Daniel Grossmann

Transverse pump-probe microscopy of moving breakdown, filamentation and self-organized absorption in alkali aluminosilicate glass using ultrashort pulse laser.

We present time and space resolved transverse pump-probe measurements of the free electron and defect generation induced by nonlinear absorption of ultra short pulsed laser radiation in unhardened Corning Gorilla glass. The applied setup exhibits a 100 fs probe pulse duration and an independent pump pulse duration up to 5 ps. Hence, our work comprises the absorption of ultra short pulsed laser radiation at a wavelength of 800 nm and pulse energies from 10 μJ to 50 μJ up to a delay of 6 ns. Our investigations reveal different absorption regimes like filamentation and moving breakdown as well as the formation of permanent modifications. Finally, the deposition of multiple pulses in the incubation regime is examined, observing a self-organizing absorption effect.

Beam shaping and in-situ diagnostics for development of transparent materials processing

Several applications based on laser machining of transparent materials by nonlinear induced absorption of ultra short pulses are meanwhile well established. We apply in-situ diagnostics for the development auf new and improved processing techniques. The novel pump-probe system offers flexible pulse duration, burst options, beam shaping and repetition rates up to 2 MHz at an extended range of probe delay. This allows a deep inside into the spatial and temporal characteristics of nonlinear absorption and subsequent relaxation. By including effects of incubation and accumulation, mechanisms on multiple temporal and spatial scales can be addressed. This is exemplified by results achieved for ablation, welding and modification cutting by elongated beam shapes. At a probe delay in the ns-range, pressure waves can be observed. Applying fluence near threshold, a remarkable influence of accumulation on the absorption becomes obvious even at repetition rates down to 10 kHz. Increasing the repetition rate results in thermal load on a zone by far extending the initial absorption region, as can be seen by pump-probe polarization microscopy. Pump-probe diagnostics support aberration correction for improved modification cutting by Bessel-like beams. The examples on processing results highlight the achievements enabled by thorough consideration of the plurality of relevant effects.

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.

Advanced welding of transparent materials by ultrashort laser pulses

The welding of transparent materials by ultrashort laser pulses has gained particular interest in recent years. While the short pulse duration enables to locally modify glasses within the bulk, high pulse repetition rates facilitate to accumulate heat from pulse to pulse leading to local melting. After resolidification strong covalent bonds are formed providing high stability of the joined partners without the need of additional material such as glue. In particular dissimilar glasses can be welded with breaking strengths in the range of the volume material while the weld seams are gas dense and long term stable. However, industrial applications demand for enhanced throughput. Scaling the process speed requires advanced concepts for temporal and spatial tailoring the laser induced heat. By using short laser pulse trains, so-called bursts allows to reduce or redistribute the induced stress and hence increase the breaking strength of welded samples in the range of the volume material. Besides the laser parameters used, also the surface quality and eventual gaps denote a decisive issue for laser welding under industrial conditions. In this framework time-resolved pump-probe measurements are used to analyze the evolution of weld seams, in particular the melt transfer in between the samples facilitating process development. By extending the time-resolved pump-probe setup with polarization optics even allows for quantitatively investigating laser induced stress which serves to optimize the achievable breaking strength.

Advanced in-situ diagnostics of ultra short pulsed micromachining in glass

Micro structuring of transparent materials with ultra short pulsed laser radiation is nowadays an established and widely used processing method. However, process optimization, such as the reduction of cracks and defects as well as achieving an increased throughput, remains a challenging task. A general approach requires a detailed knowledge of the underlying mechanism of the laser material interaction. For this purpose, in-situ microscopy offers comprehensive insight into the spatial and temporal characteristics of the nonlinear absorption and subsequent thermalization or relaxation phenomena, respectively. To pursue this approach and analyze various damage mechanisms in a subtractive micromachining process, we apply a novel pump probe microscopy setup, which enables us for the first time to examine an extended parameter range. We present in-situ data of the nonlinear interaction region in glass on a micrometer scale with a temporal resolution of approximately 200 fs comprising the laser material interaction from femtoseconds to microseconds. Our investigations are carried out for incubation and accumulation processing regimes up to a repetition rate of 1 MHz. Additionally, pump pulse durations between 300 fs to 20 ps, as well as several burst operation modes are accessible with our experimental setup. Our extensively automated pump probe setup enables us to reconstruct the material extinction response to analyze the complex absorption profiles. In this context, we report on flexible processing strategies and exemplarily processing results.

Glass cutting optimization with pump-probe microscopy and Bessel beam profiles

The processing of transparent materials by ultrashort pulses has opened diverse promising and fast-growing application areas such as the cutting of glasses for consumer electronics. One of the major benefits is the precise energy deposition, which may result in local weakening of the glass and hence defines a preferential direction for the separation path. Due to the vast variety of possible uses for different displays, research in this field is needed for more complex shapes and glasses of various thicknesses. Bessel-Gaussian beams with their elongated, thin focus profile and self-healing nature are an excellent fit, even for glasses up to several millimeters. Additional development to more complex beam profiles allows precise tailoring with respect to the mandatory specifications of the cutting process such as process speed or the realization of inner contours. One concept for the latter is the use of tilted Bessel-Gaussian beams to achieve both high quality and easy separation. Further approaches include the usage of higher-order Bessel beams or modified Gauss-Bessel beams. We employ digital holographic techniques to create the various profiles with the desired absorption distribution. Traditional microscopes fail to characterize these sensible changes in the interaction region, since they are limited to visualize permanent changes (ex situ) of the glass structure only. We take advantage of pump-probe microscopy to receive concise recordings of the extinction mechanisms of the beam-material-interaction. With both, high temporal and spatial resolution of in-situ diagnostics we gain access to the entire process window which enables us to develop optimized processing parameters for high-quality glass cuttings.

In-situ microscopy of front and rear side ablation processes in alkali aluminosilicate glass using ultra short pulsed laser radiation

The visualization of the nonlinear absorption, the subsequent relaxation of excited states and the formation of defects enables the investigation of fundamental laser-material-interaction as well as the identification of process windows for micromachining of transparent materials with ultra short pulsed laser radiation. In this work, time resolved pump probe microscopy is applied to analyze the laser-material-interaction and to reduce damage inside the material during front- and rear side ablation of nonstrengthened Corning Gorilla glass. The experiments give an insight into the pulse duration dependence of the absorption zone, the influence of the surface geometry, in-volume damage and the formation of transient visible cracks.

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.