Ultrafast acousto-plasmonics in gold nanoparticle superlattices

We report the investigation of the generation and detection of GHz coherent acoustic phonons in plasmonic gold nanoparticle superlattices (NPSs). The experiments have been performed with an optical femtosecond pump-probe scheme across the optical plasmon resonance of the superlattice. Our experiments allow us to estimate first the fundamental mechanical parameters such as the collective elastic response (sound velocity) of the NPS and the nanocontact elastic stiffness. Furthermore, it appears that the light-induced coherent acoustic-phonon pulse has a typical in-depth spatial extension of about 45 nm which is roughly four times the optical skin depth in gold. The modeling of the transient optical reflectivity indicates that the mechanism of phonons generation is achieved through ultrafast heating of the NPS assisted by light excitation of the volume plasmon polariton. Based on these results, we demonstrate that it is possible to map the photon-electron-phonon interaction in subwavelength nanostructures which, in particular, provides insights on the fundamental properties of these nanometamaterials.

Figure 1

(a) Transmission electron diffraction (TED) pattern showing the Debye Scherrer rings (top figure) coming from the crystallized cubic gold nanoparticles. In the middle figure, the transmission electron microscopy image (TEM) shows the mesoscopic hexagonal arrangement (hcp) with the two neighbor plane distance of 4.4 nm. The hcp arrangement is also well evidenced with small angle electron diffraction revealing the 6-fold axis (bottom figure). (b) Sketch of the gold nanoparticle superlattices connected via mercaptosuccinic acid (MSA) molecules. (c) Optical microscopy view of a superlattice deposited onto a silicon substrate. (d) Atomic force microscopy (AFM) profile of the hexagonal shape superlattice (height $H=180$ nm). (e) Sketch of the pump and probe experiments performed on these nanoparticle superlattice where coherent acoustic phonons are generated and detected in the front-front configuration. (f) Top panel: plasmon resonance of the gold superlattice from [9] fitted with Eq. (1). Middle and bottom panels: refractive index (real $n$ and imaginary parts $k)$ of the gold superlattice compared to those of bulk gold [31]. The red symbols (square and circles) are the refractive index values estimated with the photoacoustic response [see Eq. (2) and details in the text].

Figure 2

Coherent acoustic phonons generated and detected in the silicon substrate only revealing the expected coherent acoustic-phonon Brillouin mode at 212 GHz (pump at 1.49 eV, probe at 2.99 eV).

Figure 3

(a) Time derivative of transient optical reflectivity revealing the coherent acoustic-phonon signal (blue curve) with numerical adjustment (red curve) containing information on the acousto-plasmonic processes of the generation and detection of the acoustic phonons [see Eq. (2)] (inset shows the fast electronic response of the superlattice). As a comparison a simulation of the coherent acoustic-phonon signal is given when the pump light energy is converted only within the gold crystal skin depth (i.e., 11 nm at 1.59 eV without hot electrons diffusion) and probed with standard optical properties of gold crystal (black curve). (b) Corresponding coherent acoustic-phonon spectrum obtained by a fast Fourier transform (FFT) of the experimental time derivative transient signal shown in (a). Four harmonics of the mechanical resonance of the superlattice appear (inset).

Figure 4

(a) Time derivative of transient optical reflectivity revealing the coherent acoustic-phonon signal (blue curve) with numerical adjustment with a damped sinus (red curve) (inset shows the fast electronic response of the superlattice). (b) Corresponding coherent acoustic-phonon spectrum obtained by a fast Fourier transform (FFT) of the experimental time derivative transient signal shown in (a).

Figure 5

(a) The full (dotted) line curve represents the photoinduced coherent acoustic-phonon spectrum with (without) phonon attenuation used to adjust the transient photo-acoustic response shown in Fig. 3. The inset shows the emitted coherent acoustic-phonon strain (ultrafast heating of nanoparticles) with a spatial extension of 42 nm used for the simulation. This spatial extension is sketched by the red curve in (b) where it is compared to the penetration depth of light into bulk gold.