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Enhancement and Inversion of Absorptive Nonlinearity Induced by Topochemically Controlled Insulator-to-Metal Transition

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journal contribution
posted on 25.11.2021, 16:34 by Di Zhao, Duoduo Zhang, Yuting Yang, Xiaojie Yin, Xiaofeng Liu, Jianrong Qiu
The nonresonant, nonlinear optical (NLO) absorption of photons with sub-bandgap energies by gapped insulators is often dominated by a two-photon or multiphoton process, which is characterized by a positive NLO absorption coefficient and manifests as reverse saturable absorption. We show here that this absorptive nonlinearity can be reversed to saturable absorption through controlled insulator-to-metal transition. As exemplified with the model transition metal oxide TiO2, we demonstrate a crossover of NLO absorption coefficients from positive to negative values accompanied by a dramatic enhanced subpicosecond NLO response in the near-infrared (NIR) sub-bandgap spectral region, which is realized by topochemical engineering of oxygen stoichiometry. From first-principles electronic structure calculations, the inversion of absorption nonlinearity is associated with the oxygen deficiency-controlled filling of the Ti-3d band that drives a transition from the gapped insulating phase to a gapless metallic phase. We show further that the resultant SA behavior can be leveraged to drive a NIR optical switch that enables both Q-switched and mode-locked laser pulse generation. The methodology for engineering the NLO response by controlling the energetics and d-band filling could be extended for similar TM oxides and exploited further for ultrafast photonics applications.