Acoustic Far-Field Hypersonic Surface Wave Detection with Single Plasmonic Nanoantennas

The optical properties of small metallic particles allow us to bridge the gap between the myriad of subdiffraction local phenomena and macroscopic optical elements. The optomechanical coupling between mechanical vibrations of Au nanoparticles and their optical response due to collective electronic oscillations leads to the emission and the detection of surface acoustic waves (SAWs) by single metallic nanoantennas. We take two Au nanoparticles, one acting as a source and the other as a receptor of SAWs and, even though these antennas are separated by distances orders of magnitude larger than the characteristic subnanometric displacements of vibrations, we probe the frequency content, wave speed, and amplitude decay of SAWs originating from the damping of coherent mechanical modes of the source. Two-color pump-probe experiments and numerical methods reveal the characteristic Rayleigh wave behavior of emitted SAWs, and show that the SAW-induced optical modulation of the receptor antenna allows us to accurately probe the frequency of the source, even when the eigenmodes of source and receptor are detuned.

Figure 1

(a) Schematic representation of pump-probe experiment. NLC: nonlinear crystal, ODL: optical delay line, AOM: acousto-optic modulator, O: microscope objective, D: photodetector, TP: telescope and position adjustment. (inset) SEM image of a rod source and a disk receptor $1.5\mu \mathrm{m}$ distant. Scale bar corresponds to 500 nm. (b) FEM average displacement at the edge of the simulation region of a $140\times 60\times 35\mathrm{nm}$ rod (blue, 0–500 ps) and a $140\times 35\mathrm{nm}$ disk (red, 0–100 ps). Rod orientation is shown on top. (c) Substrate displacement at $t=50\mathrm{ps}$. Antennas omitted for clarity. (d) Modulated probe transmission of single antennas. (inset) Fourier transform of probe signals (solid-square lines) and FEM size variations along $x$ axis (dashed-circle lines).

Figure 2

Transient probe transmission for (a) a rod and (b) disk receptors ($R$) at varying distances ($d$) from the source ($S$). (c) SAW arrival time at varying distances with corresponding linear fits (black lines) for both receptor geometries.

Figure 3

(a) xz-plane cross section of displacement (color scale) and polarization (black arrows) of the extensional mode of a $140\times 60\times 35\mathrm{nm}$ Au rod (top left of image) at 8.3 GHz. (b) Normalized amplitudes at $1\mu \mathrm{m}$ of the extensional mode displacement at the surface [grey squares indicated by arrow in (a)] and transient probe transmission for disk (green circles) and rod (orange triangles) receptors. Black dashed line shows the $d^{-0.5}$ expected decay behavior of a Rayleigh wave away from its source.

Figure 4

(a) Averaged experimental frequencies of rod sources (60 nm width, 30 nm height—green triangles), and rod (60 nm width, 30 nm height, top) or disk (30 nm height, bottom) receptors ($R$—red circles) of indicated main-axis size (diameter for disks). Stronger colors correspond to larger sizes. Blue squares indicate the measured SAW-induced probe modulation frequencies at the receptor when the pump beam is set at the source. (b) FEM time-domain Fourier components of the size variation along the axes parallel to the probe beam polarization (black arrows) for the rod source (top), and rod (middle), and disk (bottom) receptors. Vertical dashed lines correspond to extensional modes of the sources. Disk radial modes are shown in red dashed lines and circles (bottom).