Unveiling and engineering of third-order optical nonlinearities in NiCo₂O₄ nanoflowers

In this Letter, we demonstrate for the first time, to the best of our knowledge, NiCo₂O₄ (NCO) as a novel nonlinear optical material with straightforward potential applications in optical limiting. For the 532 nm nanosecond laser, excited state absorption (ESA) and free-carrier absorption give rise to large ESA coefficient (β) and positive nonlinear n₂. On the other hand, when excited with the 800 nm femtosecond laser, two-photon absorption (TPA) takes place, and bound carriers induce strong negative n₂. The values of β and n₂ obtained for NCO are found to be higher compared to other conventional transition metal oxides and, therefore, are promising for optics and other photonics applications.