10 Hz) and high (i.e., 80 MHz) repetition rate femtosecond (fs) laser excitation conditions, using
Z
-scan, optical Kerr effect (OKE), and a combination of
Z
-scan and thermal lensing techniques. It is shown, that the NLO response of graphene dispersions is negligible under low repetition rate fs laser excitation, while it becomes very large under high repetition rate laser excitation. In the latter case, it is shown that the observed very large NLO response arises entirely from thermal cumulative effects.
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