Ultrafast acousto-plasmonics in gold nanoparticle superlattices

Abstract

We report the investigation of the generation and detection of GHz coherent acoustic phonons in plasmonic gold nanoparticles superlattices (NPS). The experiments have been performed from an optical femtosecond pump-probe scheme across the optical plasmon resonance of the superlattice. Our experiments allow to estimate the collective elastic response (sound velocity) of the NPS as well as an estimate of the nano-contact elastic stiffness. It appears that the light-induced coherent acoustic phonon pulse has a typical in-depth spatial extension of about 45 nm which is roughly 4 times the optical skin depth in gold. The modeling of the transient optical reflectivity indicates that the mechanism of phonon generation is achieved through ultrafast heating of the NPS assisted by light excitation of the volume plasmon. These results demonstrate how it is possible to map the photon-electron-phonon interaction in subwavelength nanostructures.PACS numbers: 78.67. Pt, 73.20.Mf, Plasmon assisted subwavelength light transmission is a key physical mechanism for modern nano-optics and nano-plasmonics [1]. The propagation of plasmonpolariton waves in nano particles nanostructures (chains, arrays) is at the core of intense fundamental investigations and has led to numerous reports [2-6]. For plasmonic applications, it is obviously crucial to understand and control the damping of these plasmonpolariton waves, i.e. the collective propagation distance. This propagation distance as well as a more general description of the dispersion curves for both longitudinal and transverse light electric field have already been obtained for supported 1D chains [2][3][4][5]. The propagation distance is currently limited by the intrinsic electronphonon, electron-defect interaction within each particle as well as radiation loss. In some particular 2D optical spectroscopy imaging, it has been shown recently that the propagation/distribution of the plasmon-polariton could even be mapped for 1D nanoparticles chains [2,6]. The visualization of these plasmons at the interface air/nanostructure with near-field imaging [5] or with an electron beam imaging [7] were also reported. This stateof-art 2D imaging is however limited to 1D or 2D systems, i.e. to surface plasmonics and no local measurement of volume plasmon-polariton propagation has been reported for 3D nanoparticles arrays so far. Most of the plasmonic responses in nanoparticles superlattices are obtained indeed from far field optical absorbance measurements [8,9]. Because of the difficulty to probe the innner part of a plasmonic superlattices, no quantitative depth profiling description of the plasmon-polariton propagation have been reported to date. However, it is known that ultrafast optical techniques can provide a picture in time and space of the light-matter interaction [10][11][12]. In particular, in case of femtosecond light pulses, the light-matter coupling can lead to the emission of coherent acoustic phonons that occurs all over the spatial extension where the light energy is converted in...

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