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. 2014 May 6;1(3):034301.
doi: 10.1063/1.4875347. eCollection 2014 May.

Measurement of transient atomic displacements in thin films with picosecond and femtometer resolution

M Kozina, T Hu, J S Wittenberg  1 , E Szilagyi, M Trigo, T A Miller, C Uher  2 , A Damodaran  3 , L Martin  3 , A Mehta  4 , J Corbett  4 , J Safranek  4 , D A Reis, A M Lindenberg
Affiliations

Measurement of transient atomic displacements in thin films with picosecond and femtometer resolution

M Kozina et al. Struct Dyn. .

Abstract

We report measurements of the transient structural response of weakly photo-excited thin films of BiFeO3, Pb(Zr,Ti)O3, and Bi and time-scales for interfacial thermal transport. Utilizing picosecond x-ray diffraction at a 1.28 MHz repetition rate with time resolution extending down to 15 ps, transient changes in the diffraction angle are recorded. These changes are associated with photo-induced lattice strains within nanolayer thin films, resolved at the part-per-million level, corresponding to a shift in the scattering angle three orders of magnitude smaller than the rocking curve width and changes in the interlayer lattice spacing of fractions of a femtometer. The combination of high brightness, repetition rate, and stability of the synchrotron, in conjunction with high time resolution, represents a novel means to probe atomic-scale, near-equilibrium dynamics.

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Figures

FIG. 1.
FIG. 1.
(a) Beam position monitor illustrating time structure of x-ray bunches. There are four pulse trains per synchrotron period (781 ns) consisting of pulses 2.1 ns apart along with one lone bunch separated by a ∼60 ns gap on either side. This bunch alone is used for timing. The black arrow shows one round trip of the synchrotron. (b) Zoomed-in portion of (a) centered around the timing pulse; fine time structure is an artifact of the detection mechanism.
FIG. 2.
FIG. 2.
(a) 2D plot of the Bi 222 diffraction peak shift induced by 210 μJ/cm2 of 1030 nm laser light. The vertical axis is a summation across the area detector mapping the 2D detector to an angular lineout. The horizontal axis is the time delay between laser and x-ray pulses (negative time means x rays arrive first). The intensity axis is the difference (normalized to ±1) between the averaged lineout with x rays very early (before any excitation, i.e., negative time) and the lineout at each particular time point. (b) Fractional change in Bi lattice spacing Δd∕d extracted from data in (a). The dashed red line is an error function (“Erf”) fit for the initial change. The dotted green line is an exponential (“Exp”) decay fit on long time-scales. The inset shows the fractional change in the square root of the variance of the x-ray distribution on the detector along the scattering plane direction.
FIG. 3.
FIG. 3.
(a) 2D plot of the BFO film 220 diffraction peak shift upon excitation by 343 nm optical pulse (25 μJ/cm2 absorbed). The vertical axis is a summation across the area detector mapping the 2D detector to an angular lineout. The horizontal axis is time delay between laser and x rays. The intensity axis is the difference (normalized to ±1) between the average lineout with x rays very early (before any excitation) and the lineout at each particular time point. (b) 2D plot similar to (a) for the STO substrate. (c) Fractional change in BFO film (blue) and STO substrate (red) lattice spacings. Red (purple) dashed line is error function fit to the BFO (STO) rise time. There is a clear delay between the temporal overlap of the x-ray and laser pulses (∼t = 0) and the actual onset of a change in the STO, while the change in the BFO happens immediately. Note the STO data are scaled by a factor of 3 for ease of comparison with the BFO data and are averaged over several neighboring times for smoothing.
FIG. 4.
FIG. 4.
Fractional changes in (a) PZT, (b) Bi, and (c) BFO lattice spacings Δd∕d measured with the 003, 222, and 220 reflections, respectively, collected in the short-pulse, low-α mode. The inset to (a) shows streak camera data measuring the x-ray pulse duration in the low-α mode. The BFO and PZT samples were pumped with an absorbed fluence of 25 μJ/cm2 of 343 nm light and the Bi sample with 75 μJ/cm2 of 515 nm light. The red dashed line is a fit to an error function for all three samples.
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