Unusual higher-order nonlinear optical properties in Au-coated triangular Ag-Au nanostructures.

Here, we report hitherto unobserved local field (LF)-assisted pump wavelength-dependent nonlinear optical (NLO) effects of three-photon (3PA)-induced four-photon absorption (4PA) at 532 nm and two-photon-induced 3PA at 730 nm in triangular-shaped core–shell Ag-Au nanoparticles (TrAg@Au) by femtosecond Z-scan. The shell thickness-dependent enhancement in the LF is observed by a COMSOL simulation. The intensity-dependent interplay between saturable and reverse-saturable absorptions along with switching of nonlinear (NL) phase is reported at 730 nm, showing the superiority of TrAg@Au in optical switching (OS). The optical limiting (OL) threshold (Fth) of 5.9(6.5)mJ/cm2 at 730 (532) nm boost their potential over benchmarked materials.