(Sub-)Picosecond Surface Correlations of Femtosecond Laser Excited Al-Coated Multilayers Observed by Grazing-Incidence X-ray Scattering.

Femtosecond high-intensity laser pulses at intensities surpassing 1014 W/cm2 can generate a diverse range of functional surface nanostructures. Achieving precise control over the production of these functional structures necessitates a thorough understanding of the surface morphology dynamics with nanometer-scale spatial resolution and picosecond-scale temporal resolution. In this study, we show that single XFEL pulses can elucidate structural changes on surfaces induced by laser-generated plasmas using grazing-incidence small-angle X-ray scattering (GISAXS). Using aluminium-coated multilayer samples we distinguish between sub-picosecond (ps) surface morphology dynamics and subsequent multi-ps subsurface density dynamics with nanometer-depth sensitivity. The observed subsurface density dynamics serve to validate advanced simulation models representing matter under extreme conditions. Our findings promise to open new avenues for laser material-nanoprocessing and high-energy-density science.

Release dynamics of nanodiamonds created by laser-driven shock-compression of polyethylene terephthalate.

Laser-driven dynamic compression experiments of plastic materials have found surprisingly fast formation of nanodiamonds (ND) via X-ray probing. This mechanism is relevant for planetary models, but could also open efficient synthesis routes for tailored NDs. We investigate the release mechanics of compressed NDs by molecular dynamics simulation of the isotropic expansion of finite size diamond from different P-T states. Analysing the structural integrity along different release paths via molecular dynamic simulations, we found substantial disintegration rates upon shock release, increasing with the on-Hugnoiot shock temperature. We also find that recrystallization can occur after the expansion and hence during the release, depending on subsequent cooling mechanisms. Our study suggests higher ND recovery rates from off-Hugoniot states, e.g., via double-shocks, due to faster cooling. Laser-driven shock compression experiments of polyethylene terephthalate (PET) samples with in situ X-ray probing at the simulated conditions found diamond signal that persists up to 11 ns after breakout. In the diffraction pattern, we observed peak shifts, which we attribute to thermal expansion of the NDs and thus a total release of pressure, which indicates the stability of the released NDs.

Transonic dislocation propagation in diamond.

The motion of line defects (dislocations) has been studied for more than 60 years, but the maximum speed at which they can move is unresolved. Recent models and atomistic simulations predict the existence of a limiting velocity of dislocation motion between the transonic and subsonic ranges at which the self-energy of dislocation diverges, though they do not deny the possibility of the transonic dislocations. We used femtosecond x-ray radiography to track ultrafast dislocation motion in shock-compressed single-crystal diamond. By visualizing stacking faults extending faster than the slowest sound wave speed of diamond, we show the evidence of partial dislocations at their leading edge moving transonically. Understanding the upper limit of dislocation mobility in crystals is essential to accurately model, predict, and control the mechanical properties of materials under extreme conditions.

Diamond formation kinetics in shock-compressed C─H─O samples recorded by small-angle x-ray scattering and x-ray diffraction.

Extreme conditions inside ice giants such as Uranus and Neptune can result in peculiar chemistry and structural transitions, e.g., the precipitation of diamonds or superionic water, as so far experimentally observed only for pure C─H and H2O systems, respectively. Here, we investigate a stoichiometric mixture of C and H2O by shock-compressing polyethylene terephthalate (PET) plastics and performing in situ x-ray probing. We observe diamond formation at pressures between 72 ± 7 and 125 ± 13 GPa at temperatures ranging from ~3500 to ~6000 K. Combining x-ray diffraction and small-angle x-ray scattering, we access the kinetics of this exotic reaction. The observed demixing of C and H2O suggests that diamond precipitation inside the ice giants is enhanced by oxygen, which can lead to isolated water and thus the formation of superionic structures relevant to the planets' magnetic fields. Moreover, our measurements indicate a way of producing nanodiamonds by simple laser-driven shock compression of cheap PET plastics.

Spatially resolved single-shot absorption spectroscopy with x-ray free electron laser pulse.

A new method of spatially resolved single-shot absorption spectroscopy for an x-ray free electron laser (XFEL) pulse has been developed by using a dispersive spectrometer and an elliptical mirror to enhance the spatial resolution. As a demonstration, we performed x-ray absorption near-edge structure measurement of Cu with a pump-probe scheme combining an XFEL pulse and a high-power femtosecond laser pulse. In the experiment, changes of an absorption spectrum in a plasma generated with a laser shot were successfully observed. The method will be a powerful tool for experiments requiring a spatial resolution and/or a single-shot measurement, such as high energy density science using a high-power laser pulse.

Ultrafast olivine-ringwoodite transformation during shock compression.

Meteorites from interplanetary space often include high-pressure polymorphs of their constituent minerals, which provide records of past hypervelocity collisions. These collisions were expected to occur between kilometre-sized asteroids, generating transient high-pressure states lasting for several seconds to facilitate mineral transformations across the relevant phase boundaries. However, their mechanisms in such a short timescale were never experimentally evaluated and remained speculative. Here, we show a nanosecond transformation mechanism yielding ringwoodite, which is the most typical high-pressure mineral in meteorites. An olivine crystal was shock-compressed by a focused high-power laser pulse, and the transformation was time-resolved by femtosecond diffractometry using an X-ray free electron laser. Our results show the formation of ringwoodite through a faster, diffusionless process, suggesting that ringwoodite can form from collisions between much smaller bodies, such as metre to submetre-sized asteroids, at common relative velocities. Even nominally unshocked meteorites could therefore contain signatures of high-pressure states from past collisions.

Coupling Effects in Multistage Laser Wake-field Acceleration of Electrons.

Staging laser wake-field acceleration is considered to be a necessary technique for developing full-optical jitter-free high energy electron accelerators. Splitting of the acceleration length into several technical parts and with independent laser drivers allows not only the generation of stable, reproducible acceleration fields but also overcoming the dephasing length while maintaining an overall high acceleration gradient and a compact footprint. Temporal and spatial coupling of pre-accelerated electron bunches for their injection in the acceleration phase of a successive laser pulse wake field is the key part of the staging laser-driven acceleration. Here, characterization of the coupling is performed with a dense, stable, narrow energy band of <3% and energy-selectable electron beams with a charge of ~1.6 pC and energy of ~10 MeV generated from a laser plasma cathode. Cumulative focusing of electron bunches in a low-density preplasma, exhibiting the Budker-Bennett effect, is shown to result in the efficient injection of electrons, even with a long distance between the injector and the booster in the laser pulse wake. The measured characteristics of electron beams modified by the booster wake field agree well with those obtained by multidimensional particle-in-cell simulations.

An experimental platform using high-power, high-intensity optical lasers with the hard X-ray free-electron laser at SACLA.

An experimental platform using X-ray free-electron laser (XFEL) pulses with high-intensity optical laser pulses is open for early users' experiments at the SACLA XFEL facility after completion of the commissioning. The combination of the hard XFEL and the high-intensity laser provides capabilities to open new frontiers of laser-based high-energy-density science. During the commissioning phase, characterization of the XFEL and the laser at the platform has been carried out for the combinative utilization as well as the development of instruments and basic diagnostics for user experiments. An overview of the commissioning and the current capabilities of the experimental platform is presented.

Scattering pulse-induced temporal contrast degradation in chirped-pulse amplification lasers.

Herein, a theory for modeling the problem of scattering pulse-induced temporal contrast degradation in chirped-pulse amplification (CPA) lasers is introduced. Using this model, the temporal evolutions of the scattering and signal pulses were simulated, the temporal contrasts for different cases were compared, and finally the theoretical prediction was verified by an experimental demonstration. The result shows that the picosecond and the nanosecond temporal contrast is mainly determined by the scattering pulses generated in the stretcher and the compressor, respectively. In addition, the B-integral accumulation will further degrade the temporal contrast, especially the picosecond temporal contrast. We believe it is helpful for solving the problem of the picosecond pedestal contrast (i.e., noise limit). With reference to these results, some suggestions for the temporal contrast improvement are presented.

Decomposition of powerful axisymmetrically polarized laser pulses in underdense plasma.

Interaction of relativistically intense axisymmetrically polarized (radially or azimuthally polarized) laser pulses (RIAPLP) with underdense plasma is shown experimentally and theoretically to be essentially different from the interaction of conventional Gaussian pulses. The difference is clearly observed in distinct spectra of the side-scattered laser light for the RIAPLP and Gaussian pulses, as well as in the appearance of a spatially localized strong side emission of second harmonic of the laser pulse in the case of RIAPLP. According to our analysis based on three-dimensional particle-in-cell simulations, this is a result of instability in the propagation of RIAPLP in uniform underdense plasma.

Ultrafast time-resolved pump-probe spectroscopy of PYP by a sub-8 fs pulse laser at 400 nm.

Impulsive excitation of molecular vibration is known to induce wave packets in both the ground state and excited state. Here, the ultrafast dynamics of PYP was studied by pump-probe spectroscopy using a sub-8 fs pulse laser at 400 nm. The broadband spectrum of the UV pulse allowed us to detect the pump-probe signal covering 360-440 nm. The dependence of the vibrational phase of the vibrational mode around 1155 cm(-1) on the probe photon energy was observed for the first time to our knowledge. The vibrational mode coupled to the electronic transition observed in the probe spectral ranges of 2.95-3.05 and 3.15-3.35 eV was attributed to the wave packets in the ground state and the excited state, respectively. The frequencies in the ground state and excited state were determined to be 1155 ± 1 and 1149 ± 1 cm(-1), respectively. The frequency difference is due to change after photoexcitation. This means a reduction of the bond strength associated with π-π* excitation, which is related to the molecular structure change associated with the primary isomerization process in the photocycle in PYP. Real-time vibrational modes at low frequency around 138, 179, 203, 260, and 317 cm(-1) were also observed and compared with the Raman spectrum for the assignment of the vibrational wave packet.

Experimental demonstration of spatially coherent beam combining using optical parametric amplification.

We experimentally demonstrated coherent beam combining using optical parametric amplification with a nonlinear crystal pumped by random-phased multiple-beam array of the second harmonic of a Nd:YAG laser at 10-Hz repetition rate. In the proof-of-principle experiment, the phase jump between two pump beams was precisely controlled by a motorized actuator. For the demonstration of multiple-beam combining a random phase plate was used to create random-phased beamlets as a pump pulse. Far-field patterns of the pump, the signal, and the idler indicated that the spatially coherent signal beams were obtained on both cases. This approach allows scaling of the intensity of optical parametric chirped pulse amplification up to the exa-watt level while maintaining diffraction-limited beam quality.

Control of spatial polarization by use of a liquid crystal with an optically treated alignment layer and its application to beam apodization.

We have investigated the alignment of a liquid crystal whose orientation is controlled by photoisomerization reaction for use in developing optical devices to improve beam quality. A glass window of a liquid-crystal cell that is coated with poly(vinyl alcohol) doped with azo dye was illuminated with a Hg lamp. We confirmed the dependence of the spatially controlled alignment direction of a liquid crystal on the irradiation time of this ultraviolet light. The new azo dye used in this study substantially reduced the illumination energy density required for aligning liquid-crystal molecules. We have demonstrated the control of polarization and successfully fabricated a serrated apodizing aperture and a soft aperture.