Ultrafast nonlinear absorption with multiple transformations and transient dynamics of gold nanobipyramids

The process and condition of saturable absorption (SA) and reverse saturable absorption (RSA) of ultrafast nonlinear optics in metal nanoparticles are essential for applications including light generation, amplification, modulation, and switching. Here, we first discover and explore the multiple transformations (SA-RSA-SA) of ultrafast nonlinear absorption behavior of metal nanoparticles in femtosecond pulses. Correspondingly, the energy level model and fitting formula of multiple transformations are established to illustrate the process of optical response. The femtosecond transient absorption spectra provide information about their ultrafast dynamics process and vibrational mode, which further reveals the multiple transformation mechanisms of nonlinear absorption in gold nanobipyramids (Au-NBPs). Furthermore, Au-NBPs exhibit a significantly higher SA modulation depth up to 42% in the femtosecond, which is much higher than the reported values of other nanomaterials. Our results indicate that Au-NBPs can be used as broadband ultrafast Q-switching and mode-locking, and the conversion offers new opportunities for metal nanostructures in applications of optical switching. (c) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Automated summary

The process and condition of saturable absorption (SA) and reverse saturable absorption (RSA) in metal nanoparticles play a crucial role in applications such as light generation, amplification, modulation, and switching. This study discovers and explores the multiple transformations of ultrafast nonlinear absorption behavior in metal nanoparticles and establishes an energy level model and fitting formula to illustrate the optical response process. The results indicate that gold nanobipyramids (Au-NBPs) exhibit a significantly higher SA modulation depth and offer new opportunities for optical switching applications.