Abstract
Localized surface plasmon resonance (LSPR) is essentially a collective oscillation of free electrons in nanostructured metals. Interband excitation may also produce conduction-band electrons above the Fermi level. However, a question here is whether these excited electrons can take part in plasmonic oscillation. To answer this question, femtosecond pump-probe measurements on gold nanoparticles were performed using interband excitation, where the pump pulse produced a large amount of electrons in the sp-conduction band and left holes in the d-band. Probing by transient absorption spectroscopy, we resolved an induced LSPR feature located at a red-shifted spectrum. This feature cannot be observed for a pumping photon energy lower than the threshold for interband transition. The commonly observed red-shift or broadening of LSPR spectrum due to electron-electron and electron-phonon scattering under strong optical excitation can be ruled out for understanding this feature by a comparison between the plasmonic dynamics at a pump above and below the interband-transition threshold. In particular, a “holding” time of about 1 ps was resolved for the interband-excitation-induced electrons to relax to the LSPR oscillation.
Highlights
Ultrafast electronic relaxation performance is the basis for the application of localized surface plasmon resonance (LSPR) in optical switching devices based on nanostructured metals[1,2,3,4,5]
An intraband relaxation and an adaptive phase/frequency adjustment are needed for the newly generated electrons to take part in the plasmonic oscillation process, which leads to a time delay of the induced Localized surface plasmon resonance (LSPR), as indicated by Δτ in Fig. 1(a). (3) The new generation or the increased density of the conduction-band electrons will lead to stronger interaction between the plasmonic electrons
The above mechanisms were verified by the experimental results presented in section 3, which could not be observed for a pump photon energy (e.g. at 1.55 eV, as shown in Fig. 1(a)) lower than the threshold for the interband transition
Summary
We observed a transient plasmon feature induced by interband excitation of gold nanoparticles, which is located in a red-shifted spectrum with respect to the intrinsic mode. Such feature was not observed when the excitation photon energy is lower than the threshold for interband transition, confirming that the new plasmon feature resulted from modification on the conduction-band electron dynamics by interband-excitation. The increased electron density enhanced electron-electron scattering processes and slowed down the damping rate of the plasmonic electrons. This is equivalent to a longer-distance travelling of the electrons within the same nanostructures under optical electric field. A holding time of about 1 ps was resolved for the interband electrons to relax into plasmonic oscillation, which was observed as a much slower decay dynamics than the intrinsic plasmon
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