Martin Hermans

In situ measurement of plasma and shock wave properties inside laser-drilled metal holes

High-speed imaging of shock wave and plasma dynamics is a commonly used diagnostic method for monitoring processes during laser material treatment. It is used for processes such as laser ablation, cutting, keyhole welding and drilling. Diagnosis of laser drilling is typically adopted above the material surface because lateral process monitoring with optical diagnostic methods inside the laser-drilled hole is not possible due to the hole walls. A novel method is presented to investigate plasma and shock wave properties during the laser drilling inside a confined environment such as a laser-drilled hole. With a novel sample preparation and the use of high-speed imaging combined with spectroscopy, a time and spatial resolved monitoring of plasma and shock wave dynamics is realized. Optical emission of plasma and shock waves during drilling of stainless steel with ns-pulsed laser radiation is monitored and analysed. Spatial distributions and velocities of shock waves and of plasma are determined inside the holes. Spectroscopy is accomplished during the expansion of the plasma inside the drilled hole allowing for the determination of electron densities.

In-situ diagnostics on fs-laser-induced modification of glasses for selective etching

In-situ observation of the in-volume modification of glasses by focused ultra-short pulsed laser radiation with an interferometer microscope allows for the spatially resolved measurement of the transient optical path difference (OPD) in the surrounding of the laser-induced modification. By the relation of refractive index and temperature an estimation of temperature during modification process is possible. The absorption of the laser radiation is measured and is, together with the estimation of processing temperature during modification, a first step towards a process model for the induced modifications of the transparent material.

Selective Laser-Induced Etching of 3D Precision Quartz Glass Components for Microfluidic Applications—Up-Scaling of Complexity and Speed

By modification of glasses with ultrafast laser radiation and subsequent wet-chemical etching (here named SLE = selective laser-induced etching), precise 3D structures have been produced, especially in quartz glass (fused silica), for more than a decade. By the combination of a three-axis system to move the glass sample and a fast 3D system to move the laser focus, the SLE process is now suitable to produce more complex structures in a shorter time. Here we present investigations which enabled the new possibilities. We started with investigations of the optimum laser parameters to enable high selective laser-induced etching: surprisingly, not the shortest pulse duration is best suited for the SLE process. Secondly we investigated the scaling of the writing velocity: a faster writing speed results in higher selectivity and thus higher precision of the resulting structures, so the SLE process is now even suitable for the mass production of 3D structures. Finally we programmed a printer driver for commercial CAD software enabling the automated production of complex 3D glass parts as new examples for lab-on-a-chip applications such as nested nozzles, connectors and a cell-sorting structure.

High speed micro scanner for 3D in-volume laser micro processing

Using an in-house developed micro scanner three-dimensional micro components and micro fluidic devices in fused silica are realized using the ISLE process (in-volume selective laser-induced etching). With the micro scanner system the potential of high average power femtosecond lasers (P > 100 W) is exploited by the fabrication of components with micrometer precision at scan speeds of several meters per second. A commercially available galvanometer scanner is combined with an acousto-optical and/or electro-optical beam deflector and translation stages. For focusing laser radiation high numerical aperture microscope objectives (NA > 0.3) are used generating a focal volume of a few cubic micrometers. After laser exposure the materials are chemically wet etched in aqueous solution. The laser-exposed material is etched whereas the unexposed material remains nearly unchanged. Using the described technique called ISLE the fabrication of three-dimensional micro components, micro holes, cuts and channels is possible with high average power femtosecond lasers resulting in a reduced processing time for exposure. By developing the high speed micro scanner up-scaling of the ISLE process is demonstrated. The fabricated components made out of glass can be applied in various markets like biological and medical diagnostics as well as in micro mechanics.

Selective laser-induced etching (SLE): A scalable subtractive 3D printing process for transparent glasses and crystals (Conference Presentation)

By modification of transparent glasses and crystals with ultrafast laser radiation and subsequent wet-chemical etching (here named SLE = selective laser-induced etching), precise 3D structures have been produced, especially in quartz glass (fused silica), for more than a decade. By the combination of a high precision three-axis system to move the glass sample and a fast 3D beam steering system to move the laser focus, the SLE process is now suitable to produce more complex structures in a shorter time. We have programmed a printer driver for commercial CAD software and flexible machine software enabling the automated production of complex 3D glass parts with the LightFab 3D Printer. New examples of 3D precision glass parts e.g. for lab-on-a-chip applications (cell-sorting microfluidics), electronics (glass via and connectors), semiconductor (quartz chucks), optics and precision mechanics are presented. The SLE process is very scalable for high throughput since a faster writing speed results in higher selectivity and thus larger precision of the resulting structures. Thus SLE is a process which is suitable for mass production of 3D structures in glasses. Some examples of rapidly produced structures using our high speed beam deflection modules are demonstrated, which are the basis of our special machines enabling mass-production. Therefore, 3D printing of glasses is not only a niche technology for prototypes anymore.

High-power ultra-short pulse laser radiation: New sources as key enablers for emerging applications

Ultrafast laser sources with pulse durations in the sub-picosecond regime enable a precise machining of various materials. Pulse durations shorter than the electron phonon-coupling time lead to a low thermal load or even non-thermal ablation processes. Exploiting non-linear absorption processes, the absorption becomes nearly material independent when laser pulses of several microjoule energy and high beam quality are focused on the materials surface. Low pulse energies and intensities well above the vaporization threshold and therefore an eduction of the absorbed energy within the ablation product enables a high-precision cutting, ablation and drilling of, even weakly absorbing materials, multi-component and multi-layer systems. Additional, the focusing of ultrafast laser pulses in the volume of transparent dielectrics allows a localized modification of the bulk material. Specifically, defined refractive index changes in glasses and crystals can be utilized for waveguiding and beam-forming applications. A combined approach of material modification followed by chemical etching provides the possibility to manufacture micro-channels or 3D-micro mechanical parts. The 3D-capability of the in-volume material processing originates from the non-linear absorption of light in the initially transparent material.To achieve high process efficiencies in material processing, laser sources delivering high average power are necessary. High average power is achieved either by high pulse energies and low repetition rates or high pulse repetition rates and moderate pulse energies. The optimum set of parameters is strongly depending on the process, the material and the application. In this paper, we present compact laser sources with a high flexibility in pulse energy and pulse repetition rate and an average power of several hundreds of Watt. Additional, a broad range of applications, from micro- and nanostructuring of various materials to volume processing of dielectrics will be presented.Ultrafast laser sources with pulse durations in the sub-picosecond regime enable a precise machining of various materials. Pulse durations shorter than the electron phonon-coupling time lead to a low thermal load or even non-thermal ablation processes. Exploiting non-linear absorption processes, the absorption becomes nearly material independent when laser pulses of several microjoule energy and high beam quality are focused on the materials surface. Low pulse energies and intensities well above the vaporization threshold and therefore an eduction of the absorbed energy within the ablation product enables a high-precision cutting, ablation and drilling of, even weakly absorbing materials, multi-component and multi-layer systems. Additional, the focusing of ultrafast laser pulses in the volume of transparent dielectrics allows a localized modification of the bulk material. Specifically, defined refractive index changes in glasses and crystals can be utilized for waveguiding and beam-forming applications. A...

3D nano and micro structures in transparent materials by in-volume femtosecond laser processing

Nano as well as micro structuring in the volume of transparent materials is enabled by ultrafast laser radiation. By laser radiation with pulse durations in the fs and ps regime multi photon processes are efficiently induced resulting in a high resolution of less than 1 µm3, a very low heat input and a high writing flexibility in all three dimensions. High transparent materials such as sapphire and glasses are locally modified in the volume to change the refractive index for optical applications or to increase the corrodibility selectively for the manufacturing of micro channels and micro structured parts for the use in micro systems and medical technology.The miniaturization of products for micro optics, the medical technology and micro systems engineering requires transparent components with structure sizes in the micrometer range and accuracies of approx. 100 nm. In-volume Selective Laser-induced Etching (ISLE) is an appropriate manufacturing process for micro machining of transparent materials such as sapphire and glasses, e.g. fused silica. By focusing the laser radiation (wavelength 1040 nm, pulse duration 500 fs, repetition rate 0.1-5 MHz, pulse energy <1 µJ) in the volume the material is locally modified. By scanning the laser focus with pulse overlap inside the material, connected volumes of modified material are created. The modified volumes are subsequently removed by chemical etching in a second processing step using aqueous solution of e.g. HF or KOH.Periodical nano structures (ripples, nanoplanes) are fabricated inside transparent materials exploiting fs-laser induced nano optics with potential applications such as nano sieves for filtering, optical gratings or functionalized internal surfaces.Direct laser writing of waveguides in transparent materials by fs laser radiation is well known. Further applications include crack-free 3D micro markings in transparent materials consisting e.g. of micro-gratings, which resu lt in a colorful experience in the observers eye due to diffraction and interference.To exploit the high manufacturing velocities possible with ultrafast lasers with repetition rates of several MHz a high speed scanning system with large numerical aperture and pre-compensation of spherical aberrations has been developed for in-volume nano and micro structuring. With this system high writing velocities (50-400 mm/s), small focus size (0.6-2.2 mm) and high precision (100-400 nm) are combined for the first time on rat her large scanning fields (0.6-1.5 mm) and material depth of up to 2 mm.Nano as well as micro structuring in the volume of transparent materials is enabled by ultrafast laser radiation. By laser radiation with pulse durations in the fs and ps regime multi photon processes are efficiently induced resulting in a high resolution of less than 1 µm3, a very low heat input and a high writing flexibility in all three dimensions. High transparent materials such as sapphire and glasses are locally modified in the volume to change the refractive index for optical applications or to increase the corrodibility selectively for the manufacturing of micro channels and micro structured parts for the use in micro systems and medical technology.The miniaturization of products for micro optics, the medical technology and micro systems engineering requires transparent components with structure sizes in the micrometer range and accuracies of approx. 100 nm. In-volume Selective Laser-induced Etching (ISLE) is an appropriate manufacturing process for micro machining of transparent materials such as...

Micro structuring of sapphire and fused silica by high speed and high precision in-volume selective laser etching

Micro structuring of sapphire and fused silica [1] is an important market e.g. the production of microfluidic devices and sensors that combine optical and microfluidic functions [2] as well as freeform microlense systems. New techniques which enable the processing of sapphire and fused silica are requested. The In-volume Selective Laser Etching (ISLE) technique is a two step process for the micro machining of sapphire and fused silica. First the sample is irradiated using fs-laser direct writing inducing material modifications within the focal volume by non-linear absorption processes. Subsequently the sample is wet etched. The use of high repetition rate fs-lasers, e.g. FCPA lasers with f=0.1-5 MHz, potentially allows for a high productivity, but can not be achieved with nowadays handling systems concerning speed without sacrificing precision and/or numerical aperture. To overcome the limitations mentioned above a scanning optics for fs-laser writing with high precision Δx=100-300 nm), high speed (v=100-300 mm/s) and large numerical aperture (NA=0.4-1.2) is developed. New results which are achieved with ISLE method in combination with the developed scanning optics will be shown and discussed.Micro structuring of sapphire and fused silica [1] is an important market e.g. the production of microfluidic devices and sensors that combine optical and microfluidic functions [2] as well as freeform microlense systems. New techniques which enable the processing of sapphire and fused silica are requested. The In-volume Selective Laser Etching (ISLE) technique is a two step process for the micro machining of sapphire and fused silica. First the sample is irradiated using fs-laser direct writing inducing material modifications within the focal volume by non-linear absorption processes. Subsequently the sample is wet etched. The use of high repetition rate fs-lasers, e.g. FCPA lasers with f=0.1-5 MHz, potentially allows for a high productivity, but can not be achieved with nowadays handling systems concerning speed without sacrificing precision and/or numerical aperture. To overcome the limitations mentioned above a scanning optics for fs-laser writing with high precision Δx=100-300 nm), high speed (v=100-...