Tobias Ullsperger

Perfect X-ray focusing via fitting corrective glasses to aberrated optics

Due to their short wavelength, X-rays can in principle be focused down to a few nanometres and below. At the same time, it is this short wavelength that puts stringent requirements on X-ray optics and their metrology. Both are limited by todays technology. In this work, we present accurate at wavelength measurements of residual aberrations of a refractive X-ray lens using ptychography to manufacture a corrective phase plate. Together with the fitted phase plate the optics shows diffraction-limited performance, generating a nearly Gaussian beam profile with a Strehl ratio above 0.8. This scheme can be applied to any other focusing optics, thus solving the X-ray optical problem at synchrotron radiation sources and X-ray free-electron lasers.

Influence of ambient pressure on the hole formation process in ultrashort pulse laser deep drilling

We investigate the influence of the ambient pressure on the hole formation process during percussion drilling of silicon by applying an in-situ imaging technique. In this study the pressure is varied from atmospheric conditions down to medium vacuum of 10 !bar. Drilling was performed using an ultrashort pulse system providing 8 ps pulses with up to 125 μJ at 1030 nm. At this wavelength, the ablation behavior of silicon is comparable to metals. At the beginning of the drilling process, we observe an increased drilling efficiency by 40% already for a moderate pressure decrease to 100 mbar. The formation of an ideally shaped hole lasts for approximately 200 pulses instead of only 100 as for atmospheric conditions and therefore leads to 3 times the depth at this point. The effect can be enhanced by increasing the pulse energy, but not by decreasing pressure further. However, the number of pulses till the end of the drilling process is extended by decreasing the pressure further. For a low ambient pressure of 10 μbar, this is accompanied by an increase of the maximum achievable depth of more than 100%. Simultaneously the hole shape changes from a few ends and bulges at atmospheric conditions to numerous branches over the complete lower part of the hole at low pressure. This drilling behavior can be attributed to a better removal of ablated particles from the hole capillary with decreasing pressure, which leads to lower scattering losses for the pulse propagation inside the hole.

Selective laser melting of glass using ultrashort laser pulses

Within the field of laser assisted additive manufacturing, the application of ultrashort pulse lasers for selective laser melting came into focus recently. In contrast to conventional lasers, these systems provide extremely high peak power at ultrashort interaction times and offer both the opportunity of nonlinear absorption and the potential to control the thermal impact at the vicinity of the processed region by tailoring the pulse repetition rate. Consequently, transparent materials like borosilicate glass or opaque materials with extremely high melting points like copper, tungsten or even special composites like AlSi40 can be processed. In this publication, we present the selective laser melting of glass by using 500 fs laser pulses at MHz repetition rates emitted at a center wavelength of 515 nm. In order to identify an appropriate processing window, a detailed parameter study was performed. We demonstrate the fabrication of porous bulk glass parts as well as the realization of structures featuring thicknesses below 30 μm, which is below typical achieved structural sizes using pulsed or CO2 laser [1]. In contrast to alternative approaches [2], due to the nonlinear absorption and therefore complete melting of the material, there was no need for binding materials. This work demonstrates the potential for 3D printing of glass using the powder bed approach.

Selective laser melting of copper using ultrashort laser pulses at different wavelengths

Additive manufacturing gained increasing interest during the last decade due to the potential of creating 3D devices featuring nearly any desired geometry. One of the most widely used methods is the so-called powder bed method. In general, conventional cw and pulsed laser sources operating around 1030 nm and CO2 lasers at 10.6 μm are usually applied. Among other materials like polymers, these systems are feasible for several metals, alloys and even ceramics, but easily reach their limitation at a wide range of other materials, regarding required absorption and intensity. In order to overcome these limits, ultrashort pulse laser systems are one approach. Due to the increased peak power and ultrashort interaction times within the femtosecond and picosecond time range, materials with extraordinary high melting points, increased heat conductivity or new composites with tailored specifications are coming into reach. Moreover, based on the nonlinear absorption effect, also transparent materials can be processed. Here, we present the selective laser melting of pure copper using ultrashort laser pulses. This work involves a comparative study using 500 fs pulses at processing wavelengths of 515 nm and 1030 nm. The repetition rate of the applied laser system was varied within the MHz range in order to exploit heat accumulation. By using the ultrashort interaction times and tailoring the repetition rate, the induced melt pool can be significantly optimized yielding robust copper parts revealing thin-wall structures in the range below 100 μm.

Aberration Correction for Hard X-ray Focusing at the Nanoscale

We developed a corrective phase plate that enables the correction of residual aberration in reflective, diffractive, and refractive X-ray optics. The principle is demonstrated on a stack of beryllium compound refractive lenses with a numerical aperture of 0.49 10-3 at three synchrotron radiation and x-ray free-electron laser facilities, where we corrected spherical aberration of the optical system. The phase plate improved the Strehl ratio of the optics from 0.29(7) to 0.87(5), creating a diffraction-limited, large aperture, nanofocusing optics that is radiation resistant and very compact.