complex-conjugate pole-residue pair-based fdtd method for assessing ultrafast transient plasmonic near field

The study of the optical properties of plasmonic nanostructures in the stationary regime has greatly benefited from the development of numerical methods, among which finite difference time domain (fdtd) is popular. in contrast, the use of these numerical tools for assessing the transient plasmonic optical response triggered by ultrashort laser pulses is hampered by the difficulty to address small variations of the material optical properties with reasonable computational time. yet, many of the developments based on this ultrashort response rely on the dynamics of the near-field topography around the nanostructures. in this article, we present a way to bridge this gap with the complex-conjugate pole-residue pair (ccprp) approach. a ccprp-based fdtd simulator has been developed. first, a simple methodology to check the end-to-end accuracy of the fdtd simulation is provided. then, in conjunction with a three-temperature model, the approach enables us to calculate the ultrafast transient near field inside and around a gold nanoparticle (aunp) upon absorption of a subpicosecond laser pulse. the transient variation of the field intensity inside and around the aunp is compared with the one determined by the mie theory. the dependence of the transient field intensity on the distance away from the nanoparticle surface and on the delay time after laser pulse absorption is finally analyzed.