Size-dependent two-photon absorption and ultralow optical-limiting response in atomically-thin rhodonite.

Atomically-thin materials continue to captivate researchers due to their extraordinary physical properties that often surpass those of their bulk forms. Among them, two-dimensional (2D) silicates hold particular promise, yet their nonlinear optical characteristics remain largely underexplored. This study provides an in-depth analysis of the size-dependent nonlinear optical response and optical limiting characteristics of 2D rhodonite nanoflakes, a non-layered silicate mineral, under femtosecond laser excitation. A pronounced enhancement in two-photon absorption is observed as the material transitions from large flakes (∼40 nm thickness) to few-layer structures (∼2.5 nm thickness), with the two-photon absorption coefficient increasing from the 10 to 10 cm GW range, highlighting the influence of dimensional tuning. Few-layer rhodonite exhibits an ultralow optical limiting threshold of 0.38 mJ cm, outperforming many benchmark 2D materials, including graphene, TMDCs and MXenes. Density functional theory analysis indicates that the enhanced two-photon absorption in 2D rhodonite arises from the contributions of Fe orbitals originating from electronic states near the Fermi level. In addition, the increased probability of two-photon absorption can also be attributed to transitions between orbitals of similar character with strong contributions, which occur as a result of the hybridization between Si and O p orbitals. These findings position 2D rhodonite as a highly promising candidate for next-generation photonic technologies, including optical switching, 3D microfabrication, and quantum information processing.