Unravelling Carrier Dynamics and Intensity-Dependent Multi-photon Absorption of Gold Nanorods

In this study, we have investigated the hot carrier dynamics and femtosecond (fs) third-order nonlinear optical (NLO) properties of excited plasmonic modes in gold nanorods (Au NRs). Au NRs were synthesized via a seed-mediated growth method, exhibiting an aspect ratio of ~3.4. The plasmonic local field enhancement of Au NRs was simulated, which undergoes the non-radiative decay to generate hot carriers. The ultrafast dynamics of these generated carriers associated with the LSPR and TSPR bands are investigated using fs transient absorption spectroscopy (TAS) measurements. The results reveal insight into carrier relaxation after photoexcitation, demonstrating three decay stages: electron-electron scattering in fs, followed by electron-phonon and phonon-phonon interaction in ps, evolving the final energy into the environment. LSPR mode shows higher relaxation times than TSPR mode at three different fluences. Next, the FFT of transient absorption traces revealed that the breathing vibrational frequency of Au NRs decreases with increasing pump fluence. Moreover, the NLO response of these NRs is explored using the fs Z-scan technique at an excitation wavelength of 800 nm at different peak intensities of 17.4 to 174 GW/cm 2 . The open-aperture studies revealed a transition in nonlinear absorption behavior from saturable absorption (SA) to reverse saturable absorption (RSA) with increasing incident laser intensity, indicating strong intensity-dependent nonlinear absorption characteristics. The optical limiting threshold of Au NRs was estimated at ~14 mJ/cm 2 . The observed NLO response, combined with ultrafast plasmonic dynamics, highlights the potential of this material for applications in nonlinear photonics and optical limiters.