Intensity-driven resurgence of reverse saturable absorption and vibrational modes in hexagonal silver nanoplates

Abstract

We report a correlation between the unique third-order optical nonlinearity and mechanical vibrational modes in hexagonal silver nanocrystals (AgNCs-550) having localized surface plasmon resonance (LSPR) at 550 nm. By femtosecond laser pulses, we measured nonlinear absorption (NLA) and nonlinear refraction (NLR), revealing the dual resurgence of reverse saturable absorption (RSA) as a function of pump intensity. Both on-resonant and off-resonant pumping of AgNCs-550 exhibit saturable absorption (SA) with an RSA–SA–RSA feature. The intensity threshold of the reappearance of the RSA effect is lower with the red edge pumping of the LSPR band. SA can be attributed to both single-photon and two-photon absorption saturation effects, whereas RSA can be attributed to two-photon absorption (2PA) and three-photon absorption (3PA). Femtosecond transient absorption (TA) spectroscopy further revealed mechanical vibrational modes with distinct quality factors, in line with finite-element modeling (FEM). In the case of TA, red-edge pumping shows an intensity-dependent change in vibrational modes, possibly due to lattice softening. Temperature enhancement calculations in AgNCs-550 reveal that higher laser intensity leads to a significant temperature rise, further supporting its correlation with lattice softening, particularly under red-edge pumping. Additionally, utilizing FEM, we demonstrate local field enhancement (LFE) at the corners and edges of the nanocrystals. We observe the enhancement of the nonlinear refractive index in the off-resonant excitation due to higher-order nonlinearities. Femtosecond (fs) TA, along with resonant and off-resonant third-order optical nonlinearity, suggest the suitability of AgNCs-550 as an excellent candidate for nanophotonic and optoelectronic applications.