High average laser powers can have a serious adverse impact on the ablation quality in ultra-short pulsed laser material processing of metals. With respect to the scanning speed, a sharp transition between a smooth, reflective and an uneven, dark ablated surface is observed. Investigating the influence of the sample temperature, it is experimentally shown that this effect stems from heat accumulation. In a numerical heat flow simulation, the critical scanning speed indicating the change in ablation quality is determined in good agreement with the experimental data.

A linear to radial and/or azimuthal polarization converter (LRAC) has been inserted into the beam delivery of a micromachining station equipped with a picosecond laser system. Percussion drilling and helical drilling in steel have been performed using radially as well as azimuthally polarized infrared radiation at 1030 nm. The presented machining results are discussed on the basis of numerical simulations of the polarization-dependent beam propagation inside the fabricated capillaries.

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Laser processing with ultra-short double pulses has gained attraction since the beginning of the 2000s. In the last decade, pulse bursts consisting of multiple pulses with a delay of several 10 ns and less found their way into the area of micromachining of metals, opening up completely new process regimes and allowing an increase in the structuring rates and surface quality of machined samples. Several physical effects such as shielding or re-deposition of material have led to a new understanding of the related machining strategies and processing regimes. Results of both experimental and numerical investigations are placed into context for different time scales during laser processing. This review is dedicated to the fundamental physical phenomena taking place during burst processing and their respective effects on machining results of metals in the ultra-short pulse regime for delays ranging from several 100 fs to several microseconds. Furthermore, technical applications based on these effects are reviewed.

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Nuclear-spin relaxation rates are usually measured by pulsed radio frequency excitation of nuclear-spin transitions. When the number of spins is too small for direct detection with conventional nuclear-magneticresonance spectrometers, pulsed excitation may be combined with optical detection of the nuclear spins for increased sensitivity. Here we demonstrate that such measurements can also be done purely optically, without radio frequency fields. This is achieved by preparing nonthermal populations by optical hole-burning and time-resolved probing of the populations as a function of the decay time. Measurements of the nuclear-spin relaxation of 141 Pr in the electronic ground state of Pr 3ϩ :YAlO 3 show that the relaxation rates for the three transitions differ by a factor of 3.

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