Klaus Bergner

Comparison of different processes for separation of glass and crystals using ultrashort pulsed lasers

We investigate cutting of transparent materials using ultra short laser pulses with pulse durations in the sub to a few ps regime. All compared methods base on nonlinear absorption including ablation cutting and cleaving or selective etching supported by laser induced modification inside the bulk material. For most of the experiments samples of hardened glass (Corning Gorilla®) with thickness up to 700 μm were used, ablation cutting of sapphire is presented additionally. Absorption and modification inside the volume is analyzed in detail, aiming for tailored modifications. Besides optical microscopy a pump probe setup was used. We show results of time resolved absorption measurements of 6 ps pulses focused into the volume. We observe shielding due to the interaction region and accumulation effects influencing the modifications. First results on inscribing and cutting by using beam shaping indicate the importance of tailoring the shape and arrangement of the pulses temporally and spatially. The results presented for the different cutting methods supports an assessment of the individual potential and a selection of the applicable method based on the requirements.

Higher-order Bessel-like beams for optimized ultrafast processing of transparent materials

The controlled energy deposition by nonlinear absorption of ultrashort laser pulses offers a variety of different processing strategies for the machining of wide-bandgap materials. Considering laser-glass cutting applications, efficient single pass processes with volume modifications along the entire substrate thickness become possible using adapted focal field distributions [1]. The required extreme aspect ratios of longitudinal (given by glass thickness) to transverse (diffraction limit) beam dimensions are met by the class of Bessel-like beams that can be generated efficiently using phase-only spatial light modulators (SLMs) [2]. Simple multiplexing π-phase jumps or phase vortices ∝ exp (iℓθ) into the Fresnel-axicon-type phase mask [cf. Fig. 1(a)] enables to generate Bessel-like beams exhibiting ring- and petal-like transverse intensity distributions, respectively, while keeping the non- diffracting and self-healing beam properties [3]. By using pump-probe microscopy we proof that the resulting absorption distribution and, thus, the spatial energy deposition inside the material follows accurately the beams simulated intensity profile.

Throughput scaling by spatial beam shaping and dynamic focusing

With availability of high power ultra short pulsed lasers, one prerequisite towards throughput scaling demanded for industrial ultrafast laser processing was recently achieved. We will present different scaling approaches for ultrafast machining, including raster and vector based concepts. The main attention is on beam shaping for enlarged, tailored processed volume per pulse. Some aspects on vector based machining using beam shaping are discussed. With engraving of steel and full thickness modification of transparent materials, two different approaches for throughput scaling by confined interaction volume, avoiding detrimental heat accumulation, are exemplified. In Contrast, welding of transparent materials based on nonlinear absorption benefits from ultra short pulse processing in heat accumulation regime. Results on in-situ stress birefringence microscopy demonstrate the complex interplay of processing parameters on heat accumulation. With respect to process development, the potential of in-in-situ diagnostics, extended to high power ultrafast lasers and diagnostics allowing for multi-scale resolution in space and time is addressed.

Simultaneous spatial and temporal focusing: a route towards confined nonlinear materials processing

Ultrashort pulse lasers enable reliable and versatile high precision ablation and surface processing of various materials such as metals, polymers and semiconductors. However, when modifications deep inside the bulk of transparent media are required, nonlinear pulse material interactions can decrease the precision, since weak focusing and the long propagation of the intense pulses within the nonlinear media may induce Kerr self-focusing, filamentation and white light generation. In order to improve the precision of those modifications, simultaneous spatial and temporal focusing (SSTF) allows to reduce detrimental nonlinear interactions, because the ultrashort pulse duration is only obtained at the focus, while outside of the focal region the continuously increasing pulse duration strongly reduces the pulse intensity. In this paper, we review the fundamental concepts of this technology and provide an overview of its applications for purposes of multiphoton microscopy and laser materials processing. Moreover, numerical simulations on the nonlinear pulse propagation within transparent media illustrate the linear and nonlinear pulse propagation, highlighting the differences between conventional focusing and SSTF. Finally, fs-laser induced modifications in gelatine are presented to compare nonlinear side-effects caused by conventional focusing and SSTF. With conventional focusing the complex interplay of self-focusing and filamentation induces strongly inhomogeneous, elongated disruptions. In contrast, disruptions induced by SSTF are homogeneously located at the focal plane and reduced in length by a factor >2, which is in excellent agreement with the numerical simulations of the nonlinear pulse propagation and might favor SSTF for demanding applications such as intraocular fs-laser surgery.

Time-resolved tomography of ultrafast laser-matter interaction

We demonstrate time-resolved tomography with 200 fs resolution for the three-dimensional analysis of the non-linear dynamics of ultrafast laser-matter interaction inside the volume of transparent materials. We reconstruct as an example the three-dimensional spatial distribution of the transient extinction coefficient induced by focusing higher-order Bessel-Gaussian-beams into Gorilla glass. This approach can be employed to gaseous, liquid and transparent solid state matter which interact with laser light.

Scaling ultrashort laser pulse induced glass modifications for cleaving applications

Ultrashort laser pulses allow for in-volume processing of glass through non-linear absorption. This results in permanent material changes, largely independent of the processed glass, and it is of particular relevance for cleaving applications. In this paper, a laser with a wavelength of 1030 nm, pulse duration of 19 ps, repetition rate of 10 kHz, and burst regime consisting of either four or eight pulses, with an intra-burst pulse separation of 12.5 ns, is used. Subsequently, a Gaussian-Bessel focal line is generated in a fused silica substrate with the aid of an axicon configuration. We show how the structure of the modifications, including the length of material disruptions and affected zones, can be directly influenced by a reasonable choice of focus geometry, pulse energy, and burst regime. We achieve single-shot modifications with 2 μm in diameter and 7.6 mm in length, exceeding an aspect ratio of 1:3800. Furthermore, a maximum length of 10.8 mm could be achieved with a single shot.

Time-resolved microscopy using variable probe wavelengths for ultra-short pulse interaction (Conference Presentation)

Glass processing with ultrashort laser pulses allow for different material modifications, ranging from smooth refractive index changes which can be used for the generation of waveguides up to large disruptions due to accumulates stress for glass separation. These disruptions, generated by a dense electron plasma, are favored for glass dicing applications. To tailor the resulting material response a fundamental understanding of the laser/material interaction is of interest. Therefore, we analyze the spatio-temporal evolution of free carriers induced by ultrashort laser pulses using a pump-probe setup with high temporal and spatial resolution and various probe wavelengths. Single laser pulses with 1026nm wavelength, 6ps (FWHM) pulse duration and 200μJ pulse energy were applied to fused silica, Borofloat 33 and Gorilla glass. Electron densities around 1 x 1020cm-3 in the focal plane and 1 x 1019cm-3 in front of the focus are obtained, independent from the glass type used. The free carriers slowly decay within several ns, while the decay time depends on both the maximum electron densities reached and glass species. In this process a part of the excited electrons relax within several 10ps into a long-living stage where a transient effect is observed. Here, various probe wavelengths show differences in the recorded signal. A further carrier relaxation leads to permanent (stress, voids) and non-stable (color center) modifications crucial for precise glass dicing applications.

CIGS P3 scribes using ultra-short laser pulses and thermal annealing

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.

Ultrafast laser processing of transparent materials supported by in-situ diagnostics

For the development of industrial NIR ultrafast laser processing of transparent materials, the absorption inside the bulk material has to be controlled. Applications we aim for are front and rear side ablation, drilling and inscription of modifications for cleaving and selective laser etching of glass and sapphire in sheet geometry. We applied pump probe technology and in situ stress birefringence microscopy for fundamental studies on the influence of energy and duration (100 fs – 20 ps), temporal and spatial spacing, focusing and beam shaping of the laser pulses. Applying pump probe technique we are able to visualize differences of spatio-temporal build up of absorption, self focusing, shock wave generation for standard, multispot and beam shaped focusing. Incubation effects and disturbance of beam propagation due to modifications or ablation can be observed. In-situ imaging of stress birefringence gained insight in transient build up of stress with and without translation. The results achieved so far, demonstrate that transient stress has to be taken into account in scaling the laser machining throughput of brittle materials. Furthermore it points out that transient stress birefringence is a good indicator for accumulation effects, supporting tailored processing strategies. Cutting results achieved for selective laser etching by single pass laser modification exemplifies the benefits of process development supported by in situ diagnostics.

Ultrafast Laser Machining of Transparent Materials: Process Development Supported by In-Situ Diagnostics

Controlling the nonlinear absorption and accumulation effects is crucial for machining of transparent materials by ultrafast lasers. Examples illustrate the benefit of in-situ diagnostics for the process development using spatial and temporal beam shaping.