Resonant and Off-Resonant Light-Driven Plasmons in Metal Nanoparticles Studied by Femtosecond-Resolution Third-Harmonic Generation

B. Lamprecht, J. R. Krenn, A. Leitner, F. R. Aussenegg
1999 Physical Review Letters  
We report on a femtosecond-resolution study of the plasmon fields in gold nanoparticles using third-harmonic generation. Controlled resonant and off-resonant plasmon excitation is achieved by tailoring the nanoparticle sample by an electron-beam-lithographic method. Comparing the measured third order interferometric autocorrelation function of the plasmon field with simulations based on a simple harmonic oscillator model we extract the temporal characteristic of the plasmon oscillation. For
more » ... scillation. For off-resonant excitation of particle plasmons we find a beating between the driving laser field and the plasmon field which demonstrates clearly the nature of the plasmon as a collective electron oscillation. PACS numbers: 78.47. + p, 61.46. + w, 73.20.Mf In metal nanoparticles resonant collective oscillations of the conduction electrons (particle plasmons) can be excited by light in the visible spectral range. The resonance frequency of a particle plasmon is determined mainly by the dielectric functions of the metal and the surrounding medium, respectively, and by the particle shape, i.e., the ratio of the particle principal axes. Resonance leads to a narrow spectrally selective absorption and an enhancement of the local light field confined on and close to the surface of the metal particle [1]. The spectral width of absorption and near-field enhancement is given by the decay time of the particle plasmons. This decay time is determined by various damping mechanisms acting on the oscillating electrons, as the electronic conductivity of the metal at light frequencies and radiation damping in the case of particle diameters exceeding the Rayleigh limit. Particle plasmons have been a subject of extensive experimental and theoretical work, resulting in a quite comprehensive understanding of the steady state properties of this phenomenon. Recently also the dynamic aspects of particle plasmons have been addressed by fs timeresolved measurements based on second harmonic generation (SHG) [2] [3] [4] [5] . As SHG enhancement originates from the enhanced near field of the plasmon oscillation, SHG can be considered a suitable noninvasive probe of plasmon dynamics. The experiments revealed that the decay of particle plasmons which can be described as the loss of coherence of the oscillating electrons (dephasing) occurs on a sub-10 fs time scale [5] . In this Letter we report on first fs time-resolved measurements of the particle plasmon dynamics relying on third harmonic generation (THG) as a noninvasive monitor for particle plasmon dynamics. By using THG we eliminate a major drawback of SHG-based methods which are restricted to noncentrosymmetric particle shapes. Furthermore we achieve an unprecedented control of resonant and off-resonant plasmon excitation conditions by tailoring nanoparticle samples by a lithographic method [6] . This enables us to deduce the temporal characteristics and the decay time of particle plasmons for resonant and offresonant excitation, respectively, and thus to gain deepened insight into the physics of particle plasmons. To model the time dependent particle plasmon field we apply a damped harmonic oscillator driven by a given force K͑t͒. The harmonic approach is justified as the anharmonicity of the plasmon oscillation producing THG is sufficiently small not to influence the linear temporal behavior of the plasmon oscillation. It follows that the particle plasmon field E Pl ͑t͒ is proportional to [7] with v 0 2pc͞l res and g 1͞2t, where l res denotes the resonance wavelength which corresponds to the resonance frequency v 0 , c is the speed of light, and t is the oscillator energy decay time. Experimentally we determine the third order interferometric autocorrelation function (ACF) [8, 9] of E Pl ͑t͒ by measuring the frequency tripled radiation from the nanoparticles after exciting the plasmon oscillations by two identical femtosecond laser pulses separated by a variable time delay t 0 . Thus in the calculation K͑t͒ is set proportional to E pulse ͑t͒ 1 E pulse ͑t 1 t 0 ͒, i.e., the sum of two laser pulse fields E pulse ͑t͒ separated by the time delay t 0 . The calculation of THG intensities is performed by taking the sixth power of E Pl ͑t͒ and integrating the result in time due to the finite response time of the detector. To obtain the third order ACF we perform this calculation procedure for different t 0 values covering the whole experimentally relevant time delay scan range. The resulting shape of the envelope of the third order ACF is unambiguously determined by the time characteristic of E Pl ͑t͒. We find this temporal characteristic of the plasmon field by fitting the experimentally determined ACF with calculated ACFs with t as the only fit parameter. In the experiment we use a Kerr-lens-mode locked Ti:sapphire laser for excitation, which delivers 15 fs pulses at a fixed center wavelength of 774 nm [10] . For a pulse 0031-9007͞99͞83(21)͞4421(4)$15.00
doi:10.1103/physrevlett.83.4421 fatcat:rmvor3oynzbzdfq7nnkcyd2m3a