Laser-induced picosecond lattice oscillations in submicron gold crystals

We report direct observations of structural changes in submicron-sized gold crystals and gold films on a fused-quartz substrate using picosecond time-resolved x-ray diffraction. The femtosecond laser-induced expansion waves in 400 and 800 nm irradiation generated a lattice vibration inside the gold nanocrystals. This laser-induced strain wave repeatedly propagated and reflected to the finite-sized gold nanocrystals on the substrate. These longitudinal strain waves generated lattice vibrations in corresponding appropriately sized gold nanocrystals. In addition, each of the two laser-induced lattice oscillation modes in the polydispersed size distribution for 400 and 800 nm excitation was shown over a duration of a few hundred picoseconds.

Figure 1

(a) Preparation procedure for the gold film and gold nanocrystals on the fused-quartz substrate. The gold nanocrsytals were annealed with the evaporated gold film for 12 h at $500\hspace{0.5em}{}^{ˆ}\mathrm{C}$. The absorption spectra of the evaporated gold film (b) and gold nanocrystals (c) on the substrate.

Figure 2

Schematic of the picosecond pump-probe time-resolved x-ray diffraction system. The pulse widths of the laser and x ray are 150 and 100 ps. The thickness and size of the fused-quartz substrate are 70 $\mu$m and 70 $\times$ 70 mm. Sample substrate was placed perpendicular to the x-ray path. The laser path was about ${10}^{°}$ to ${15}^{°}$ normal to the sample surface.

Figure 3

(a) X-ray diffraction profile of the gold nanocrystals on the fused-quartz substrate for ambient condition. (111), (200), (220), (311), and (222) diffraction peaks were observed in the background from x-ray scattering from fused quartz. (b) Profile of the (222) reflection for the gold nanocrystals for delay times of $-150$ ps (dotted line) and 180 ps (solid line). The background from the fused-quartz x-ray scattering was subtracted. The peak at 180 ps shifted to a higher angle as a result of the lattice in the gold nanocrystals being expanded by laser irradiation (8 mJ/cm${}^2$).

Figure 4

(a) Relative lattice expansion determined from the (111), (200), (220), (311), and (222) diffraction peaks of the gold nanocrystals as a function of the delay time after irradiation. (b) Relative lattice expansion from the (222) peak for the evaporated gold film as a function of the delay time after laser irradiation.

Figure 5

Time dependence of the (222) lattice constant of the gold nanocrystals for 400 nm (open circle) and 800 nm (filled triangle) excitation. The solid lines are the fit of the time-dependent data using Eq. (1).

Figure 6

AFM images of evaporated gold film (60-nm thickness) (a) and gold nanocrystals (b) on the fused-quartz substrate. The thickness of the evaporated gold film was 60 nm. All gold nanocrystals were fitted using ellipses to estimate their size distribution. The size distribution of the major (c) and minor (d) axes for the gold nanocrystals on the fused-quartz substrate. The size distribution of the minor axis was fit to a Gaussian distribution function (red curve). The size distribution of the major axis was fit to three Gaussian distributions (red curve). The fitting parameters on the major axis were used to estimate the length of the gold nanocrystals from the time-dependent data in Fig. 5.