@article{RungBokanRutschetal.2018, author = {Rung, Stefan and Bokan, Kevin and Rutsch, Kevin and Schwarz, Simon and Esen, Cemal and Hellmann, Ralf}, title = {Laser Induced Periodic Surface Structures on 100Cr6 Steel for Modification of Friction Demonstrated with Stribeck Test}, series = {Journal of Laser Micro/Nanoengineering}, volume = {13}, journal = {Journal of Laser Micro/Nanoengineering}, number = {3}, doi = {10.2961/jlmn.2018.03.0004}, pages = {160 -- 165}, year = {2018}, abstract = {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.}, subject = {Laser}, language = {en} } @article{SchwarzRungEsenetal.2018, author = {Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Fabrication of a high-quality axicon by femtosecond laser ablation and CO2 laser polishing for quasi-Bessel beam generation}, series = {Optics Express}, volume = {26}, journal = {Optics Express}, number = {18}, pages = {23287 -- 23294}, year = {2018}, subject = {Femtosekundenlaser}, language = {en} } @article{SchwarzRungEsenetal.2018, author = {Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Influence of pulse duration on high-precision manufacturing of 3D geometries}, series = {Journal of Laser Micro/Nanoengineering}, volume = {13}, journal = {Journal of Laser Micro/Nanoengineering}, number = {3}, pages = {292 -- 295}, year = {2018}, subject = {Dreidimensionale Geometrie}, language = {en} } @article{SchwarzRungEsenetal.2018, author = {Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Homogeneous low spatial frequency LIPSS on dielectric materials generated by beam-shaped femtosecond pulsed laser irradiation}, series = {Journal of Laser Micro/Nanoengineering}, volume = {13}, journal = {Journal of Laser Micro/Nanoengineering}, number = {2}, pages = {90 -- 94}, year = {2018}, subject = {Femtosekundenlaser}, language = {en} } @article{SchwarzRungEsenetal.2020, author = {Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Rapid fabrication of precise glass axicon arrays by an all laser-based manufacturing technology}, series = {Journal of Laser Applications}, volume = {32}, journal = {Journal of Laser Applications}, number = {1}, pages = {012001 -- 012001}, year = {2020}, subject = {Femtosekundenlaser}, language = {en} } @article{SchwarzRungEsenetal.2018, author = {Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Surface Plasmon Polariton Triggered Generation of 1D-Low Spatial Frequency LIPSS on Fused Silica}, series = {Applied Sciences}, volume = {8}, journal = {Applied Sciences}, number = {9}, pages = {1624 -- 1624}, year = {2018}, subject = {Femtosekundenlaser}, language = {en} } @article{RothEsenHellmann2018, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {Control of femtosecond laser generated microfluidic channels inside poly (methyl methacrylate)}, series = {Journal of Laser Applications}, volume = {30}, journal = {Journal of Laser Applications}, number = {3}, pages = {032016-1 -- 032016-6}, year = {2018}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothEsenHellmann2019, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {Circular microchannels inside bulk polymethylmethacrylate generated by femtosecond laser using slit beam shaping}, series = {Journal of Laser Applications}, volume = {31}, journal = {Journal of Laser Applications}, number = {2}, pages = {022603-1 -- 022603-5}, year = {2019}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothEsenHellmann2018, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {Vertical Microchannels for Microfluidic Multilayer Interconnections in PMMA}, series = {Journal of Laser Micro/Nanoengineering}, volume = {13}, journal = {Journal of Laser Micro/Nanoengineering}, number = {3}, pages = {155 -- 159}, year = {2018}, abstract = {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.}, subject = {Polymethylmethacrylate}, language = {en} } @inproceedings{RothEsenHellmann2020, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {Internal micro structuring of transparent optical polymers by fs laser}, series = {Laser-based Micro-and Nanoprocessing XIV 2020}, volume = {11268}, booktitle = {Laser-based Micro-and Nanoprocessing XIV 2020}, publisher = {International Society for Optics and Photonics}, doi = {10.1117/12.2543487}, pages = {112681L}, year = {2020}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothRungEsenetal.2020, author = {Roth, Gian-Luca and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Microchannels inside bulk PMMA generated by femtosecond laser using adaptive beam shaping}, series = {Optics Express}, volume = {28}, journal = {Optics Express}, number = {4}, pages = {5801 -- 5811}, year = {2020}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothEsenHellmann2019, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {A New Approach to Seal Polymer Microfluidic Devices Using Ultrashort Laser Pulses}, series = {Journal of Laser Micro/Nanoengineering}, volume = {14}, journal = {Journal of Laser Micro/Nanoengineering}, number = {1}, pages = {49 -- 53}, year = {2019}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @inproceedings{RothWolfEsenetal.2019, author = {Roth, Gian-Luca and Wolf, Bastian and Esen, Cemal and Hellmann, Ralf}, title = {Generation of internal 3D microfluidic architectures in polymers by fs laser processing}, series = {Lasers in Manufacturing 2019}, booktitle = {Lasers in Manufacturing 2019}, year = {2019}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothEsenHellmann2018, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {Laser beam microwelding of transparent plastics}, series = {Joining Plastics}, volume = {12}, journal = {Joining Plastics}, number = {3/4}, pages = {2 -- 8}, year = {2018}, abstract = {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.}, subject = {Laserschweißen}, language = {en} } @article{RothEsenHellmann2017, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {Femtosecond laser direct generation of 3D-microfluidic channels inside bulk PMMA}, series = {Optics Express}, volume = {25}, journal = {Optics Express}, number = {15}, pages = {18442 -- 18450}, year = {2017}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} }