Ingolf V. Hertel

Collisional alignment and orientation of atomic outer shells. II. Quasi-molecular excitation, and beyond

Abstract We present a comprehensive and critical review on atomic alignment and orientation after collisional excitation including a full reevaluation and discussion of data published to date in the literature. The present Vol. II focuses on the quasimolecular low-energy regime in ion-atom and atom-atom processes. The discussion centers on experimental and theoretical data for planar scattering geometry in which the alignment and orientation parameters are determined as a function of scattering angle (particle-photon coincidence or scattering from laser-excited atoms, the so-called third-generation of experiments). In addition, alignment studies with cylindrical symmetry (second-generation experiments) and integral cross sections (first-generation experiments) are also discussed wherever this clarifies the understanding of the relevant processes. A unified set of parameters is used with reference to the so-called natural coordinate frame (having its z -axis perpendicular to the collision plane): the angular momentum transfer L ⊥ , the alignment angle γ, the linear polarization P + and the symmetry-changing probability ϱ00. This parametrization allows one an intuitive interpretation of these otherwise somewhat abstract quantities. After reviewing the theoretical framework the individual model systems are described, starting with excitation and charge transfer in the genuine one-electron system H + + H and ending with He + + Rg ( Rg = raregas atom) collisions involving many electrons and demonstrating the explicit influence of spin-orbit coupling. In several appendices a compilation of the relevant classical deflection functions and many useful formulae for the interpretation of alignment and orientation parameters are reported.

Collisional alignment and orientation of atomic outer shells. III. Spin-resolved excitation

Abstract The review generalizes the formalism of the first part of this series [N. Andersen, J.W. Gallagher and I.V. Hertel, Phys. Rep. 165 (1988) 1–188] on electron-atom collisions to cases where the spin polarization of the electron is investigated at least once, i.e., before and/or after the collision. In addition, the target atom may be spin polarized. The preparation of initially polarized beams and/or the analysis of photon and spin polarization in the final state significantly increase the number of cases where a “perfect scattering experiment” can be performed. The connection between the scattering amplitudes and the generalized Stokes and STU parameters is analyzed. Favorable scattering geometries for perfect experiments and possibilities for consistency checks are pointed out. Recommendations are made about directions of future work.

Surface and bulk ultrashort-pulsed laser processing of transparent materials

Ultrashort pulsed laser ablation of dielectrics has been investigated using ex-situ morphological examinations in combination with in-situ time-of-flight mass spectrometry of the ablated species. Analysis of the energy spectrum of the ablation products provides a wealth of information on the processes occurring during femtosecond laser ablation of materials. The presentation will focus on the case of sapphire (Al2O3) and discuss the fundamental processes in ultrashort pulsed laser sputtering. Two different ablation phases have been identified, a gentle phase with low ablation rates and a strong etch phase with higher ablation rates, but with limitation in structure quality. A comparison of the energy and momentum distributions of ejected ions, neutrals and electrons allows one to distinguish between non-thermal and thermal processes that lead to the macroscopic material removal. Fast positive ions with equal momenta are resulting from Coulomb explosion of the upper layers at low fluence and low number of irradiating laser pulses (gentle etch phase). Pump-probe studies with fs laser pulses reveal the dynamics of excitation and electron mediated energy transfer to the lattice. At higher laser fluences or after longer incubation, evidence for phase explosion can be derived from both the morphology of the surface and the results of the in-situ experiments.

Adaptive control of ion beams produced by ultrafast laser ablation of silicon (Invited Paper)

In a context where ultrafast lasers have become ideal tools for material probing and processing we present various concepts for process control and optimization. Temporal tailoring of ultrashort laser pulses enables synergies between radiation and material and, therefore, new opportunities for optimal processing of materials. The concept of optimizing laser interactions is based on the possibility to adjust energy delivery so that control of laser-induced processes can be achieved and particular states of matter can be accessed. We present recent results related to the implementation of adaptive feedback loops based on temporal shaping of ultrafast laser pulses to control laser-induced phenomena for practical applications. The chosen example indicates the possibility to manipulate the kinetic properties of ions emitted from ultrafast laser irradiated semiconducting samples, using excitation sequences synchronized with the phase-transformation characteristic times. Versatile sub-keV ion beams are obtained exploiting transitions to supercritical fluid states with minimal energetic expenses, while achieving very efficient energy coupling and thermodynamic paths towards highly volatile states. Temporally selective irradiation can thus open up efficient thermodynamic paths towards critical points, delivering at the same time an extended degree of control in material processing.

Adaptive spatio-temporal techniques for smart ultrafast laser processing of optical glasses

Ultrafast lasers emerged as efficient tools to process transparent materials on minimal scales. Localized refractive index changes can serve as building blocks for embedded optical functions. The requirements of a desired photonic response involve precise adjustments of the refractive index which usually depends on the material relaxation paths. Advanced strategies are then required to improve the irradiation results. Recently, new beam manipulation concepts were developed which allow a modulation of the energy feedthrough according to the material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal results. Considering the potential of optical functionalization, we discuss the possibility of controlling laser-induced modifications of transparent materials employing automated temporal pulse shaping. Examples of adaptive design of refractive index changes in glasses will be shown, accompanied by concepts of efficient processing approaches. This involves an engineering aspect related to simultaneous processing of structural modifications in 3D arrangements where a feasible solution is represented by dynamic spatial beam shaping techniques. The approach has a dual aspect and includes corrections for beam propagation errors and spatial intensity distributions in desired forms for parallel processing. Adding the possibility of laser-induced birefringence, photowritten structures can be arranged in patterns generating complex propagation and polarization effects.Ultrafast lasers emerged as efficient tools to process transparent materials on minimal scales. Localized refractive index changes can serve as building blocks for embedded optical functions. The requirements of a desired photonic response involve precise adjustments of the refractive index which usually depends on the material relaxation paths. Advanced strategies are then required to improve the irradiation results. Recently, new beam manipulation concepts were developed which allow a modulation of the energy feedthrough according to the material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal results. Considering the potential of optical functionalization, we discuss the possibility of controlling laser-induced modifications of transparent materials employing automated temporal pulse shaping. Examples of adaptive design of refractive index changes in glasses will be shown, accompanied by concepts of efficient processing approaches. This invol...

Adaptive optimization in ultrafast laser material processing (Plenary Paper)

Ultrafast lasers promise to become attractive and reliable tools for material processing on micro- and nanoscale. The additional possibility to temporally tailor ultrashort laser pulses by Fourier synthesis of spectral components enables extended opportunities for optimal processing of materials. An experimental demonstration of the technique showing the possibility to design particular excitation sequences tailored with respect to the individual material response will be described, laying the groundwork for adaptive optimization in materials structuring. We report recent results related to the implementation of self-learning, adaptive loops based on temporal shaping of the ultrafast laser pulses to control laser-induced phenomena for practical applications. Besides the fundamental interest, it is shown that under particular excitation conditions involving modulated excitation, the energy flow can be controlled and the material response can be guided to improve processing results. Examples are given illuminating the possibility to control and manipulate the kinetic properties of ions emitted from laser irradiated semiconductor samples using excitation sequences synchronized with the phase transformation characteristic times.

Fundamentals and advantages in ultrafast microstructuring of transparent materials

Time resolved studies using femtosecond laser pulses at 800 nm illuminate the distinctions in the dynamics of ultrafast processing of dielectrics compared to semi-conductors and metals. Dielectric materials are strongly charged at the surface on the sub-ps time scale and undergo an impulsive Coulomb explosion prior to thermal ablation. Provided the laser pulse width remains in the ps or sub-ps time domain this effect can be exploited for processing. Otherwise, the high localization of energy accompanied by ultrafast laser micro structuring is of great advantage also for high quality processing of thin metallic or semi-conductive layers, where the surface charge is effectively quenched.