The energy levels of hydrogen-like atomic systems can be calculated with great precision. Starting from their quantum mechanical solution, they have been refined over the years to include the electron spin, the relativistic and quantum field effects, and tiny energy shifts related to the complex structure of the nucleus. These energy shifts caused by the nuclear structure are vastly magnified in hydrogen-like systems formed by a negative muon and a nucleus, so spectroscopy of these muonic ions can be used to investigate the nuclear structure with high precision. Here we present the measurement of two 2S–2P transitions in the muonic helium-4 ion that yields a precise determination of the root-mean-square charge radius of the α particle of 1.67824(83) femtometres. This determination from atomic spectroscopy is in excellent agreement with the value from electron scattering¹, but a factor of 4.8 more precise, providing a benchmark for few-nucleon theories, lattice quantum chromodynamics and electron scattering. This agreement also constrains several beyond-standard-model theories proposed to explain the proton-radius puzzle^2–5, in line with recent determinations of the proton charge radius^6–9, and establishes spectroscopy of light muonic atoms and ions as a precise tool for studies of nuclear properties.

The development of high-power diode lasers enabled new solid-state laser concepts such as thin-disk, fiber, and Innoslab lasers based on trivalent ytterbium as the laser-active ion, which resulted in a tremendous increase in the efficiency and beam quality of cw lasers compared to previously used lamp-pumped rod or slab lasers and the realization of ultrafast lasers with several 100 W or even kilowatts of average power. In addition to their beneficial thermo-optical properties, these architectures offer characteristic benefits making them especially suitable to obtain dedicated laser properties. This review article comprises milestone developments, characteristic challenges, and benefits, and summarizes the state of the art of high-power solid-state lasers with the focus on ultrafast lasers.

The grain structure of laser welded AlMgSi-alloys was varied by adapting the welding parameters and alloying additives. The hot cracking susceptibility was determined by the critical strain rate, measured by the application of digital image correlation in a self-restrained hot cracking test. A direct relation was found: The hot cracking susceptibility decreases with increasing number of grain boundaries across the width of the weld seam, i.e. along the effective load direction of transverse strain rates. It was shown, that the hot cracking susceptibility can be reduced by an optimisation of both the welding parameters and the alloying additives, which promote nucleation.

Spatial beam oscillation during laser beam welding of aluminum to copper was investigated. The beam was spatially oscillated perpendicular to the direction of feed in a sinusoidal mode. The influence of the oscillation amplitude and frequency on the weld seam geometry and the implications on the electrical resistance of the joints was investigated. It was found that spatial beam oscillation allows to set the welding depth and seam width virtually independent of each other. Furthermore, low welding depths into the lower copper sheet in combination with high ratios of seam width at the interface of the two sheets to welding depth into the lower copper sheet result in low electrical resistances of the welds. Low electrical resistances were found to correlate with high mechanical strengths of the welds.

Hydrogen-like light muonic ions, in which one negative muon replaces all of the electrons, are extremely sensitive probes of nuclear structure. Using pulsed laser spectroscopy, we have measured three 2 S -2 P transitions in the muonic helium-3 (μ ³ He ⁺ ) ion, an ion formed by a negative muon and bare helium-3 nucleus. This allowed us to extract the Lamb shift, the 2 P fine structure splitting, and the 2 S -hyperfine splitting in μ ³ He ⁺ . Comparing these measurements with theory, we determined the root-mean-square charge radius of the helion ( ³ He nucleus) to be r _h = 1.97007(94) fm, in good agreement with the value from elastic electron scattering but a factor 15 more accurate. Our results represent benchmarks for few-nucleon theories and open the way for precision quantum electrodynamics tests in He atoms and ions.