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Ultrafast lasers of subpicosecond pulse duration have thus far been investigated for ablation, drilling, and cutting processes. Ultrafast lasers also have the potential for laser welding of small components of the order of microns and for laser shock peening to enhance the peening depth. In this paper, the potential for welding is investigated using the two-temperature model to analyze laser-matter interaction for ultrafast lasers. The model is implemented in a general-purpose commercial finite element method package, ABAQUS, tO enable broad based application of the two-temperature model in practical engineering problems. The implementation is validated by comparison with linear solutions obtained using separation of variables. It is then used to investigate the potential for microwelding using ultrafast laser pulse for both low and high fluence laser input. [DOI: 10.1115/1.3075869] Keywords: ultrashort laser pulses, femtosecond lasers, FEM, ABAQUS, two-temperature model laser microwelding

Ultrahigh-intensity lasers have a place in nonlinear optics in the relativistic regime for future applications in time-resolved chemistry. Ability to amplify pulses to extreme levels makes possible fundamentally new applications in science and technology. Interactions of lasers with matter in the ultrahigh-intensity regime makes generation of energetic photons and particles in the giga-electron-volt and X-ray area possible. That can make it possible to investigate ultrafast phenomena in chemistry, biology and physics with compact, fairly inexpensive systems such as those at universities.

Absorbing structures such as enzymes, organelles or cells having pulse energies lower than that of single-pulse damage threshold energy suffer damage from repeated exposure to short laser pulses. The retinal pigment epithelium suffer selective damage, while the neural retinal are unaffected. The retinal pigment epithelium is damaged by thermal interaction mechanisms and also by thermomechanical effects.

Crystalline gallium arsenide (GaAs) is a suitable system for quantitative investigations using optical pumping and X-ray probing. Information on bulk dynamics in absorbing materials has not been obtained due to the short penetration depth of visible light. A coherent acoustic phonon propagation in crystalline GaAs has been reported, using a non-thermal, ultrafast-laser-driven plasma. A 100-ps coherent acoustic pulse was followed, propagating through the x-ray penetration depth.

Studies show that a timescale of under a picosecond can be achieved in monitoring changes in an organic film's X-ray diffraction pattern after laser pulse heating. A Langmuir-Blodgett multilayer cadmium arachidate film's response to laser heating is observed, with intensity seen to decrease significantly following one picosecond of heating. Thin film diffraction is shown to switch off ultra quickly, demonstrating that large structural rearrangements can occur in condensed matter on the femtosecond timescale.

The excitation of a relativistic plasma wave to the breaking point by a short and high-intensity laser pulse, using the Raman forward-scattering instability, accelerates the electrons and enhances their number. The acceleration of electrons causes a number of scattered frequencies and the coherence of laser light scattered by the wave is lost. An increase in plasma density increases the maximum electron energy (up to 44 MeV) and results in a tremendous broadening of each anti-Stokes peak at the corresponding high density.

A study was conducted to analyze the single burst ablation of dielectrics by a femtosecond pulse train that consists of one or multiple pulses. By using constant-ablation-depth zones with respect to fluences in the femtosecond pulse-train technology, repeatable nanostructures at the desired dimension could be obtained, even when the laser system is subject to fluctuations in laser fluences.

We present the results of two-pump and probe femtosecond experiments designed to follow the relaxation dynamics of the lowest excited state ([S.sub.1]) populated by different modes. In the first mode, a direct ([S.sub.0] [right arrow] [S.sub.1]) radiative excitation of the ground state is used. In the second mode, an indirect excitation is used where the [S.sub.1] state is populated by the use of two femtosecond laser pulses with different colors and delay times between them. The first pulse excites the [S.sub.0] [right arrow] [S.sub.1] transition whereas the second pulse excites the [S.sub.1] [right arrow] [S.sub.n], transition. The nonradiative relaxation from the [S.sub.n] state populates the lowest excited state. Our results suggest that the [S.sub.1] state relaxes faster when populated nonradiatively from the [S.sub.n], state than when pumped directly by the [S.sub.0] [right arrow] [S.sub.1] excitation. Additionally, the [S.sub.n] [right arrow] [S.sub.1] nonradiative relaxation time is found to change by varying the delay time between the two pump pulses. The observed dependence of the lowest excited state population as well as its dependence on the delay between the two pump pulses are found to fit a kinetic model in which the [S.sub.n] state populates a different surface (called [S'.sub.1]) than the one being directly excited ([S.sub.1]). The possible involvement of the [A.sub.g] type states, the J intermediate, and the conical intersection leading to the [S.sub.0] or to the isomerization product (K intermediate) are discussed in the framework of the proposed model.

Physicists have used lasers to increase the amount of energy that electrons gain per metre by more than two orders of magnitude compared with traditional accelerators. Conventional colliders can accelerate [...]

Physicists have known for centuries that one of the surest ways to make new discoveries is to go to extremes: shorter timescales, greater intensities, higher energies, smaller distances. Going to [...]

The ability to generate optical pulses with durations under 100 fs has revolutionized spectroscopy and opened up many exciting possibilities in optical communications. Recent advances with ultrafast laser sources and [...]

The advent of femtosecond chemistry and its impact on chemistry research are discussed. Scientists are pushing the frontiers of chemistry by using lasers that generate ultra-short impulses to directly observe the behavior of atoms as molecules undergo chemical reactions. The possibilities of femtosecond chemistry are demonstrated most clearly by rotational coherence spectroscopy to study large molecules, dimers and clusters that have large moments of inertia. Some scientists are wondering if femtosecond chemistry represents the limit of time-resolved measurements of molecular interactions.

It may become possible to dissociate molecules in a controlled manner through the use of chirped laser pulses. Dutch researchers have excited the transition of rubidium's valence electron from the ground to the first two excited states using a chirped laser pulse. As the frequency of the laser increases, the first transition is stimulated and then the slightly less energetic second transition. Surprisingly, pulses of decreasing frequency populate the higher energy level while bypassing the intermediate one.