Third-order nonlinear optical response and optical limiting of colloidal carbon dots.

In this work, the nonlinear optical response of some organophilic and hydrophilic carbon dots derived from gallate precursors is studied under 4 ns and 35 ps, visible (532 nm) and infrared (1064 nm) laser excitation conditions by the Z-scan technique. The prepared carbon dots were found to exhibit considerable nonlinear optical response in the visible, the organophilic ones exhibiting stronger response in the infrared and, in general, significantly larger response than their hydrophilic counterparts. In all cases, the corresponding nonlinear optical parameters have been determined. In particular, it was found that both carbon dots exhibited important negative nonlinear refractivity, under all excitation conditions tried, corresponding to self-defocusing, while negligible nonlinear absorption was found in the ps regime. Oppositely, both types of carbon dots were found to exhibit sizeable nonlinear absorption under ns excitation, indicating their potential for optical limiting applications. The present results are discussed and compared with other results concerning similar carbon based nanostructures reported in the literature.