Vacancy-Enhanced Biotite Nanosheets for Optical Limiters

The advancement of high-power lasers necessitates the development of effective optical limiting materials to prevent the radiation damage of both sensors and optical vision. Atomically thin two-dimensional (2D) silicates, by virtue of their unique optical properties, have enabled several applications, including optical limiting. This study demonstrates the thickness-dependent nonlinear absorption and optical limiting capabilities of 2D biotite, a silicate mineral, by using femtosecond laser pulses. With decreasing thickness from a few layers to a monolayer, both the third-order nonlinear susceptibility and the two-photon absorption coefficient of this material manifest an increase by 2 orders of magnitude. Monolayer biotite exhibits an optical limiting threshold of 1.51 mJ/cm², surpassing the respective values for other conventional two-dimensional systems like graphene and transition metal dichalcogenides. The enhancement of the two-photon absorption is intimately tied up with the quantum confinement of the 2D system and its underlying intrinsic lattice defects. An elaborate first-principles investigation of the layer-dependent electronic structure of 2D biotite with various possible defect geometries indicates the presence of highly localized midgap levels primarily associated with Si and O-p orbitals, which may result in improved two-photon absorption in this system. DFT analyses indicate that disruption of the potassium layer (K-layer) and generation of the oxygen vacancies as a result of the liquid-phase exfoliation process may account for the generation of midgap states leading to an enhanced two-photon absorption in systems subjected to prolonged exfoliation periods. Our findings can be extremely beneficial for the development of highly efficient optical limiters utilizing 2D silicates at relatively much lower fluences.