Ultrafast dynamics of silver nanoparticle shape transformation studied by femtosecond pulse-pair irradiation

Abstract

Spherical silver nanoparticles embedded in glass were irradiated by pairs of time-delayed laser pulses with equal intensities resulting in delay-dependent nanoparticle shape transformations. The corresponding persistent changes in the surface-plasmon extinction bands are analyzed as a function of time delay and relative polarization of the pulse pairs. We find that the strongest nanoparticle shape changes, i.e., the highest aspect ratios, are achieved when the delay between pulse pairs is less than 3 ps. After 10 ps the dichroism is strongly reduced in the case of pulse pairs having identical polarization and vanishes using pulse pairs with orthogonal polarization. Using an extended two-temperature model, the time dependence of directed and isotropic thermal electron emissions was estimated to decay on a time scale of a few picoseconds. Our results strongly suggest that the electron and following ion emission from the nanoparticles are finished within less than 20 ps, and the directional memory is favorably defined by directed emission of hot electrons interacting with the laser field.