This PDF file contains the front matter associated with SPIE Proceedings Volume 8968, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

We perform structural characterisation of direct laser write (DLW) waveguides. Quantitative phase microscopy, based on solution of the transfer of intensity equation, is used to measure the cumulative refractive index change through a waveguide perpendicular to its axis. Results are compared with interferometry, cross-sectional measurements using third harmonic microscopy, and analysis of the near-field image of the mode propagating in the waveguide. We show that in many situations, notably in the presence of depth dependent spherical aberrations, the cross-section for DLW waveguides may not be assumed symmetric about the waveguide axis. This is particularly important when fabricating at depths greater than 2 mm in fused silica. Therefore additional measurements are required to fully characterise the refractive index profile.

Self-Q-switched microchip lasers are attractive alternative to femtosecond lasers for micromachining in transparent materials. They can easily reach pulse peak powers needed to trigger ablation in all materials, including diamond, ceramics, plastics, and glasses. In addition, they are low cost with compact and rugged design. In this work, we report on using microchip lasers for micro-engraving different types of transparent materials. Micro-size marking is demonstrated on the surface of borosilicate glass. Microfluidic channels are engraved on BK-7 glass microchips with ion-doped waveguides. Arrays of dense micro-channels are fabricated at the surface of thermoplastics with a zone affected by thermal effects limited to the micron range.

Lithium manganese oxide composite cathodes are realized by laser-printing. The printed cathode is a composite and consists of active powder, binder and conductive agents. Laser-printed cathodes are first calendered and then laser structured using femtosecond-laser radiation in order to form three-dimensional (3D) micro-grids in the cathode material. Three-dimensional micro-grids in calendered/laser structured cathodes exhibit improved discharge capacity retention at a 1 C discharging rate. Calendered but unstructured cathodes indicate the poorest cycling behavior at 1 C discharge. The improved capacity retention and the reduced degradation of calendered/structured cathodes can be attributed to both the increased electrical contact through calendering as well as shortened Li-ion pathways due to laser-induced 3D microgrids.

Surface micro-texturing has been widely theoretically and experimentally demonstrated to be beneficial to friction reduction in sliding contacts under lubricated regimes. Several microscopic mechanisms have been assessed to concur to this macroscopic effect. In particular, the micro-textures act as lubricant reservoirs, as well as traps for debris. Furthermore, they may produce a local reduction of the shear stress coupled with a stable hydrodynamic pressure between the lubricated sliding surfaces. All these mechanisms are strongly dependent both on the micro-texturing geometry and on the operating conditions. Among the various micro-machining techniques, laser ablation with ultrashort pulses is an emerging technology to fabricate surface textures, thanks to the intrinsic property of laser light to be tightly focused and the high flexibility and precision achievable. In addition, when using sub-ps pulses, the thermal damage on the workpiece is negligible and the laser surface textures (LST) are not affected by burrs, cracks or resolidified melted droplets, detrimental to the frictional properties. In this work several LST geometries have been fabricated by fs-laser ablation of steel surfaces, varying the diameter, depth and spacing of micro-dimples squared patterns. We compared their frictional performance with a reference nontextured sample, on a range of sliding velocities from the mixed lubrication to the hydrodynamic regime. The measured Stribeck curves data show that the depth and diameter of the microholes have a huge influence in determining the amount of friction reduction at the interface. Different theoretical interpretations to explain the experimental findings are also provided.

We report a non-digitized diffractive beam splitter with a split count of 45, a 95% splitting efficiency, and a 0.90 splitting uniformity. The splitter was iteratively designed and was created on fused silica by laser writing lithography. Antireflection coatings were added to the splitter to ensure high efficiency. This splitter was applied to the manufacture of inkjet printer heads, in which silicon wafers were drilled with a 532-nm, nanosecond pulse laser with an average output of 10 W and were wet-etched to produce microfluidic channels. We also discuss large beam arrays for process throughput and subwavelength structures formed on the splitter for efficient laser power use.

The authors present a direct UV writing approach to fabricate fiber Bragg gratings (FBGs) and gratings in photolithographic waveguides. The technique uses two coherent UV beams, which are focused to a small spot (~7μm diameter) at the point at which they overlap. The resulting interference pattern at the foci consists of fringes which are used to define several grating planes per exposure, giving greater design flexibility and a significantly larger accessible spectral range compared to traditional approaches. The typical index contrast during grating fabrication is observed to be 4.7×10^-3, at writing speeds of 8 mm/min.

In this research the influence of laser micromachining on physicochemical properties of bioabsorbable polymer was investigated. Poly(l-lactide) (PLLA), commonly used for manufacturing non-permanent biomedical devices, was irradiated with varying fluences by CO₂ laser and by KrF excimer laser. To evaluate modification of the material, several analytical techniques were used: ATR (attenuated total reflection), XPS (X-ray photoelectron spectroscopy) and DSC (differential scanning calorimetry). We found that the laser-affected material has lower glass transition (T_g) and melting (T_m) temperatures. CO₂ and KrF excimer lasers can be successfully used for cutting and drilling of polylactide.

Periodic patterned surfaces can be used to provide unique surface properties in applications, such as biomaterials, surface engineering, photonics and sensor systems. Such periodic patterns can be produced using laser processing tools, showing significant advantages due to a precise modification of the surfaces without contamination, remote and contactless operation, flexibility, and precise energy deposition. On the other hand, the resolution of such laser based surface structuring methods, like direct laser writing, is generally inversely proportional to the fabrication speed. Therefore, the development of new laser structuring technologies as well as strategies offering both high speed and resolution is necessary. In this study, the fabrication of spatially ordered structures with micrometer and submicrometer lengthscales at high surface processing fabrication speed is demonstrated. The procedures shown here are applied to process both planar surfaces and also three dimensional components. Different application examples of structured surfaces on different materials are also described. The applications include the development of thin film structured electrodes to improve the efficiency of organic light emitting diodes (OLEDs) as well as the direct fabrication of decorative elements on technological steels. Finally, an example of fabrication at high fabrication speed is shown.

Three-dimensional (3D) battery architectures are under current scientific investigation since they can achieve large areal energy capacities while maintaining high power densities. A main objective of surface patterning is the enhancement of lithium-ion diffusion which is often a limiting factor in lithium-ion cells. By using a rather new approach, laser material processing of thick-film electrodes has been investigated for the precise adjustment of 3D surface topography. Besides lithium-ion diffusion in electrode materials as an electrochemically limited process, a critical step in lithium-ion pouch cell manufacturing is the homogeneous electrolyte wetting of stacked electrodes and separators. This process requires cost expensive and time-consuming vacuum and storage processes at elevated temperatures. A new and cost efficient laser process has been successfully applied in order to significantly improve the electrode wetting and the battery operation. Preliminary investigations for testing the process on pouch cell geometry revealed higher capacities and increased cell life-time compared to standard cells without storage processes at elevated temperatures. The laser structuring process can be applied to commercial electrode materials and integrated into existing production lines.

There are a growing number of unique self-organized micro/nanostructures created using femtosecond laser surface processing that have been demonstrated. Although researchers have provided insight into the formation processes for distinctive morphologies on specific materials, there is a need for a broader understanding of the physics behind the formation of a wide range of morphologies and what parameters affect their formation. In this work, the formation processes for mound structures on 316 stainless steel (SS) with growth above the original sample surface are studied. The formation process for the structures on 316 SS is compared to similar structures formed on nickel using the same technique. The structures are formed using 800 nm, 50 fs laser pulses, and are self-organized, meaning the structure dimensions are much smaller than the spot size of the pulses used to create them. The formation dynamics were studied using a stop-motion scanning electron microscope (SEM) technique, where the same location of an irradiated sample was imaged in the SEM at various pulse counts. The result is a series of images showing the developmental progress with increasing pulse counts. The structures form through a combination of fluid flow of the surface melt that results after irradiation, preferential ablation of the center of the pits between structures, and material/nanoparticle redeposition.

Laser-induced periodic surface structures (LIPSSs) consist of regular wavy surface structures, or ripples, with amplitudes and periodicity in the sub-micrometer range. A summary of experimentally observed LIPSSs is presented, as well as our model explaining their possible origin. Linearly polarized continuous wave (cw) or pulsed laser light, at normal incidence, can produce LIPSSs with a periodicity close to the laser wavelength, and direction orthogonal to the polarization on the surface of the material. Ripples with a periodicity (much) smaller than the laser wavelength develop when applying laser pulses with ultra-short durations in the femtosecond and picosecond regime. The direction of these ripples is either parallel or orthogonal to the polarization direction. Finally, when applying numerous pulses, structures with periodicity larger than the laser wavelength can form, which are referred to as “grooves”. The physical origin of LIPSSs is still under debate. The strong correlation of the ripple periodicity to the laser wavelength, suggests that their formation can be explained by an electromagnetic approach. Recent results from a numerical electromagnetic model, predicting the spatially modulated absorbed laser energy, are discussed. This model can explain the origin of several characteristics of LIPSSs. Finally, applications of LIPSSs will be discussed.

In medical device manufacturing there is an increasing interest to enhance machining of biocompatible materials on a micrometer scale. Obviously there is a trend to generate smaller device structures like cavities, slits or total size of the device to address new applications. Another trend points to surface modification, which allows controlling selective growth of defined biological cell types on medical implants. In both cases it is interesting to establish machining methods with minimized thermal impact, because biocompatible materials often show degradation of mechanical properties under thermal treatment. Typical examples for this effect is embrittlement of stainless steel at the edge of a cutting slit, which is caused by oxidation and phase change. Also for Nitinol (NiTi alloy) which is used as another stent material reduction of shape-memory behavior is known if cutting temperature is too high. For newest biodegradable materials like Polylactic acid (PLA) based polymers, lowest thermal impact is required due to PLA softening point (65°C) and melting temperature (~170 °C ). Laser machining with ultra-short pulse lasers is a solution for this problem. In our work we demonstrate a clean laser cut of NiTi and PLA based polymers with a high repetition-rate 1030 nm, 400-800 fs laser source at a pulse energy of up to 50 μJ and laser repetition rate of up to 500 kHz.

We report on the experimental results of micro- and nanostructures fabricated on the surface of fused silica by a train of two femtosecond laser pulses, a tightly focused 266 nm (ultraviolet, UV) pulse followed by a loosely focused 800 nm (infrared, IR) pulse. By controlling the fluence of each pulse below the damage threshold, micro- and nanostructures are fabricated using the combined beams. The resulting damage size is defined by the UV pulse, and a reduction of UV damage threshold is observed when the two pulses are within ~ 1 ps delay. The effects of IR pulse duration on the UV damage threshold and shapes are investigated. These results suggest that the UV pulse generates seed electrons through multiphoton absorption and the IR pulse utilizes these electrons to cause damage by avalanche process. A single rate equation model based on electron density can be used to explain these results. It is further demonstrated that structures with dimensions of 124 nm can be fabricated on the surface of fused silica using 0.5 NA objective. This provides a possible route to XUV (or even shorter wavelength) laser nano-machining with reduced damage threshold.

Laser induced forward transfer (LIFT) process was used to print thick-film electrodes (LiCoO2 cathode and carbon anode) and solid-state polymer membranes for Li-ion microbatteries. Their electrochemical behaviors were characterized by cyclic voltammograms, capacity measurement and cycling performance. Microbatteries based on these laser-printed thick-film electrodes showed significantly higher discharge capacities than those made by sputter-deposited thin film techniques. This enhanced performance is attributed to the high surface area porous structure of the laser-printed electrodes that allows improved diffusion of the Li-ions across the 100 μm-thick electrodes without a significant internal resistance. In addition, a laser structuring process was used to prepare three-dimensional microstructures on the laserprinted thick-film electrodes to further improve battery performance by increasing the active surface area. These results indicate that the laser processing techniques are a viable approach for developing Li-ion microbatteries in microelectronic devices. This paper will show examples of Li-ion microbatteries fabricated with various polymer separators and structured electrodes using a combination of LIFT and excimer laser structuring processes.

LiFePO₄ is a very promising material to be used as positive electrode for future lithium-ion batteries. Nevertheless, a reduced rate capability at high discharging and charging currents is the main drawback. In this work, a 3D structure was made in LiFePO₄ composite electrodes by applying ultrafast laser ablation. The change of the electrochemical properties in a lithium-ion half-cell due to laser structuring was studied in detail and will be discussed. The main challenging goal is to correlate cell properties such as capacity retention with laser parameters and laser generated microstructure. For microstructuring electrode materials an ultrafast as well as a ns fiber laser were used. The pulse duration was varied in the range from 350 fs to 200 ns. With ultrashort laser radiation, the ablation efficiency was increased. Electrochemical characterisations were performed. For this purpose, Swagelok® test cells with lithium metal as counter electrode were assembled. Main electrochemical parameters such as specific capacity and cycle stability were determined for the cells with structured and unstructured cathodes. It was shown that the rate capability for the cells with structured cathodes in comparison to cells with unstructured cathodes was significantly enhanced, especially for high charging and discharging rates.

We report on recent results on selective structuring of photoresist with femtosecond laser pulses in combination with conventional UV photolithography. The advantages of both processes can be combined to generate structures covering lateral dimension from the micron scale up to patterns of millimeter size with high quality in a photoresist double layer system. The fabrication process is based on a photoresist multilayer system where a negative photoresist is placed on a thick SU-8 layer. The negative resist layer is patterned by photolithography and the SU-8 layer by means of selective laser ablation, respectively. An additional thin sacrificial layer of photoresist on the top surface serves as a protective coating and enables the removal of debris which is deposited on the top surface during laser structuring. After resist structuring the process parameters of the femtosecond laser is adapted to enable processing of the glass substrate where drilling of vias and the formation of cavities within the glass substrate is carried out, respectively. This enables resist patterning and substrate processing within one laser step offering a fast and flexible process. Laser processing experiments were carried out with a pulse duration of 400 fs and a wavelength of 520 nm. Photolithography was carried out with a standard mask aligner (MA6, SUESS).

The laser sintering employing a CW DPSS laser was applied to the fabrication of transparent conductive films using silver and indium tin oxide (ITO) nanoparticle inks. The laser sintering of an Ag nanoparticle thin film gave a transparent conductive film with a thickness of ca. 10 nm, whereas such a thin film fabricated by conventional heat treatment using an electronic furnace was insulator because of the formation of isolated silver grains during the slow heating process The influences of the laser sintering conditions such as laser scan speed on the conductivity and the transparency were studied. The laser sintering using ITO nanoparticle ink gave a high transparent conductive film by one step scanning of laser beam in air.

Femtosecond Laser Surface Processing (FLSP) is a powerful technique for the fabrication of self-organized multiscale surface structures on metals that are critical for advanced control over energy transfer at a liquid/solid interface in applications such as electrolysis. The efficiency of the hydrogen evolution reaction on stainless steel 316 electrodes in a 1 molar potassium hydroxide solution is used to analyze the role of surface geometry to facilitate the phase conversion of the liquid to a gaseous state in the vicinity of the interface. It is found that the efficiency of the electrolysis process is directly related to the separation of micro-scale features on an electrode surface. The enhancement is attributed to the size of the valleys between microstructures controlling the contact between an evolving vapor bubble and the electrode surface. The results suggest an alternative pathway for the tailoring of interfacial energy transfer on structured surfaces separate from traditional benchmarks such as surface area and contact angle.

An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers. Laser energy is used to heat the a-Si thin film to change the microstructure to poly-Si. Thin film samples of a-Si were irradiated with a CW-green laser source. Laser irradiated spots were produced by using different laser powers and irradiation times. These parameters are identified as key variables in the crystallization process. The power threshold for crystallization is reduced as the irradiation time is increased. When this threshold is reached the crystalline fraction increases lineally with power for each irradiation time. The experimental results are analysed with the aid of a numerical thermal model and the presence of two crystallization mechanisms are observed: one due to melting and the other due to solid phase transformation.

In this contribution we evaluate laser scanning approaches for the interconnection of the 31 electrical contacts of a MWT back-contact solar cell. The selective energy deposition with a laser system allows the minimization of thermomechanical stress in the wafer, but requires adapted scanning strategies to prevent damages in the solar cell top layer. The laser scanning process of the MWT solar cell is conducted in combination with a composite foil as an interconnector and a pre-dispensed solder paste. This approach is evaluated regarding the joining quality of the interconnection as a function of the scanning strategy.

Semiconductors such as Si and GaAs are transparent to infrared laser radiation with wavelengths >1.2 μm. Focusing laser light at the back surface of a semiconductor wafer enables a novel processing regime that utilizes this transparency. However, in previous experiments with ultrashort laser pulses we have found that nonlinear absorption makes it impossible to achieve sufficient optical intensity to induce material modification far below the front surface. Using a recently developed Tm:fiber laser system producing pulses as short as 7 ns with peak powers exceeding 100 kW, we have demonstrated it is possible to ablate the “backside” surface of 500-600 μm thick Si and GaAs wafers. We studied laser-induced morphology changes at front and back surfaces of wafers and obtained modification thresholds for multipulse irradiation and surface processing in trenches. A significantly higher back surface modification threshold in Si compared to front surface is possibly attributed to nonlinear absorption and light propagation effects. This unique processing regime has the potential to enable novel applications such as semiconductor welding for microelectronics, photovoltaic, and consumer electronics.

The generation of high power in active fiber application and the transmission of high laser power via fiber cables both require protection from misdirected laser light. The following paper presents a new approach to removing this unwanted part of light. The deposition of fused silica material on the fiber cladding applied with CO₂ laser processes constitutes a robust cladding light stripper suitable for high power levels. The CO₂ laser processes are easy to apply, obviate the need for any dangerous liquids and promise greater mechanical stability in handling and assembly.

Fresnel reflectivity at dielectric boundaries between optical components, lenses, and windows is a major issue for the optics community. The most common method to reduce the index mismatch and subsequent surface reflection is to apply a thin film or films of intermediate indices to the optical materials. More recently, surface texturing or roughening has been shown to approximate a stepwise refractive index thin-film structure, with a gradient index of refraction transition from the bulk material to the surrounding medium. Short-pulse laser ablation is a recently-utilized method to produce such random anti-reflective structured surfaces (rARSS). Typically, high-energy femtosecond pulsed lasers are focused on the surface of the desired optical material to produce periodic or quasi-periodic assemblies of nanostructures which provide reduced surface reflection. This technique is being explored to generate a variety of structures across multiple optical materials. However, femtosecond laser systems are relatively expensive and more difficult to maintain. We present here a low power and low-cost alternative to femtosecond laser ablation, demonstrating random antireflective structures on the surface of Cleartran ZnS windows produced with a continuous-wave laser. In particular, we find that irradiation with a low-powered (<10 mW), defocused, CW 325nm-wavelength laser produces a random surface with significant roughness on ZnS substrates. The transmission through the structured ZnS windows is shown to increase by up to 9% across a broad wavelength range from the visible to the near-infrared.

Laser induced breakdown spectroscopy (LIBS) and endoscope observation were combined to design a remote probing device. We use this probing device to inspect a crack of the inner wall of the heat exchanger. Crack inspection requires speed at first, and then it requires accuracy. Once Eddy Current Testing (ECT) finds a crack with a certain signal level, another method should confirm it visually. We are proposing Magnetic particle Testing (MT) using specially fabricated the Magnetic Particle Micro Capsule (MPMC). For LIBS, a multichannel spectrometer and a Q-switch YAG laser were used. Irradiation area is 270 μm, and the pulse energy was 2 mJ. This pulse energy corresponds to 5-2.2 MW/cm². A composite-type optical fiber was used to deliver both laser energy and optical image. Samples were prepared to heat a zirconium alloy plate by underwater arc welding in order to demonstrate severe accidents of nuclear power plants. A black oxide layer covered the weld surface and white particles floated on water surface. Laser induced breakdown plasma emission was taken into the spectroscope using this optical fiber combined with telescopic optics. As a result, we were able to simultaneously perform spectroscopic measurement and observation. For MT, the MPMC which gathered in the defective area is observed with this fiber. The MPMC emits light by the illumination of UV light from this optical fiber. The size of a defect is estimated with this amount of emission. Such technology will be useful for inspection repair of reactor pipe.

Micro-structured dielectric surfaces in combination with electrode structures are promising in the field of rapid prototyping of micro-sensors. In this work laser-induced back side etching and back side deposition using aqueous copper sulfate in form of a tartrate complex with formaldehyde as absorber liquid has been investigated regarding this aim. Results obtained with different laser systems ranging from UV to Near-IR and with pulse lengths from femtoseconds to nanoseconds will be presented, in order to give a wide-spread overview of the different observable effects. Depending on the specific setup and laser parameters, either well-defined compact Cu deposits, micro- or nanoscaled Cu droplets or ablation of the dielectric substrate was observed. Best quality crystalline and conducting Cu structures were achieved using ns pulses at 532 nm wavelength. Droplet formation with UV excimer laser was observed. Parameters influencing each configuration will be discussed.

In this study, Graphene patterns using laser-induced chemical vapor deposition (LCVD) with a visible CW laser (λ = 532 nm) irradiation at room temperature was investigated. Optically-pumped solid-state laser with a wavelength of 532 nm irradiates a thin nickel foil to induce a local temperature rise, thereby allowing the direct writing of graphene patterns about ~10 μm in width with high growth rate on precisely controlled positions. It is demonstrate that the fabrication of graphene patterns can be achieved with a single scan for each graphene pattern using LCVD with no annealing or preprocessing of the substrate. The scan speed reaches to about ~200 μm/s, which indicates that the graphene pattern with an unite area (10×10 μm) can be grown in 0.05 sec. The number of graphene layers was controlled by laser scan speed on a substrate. The fabricated graphene patterns on nickel foils were directly transferred to desired positions on patterned electrodes. The position-controlled transfer with rapid single-step fabrication of graphene patterns provides an innovative pathway for application of electrical circuits and devices.

Aluminum nitride (AlN) ceramics has a unique characteristic, namely the ability to form conductive structures on its surface directly by laser-induced decomposition of the base material. Various research has been carried out on obtaining low-ohmic structures depending on process parameters such as the laser power, overlap of subsequent pulses and the type of shielding gas (air, nitrogen and argon). This paper focuses on explaining which factors have the greatest impact on the resistance (resistivity) value of obtained structures. In order to explain the effect of the laser fluence (below and above the ablation threshold of aluminum nitride) on the chemical structure of the conductive layers, qualitative EDX analyses were performed. Optimization of the process allowed obtaining a resistivity of the conductive layers at a level of ρ = 0.64·10^-6 Ω·m, with a thickness of aluminum up to 10 μm (sheet resistance R_S = 10 mΩ/Sr). This technology can be useful in making printed circuit boards (PCB), various types of sensors as well as radio-frequency identification (RFID) and Lab-On-a-Chip (LOC) structures. This technology can also be useful for the production of metamaterials.

Within the plastic industry laser transmission welding ranks among the most important joining techniques and opens up new application areas continuously. So far, a big disadvantage of the process was the fact that the joining partners need different optical properties. Since thermoplastics are transparent for the radiation of conventional beam sources (800- 1100 nm) the absorbance of one of the joining partners has to be enhanced by adding an infrared absorber (IR-absorber). Until recently, welding of absorber-free parts has not been possible. New diode lasers provide a broad variety of wavelengths which allows exploiting intrinsic absorption bands of thermoplastics. The use of a proper wavelength in combination with special optics enables laser welding of two optically identical polymer parts without absorbers which can be utilized in a large number of applications primarily in the medical and food industry, where the use of absorbers usually entails costly and time-consuming authorization processes. In this paper some aspects of the process are considered as the influence of the focal position, which is crucial when both joining partners have equal optical properties. After a theoretical consideration, an evaluation is carried out based on welding trials with polycarbonate (PC). Further aspects such as gap bridging capability and the influence of thickness of the upper joining partner are investigated as well.