Picosecond acoustic response of a laser-heated gold-film studied with time-resolved x-ray diffraction

Abstract

We apply time-resolved x-ray diffraction using ultrashort x-ray pulses from a laser-produced plasma to probe the picosecond acoustic response of a thin laser-heated gold film. Measurements of the temporal changes in the angular distribution of diffracted x-rays provide direct quantitative information on the transient evolution of lattice strain. This allows to disentangle electronic and thermal pressure contributions driving lattice expansion after impulsive laser excitation. The electron-lattice energy equilibration time τE=(5±0.3) ps as well as the electronic Grüneisen parameter γe=(1.48±0.3) have been determined.

Reconstructing longitudinal strain pulses using time-resolved x-ray diffraction

“…Upon ultrafast excitation, a shift in the diffraction peak to lower angles is clearly observed. Fitting the x-ray diffraction peaks to a series of gaussian curves, we determined that the gold lattice has expanded by ∼0.1% in under 10 ps, consistent with previously published results 16 . Using the accepted value of the lattice expansion coefficient of bulk gold (1.4 × 10 −5 K −1 ), the resulting lattice expansion indicates that the gold film has increased in temperature by ∼100 K. At intermediate time delays (10 ps< ∆T <100 ps), the location of the gold diffraction peak changes minimally, and by 650 ps, the peak has returned to its pre-pumped position indicating the cooling process takes on order of 500 ps.…”

“…The observation of this spatio-temporal elongation has not been reported in most time-domain optical [8][9][10]13 or TRXRD studies 16,24 . Part of this spatial dilation is likely due to the sound speed difference in gold (∼3300 m/s) versus germanium (∼5400 m/s), which results in spatially stretching the acoustic pulse by almost a factor of two.…”

“…Coupling this effect with the change in acoustic velocities at the gold/Ge interface, will generate a longitudinal acoustic pulse that is ∼ 250nm wide. The final discrepancy between the time-domain optical and TRXRD measurements may be due to a slight enhancement of the low frequency modes from the residual < 10K surface heating of the crystalline substrate and/or a difference in the electron-phonon coupling time than was previously reported 16 .…”

“…The central wavelength of this strain pulse can be as small as a few nanometers, providing a method to directly image of nanometer scale structures and biomaterials 7,8 . Direct comparisons of theoretical models with experiments can lead to an understanding of electron-phonon interactions and thermal properties of materials 1,[9][10][11][12][13][14][15][16] .…”

“…With the development of high-brightness, short pulse, hard x-ray sources in the last 25 years, TRXRD has rapidly grown to study a whole range of laser initiated structural dynamics 1,3,5,11,12,[16][17][18][19][20][21][22][23] . Like traditional optical pump-probe techniques, TRXRD typically utilizes an intense optical pump pulse to rapidly initiate structural change through the rapid heating of the material.…”