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Electronic correlations in nanostructures


Start date: 01.01.2005
Funded by: Universität Augsburg
Local project leader: Gert-Ludwig Ingold
External scientists / cooperations: Rodolfo A. Jalabert (Strasbourg)
Guillaume Weick (Strasbourg)
Dietmar Weinmann (Strasbourg)
Publications: Publication list

Abstract

For the optical properties of metallic nanoparticles, a collective electronic excitation plays a prominent role: the so-called surface plasmon. In this project, properties of the surface plasmon like its frequency and width are studied. Of particular interest is the relaxation dynamics on the femtosecond scale accessible to modern pump-probe experiments.

Description

Within classical electrodynamics a nanoparticle in vacuum displays a resonance at the so-called Mie frequency which can be related with a collective electronic excitation, the surface plasmon. In reality, however, one finds a resonance which is red-shifted with respect to the Mie frequency. This shift has two reasons: the spill-out effect, i.e. the possibility to find electrons outside the nanoparticle, and the coupling to other electronic degrees of freedom.

In WIJW06 mainly semiclassical methods were employed to determine the position and the line width of the surface plasmon resonance as a function of the size of the nanoparticle and of temperature at low temperatures as well as the fraction of electrons outside of the nanoparticle. Good agreement with results of local density approximation calculations is obtained if one accounts for an effective radius obtained from the selfconsistent mean-field potential. The results of the semiclassical calculation further demonstrate that the coupling to the other electronic degrees of freedom is not negligible with respect to the spill-out effect. For very small nanoparticles, a nonmonotonic dependence of the red shift induced by the electronic environment on the size of the nanoparticle is found which allows to discriminate against the red shift due to the spill-out effect. These results can be employed to analyze the differential transmission in time-resolved pump-probe experiments.

In such experiments on copper nanoparticles, a slow-down of the relaxation of the differential transmission close to the surface plasmon frequency was found. Surprisingly, this effect survives far beyond the lifetime of the surface plasmon. Based on results obtained in WIJW06 it was shown in WWIJ07 that this effect can be explained in terms of the temperature dependence of position and width of the surface plasmon resonance. Then this anomaly in the relaxation should not only appear for nobel metal nanoparticles but also for alkali nanoparticles. In the latter nanoparticles no anomaly would occur if the effect depends on interband transmissions invoked within an alternative explanation.

In order to achieve a better understanding of time-resolved experiments, the relaxation dynamics was studied in WIWJ07 by means of a master equation for weak as well as for strong excitation. It was found that a two-level description of the surface plasmon is sufficient. Furthermore, the appearence of sidebands in the absorption spectrum of the probe beam was predicted for strong excitation.