High‐Order Nonlinear Optical Properties Generated by Different Electron Transition Processes of NiO Nanosheets and Applications to Ultrafast Lasers

Manipulation of the electron transition process is the foundation of modern photonics and electronics and its progress will boost the advancement of science. Associated with strongly electron correlated properties, this study reports on the manipulation of electron transition processes in NiO nanosheets. This study finds that these transition processes evoke different optical responses that bestow multifunctional photonics properties, including optical limiting, three‐photon absorption and emission, and saturable absorption. The thickness‐dependence of electron transitions is investigated and experimentally connected to an increase in the thickness of the nanosheet. NiO optical behavior switches from saturable absorption to reverse saturable absorption and then back to saturable absorption again, a phenomenon which is the result of dynamic balancing among intraband transitions, Pauli blocking, and three‐photon interband transitions and emission. With a sample optimized for saturable absorption, broadband mode‐locked lasers with wavelengths of 1.06 and 1.34 µm are successfully realized. This study shows that strongly correlated electron materials are promising candidates for the manipulation of the electron transition process, expands these materials into optoelectronics, and provides a multifunctional device design for ultrafast photonics. Moreover, the present mechanism may be helpful for future photonic and electronic device design.