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Our simulations and experiments demonstrate a new physical mechanism for detecting acoustic waves of THz frequencies. Detectable THz-frequency radiation is generated when the acoustic wave passes a piezoelectric interface.

Publisher Summary This chapter describes the various time-resolving electron optical techniques that are developed for a fast study of transient effects in freestanding films and on surfaces of bulk materials down to the nanosecond time scale. Hydrodynamic instabilities in confined laser pulse-produced melts and their solidification and evaporation are discussed in the chapter, as they are of major concern to micromachining with laser pulses. The mechanisms uncovered by high-speed electron microscopy are described. There are three different time-resolving techniques distinguished by the number of spatial coordinates in the image: (1) short-time-exposure imaging, (2) streak imaging, and (3) image intensity tracking. There are several types of electron microscopes that probe different zones of the specimen. Transmission microscopes uncover the volume processes of free-standing films that often mimic bulk material. Properties of the top layers of a surface are successfully studied by the photoelectron microscope. Reflection electron microscopy gives access to the space above the surface of the specimen. These three types of electron microscopes are adapted to investigations of fast processes in their specific domain.

We discover that strain waves of THz frequencies can coherently generate radiation when they propagate past an interface between materials with different piezoelectric coefficients. By considering AlN/GaN heterostructures, we show that the radiation is of detectable amplitude and contains sufficient information to determine the time‐dependence of the strain wave with potentially unprecedented nearly atomic time and space resolution. We demonstrate this phenomenon within the context of high amplitude THz frequency strain waves that spontaneously form at the front of shock waves in GaN crystals. We have performed proof of principle experiments that demonstrate THz signals that correlate with strain wave propagation times across Al thin films.

A new concept is introduced whereby thermal stresses generated through the pumping process are canceled by an applied force. By use of a Nd:YVO(4) disk laser as a model system, significant reduction of thermal lensing and deformation is demonstrated as a function of applied pressure, and the output power (19 W TEM(00)) is limited only by the available pump power.

The Dynamic Transmission Electron Microscope (DTEM) project is developing an in situ electron microscope with nanometer- and nanosecond-scale resolution for the study of rapid laser-driven processes in materials. We report on the results obtained in a year-long LDRD-supported effort to develop DTEM techniques and results for phase transitions in molecular crystals, reactive multilayer foils, and melting and resolidification of bismuth. We report the first in situ TEM observation of the HMX {beta}-{delta} phase transformation in sub-{micro}m crystals, computational results suggesting the importance of voids and free surfaces in the HMX transformation kinetics, and the first electron diffraction patterns of intermediate states in fast multilayer foil reactions. This project developed techniques which are applicable to many materials systems and will continue to be employed within the larger DTEM effort.

Background and Objective Few studies have investigated femtosecond (fs) lasers for cutting bone tissue. Study Design/Materials and Methods A 775 nm, 1 kHz, 200 femtosecond, up to 400 µJ laser system was used to irradiate in vitro calcified cortical bone samples and bone tissue culture samples. Results The ablation threshold in cortical bone was 0.69±0.08 J/cm2 at 775 nm and 0.19±0.05 J/cm2 at 387 nm. Plasma shielding experiments determined that the ablation plume and the plasma significantly affect material removal at high repetition rates and appear to generate thermal transients in calcified tissue. Confocal analysis revealed intact enzymatic activity on the surface of cells immediately adjacent to cells removed by fs laser irradiation. Conclusions These experiments demonstrate that fs lasers used for bone tissue cutting do not appear to generate significant temperature transients to inactivate proteins and that cellular membrane integrity is disrupted for only a few cell layers. Lasers Surg. Med. 39:273–285, 2007. © 2007 Wiley-Liss, Inc.

Recent theoretical studies of 2,6-diamino-3,5-dinitropyrazine-1-oxide (C4H4N6O5 Lawrence Livermore Molecule No. 105, LLM-105) report unreacted high pressure equations of state that include several structural phase transitions, between 8 and 50 GPa, while one published experimental study reports equation of state (EOS) data up to a pressure of 6 GPa with no observed transition. Here we report the results of a synchrotron-based X-ray diffraction study and also ambient temperature isobaric-isothermal atomistic molecular dynamics simulations of LLM-105 up to 20 GPa. We find that the ambient pressure phase remains stable up to 20 GPa; there is no indication of a pressure induced phase transition. We do find a prominent decrease in b-axis compressibility starting at approximately 13 GPa and attribute the stiffening to a critical length where inter-sheet distance becomes similar to the intermolecular distance within individual sheets. The ambient temperature isothermal equation of state was determined through refinements of measured X-ray diffraction patterns. The pressure-volume data were fit using various EOS models to yield bulk moduli with corresponding pressure derivatives. We find very good agreement between the experimental and theoretically derived EOS.

Pressure dependent angle-dispersive x-ray powder diffraction measurements of alpha-phase aluminum trifluoride (α-AlF3) and separately, aluminum triiodide (AlI3) were conducted using a diamond-anvil cell. Results at 295 K extend to 50 GPa. The equations of state of AlF3 and AlI3 were determined through refinements of collected x-ray diffraction patterns. The respective bulk moduli and corresponding pressure derivatives are reported for multiple orders of the Birch-Murnaghan (B-M), finite-strain (F-f), and higher pressure finite-strain (G-g) EOS analysis models. Aluminum trifluoride exhibits an apparent isostructural phase transition at approximately 12 GPa. Aluminum triiodide also undergoes a second-order atomic rearrangement: applied stress transformed a monoclinically distorted face centered cubic (fcc) structure into a standard fcc structural arrangement of iodine atoms. Results from semi-empirical thermochemical computations of energetic materials formulated with fluorine containing reactants were obtained with the aim of predicting the yield of halogenated products.

The materials science community is poised to take advantage of new technologies that add unprecedented time resolution to already existing spatial-resolution capabilities. In the same way that chemists and biologists are using ultrafast optical, photon, and particle techniques to reveal transition pathways, materials scientists can expect to use variations of these methods to probe the most fundamental aspects of complex transient phenomena in materials. The combination of high-spatial-resolution imaging with high time resolution is critical because it enables the observation of specific phenomena that are important to developing fundamental understanding. Such a capability is also important because it enables experiments that are on the same time and length scales as recent high-performance computer simulations. This article describes several new techniques that have great potential for broader application in materials science, including electron, x-ray, and γ-ray imaging.

We have investigated the effects of femtosecond (fs) laser irradiation on bone samples in in vitro and on ex vivo living bone samples. Ablation threshold, material removed per pulse and plasma shielding were examined using in vitro samples. Ablation threshold was found to be 0.9J/cm2 at 775nm and 0.3J/cm2 at 367nm using 200fs pulses. Material removal was found to vary non-linearly with pulse energy. Using in vivo samples we have demonstrated intact enzymatic activity on the surface of cells immediately adjacent to cells removed by fs laser irradiation suggesting no thermal damage.

We have measured the transient events of the α-β martensitic transformation in nanocrystalline Ti films via single shot electron diffraction patterns with 1.5 ns temporal resolution. This was accomplished with a newly constructed dynamic transmission electron microscope (DTEM), which combines pulsed laser systems and pump-probe techniques with a conventional TEM. The DTEM thereby enables studies of transformations that are (1) far too fast to be captured by conventional bulk techniques, and (2) difficult to study with current ultrafast electron diffraction (UED) instruments (which typically require an accumulation of multiple shots for each diffraction pattern). Martensitic transformations in nanocrystalline materials meet both criteria, with their rapid nucleation, characteristic interface velocities ∼1 km/s, and significant irreversible microstructural changes. Free-standing 40-nm-thick Ti films were laser-heated at a rate of ∼1010K/s to a temperature above the 1155 K transition point, then probed at various time intervals with a 1.5-ns-long intense electron pulse. Diffraction patterns show an almost complete transition to the β phase within 500 ns. Post-mortem analysis (after the sample is allowed to cool) shows a reversion to the α phase coupled with substantial grain growth, lath formation, and texture modification. The cooled material also shows a complete lack of apparent dislocations, suggesting the possible importance of a "massive" short-range diffusion mechanism.

Heme proteins form the cornerstone for our understanding of the general phenomena of molecular cooperativity; where the binding affinity of diatomic ligands in hemoglobin and associated changes in quaternary structure with ligation state is the best-characterized example. At the heart of this problem is an understanding how the initially localized reaction forces couple to the global protein coordinate to execute these functionally relevant motions. The energetics for this process are derived from one or a few chemical bonds and at the instant the bond is formed or broken, the motions are necessarily localized over atomic length scales at the active site and subject to quantum effects. From a practical standpoint, the heme proteins provide ideal model systems for studying the relationship between protein structure and function since they are well-characterized and their ligands can be photo-dissociated with unit quantum efficiency on the truly femtosecond timescale. This chapter discusses the results of ultrafast pump-probe spectroscopy of carboxymyoglobin (MbCO) to determine the initial structure changes of the ligand dissociation process using pulses of less than 10 fs in the 500-600 nm spectral regions.

Summary form only given. In the past decade, there have been significant strides in the development of high power diode-pumped solid-state lasers (DPSSL). DPSSL's are already available in commercial products employed in a variety of applications. Despite this progress, scaling to higher power with good efficiency still remains an obstacle. There have been a number of schemes developed to reduce the magnitude of the thermal lens, including slabs, disk designs, and end-pumping of Nd:YLF. Although these systems have demonstrated high average power, Nd:YLF is desirable in some industrial Q-switching applications for its less severe thermal lensing and high capacity energy storage. To summarize, we have developed a diode-pumped Nd:YLF laser with TEM/sub 00/ output power greater than 20 W and exceptional dynamic range. The laser system has also been successfully Q-switched at repetition rates as high as 20 KHz.

Amplified few cycle pulses were used to observe the initial dynamics of the pho todissociation process of carboxy myoglobin to specifically probe the issue of reaction driven modes in this system. Significant amplitude was observed at distinct modulation frequencies corresponding to deoxymyoglobin providing insight into the microscopic details of the reactive crossing.

We dynamically compress solid deuterium over

We have used a table-top scale laser to dynamically compress iron at strain rates in excess of 109 s−1. Using an embedded ultrafast interferometer, we have measured corresponding free surface histories with a time resolution of approximately 10 ps. We have analyzed the surface histories using a method that accounts for nonsteady wave propagation and time-dependent material behavior. We show that at these strain rates, the α→ϵ polymorphic transition begins within 100 ps after an initial very large (∼10 GPa) and mostly elastic compression and appears largely complete within a similar time thereafter. The corresponding deviatoric stress before the transition begins can exceed 3 GPa, while the transition stress itself is up to 25 GPa, nearly twice the value measured at low strain rates. We use these results to propose a systematic variation with loading time of the normal-stress/relative-volume curve followed by iron during rapid compression.

The development of high power diode-pumped solid-state lasers (DPSSL's) with diffraction limited beam quality has been significantly hampered by pump induced inhomogeneous temperature profiles in the laser gain medium which results in thermal lensing and birefringence.

Laser driven dynamic compression experiments may, in materials with picosecond equilibration times, be possible with orders of magnitude less drive energy than currently used. As we show, the compression energy for geometrically similar experiments varies as the third power of the time scale of compression. For materials which equilibrate and can be characterized on picosecond time scales, the compression energy can be orders of magnitude smaller than the 1–100 ns scale time scale of many current experiments. The use of substantially lower compression energy is a great practical advantage in such experiments, potentially enabling the observation of extreme states of matter with table top scale laser systems.

We apply ultrafast single shot interferometry to determine the pressure and density of argon shocked from up to 7.8 GPa static initial pressure in a diamond anvil cell. This method enables the observation of thermodynamic states distinct from those observed in either single shock or isothermal compression experiments. In particular, this method enables access to high density, relatively low temperature states of light materials, such as isentropically compressed states of giant planets. Further, since excitation by a shock wave is intrinsically ultrafast and this method has picoseconds time resolution, it has the potential to observe the collective dynamics of materials undergoing shock induced phase transitions and chemistry on ultrafast time scales. We also present a straightforward method for interpreting ultrafast shock wave data which determines the index of refraction at the shock front, and the particle and shock velocities for shock waves in transparent materials. Based on these methods, we observ...

Many pivotal aspects of material science, biomechanics, and chemistry would benefit from nanometer imaging with ultrafast time resolution. Here we demonstrate the feasibility of short-pulse electron imaging with t10 nanometer/10 picosecond spatio-temporal resolution, sufficient to characterize phenomena that propagate at the speed of sound in materials (1-10 kilometer/second) without smearing. We outline resolution-degrading effects that occur at high current density followed by strategies to mitigate these effects. Finally, we present a model electron imaging system that achieves 10 nanometer/10 picosecond spatio-temporal resolution.

A dynamic transmission electron microscope (DTEM) has been designed and implemented to study structural dynamics in condensed matter systems. The DTEM is a conventional in situ transmission electron microscope (TEM) modified to drive material processes with a nanosecond laser, “pump” pulse and measure it shortly afterward with a 30-ns-long probe pulse of ∼107 electrons. An image with a resolution of

We have compressed the output from a beta-barium borate noncollinear optical parametric amplifier to ~7-fs pulse durations, using a micromachined deformable mirror with an efficient search algorithm. This compression method allows phase compensation of both material and gain dispersion, which produces an optimized wavelength-tunable pulse shape for ultrahigh-resolution time-domain spectroscopy.