Femtosecond-Laser-Induced Saturable Absorption and Optical Limiting of Hollow Silver Nanocubes: Implications for Optical Switching and Bioimaging

The effects of shape, size, and surface plasmon resonance (SPR) peak position (λSPR) on nonlinear absorption (NLA) were investigated at 808 nm, using a femtosecond laser. We found that NLA behavior is the result of competition between saturable absorption (SA) and reverse saturable absorption (RSA). SA behavior, i.e., optical transparency in silver nanocrystals, is introduced by the change in shape and structure from solid nanospheres to hollow nanocubes. For the latter, SA behavior dominates at low input irradiance, while RSA dominates at higher laser powers. A series of hollow silver nanocubes (HAgNCs) was synthesized that exhibit SPR peaks at 510, 550, 590, and 630 nm, hence named as HAgNC-510, HAgNC-550, HAgNC-590, and HAgNC-630, respectively. The edge lengths (l) of HAgNC-510, HAgNC-550, HAgNC-590, and HAgNC-630 were determined to be 33 ± 4, 45 ± 8, 70 ± 10, and 100 ± 15 nm. The switching of SA to RSA occurred at lower laser power for larger HAgNCs as compared to the smaller ones. The saturation intensity (Isat) was found to vary linearly with the size of HAgNCs. Moreover, for a four-times rise in input pulse intensity (63 → 252 GW/cm2), a huge increase of 3650% in the NLA coefficient, βeff, was observed for HAgNC-590, whereas merely 19% increase was found for the solid silver nanoparticles (SAgNP-400). The extraordinary enhancement in βeff is attributed to the enhanced electric field at the sharp edges and corners of the nanocubes. The red-tailing of SPR peaks also had a significant effect on the values of βeff. The outcomes, i.e., optical transparency and SA-RSA switching, will open vistas for a variety of diverse applications such as optical switching, limiting, and bioimaging.