Enhancement and Inversion of Absorptive Nonlinearity Induced by Topochemically Controlled Insulator-to-Metal Transition
journal contributionposted 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.
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ultrafast photonics applicationssimilar tm oxidesresultant sa behaviornegative values accompaniedgapless metallic phasegapped insulating phasebandgap spectral regionband filling couldtopochemically controlled insulatorabsorptive nonlinearity inducedreverse saturable absorptionnir optical switchnlo absorption coefficientscontrolled insulatorcontrolled fillingabsorptive nonlinearitysaturable absorptionabsorption nonlinearitynonlinear opticalgapped insulatorsbandgap energies3d bandoxygen stoichiometryoxygen deficiencyoften dominatednlo responsemultiphoton process