TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Ultrashort pulsed laser backside ablation of fused silica JF - Optics Express KW - Femtosekundenlaser KW - Quarzglas KW - Mikrobearbeitung Y1 - 2021 VL - 29 IS - 15 SP - 23477 EP - 23486 ER - TY - JOUR A1 - Rung, Stefan A1 - Bokan, Kevin A1 - Kleinwort, Frederick A1 - Schwarz, Simon A1 - Simon, Peter A1 - Klein-Wiele, Jan-Hendrik A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Possibilities of Dry and Lubricated Friction Modification Enabled by Different Ultrashort Laser-Based Surface Structuring Methods JF - Lubricants N2 - In this contribution we report on the possibilities of dry and lubricated friction modification introduced by different laser surface texturing methods. We compare the potential of Laser-Induced Periodic Surface Structures and Laser Beam Interference Ablation on 100Cr6 steel in a linear reciprocating ball-on-disc configuration using 100Cr6 steel and tungsten carbide balls with load forces between 50mN and 1000 mN. For dry friction, we find a possibility to reduce the coefficient of friction and we observe a pronounced direction dependency for surfaces fabricated by Laser Beam Interference Ablation. Furthermore, Laser-Induced Periodic Surface Structures result in a load-dependent friction reduction for lubricated linear reciprocating movements. This work helps to identify the modification behaviour of laser generated micro structures with feature sizes of approximately 1μm and reveals new possibilities for surface engineering KW - laser-induced periodic surface structures KW - laser beam interference ablation KW - dry friction KW - lubricated friction KW - laser surface texturing KW - Laser KW - Oberflächenstruktur Y1 - 2019 U6 - https://doi.org/10.3390/lubricants7050043 VL - 7 IS - 43 SP - 1 EP - 13 ER - TY - JOUR A1 - Rung, Stefan A1 - Bokan, Kevin A1 - Rutsch, Kevin A1 - Schwarz, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Laser Induced Periodic Surface Structures on 100Cr6 Steel for Modification of Friction Demonstrated with Stribeck Test JF - Journal of Laser Micro/Nanoengineering N2 - In this contribution, we report on surface functionalization by introducing laser induced periodic surface structures on 100Cr6 bearing steel to modify complex tribological properties. The advanced approach of this study is the measurement of the coefficient of friction by performing a ball-on-disk Stribeck test on laser structured surfaces with polytetrafluoroethylene balls in a lubricant environment. The Stribeck test reveals the modified friction behavior using translation speeds up to 106 mm/min and load forces between 100 mN and 5000 mN. Our results show increased stiction and coefficient of friction for laser structured surfaces in the regime of boundary lubrication which is attributed to the laser induced surface asperities. Decreased coefficient of friction is observed in the regime of mixed and hydrodynamic lubrication, i.e. for velocities higher than 1000 mm/min, for surfaces covered by laser induced periodic surface structures with low spatial frequency. KW - Laser KW - Oberflächenstruktur Y1 - 2018 U6 - https://doi.org/10.2961/jlmn.2018.03.0004 VL - 13 IS - 3 SP - 160 EP - 165 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Fabrication of a high-quality axicon by femtosecond laser ablation and CO2 laser polishing for quasi-Bessel beam generation JF - Optics Express KW - Femtosekundenlaser Y1 - 2018 VL - 26 IS - 18 SP - 23287 EP - 23294 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Influence of pulse duration on high-precision manufacturing of 3D geometries JF - Journal of Laser Micro/Nanoengineering KW - Dreidimensionale Geometrie Y1 - 2018 VL - 13 IS - 3 SP - 292 EP - 295 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Homogeneous low spatial frequency LIPSS on dielectric materials generated by beam-shaped femtosecond pulsed laser irradiation JF - Journal of Laser Micro/Nanoengineering KW - Femtosekundenlaser Y1 - 2018 VL - 13 IS - 2 SP - 90 EP - 94 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Rapid fabrication of precise glass axicon arrays by an all laser-based manufacturing technology JF - Journal of Laser Applications KW - Femtosekundenlaser Y1 - 2020 VL - 32 IS - 1 SP - 012001 EP - 012001 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Surface Plasmon Polariton Triggered Generation of 1D-Low Spatial Frequency LIPSS on Fused Silica JF - Applied Sciences KW - Femtosekundenlaser KW - Quarzglas Y1 - 2018 VL - 8 IS - 9 SP - 1624 EP - 1624 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Control of femtosecond laser generated microfluidic channels inside poly (methyl methacrylate) JF - Journal of Laser Applications N2 - In this study, the authors report on femtosecond laser direct generation of microchannels inside poly(methyl methacrylate) with variable dimensions ranging from a channel height of 20 to 350 μm and an aspect ratio down to 1.2. Focused ultrashort laser pulses are used to trigger a material modification in the focal area which can be selectively opened by a subsequent annealing process. A 3D microchannel architecture can easily be realized by moving the specimen using motorized stages, allowing freely chosen complex shaped channel layouts. While the laser wavelength is identified as one of the main parameters determining process effectiveness, another core of this study is the variation of channel dimensions by the numerical aperture of the applied objective to focus the laser. The authors find that both size and shape of the modified region and the resulting microchannel can be controlled by altering the numerical aperture. KW - Femtosekundenlaser KW - Polymethylmethacrylate Y1 - 2018 VL - 30 IS - 3 SP - 032016-1 EP - 032016-6 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Circular microchannels inside bulk polymethylmethacrylate generated by femtosecond laser using slit beam shaping JF - Journal of Laser Applications N2 - The authors report on the laser generation of three-dimensional microchannel architectures with fundamentally unlimited channel length by focused 514 nm femtosecond laser pulses inside polymethylmethacrylate bulk material. The three-dimensional channel layout is implemented by moving the specimen using three-dimensional motorized stages, allowing freely chosen complex shaped channel architectures. Based on nonlinear absorption of high intensities around the focal volume, a material modification, including a refractive-index shift is triggered. After laser exposure, the channel is created by a gaseous degradation of exposed areas during an annealing process. Cross section and aspect ratio of thus generated microchannels are strongly influenced by the numerical aperture of the applied objective. To obtain a circular cross-sectional shape, high focusing objectives are necessary. In this report, the authors improve the existing setup by using an elliptical input beam, transforming the intensity distribution of the focal area in the propagation direction to a circular shape even for low and medium numerical aperture focusing objectives and therefore avoiding a small working distance as a limiting factor for processing depth. The elliptical input beam is obtained by insertion of a slit mask in front of the focusing objective. The ellipticity of the incident beam defines the aspect ratio of the generated microchannels. Furthermore, the size of internal microchannels is adjustable by varying laser and annealing process parameters. KW - Femtosekundenlaser Y1 - 2019 VL - 31 IS - 2 SP - 022603-1 EP - 022603-5 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Vertical Microchannels for Microfluidic Multilayer Interconnections in PMMA JF - Journal of Laser Micro/Nanoengineering N2 - We report on laser generation of microchannels by focused femtosecond laser pulses inside PMMA bulk material. This process enables direct fabrication of three dimensional microfluidic channel architectures with generally unlimited channel length. Based on nonlinear ultrashort pulse laser absorption of high intensities around the focal volume, a material modification including a refractive index shift is triggered. After laser exposure the channel is created by a gaseous degradation of exposed areas during an annealing process. The 3D channel layout can easily be realized by moving the specimen using 3D motorized stages, allowing freely chosen complex shaped channel architectures. In this report, the creation of inner lying vertical microchannels is examined. In addition to previously shown horizontal channel architectures vertical channels are necessary to fulfill the definition of an internal three-dimensional microfluidic structure and to create interconnections in multilevel lab on chip devices. The cross section and geometry of vertical channels are strongly influenced by laser parameters such as pulse duration, laser power and writing speed. The influence of these process parameters on the channels diameter is determined and parameters are optimized regarding a controllable, stable and reproducible process. The functionality of vertical microchannels as an interconnection between multilayer microfluidic devices is demonstrated using a Rhodamine B solution. KW - Polymethylmethacrylate KW - Femtosekundenlaser Y1 - 2018 VL - 13 IS - 3 SP - 155 EP - 159 ER - TY - CHAP A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Internal micro structuring of transparent optical polymers by fs laser T2 - Laser-based Micro-and Nanoprocessing XIV 2020 N2 - Lab-on-chip systems are based on components to transport, mix, separate and analyse small volumes of different fluids. The consecutive integration of more complex functions into a single and compact chip demands on multilayer systems. As the classical production using a stacking and joining of single processed layers is elaborate and limited in terms of multilayer structures, an uprising trend to fabricate those devices is the internal, three dimensional processing of transparent substrates by using ultrashort laser pulses. In this study, we report on the generation of internal hollow architectures created by focused 514nm femtosecond laser pulses inside optical polymer bulk materials of different polymers. The three-dimensional channel layout is implemented by moving the sample using three-dimensional motorized stages, allowing arbitrary complex shaped internal channel architectures. Size and cross sectional shape of a single internal generated microchannel are determined by the intensity distribution of the focal voxel. In particular, we show a comprehensive parameter study to improve this laser process with respect to a higher processing speed and stability. KW - Femtosekundenlaser Y1 - 2020 U6 - https://doi.org/10.1117/12.2543487 VL - 11268 SP - 112681L PB - International Society for Optics and Photonics ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Microchannels inside bulk PMMA generated by femtosecond laser using adaptive beam shaping JF - Optics Express N2 - In this contribution, we report on the generation of internal microchannels with basically unlimited channel length inside of PMMA bulk material by femtosecond laser. A precisely controllable and stable circular channel cross section is obtained by using a spatial light modulator to compensate the writing depth depending spherical aberration. Furthermore, the generation of a rotatable elliptical input beam by adaptive optics ensures a fitting of the beam shaping to the writing direction. In this study, we report on both, the effect of the ellipticity of the input beam and the effect of a correction of the spherical aberration on the circularity of the resulting internal microchannels. Moreover, we demonstrate the application of this writing technique by creating microfluidic testing structures inside of a transparent standard polymer. KW - Femtosekundenlaser KW - Polymethylmethacrylate Y1 - 2020 VL - 28 IS - 4 SP - 5801 EP - 5811 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - A New Approach to Seal Polymer Microfluidic Devices Using Ultrashort Laser Pulses JF - Journal of Laser Micro/Nanoengineering N2 - We report on the joining of two different transparent thermoplastic polymers using infrared femtosecond laser pulses. Solely being based on nonlinear absorption, the developed micro-welding process for cyclo-olefin copolymers and polycarbonate does not require any intermediate absorbing layers or any surface pre-processing of the welding partners. In addition, it circumvents any heat damages at the outer surfaces of the welding partners. Femtosecond laser welding of cyclo-olefin copolymers is improved by increasing the numerical aperture of the focusing setup resulting in a wider and more homogenous welding cross-section. Welding results are classified by shear strength and fracture modes are analyzed using scanning electron microscopy. Furthermore, we demonstrate, to the best of our knowledge, first results of ultrashort pulse laser welding of transparent polycarbonate, one of the most used transparent polymers. In view of an optimized and stable micro-welding process, we study the influence of laser power and focal position on welding results. The application of ultrashort pulse laser welding for Lab-on-chip fabrication technologies is demonstrated by sealing an ultrashort pulse laser ablated microfluidic device based on a transparent polymer substrate. KW - Femtosekundenlaser KW - Laserschweißen Y1 - 2019 VL - 14 IS - 1 SP - 49 EP - 53 ER - TY - CHAP A1 - Roth, Gian-Luca A1 - Wolf, Bastian A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Generation of internal 3D microfluidic architectures in polymers by fs laser processing T2 - Lasers in Manufacturing 2019 N2 - Microfluidic lab on chip systems require components to transport, mix, separate and analyse small volumes of different fluids. In this study, we report on the laser generation of internal hollow architectures created by focused 514 nm femtosecond laser pulses inside PMMA bulk material. Size and cross-sectional shape of a single internal generated microchannel are determined by the intensity distribution inside the focal voxel and can be controlled either by the numerical aperture of the focusing objective or by laser beam shaping. As both approaches are practically limited with respect to the realizable cross-sectional shapes, we present a process based on an internal hatching to expand the achievable channel cross-sections and thereby enable the possibility to create complex 3D shaped internal structures. This process is applied to create fully internal functional microfluidic elements such as mixers which are part of most polymer lab-on-chip systems. KW - Femtosekundenlaser Y1 - 2019 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Laser beam microwelding of transparent plastics JF - Joining Plastics N2 - The laser beam welding of transparent plastic components is still a challenge according to the state of the art. A new approach for the joining of two transparent polymeric components using ultrashort laser beam pulses is presented in this report. In this respect, the high peak pulse intensity associated with these lasers permits non-linear absorption processes and thus the production of a locally well-defined heat source in the interior even of a transparent material or at the boundary layer between two transparent substrates. Welding results for transparent plastics of the cycloolefin copolymer and polycarbonate types are shown in this study. In addition to the investigation into the influences of fundamental process parameters on the weld geometry, the leak tightness is demonstrated in this study using a microfluidic system. KW - Laserschweißen Y1 - 2018 VL - 12 IS - 3/4 SP - 2 EP - 8 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Femtosecond laser direct generation of 3D-microfluidic channels inside bulk PMMA JF - Optics Express N2 - We report on laser direct generation of 3D-microchannels for microfluidic applications inside PMMA bulk material by focused femtosecond pulses. Inner lying channels with cross sectional areas from 100 µm2 to 4400 µm2 are directly created in the volume of a PMMA substrate. Using the presented process, the channel length is fundamentally unlimited. Here we demonstrate a channel length of 6 meters inside a substrate with dimensions of 20 × 20 × 1.1 mm. The formation of the micro channels is based on nonlinear absorption around the focal volume that triggers a material modification. The modified volume can be selectively opened to form the channel by a subsequent annealing process. The cross section of the channel is strongly influenced by the energy distribution and illumination around the focal volume determined by the optical setup and process design. The 3D channel layout can easily be realized by moving the specimen using 3D motorized stage, allowing freely chosen complex shaped channel architectures. Within a comprehensive parameter study, varying laser power, number of multi-passes, writing speed and writing depths, we identify an optimized process in terms of attainable channel height, width and aspect ratio, as well as process stability and reproducibility. The proof of concept for an application in three dimensional microfluidic systems is provided by florescence microscopy using a dye rhodamine B solution in isopropanol. KW - Femtosekundenlaser Y1 - 2017 VL - 25 IS - 15 SP - 18442 EP - 18450 ER - TY - JOUR A1 - Rung, Stefan A1 - Bokan, Kevin A1 - Schwarz, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Influence of laser wavelength on the modification of friction between 100Cr6 steel and polytetrafluoroethylene by femtosecond laser-induced periodic surface structures JF - Journal of Laser Applications KW - Femtosekundenlaser Y1 - 2020 U6 - https://doi.org/10.2351/7.0000036 VL - 32 IS - 3 SP - 032017-1 EP - 032017-6 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Hessler, Steffen A1 - Kefer, Stefan A1 - Girschikofsky, Maiko A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Femtosecond laser inscription of waveguides and Bragg gratings in transparent cyclic olefin copolymers JF - Optics Express N2 - We report on a femtosecond laser based fabrication technique that enables simultaneous single-step generation of optical waveguides and Bragg gratings inside bulk cyclic olefin copolymers. Due to the nonlinear absorption of focused and spatially modulated laser radiation with a wavelength of 514 nm and a pulse duration of 450 fs, a modification concluding a refractive index shift increase inside the substrate can be achieved. A sophisticated characterization of the generated waveguides by means of an elaborate cut-back method reveals a maximum attenuation of 3.2 dB/cm. Additionally, a Mach-Zehnder interferometer is used to examine the waveguide’s refractive index profile. The integrated Bragg grating structures exhibit reflectivities up to 95 % and a spectral full width at half maximum of 288 pm, at a Bragg wavelength of 1582 nm, whereas the grating period can be deliberately chosen by adapting the fabrication parameters. Thus, due to its increased flexibility and the resulting dispensability of cost-intensive phase masks, this method constitutes an especially promising fabrication process for polymer Bragg gratings inside of bulk materials. KW - Femtosekundenlaser KW - Wellenleiter KW - Bragg-Reflektor Y1 - 2020 U6 - https://doi.org/10.1364/OE.388364 VL - 28 IS - 12 SP - 18077 EP - 18084 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Haubner, Julian A1 - Kefer, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Fs-laser based hybrid micromachining for polymer micro-opto electrical systems JF - Optics and Lasers in Engineering N2 - We report on femtosecond laser direct writing of electrically conductive copper structures on transparent cyclic olefin copolymer based planar optical chips. The process is based on a laser-induced reduction of CuO nanoparticles dissolved in a water-soluble resin. Thus generated conductive copper structures are characterized with respect to their chemical composition and electrical resistivity. In addition, the application of ultrashort laser pulses enables a hybrid micromachining approach comprising ablation of polymer substrates and the fabrication of copper patterns in a single laser processing setup. A possible electro-optical application of this approach is demonstrated by employing the copper structures as an electro-thermal microheater in combination with a polymer planar optical Bragg grating sensor. This, in turn, highlights the potential of generating copious micro-opto-electro-mechanical polymer structures for numerous sensing applications ranging from Lab-on-Chip to environmental applications using the presented hybrid micromachining approach. KW - Femtosekundenlaser KW - Bragg-Reflektor Y1 - 2020 U6 - https://doi.org/10.1016/j.optlaseng.2020.106362 VL - 137 IS - 10362 SP - 1 EP - 8 ER -