Probing Impulsive Strain Propagation with X-Ray Pulses

Abstract

Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden reflections in InSb is used to follow the propagation of a coherent acoustic pulse generated by ultrafast laser-excitation. The surface and bulk components of the strain could be simultaneously measured due to the large x-ray penetration depth. Comparison of the experimental data with dynamical diffraction simulations suggests that the conventional model for impulsively generated strain underestimates the partitioning of energy into coherent modes. 78.47.+p 61.10.-i 63.20.-e The absorption of ultrafast laser pulses in opaque materials generates coherent stress when the pulse length is short compared with time for sound to propagate across an optical penetration depth [1]. The resulting strain field consists of both a surface component, static on time scales where thermal diffusion can be ignored, and a bulk component that propagates at the speed of sound (coherent acoustic phonons). This strain is typically probed by optical methods that are sensitive primarily to the phonon component within the penetration depth of the light [1,2]. However, such methods give little information about the surface component of the strain, and, moreover, they are unable to give a quantitative measure of the strain amplitude.Due to their short wavelengths, long penetration depths, and significant interaction with core electrons, x-rays are a sensitive probe of strain. We note that coherent lattice motion adds sidebands to ordinary Bragg reflection peaks due to x-ray Brillouin scattering if the momentum transfer is large compared to the Darwin width, equivalent to phonons of GHz frequency for strong reflections from perfect crystals. This effect was demonstrated many years ago with acoustoelectrically amplified phonons using a conventional x-ray tube [3].With the recent availability of high brightness short-pulse hard x-ray sources, including third generation synchrotron sources and optical laser based sources [4-6], coherent strain generation and propagation can now be probed by x-ray methods in both the frequency and time domains. Recently, time-resolved diffraction patterns of cw ultrasonically excited crystals were obtained with a synchrotron source [7]. Other experiments have employed picosecond time-resolved x-ray diffraction to study transient lattice dynamics in metals [8], organic films [9], and impulsive strain generation and melting in semiconductors [10][11][12][13][14]. In particular Rose-Petruck et al.[10] demonstrated transient ultrafast strain propagation in GaAs by laser-pump x-ray-probe diffraction. In that experiment, x-rays were diffracted far outside the Bragg peak; however, no oscillations in the diffraction efficiency were detected, and the data were consistent with a unipolar strain pulse. In a similar experiment, Lindenberg et al. [13] detected oscillations in the sidebands for an asymmetrically cut InSb crystal using a streak camera. These oscillations were due to lattice compression and were probed for discrete phonon frequencies i...

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