Author(s): Zhao, D (Zhao, Di); Zhang, DD (Zhang, Duoduo); Yang, YT (Yang, Yuting); Yin, XJ (Yin, Xiaojie); Liu, XF (Liu, Xiaofeng); Qiu, JR (Qiu, Jianrong)

Source: JOURNAL OF PHYSICAL CHEMISTRY C Volume: 125 Issue: 48 Pages: 27023-27031 DOI: 10.1021/acs.jpcc.1c08922 Published: DEC 9 2021

Abstract: 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.

Accession Number: WOS:000752819300073

ISSN: 1932-7447

eISSN: 1932-7455

Full Text: https://pubs.acs.org/doi/10.1021/acs.jpcc.1c08922