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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report on the evaluation of different pulse durations between 230 fs and 5 ps on the layer-wise ablation of fused silica. In an initial step, the polished samples are roughened by scanning the laser across the surface and ablating hatches consisting of parallel lines, where chipping and cracking is present for longer pulse durations. The ablation behavior of the further layers showed two ablation regimens with respect to the laser fluence for all pulse durations. Furthermore, for longer pulses a more efficient ablation process is found. With increasing pulse duration, the surface roughness is increased, an undesirable effect for high-precision micromachining. To demonstrate our micromachining process, we defined 3D geometries, i.e., axicon, spherical and cylindrical lenses which are ablated with 230 fs by layer-wise removal of the silica. Laser scanning microscope images show the effective fabrication of the 3D objects. Cross-sections reveal excellent agreement between the designed and the experimentally fabricated geometries confirming our high-precision micromachining process. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report on the static and dynamic contact angle on stainless steel after applying laser induced periodic surface structures and study the influence of applied femtosecond laser fluence and pulse overlap hereupon. In particular, we focus on the temporal evolution of the static contact angle, the advancing and receding contact angle, as well as on the contact angle hysteresis over a period of weeks. While directly after the generation of laser induced periodic surface structures small static contact angles and a small contact angle hysteresis are found, both properties increase after a storage of the structured specimen under ambient conditions. Particularly, the structured steel surface wettability turns into a hydrophobic regime on this time scale. [ABSTRACT FROM AUTHOR]

We report on the generation of homogenous low spatial frequency LIPSS on fused silica and sapphire with beam-shaped femtosecond pulsed laser irradiation. For the silica specimen, the periodicities of the LSFL generated by a Gaussian and a Top-Hat beam profile are compared with regard to the applied fluence, pulse overlap and angle of incidence. For both profiles, an increasing periodicity is found for an increasing pulse overlap and a decreasing periodicity for an increasing angle of incidence, in turn facilitating a fine-adjustment of the LSFL periodicity for potential applications. Furthermore, for a homogenous surface coverage with LSFL, the silica sample is rotated 40° for an angular irradiation, improving the quality of the periodic structures for Gaussian irradiation. Applying the beamshaped Top-Hat laser profile, superior LSFL with increased line width are observed. For sapphire, we focus on the generation of one-dimensional structures along straight lines with respect to different scanning directions, being either parallel or orthogonal to the laser polarization. We found an excellent quality for parallel scanning using Top-Hat beam profile as well as an increased line width as compared to Gaussian beam profile. From an applicable point of view, these high-quality structures lead to the next step texturing entire surfaces of transparent materials with homogenous microstructures for a specific adaption of the cell growth or wettability. [ABSTRACT FROM AUTHOR]

This article commemorates the 75th birthday of Prof. Dr. techn. Gustav Schweiger and his achievements in this field. Spherical microparticles are characterized by their extraordinary resonance properties. This makes it a promising tool for high sensitive measurements. Furthermore, the dependency on different physical quantities together with the possibility of targeted functionalization enables the measurement of a high variety of parameters with just one technology. [ABSTRACT FROM AUTHOR]