Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density–dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator–based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.

Abstract:The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-rayinduced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs. ©2010 Optical Society of America

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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We have observed resonant phenomena in the intense-field ionization of argon that do not easily fit into the standard models of above-threshold ionization. High-resolution angleresolved measurements of the energy region around 6-10 times the ponderomotive potential U P in the ATI electron spectrum show at least three separate series of peaks, each with a distinct intensity threshold for their onset. These series do not shift in energy and have characteristic values of intensity for their appearance, pointing to an intensity-selective resonant enhancement effect. In order to further investigate this phenomenon, we have observed VUV fluorescence photons from the focal region following ionization. Current theories for the creation of high-energy electrons do not seem to explain these phenomena without significant modifications. We discuss possible physical mechanisms and implications for extending the standard model.