Nonlinear optical switching behavior of nitrogen-doped carbon dots

Carbon dots (CDs), as an emerging class of carbon materials, have been recently demonstrated to possess nonlinear optical (NLO) behavior (e.g. nonlinear saturable absorption and reverse saturable absorption) which potentially expands CDs new application in optical limiting field. However, NLO switching behavior of CDs has not yet been reported in previous literature. In this paper, nitrogen-doped CDs (N-CDs) prepared from 2,3-diaminophenazine showed a switchable NLO behavior from saturable absorption (SA) to reverse saturable absorption (RSA) when the laser energy of incident picosecond pulse increased. By contrast, upon the irradiation of nanosecond pulse, the N-CDs exhibited only RSA behavior through excited state absorption (ESA) process. In mechanism, the switching behavior was probably resulted from competition between ground-state absorption and free carrier absorption (FCA) or two phonon absorption (TPA) processes, where both nitrogen doping and carbonyl groups of N-CDs were proved to be indispensable for the switching behavior.