Subpicosecond breathing mode excitation in metal nanoparticles

BONAFÉ, FRANCO P.; ARADI, BÁLINT; OSCAR A. DOUGLAS-GALLARDO; FRAUENHEIM, THOMAS; CRISTIAN GABRIEL SANCHEZ

Bremen

Workshop; Charge carrier dynamics in nanostructures: optoelectronic and photo-stimulated processes; 2017

CECAM

By the end of the 1990s, laser-induced mechanical oscillations were observed in metal nanoparticles using femtosecond pump-probe technique. The oscillation period was found to be larger than 2 ps for particles with radius > 3 nm and is consistent with the breathing mode frequency. The driving force for the process has been atrributed to a fast heat transfer from the electrons to the lattice, launching atomic motion impulsively.We have performed the first atomistic simulation in real time of icosahedral silver nanoparticles with diameters in the range 1-2nm under strong laser pulses tuned to the plasmon energy, using real-time, time-dependent density functional tight-binding (TDDFTB). We found breathing-like radial oscillations starting immediately after the 25-fs pulse, with increasing amplitude as the field intensity increases. The ultrafast character of this response points to a different mechanism than the one described in literature. By calculating the bond energy distribution in real time, we saw a sharp weakening of the all bonds during the laser pulse, due to the excitation of electrons to antibonding states, which leads to a uniform expansion of the nanoparticles launching the oscillations. Accuracy of the results was confirmed by comparison with time-dependent density functional theory (TDDFT) calculations.