Dynamics of colloidal crystals studied by pump-probe experiments at FLASH

We present a time-resolved infrared (IR) pump and extreme-ultraviolet (XUV) probe diffraction experiment to investigate ultrafast structural dynamics in colloidal crystals with picosecond resolution. The experiment was performed at the FLASH facility at DESY with a fundamental wavelength of 8 nm. In our experiment, the temporal changes of Bragg peaks were analyzed, and their frequency components were calculated using Fourier analysis. Periodic modulations in the colloidal crystal were localized at a frequency of about 4–5 GHz. Based on the Lamb theory, theoretical calculations of vibrations of the isotropic elastic polystyrene spheres of 400 nm in size reveal a 5.07-GHz eigenfrequency of the ground (breathing) mode.

Figure 1

Schematic view of the pump-probe experiment showing the IR pump (IR pulse) and the XUV probe (FEL pulse) separated by a time delay Δ$t$, the sample, and the detector, which is protected by the beam stop. Insets (a) and (b) show SEM images of the colloidal crystal film used in the experiment. The 11 layers of polystyrene spheres composing the colloidal crystal are visible in the inset (b).

Figure 2

Selected single-shot diffraction pattern on a logarithmic scale measured in transmission geometry at azimuthal angle orientation Δφ $=$ 0°. The family of fcc peaks ($\overline{2}\overline{2}0$) and forbidden fcc peaks (1/3(422) and 2/3(422)) are indicated. The momentum transfer vector Q and the horizontal $W_{\mathrm{x}}$ and vertical $W_{\mathrm{y}}$ size of the peaks are shown.

Figure 3

Time dependence of the momentum transfer Q (a) and (d), horizontal $W_{\mathrm{x}}$ (b) and (e) and vertical $W_{\mathrm{y}}$ (c) and (f) size (FWHM) for the selected Bragg peaks 2/3(422) (a)–(c) and $\overline{2}\overline{2}0$ (d)–(e) shown in Fig. 2. Error bars are determined as a standard deviation for ten measurements. The insets show the power spectrum of the corresponding data.