Tuning the shape anisotropy of silver nanoparticles using time-delayed laser pulse pair irradiations

摘要

Glass-embedded spherical silver nanoparticles were irradiated by pairs of delayed femtosecond laser pulses. The influence of intensity, relative polarization (par-allel or orthogonal) and time delay between the pulses was investigated. We found a delay-dependent reversal of the orientation of prolate nanoparticles produced in the low-intensity regime: at very short time delays up to 10 ps be-tween pulse pairs the polarization direction of the second-hitting pulse defines the particles' symmetry axes; in an in-termediate regime between 10 and 20 ps no optical dichro-ism is found at all; at more than 20 ps delay between the pulses, finally, the transformed nanoparticles are oriented along the polarization direction of the first-hitting pulse. Also, in the quite different situation of the high-intensity regime using parallel-polarized pulse pairs, where normally oblate particles are created, isotropic spectral changes (i.e., no dichroism) after irradiation were observed at delay times around 20 ps. The possible physical background of this ap-parently very special inter-pulse delay of around 20 ps is discussed.