Excitation and detection of surface acoustic phonon modes in $\text{Au}/{\text{Al}}_2{\text{O}}_3$ multilayers

A time-resolved pump-probe technique was used to observe the surface acoustic modes of $\text{Au}/{\text{Al}}_2{\text{O}}_3$ multilayers. As the thickness of the Au layers was increased from 2 to 5 nm, a change in the excitability of particular surface phonon modes with frequencies up to 200 GHz was observed. We compare our results with transfer-matrix theoretical calculations. In addition, we model the superlattice in a finite element simulation environment and use the data to evaluate the excitability of the surface modes. The model predictions correlate well with the experimental data. Simultaneous excitation of the surface and normal (propagating) phonon modes is observed in the $\text{Au}\left(5\text{nm}\right)/{\text{Al}}_2{\text{O}}_3\left(45\text{nm}\right)$ superlattice. The normal modes allow us to calculate the effective sound velocity of the superlattice, which agrees well with the theory calculation.

Figure 1

(Color online) Dispersion of longitudinal acoustic phonons in a $\text{Au}\left(5\text{nm}\right)/{\text{Al}}_2{\text{O}}_3\left(45\text{nm}\right)$ superlattice with first three surface modes marked with a dashed line.

Figure 2

(Color online) The optical sensitivity function $f\left(z\right)$ of $\text{Au}\left(5\text{nm}\right)/{\text{Al}}_2{\text{O}}_3\left(45\text{nm}\right)$ superlattice (a). The strain (full line) and displacement (dashed line) patterns of the first (b) and the second (c) surface mode.

Figure 3

(Color online) The measured $\Delta R\left(t\right)$ for $\text{Au}\left(4\text{nm}\right)/{\text{Al}}_2{\text{O}}_3\left(45\text{nm}\right)$ superlattice. Inset: Zoomed-in oscillatory signal for all Au layer thicknesses. The 5 nm trace was slightly offset for clarity.

Figure 4

Cross-sectional (left) and planar (right) TEM images of $\text{Au}\left(2\text{nm}\right)/{\text{Al}}_2{\text{O}}_3\left(20\text{nm}\right)$ multilayer (scale $\text{bars}=20\text{nm}$). The surface is parallel to the Au layers in the cross-sectional image (left).

Figure 5

(Color online) Optical spectra of the superlattices with various Au layer thicknesses.

Figure 6

(Color online) Zoomed-in oscillatory trace with thermal signal subtracted of $\text{Au}\left(5\text{nm}\right)/{\text{Al}}_2{\text{O}}_3\left(45\text{nm}\right)$ superlattice and its fit. Fitted curve, obtained using least-squares fitting, is decomposed into its exponentially decaying cosine function (Osci.—dotted curve) and the two echoes (Echo—dot-dashed curve).

Figure 7

(Color online) The Fourier transform spectra of the background subtracted signal of the $\text{Au}\left(5\text{nm}\right)/{\text{Al}}_2{\text{O}}_3\left(45\text{nm}\right)$ superlattice shown in Fig. 6. The dashed curve represents the Fourier transform of the echo signal only.