Mechanism studies on the superior optical limiting observed in graphene oxide covalently functionalized with upconversion NaYF₄ :Yb³⁺/Er³⁺ nanoparticles.

The increased utilization of high power laser sources has rendered great challenges for designing effi cient optical limiting (OL) materials to protect human eyes and various delicate optical devices. An ideal optical limiter should greatly attenuate an intense laser beam while exhibiting high transmittance for low-input optical intensity. [ 1 ] Up to now, numerous organic and inorganic materials have proved to be good candidates for optical limiters with different working mechanisms. Among them, carbon-based materials, such as fullerenes, carbon black, and carbon nanotubes, have exhibited very good OL performance. [ 2–5 ] As a typical representative, graphene, which consists of sp 2 -hybridized carbon atoms with single-atom thickness and 2D structure, has exhibited unique electronic, optical, and mechanical properties. [ 6 ] The interband optical transitions in graphene are independent of frequency over a wide range and depend only on the fi nestructure constant, thus it is naturally promising as a broadband optical limiter. [ 7 ] Another important advantage of graphene-based materials is that they can be easily functionalized with organic molecules and hybridized with inorganic nanomaterials via covalent bonding, to improve their OL performances through the synergistic effect. For example, the enhanced nonlinear optical properties have been reported in porphyrin-functionalized graphene, [ 8 ] organic dye ionic complex, [ 9 ] oligothiophene, [ 10 ] fullerene, [ 11 ] phthalocyanine, [ 12 ]