Mark Boyle

Laser ablation of dielectrics with temporally shaped femtosecond pulses

A significant improvement in the quality of ultrafast laser microstructuring of dielectrics is demonstrated by using temporally shaped pulse trains with subpicosecond separation. The sequential energy delivery induces a material softening during the initial steps of excitation changing the energy coupling for the subsequent steps. This leads to lower stress, cleaner structures, and provides a material-dependent optimization process.

Fragmentation dynamics of fullerenes in intense femtosecond-laser fields: Loss of small neutral fragments on a picosecond time scale

The fragmentation dynamics of C60 irradiated with intense femtosecond laser pulses is studied with one-color pump-probe spectroscopy. Small neutral fragments (C, C2, and C3) are formed by an 800-nm pump pulse which are then postionized by a delayed probe pulse. The respective ion signals detected by the time-of-flight mass spectrometry dramatically increase on a time scale of 10-20 ps.

Advanced ultrafast laser material processing using temporal pulse shaping

Phase manipulated ultrafast laser pulses and temporally tailored pulse trains with THz repetition rates are promising new tools for quality micromachining of brittle dielectric materials, allowing to adapt the laser light to the material properties for optimal processing quality. Different materials respond with specific reaction pathways to the sudden energy input depending on the efficiency of electron generation and on the ability to release the energy into the lattice. Loss and cooling mechanisms in the electron population, surface charging, as well as the strength of the electron-phonon interactions control the effectiveness of the energy deposition into the lattice. Knowledge of the response times of materials establishes a guideline for using temporally shaped pulses or pulse trains in order to optimize the structuring process with respect to efficient material removal and reduction of the residual damage. The sequential energy delivery with judiciously chosen pulse trains may induce softening of the material during the initial steps of excitation and change the energy coupling for the subsequent steps. We show, that this can result in lower stress, cleaner structures, and allow for a material-dependent optimization process.

Ultrafast laser ablation of dielectrics employing temporally shaped femtosecond pulses

Dielectric materials exposed to ultrashort laser radiation have evidenced individualized paths to deposit the energy into the lattice. Electronic and thermal mechanisms competing in the process of material removal depend on the efficiency of the electrostatic energy accumulation on the surface due to photoionization, as well as on the lattice heating which follows the electron-phonon coupling. The electrostatic surface break-up is a fast, sub-picosecond process, while thermal mechanisms start to dominate on a longer, picosecond time scale given by the electron-lattice equilibration and phase transformation time. The Coulomb- explosion induced ion ejection due to surplus charge accumulated on the surface during the photoionization process is significant only in dielectrics while in semiconductors and metals an efficient neutralization occurs. The significance of the different channels in dielectric materials can be reduced or enhanced by using laser pulses which are modulated on a time scale characteristic for the above mentioned mechanisms. Thus, amplified temporally-shaped pulses, double peaks, or pulse trains with a separation below 1 ps can have a significant effect on the quality of micromachining of transparent materials. The energy deposition can be modulated in such a way that the first pulse of properly chosen energy leads to a softening of the material associated with the onset of heating, thus changing the coupling conditions for the next pulses. This leads to less residual stress accumulation, cleaner structures, and opens the way for a material dependent optimization process.

Tailored excitation sequences for optimized laser-induced modifications in bulk transparent materials exposed to sub-ps irradiation

Tight focusing of ultrashort near infrared laser pulses in the bulk of various transparent materials induces significant modifications of the optical properties by locally changing the material refractive index. Such laser-induced phase objects are of major technological interest, notably for direct writing of embedded optical functions. While extensive studies have been reported on ultrashort pulsed laser induced modifications in several materials, especially with regard to focusing conditions, incubation effects, or the influence of the energy content of the pulse, we emphasize here the role of the temporal design of the excitation sequence. We present phase-contrast microscopy investigations of the resultant morphology and discuss the refractive index topological map induced by different temporal pulse intensity envelopes in various transparent materials. The consequences of temporal profiles generated by a pulse shaping apparatus on the morphology of the interaction zone are illustrated, emphasizing the benefits of the synchronization between the excitation temporal profile and the material response.

Laser ablation of dielectrics using ultrashort and temporally shaped laser pulses

A significant improvement in the quality of ultrafast laser micromachining of brittle dielectrics is demonstrated by using temporally shaped pulse trains with sub-ps separation, synchronized with the material specific relaxation times. The individual material response to laser radiation depends on the efficiency of electron generation and on the ability to release the energy into the lattice. Loss mechanisms in the electron population, surface charging, as well as the strength of the electron-phonon interactions control the effectiveness of the energy deposition into the lattice. Knowledge of the response times of materials establishes a guideline for using temporally shaped pulses or pulse trains in order to optimize the structuring process with respect to the efficiency of material removal and reduction of the residual damage. The sequential energy delivery induces a material softening during the initial steps of excitation changing the energy coupling for the subsequent steps. This leads to lower stress, cleaner structures, and provides a material-dependent optimization process.