Abstract:We report studies of multiphoton mechanisms of plasmon excitation and their influence on the femtosecond-laser induced subwavelength ripple generation in large-bandgap dielectric and semiconducting transparent materials. An extended Drude-Sipe formalism is applied to quantitatively estimate the real part of the dielectric function which is dependent on the carrier density. The theory is able to predict the ripple periods for selected materials in good agreement with the experimental observations. Possible limitations at very small spatial periods are also discussed.

Particle acceleration and X-ray generation in different nano-structured targets irradiated by high intensity laser pulses of high contrast have been studied. It is found that maximal energy of fast particles and its directionality can be significantly enhanced, by choosing nano-structured targets. Generation and propagation of fast electrons in laser targets consisting of nano-wires are studied. Such targets exhibit a large conversion of laser energy into electron kinetic energy. An electron bunch can propagate a long distance and can be focused by bringing wires together. The results of theory and simulations were compared with the experimental data and have shown a reasonable consistency. IntroductionLaser driven nano-plasmonics deals with optical processes in plasmas at relatively low intensities and on nanoscale, i. e. on the order of or smaller than the wavelength of the laser radiation [1]. The underlying physical process is connected with a laser field enhancement due to proper nano-structuring of a target. Laser-matter interaction including nanoscale confinement of radiation and its transformation provides attractive opportunities for both, fundamental research and technological applications. Material processing uses femtosecond laser pulses exceeding the material ablation threshold to drill micro holes, to realize micro cutting or to selectively remove a particular substance. Such an extreme precision processing adds high value [2]. Due to the ultrashort pulse durations the results benefit by very clean and defined processing areas with negligible structural changes around. This is important in surgery (e. g. ophthalmology, dermatology, dentistry), for instance, when the ultrashort laser pulse irradiation enables clean surgery without damaging the side areas of the processed tissue [3].Laser nano-plasmons produced by structured surfaces have recently gained growing attention in laser plasma ion acceleration physics, as it can significantly enhance the acceleration mechanism. Today's research on laser driven particle acceleration (ions and electrons) promises future fast ion sources for different applications. Here, a laser at relativistic intensity interacts with thin solid foils, which can be well characterized and easily handled [4]. First, the laser causes an acceleration of the ionized electron distribution, which subsequently accelerates ions up to high kinetic energies. The low emittance of such ion beams is a striking feature [5]. Nevertheless, up to now the achieved acceleration efficiency is low (around a few to ten percent of the laser energy), even if the target thickness is optimized with respect to the laser parameters, thus current research is focusing on its optimization [6].

The laser chemical machining is a non-conventional substractive processing method. It is based on the laser-activation of a material dissolution of metals in electrolyte ambient via local-induced temperature gradients and allows a gentle and smooth processing of especially temperature-sensitive metals. However, the material removal is characterized by a narrow process window and is restricted by occurring disturbances, which are supposed to be related to the localized electrolyte boiling. In order to control the removal quality and avoid disturbances, the correlation between the laser-induced temperatures and the resulting removal geometry has to be better understood. In this work an analytical modeling of the laser-induced temperatures at the surface of titanium based on a Green-function approach is presented. The main influencing factors (laser, electrolyte, material) as well as possible heat transfer into the electrolyte are included and discussed. To verify the calculated temperatures, single spot experiments are performed and characterized for titanium in phosphoric acid solution within laser irradiation of 1 s. The correlation between the temperature distribution and the resulting removal geometry is investigated based on a spatial superposition. Thereby, the bottom limit temperature is found to range between 63˚C and 70˚C whereas the upper limit is related to the nucleate boiling regime. Based on the performed correlation an indicator is identified to predict the ruling removal regime and thereby to reduce the experimental expenditure.

Laser chemical machining (LCM) is a non-conventional processing method that enables a smooth and precise micro structuring of metallic surfaces. However, a high-quality removal is limited to a laser power window of some 100 mW. This is due to the high sensibility to removal disturbances, such as the deposition of metallic salts and oxides. In this work, the dynamic process behavior around the transition from a disturbance-free to a disturbed removal is investigated for the laser chemical machining of titanium (3.7024) and stainless steel (AISI 304) in different phosphoric acid solutions. Therefore, the removal cavities are recorded using confocal scanning microscopy and characterized regarding width, depth and quality in dependence of the laser power, feed velocity and electrolyte concentration. While the removal characteristics within the disturbance-free regime are found to be material-independent, the disturbed regime is strongly dependent on the tendency of the material to gas bubble adherence. Additional CCD records of the interaction zone reveal that the transition to the disturbed regime is accompanied by significant light reflections and thereby indicate the influence of adhering gas bubbles on disturbing the removal process. Moreover, typical removal disturbances are presented and discussed with regard to the responsible mechanisms for their occurrence.

Laser chemical machining, a non-conventional processing method based on thermally activated electrochemical material dissolution, represents a promising technology for manufacturing metallic dies for micro forming applications. Prior to widespread industrial acceptance the machining quality of laser chemical machining should be characterized. For this purpose, laser chemical machining is compared with micro milling regarding both the dimensional accuracy and the surface quality. Therefore, square micro cavities exhibiting side walls between 100 μm and 400 μm in length and 60 μm in depth are machined with both manufacturing processes into the cobalt-chrome alloy Stellite 21. The geometrical features are investigated using laser-scanning confocal microscopy and scanning electron microscopy. On the one hand, laser chemical machining is more suitable for manufacturing cavities with dimensions < 200 μm due to higher shape accuracy with stable mean edge radii of (11.2 ± 1.3) μm as a result of roughing and finishing steps. On the other hand, the finish quality of micro milling with mean surface roughness Sa of 0.2 μm could not be achieved with laser chemical machining due to in-process induced waviness. Finally, the metallographic analysis of the surface-near layers reveals that both manufacturing processes ensure gentle machining without any noticeable micro structural impact.