Nanoscale physics

Kinetic theory of absorption of ultrashort laser pulses by spheroidal metal nanoparticles in the plasmon resonance region
А.А.Biliuk, O.Yu.Semchuk, O.O. Havryliuk,
Chuiko Institute of Surface Chemistry NAS of Ukraine
17 General Naumov Str., Kyiv 03164 Ukraine. 
E-mail: philosoph.tb@gmail.com

We have developed the kinetic theory which describes the features of the absorption of ultrashort laser pulses by metallic nanoparticles, depending on the particle shape and the pulse duration in the area of excitation of surface plasmon resonance. The analysis of the dependence of the energy absorbed by a spheroidal metal nanoparticles on the degree of its prolateness or oblateness is carried out for the different values of pulse duration and frequencies higher, lower, or identical to the plasmon resonance frequency. At the frequency of a carrier wave which coincides with that of the surface plasmon, the maximum absorption is observed for spherical metal nanoparticles. As soon as the carrier frequency deviates from that of the surface plasmon in a spherical particle, two maxima appear in the dependence of the absorbed energy on the ratio of spheroid semiaxes: one of them corresponds to the prolate particles, while the other – to the oblate particles. As the frequency deviates from the resonance one, the peak of the absorbed energy first decreases in absolute value, then splits and finally stabilizes for the more and more prolate or oblate particles. For the case of spheroidal metal nanoparticles subjected to the laser irradiation, with increase in the pulse duration, the peak in the dependence of the absorption on the frequency of a carrier wave also splits into two peaks located on the opposite sides of the value corresponding to the resonance frequency of a spherical particle. The peaks oscillations along or across the spheroid rotation axis. A distance from the minimum in a dip between the doublet peaks to the frequency of the plasmon resonance for a spherical particle provides the information about the degree of prolateness or oblateness of a nanoparticle. As the degree of prolateness or oblateness increases, the distance between the doublet components grows.