Here we present laser aided additive manufacturing of pure copper parts using ultrashort laser pulses. The process is based on the powder bed fusion of pure copper powder with grain sizes in the range of 5 μm – 15 μm. For processing, a fiber laser system delivering 500 fs pulses at a wavelengths of 515 nm was used. Robust 3D printed parts are generated featuring a high degree of resolution. The fabricated copper samples were characterized in terms of morphology, density distribution, thermal and electrical conductivity. The inner structure is revealed by x-ray computed tomography measurements.
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 CO<sub>2</sub> 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.<p> </p> 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.
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 CO<sub>2</sub> laser . In contrast to alternative approaches , 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.
The laser inscription of waveguides into the volume of crystalline silicon is presented. By using sub-ps laser pulses at a wavelength of 1552 nm highly localized light guiding structures with an average diameter ranging from 1 – 3 μm are achieved. The generated waveguides are characterized in terms of mode field distribution, damping losses and permanent refractive index modification. First investigations indicate an induced increase of the refractive index in the order of 10<sup>-3</sup> to 10<sup>-2</sup>. Depending on the applied laser pulse energy single-mode to multimode like propagation behavior can be observed. At optimized processing parameters, the damping losses can be estimated below 3 dB/mm.
Atmospheric-free bodies of the solar system are undergoing several processes that alter their original spectral characteristics. The whole of these processes is the so-called space weathering. The surface of such bodies is exposed to the solar wind irradiation and to the ongoing bombardment of micrometeoroids yielding material modifications at the micro- and nanometer scale. In order to understand these processes and clarify the influence on spectral reflectance and absorption, numerous experimental approaches using ion and laser irradiation have been presented so far. However, up to this date, basic damaging mechanisms are still unresolved or cannot be completely reproduced. <p> </p>In this work, we present the application of ultra-short laser pulses as a tool to reproduce space weathering, with focus on micrometeoroid impacts. In our experiments, slices of single-crystal olivine were irradiated under vacuum condition using 100 fs single-shot laser pulses. In order to perform spectral measurements, the laser-damaged regions were distributed over the sample surface within a grid geometry. After laser processing, a comprehensive study was performed by using spectroscopic measurements in the NUV-vis-NIR range, white light interferometry, SEM and TEM analysis. The cross-sections of the laser-generated craters reveal different layers including from the top to the bottom: an amorphous layer, two polycrystalline layers with different textures, and a defect-rich olivine substrate. Moreover, iron nanoparticles occur within the lower part of the amorphous layer and the polycrystalline layer. We can reproduce microcraters whose morphology, microstructure, and distribution of iron nanoparticles are similar to those found in the soil samples of the Moon or of the asteroid 25143 Itokawa.
Thin-film photovoltaic panels consist of individual solar cells which are monolithically interconnected in series. Today, these connections are commonly realized by mechanical methods. In order to increase the solar output, it is one approach to minimize the interconnection area (so called “dead area”). In this regard, recent advances in laser patterning are gaining increasing potential. However, especially high-impedance trenches realized via laser scribing generally suffer from insufficient shunt resistances. This is especially the case for the third structuring stage P3 of CIGS solar modules, which represents the isolation of nearby cells.
We report on theoretical models of the interaction of ultra-short laser pulses with multilayer structures used in thin-film solar cells. A finite-difference based optical model of light propagation within the thin-film system is used to determine the 3D-distribution of absorbed laser power in the structure. The model includes the evolution of the density of charge carriers which may be driven either by direct absorption of the laser radiation or multi-photon absorption and impact ionization of highly excited carriers.
Depending on the excitation wavelength and pulse energy absorption occurs in different depths of the structure which has a large effect on the efficiency of the laser ablation process.
InN, a novel semiconductor material, is used as THz surface emitter. The material is irradiated with fs-laser pulses at 1060 nm and 800 nm and the emitted ultrashort THz pulses are measured by phase sensitive detection. Pulsforms, amplitudes and spectra are compared to the THz emission of p-doped InAs, the standard material for THz surface emission.
Generation of InAs-surface-emitted terahertz radiation by application of an ultrashort pulse 1064 nm parabolic fiber amplifier source is reported for the first time. The fiber amplifier delivers 100 fs pulses at a repetition rate of 75 MHz and an average power of maximum 12 W. This new excitation laser for surface-emitters generates high brightness broadband THz radiation ranging from 100 GHz to over 2.5 THz. THz detection is demonstrated based on two-photon absorption at low-temperature-grown GaAs dipole receivers.