This PDF file contains the front matter associated with SPIE Proceedings Volume 11674, including the Title Page, Copyright information, and Table of Contents.

LASE Hot Topics presentation on 3D nanoprinting

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This paper presents the results of fundamental studies on the interaction of laser radiation with classical paper materials regarding the melting of the main paper components cellulose, hemicellulose and lignin. Different papers were irradiated with the laser radiation of a carbon monoxide (CO) laser. Fluencedependent interaction regimes, the dynamics of the flash pyrolysis and chemical changes due to irradiation are discussed. Using a high-speed camera, a liquid intermediate state could be observed as a result of the irradiation. This is decomposed into gaseous reaction products by a highly dynamic boiling process. In addition to the time resolved investigations, extensive FTIR studies were performed.

In the present work, we investigate the benefits and the drawbacks in using on dual-wavelength double fs-pulse laser irradiation for fused silica processing. Our purpose of this pump-pump experiment is to tune the electron dynamics in order to optimize energy deposition and then to improve ablated volume. We use green wavelength (515 nm) for the first pulse to enhance photo-ionization and near-infrared (1030 nm) for the second pulse to maximize electron heating and impact ionization. The investigated parameters are pulse-to-pulse delay (up to 20 ps), second pulse duration (1 and 10 ps) and total fluence (up to 20 J/cm²). The results will be discussed in terms of ablated volume and optical transmission. We demonstrate that (i) there is an optimum delay and (ii) the ablation behavior is intermediate between green and near-infrared single pulse irradiation. Our results are supported by a numerical model taking into account electron dynamics and absorbed energy density.

Many surfaces in nature, e.g. lotus leaf, exhibit superhydrophobicity. Some of the most attractive applications of these surfaces are based on their self-cleaning properties and anti-icing capability. Many strategies are used by researchers to replicate these natural phenomena on metallic substrates. Among them, short/ultrashort pulsed laser technologies can functionalize surfaces with micro/nano-textures enabling strong water-repellent properties and low adhesiveness, which represent a promising solution to anti-icing properties. In this work, several patterns of micro-structures were textured by femtosecond laser on metallic materials of aeronautic and aerospace interest. The wettability properties of the surfaces were investigated in terms of water contact angle (CA) under different ambient conditions. The reversibility of the sample superhydrophobicity after exposure to a highly humid environment was studied. Water-dripping tests were carried out at subzero temperature finding that, while the untreated samples were covered with ice, no frozen spot was observed on the superhydrophobic textured surfaces.

Femtosecond laser surface processing (FLSP) is a material processing technique used to produce self-organized micro/nanostructures on metals. The hierarchal structures can improve the surface properties of materials when applied to specific applications such as enhancing heat transfer. In this paper, we demonstrate a recently developed technique termed multi-material, multi-layer FLSP (3ML-FLSP). With 3ML-FLSP, micro/nanoscale features can be produced that are composed of multiple materials by processing surfaces using traditional FLSP techniques that are layered with thin foils of different materials. We demonstrate results with three layers of different metals (304 stainless steel, copper, and aluminum) clamped together during laser processing to create structures composed of all three metals. Ion beam milling is used to cross-section structures for subsurface analysis of the microstructure. The three metals did not mix within the bulk of the microstructures indicating that the microstructures were produced primarily through preferential removal of material around the structures. However, there was mixing of all three materials within the nanoparticle layer that covers the microstructures.

The use of different 3D printing technologies for pharmaceutical manufacturing provides new opportunities for personalized medicine and on-demand tailored drug products.In this work we present our recent achievements in developing a viable manufacturing process for printed personalized dosage forms of liquid-phase active substances (i.e. paclitaxel) in the nanosecond regime and the optimization of the printing process onto glass slide substrates via the Laser Induced Forward Transfer technique (LIFT). In the context of investigating the effectiveness of LIFT printing, the active pharmaceutical ingredient (API) quantification of the LIFT-printed paclitaxel films was confirmed using High-Performance Liquid Chromatography tandem Mass Spectrometry (HPLC-MS/MS) analysis.Initial experiments of APIs laser printing have shown good feasibility of this technique, highlighting LIFT as a promising method for pharmaceutical applications.

The laser beam welding process is not commonly used for joining interconnectors on metallized substrate such as Printed Circuit Board (PCB) due to the presence of the vapor capillary and high energy input which exceed the thermal destruction threshold of the substrate. To perform a welding process between interconnectors and PCBs, a metallization with sufficient thermal mass is required. The cold gas spraying process is used to spray a copper layer on a thin metallization to increase its thickness and the thermal mass. In this paper, a copper interconnector is laser-welded on a metallization of PCB which is cold gas sprayed with copper powders. The characterization and transfer of the welding technology to the joining of spray layer is investigated. The main challenges of the welding process on spray layer on substrate are the uncontrolled surface roughness and the inhomogeneous heat distribution of the spray layer compared to the bulk material. The influence of the surface roughness on the void formation is investigated by considering roughness values of the sprayed layer. A correlation between the void formation and the surface roughness is shown. Also, the void formation at the weld joint increases with the higher laser beam power. The increased laser beam power leads to a deeper vapor capillary and it is assumed that the inhomogeneous heat distribution of the lower joining partner induces varying solidifications speed. By reducing the surface roughness value and lower laser beam power, we could significantly reduce the void formation at the weld root.

In this work, we report on a single-pass method for cutting 250-μm thick Z-cut quartz plates using 200 fs laser pulses at the wavelength of 1030 nm. In particular, we delve into the influence of the process parameters, i.e. laser repetition rate, scan speed and pulse energy, on the generation of a controlled stress-induced fracture which ultimately leads to the final cut. Processing above a certain threshold pulse energy caused significant damage, resulting in poor quality cuts. Whereas, a correct combination of these parameters led to a flat and almost defect-free cut edges, in a single pass.

Laser processing of transparent materials, particularly deep hole machining, has been extensively studied from the viewpoint of its industrial use. This time, we tried to improve the processing depth by laser ablation processing for back surface of fused silica glass using a picosecond laser with a wavelength of 532 nm. The numerical aperture of lens was 0.4, and the laser was focused on the back surface of the glass. In addition, the input laser was divided into double pulses and compared with the processing results of a single pulse. As a result, it was shown that the processing depth of back surface ablation is higher than that of surface ablation. It was also found that by using a double-pulse that sets the first laser to a laser output below the fluence threshold and the second laser to the laser over the threshold, a deeper processing depth can be obtained. At the same time, when the shapes of the processing traces were observed, the processing traces were not a simple concave-shaped hole, but a donutshaped processing traces with a raised center. In double-pulse laser processing where the first laser is set above the threshold, we find that the processing traces show traces that are close to single-pulse traces.

Several fields, such as consumer electronics manufacturing, medical device packaging and microfluidics have motivated the development of techniques for adhesiveless bonding of glasses both to other glasses, and to dissimilar materials, such as metals. With the use of ultrafast lasers, through-transmission welding can be achieved using a transparent wavelength to focus tightly at the interface of the materials to be bonded. The combination of absorption initiated by high intensities near the focal point and high pulse repetition frequencies induces heat accumulation. This leads to localized melting and joining of the two materials. Other currently available laser-based methods require introduction of additional material to serve as an absorbing media at the interface to couple energy to the materials and initiate welding. Conversely ultrafast lasers benefit from their inherent ability to induce non-linear absorption localized at the focus. This allows for the potential to develop efficient microwelds that do not age, and require no intermediate layer, preserving transparency in the case of glass-to-glass welding. In this work, we present results for glass-to-glass and glass-to-metal microwelding using a high repetition rate picosecond pulsed laser. Linear joining speeds of 10’s to 100’s of mm/s are demonstrated and their geometries characterized. Welds are evaluated qualitatively by optical inspection through the glass and inspection of prepared cross-sections.

We report on ultrashort pulsed laser fabrication strategies for glass articles with customized edges and curved surfaces. To achieve single-pass, full-thickness modifications along the entire substrate processing optics are presented that allows for beam shaping of non-diffracting beams and, additionally, for aberration compensation of phase distortions occurring at the tilted or curved interfaces. The efficacy of our concepts is presented by evaluating the surface and edge qualities of separated glass tubes with complex inner and outer contours as well as glass chamfer structures.

A well-known and already having many material processing applications zero-order Bessel beam makes a great base for improvements to have even broader applicability. In our work, we analyze vector Bessel beams (VBB), which can be generated with high efficiency and quality via the use of Geometric Phase Optical Elements. The beam transverse polarization distribution enables to change intensity distribution easily, i.e. a polarizer in front of the beam will generate invariant over the propagation multi-peak ring- the shaped structure which could be very beneficial to modify material at multiple sights with a single laser shot. We analyze higher-order VBB generated modifications in thin glass and analyze the applicability for etching of large diameter holes.

We have built a fs laser welding setup with a custom built fs laser having adjustable parameters integrated to a scanning scheme including a linear translation stage and a simple stepper motor to obtain spiral scanning. We have obtained successful results with both raster scan and spiral scan using borofloat glass samples and compared them in terms of homogeneity and total elapsed time. We obtained better results in the case of spiral scanning. The modifications that have formed during spiral scanning were more homogeneous owing to the continuous motion between the start and end points and no apparent stress load were visible. We also investigated parameter space by adjusting rotation frequency and translation velocity and achieved successful welding results with 750 mm/s linear velocity, having >509.19 mm/s/W efficiency. These are the highest speed and efficiency welding results to our knowledge.

Laser Induced Periodic Surface Structures (LIPSS) and Direct Laser Writing (DLW) enable the selective fabrication of micro/nano surface structures on a broad range of materials. Such engineered surfaces can be tailored and have demonstrated various functional responses, from optical to hydrophilic/phobic and non-fouling properties. One still limiting factor to the mass production of such functional surfaces is the durability of their surface features. Indeed, surface damages can be detrimental to the attractive functional properties. In this talk, several textured surfaces (Lotus-leaf inspired hierarchical features and triangular LIPSS) were laser-fabricated on stainless steel parts using both short and ultrashort laser pulses ; and replicated on polypropylene replicas parts via injection moulding. The surface response of textured steel parts were investigated after large-area wear cycles and abrasive injection moulding. Surface hardening was used as a way to extend the lifetime of the textured surfaces. Finally, textured polypropylene replicas and their superhydrophobic responses are investigated following standardized mechanical cleaning cycles. In all cases, the degradation of surface textures had a clear impact on surface topography and thus on their functional properties.

In this paper, we demonstrated the processing of free-standing thin films by the ultrafast laser ablation, which has been difficult to process using existing nanoprocessing methods such as focused ion beam milling. First, we fabricated a holographic diffraction grating for transmission electron microscopy using a two-beam interference laser processing. We fabricated an electron phase hologram made of silicon with a thickness of 35 nm that generated electron vortex beams with high efficiency. Then, we demonstrated the laser processing of silicon nitride membranes with a thickness of 10 nm at near-threshold conditions and realized gratings with sub-100-nm structure. We believe that this technique will introduce a new nanoengineering technology using light because of its suitability for nanofilm processing and ease of use.

In recent years, various studies have described the potential of Direct Laser Interference Patterning (DLIP) technology for technical surfaces in industrial applications. The focus of the studies is currently on the variability of the structural patterns (also in combination with Direct Laser Writing processes), the increase of the aspect-ratio and the upscaling to high processing speeds as well as large area structuring using mainly laser-scanner systems. Common are spatial periods of a few microns. However, for some DLIP applications regarding the structuring of tool steel the focus is different. Namely it can be beneficial to achieve a variable spatial period while maintaining a well-defined structure. Examples are molding or embossing processes where the structured tool steel is used as a template. Again, the spatial periods ranges in the magnitude of micrometers or below. This study describes the development of an automated system technology for a high-precision DLIP setup without using a laser-scanner-system. Two coherent beams from a beam source with ultrashort laser pulses of 10 ps and a wavelength of 532 nm are superimposed on a metallic workpiece to ablate line-like periodic patterns by interference. The characteristic spatial period created in the process is dependent on the incident angle between the two beams. It can be varied in the micro- and sub-micrometer range by implementing axes to automate the beam guidance and subsequently controlling the incident angle. The setup is calibrated and fine-tuned in a process where the spatial periods structured at different incident angles are validated using an Atomic Force Microscope (AFM). Further automation is achieved by developing a user interface for intuitive control. Finally, various spatial periods are structured onto tool steel for industrial applications in the field of the automotive industry.

Femtosecond laser surface patterning is a powerful tool capable of producing hierarchical surface features with possible applications in various scientific and industrial fields. In this work, we investigate several piratical aspects of this technology. Contact angle modification for several various materials is investigated, highlighting how it can be changed from superhydrophobic to superhydrophobic. This is followed up by an inquiry into the possibility to use laser patterned surfaces for friction control. Finally, we investigate the influence of chemical polishing on surface chemistry and topography. It is relevant for possible uses in medicine. Overall, shown results give important insights into the practical implementation aspect of the produced surface patterns.

Laser-induced forward transfer (LIFT) is a direct-writing technique enabling deposition of a film. In addition, a single dot smaller than the laser wavelength can be deposited at small shot energy, and the case is called as laser-induced dot transfer (LIDT). In conventional LIDT experiments, multi-shots with step scanning have been used to form array structures, which are useful in plasmonics, pho-chemistry, light harvest, etc.. On the other hand, interference laser processing can achieve an arrayed process and generate a periodic structure in a single shot. In this presentation, the results of LIDT technique which uses a femtosecond laser interference pattern will be presented. As a result, an array of Au nanodots with 3.6 m period was successfully deposited, producing the following unit structures: a single dot, adjoining dots, and stacking dots. This new technique produces high-purity, catalyst-free nanodots in array that do not require post-cleaning or alignment processes.

Several kinds of flexible and stretchable devices were fabricated by laser direct writing. A stretchable Ag paste was employed as a conductive material, which showed the suitable resistance stability in the application to a stretchable electronic device. A stretchable Ag paste coating on a polyurethane film was applied to the fabrication of a stretchable in-plane micro-supercapacitor (MSC) consisting of a carbon electrode which was prepared by laser ablation of a carbon nanotube (CN) layer on the stretchable Ag paste coating. The cyclic voltammogram of the device showed the capacitive characteristics even during stretching. The in-plane MSC consisting of CN electrode can be worked as a power source for a digital clock during stretching up to 44% elongation. A stretchable Ag paste coating pattern was also applied to an antenna-type sensor, where a screen printing mask for the stretchable Ag paste coating was prepared by laser ablation. A black-dye containing gel film on a silk cloth was patterned by galvano-scanning of ns pulse laser beam. A dipole-type antenna pattern consisting of a stretchable Ag paste coating was prepared on a polyurethane film. A reversible change of return loss at 5.317 GHz during stretching and shrinking of the antenna pattern was observed. A flexible antenna type sensor was also applied to agri-sensing, where the sensing mechanism was based on the change of the resonant properties of the antenna-type sensor caused by the dielectric condition change around the device. A flexible spiral antenna-type sensor on a silicone elastomer sheet was fabricated by printing of a stretchable Ag paste through a gel film mask, where a spiral antenna pattern was drawn by laser ablation of a black gel film on the silicone elastomer sheet. The sensing of the drying process of a plant leaf was observed by the antenna type sensor. The change of the return loss spectrum can be attributed to the change of the water content in a plant tissue which influenced the dielectric properties of the plant leaf.

Ultrashort pulse micromachining has found a rising number of applications in a variety of scientific and industrial fields. In order to address the growing field of applications, target materials and customer requirements, a high degree of pulse parameter flexibility and ease of integration is needed. The newest generation of the TruMicro Series 2000 delivers unique features such as fast tunable pulse duration, MHz- up to GHz-burst modes in combination with flexible Pulse on Demand and elevated average power of 100W for improved productivity scaling. Three available wavelengths (343nm, 515nm, 1030nm), an integrated hollow-core fiber interface, as well as a new advanced ultrashort pulse laser control, all combined into a new one box optomechanical design with identical interfaces and dimensions opens new paths for cutting-edge applications. The improved flexibility enables fast (<800ms) and controlled (without affecting beam pointing or energy stability) tuning of pulse parameters such as pulse duration, pulse energy, pulse frequency, QCW-mode and pulse spacing up to GHz-bursts (patent pending technology). Inter- as well as intra-process parameter switching offers advanced successive parameter sequences for tailored machining. Combined processes are demonstrated that optimize both productivity (ablation rate) and quality (surface roughness, color, gloss etc.) for ablation of various metals, semiconductors and ceramics by choosing suitable timescales for energy deposition. Automated parameter studies are shown to quickly generate quantitative surface quality characteristics and foster in-depth process understanding depending on pulse parameters. Furthermore, the latest benefits for ultrafast processing employing position synchronized output and the integrated hollow-core fiber delivery with TruMicro Series 2000 are demonstrated.

Emerging applications for functionalized and smart glass surfaces have created a market pull effect for scalable digital manufacturing processes in the glass industry. Precise lateral and thickness control in additive fluid deposition processes such as inkjet printing, spray coating and drop casting rely on tuning surface wettability to match with the fluid properties. Typical industrial approaches to modifying surface wettability involve air plasma treatment and deposition of self-assembled monolayers. If patterned control of wettability is needed, these are normally followed by lithographic approaches. Here, we explore direct-write femtosecond laser patterning and shadow-masked plasma treatment as alternative approaches for rapid pattern modification of glass surface structure and surface chemistry to control wettability. We also show the use of these techniques for local removal of functional surface coatings. These capabilities can be combined to enable the digital patterning of contrasting regions of hydrophilicity-hydrophobicity on glass surfaces. We demonstrate surface confinement of liquid drops and capillary-driven spreading. The underlying mechanism of wettability control is determined through combining high-speed imaging of liquid flow with surface chemistry mapping. This research aims in the future to help enhance inkjet printed deposit adhesion, resolution and quality whilst eliminating artefacts such as the coffee ring effect.

Several beam shaping strategies have been proposed as an effective approach to fully exploit the average power delivered by the last generation of high power, industrial, femtosecond laser whilst avoiding unwanted thermal effects. Nevertheless, in the case of surface texturing, the effect of the profile modifications on the surface morphology has been barely investigated. The objective of our study is to understand more deeply how the beam profile features will impact the surface evolution. By tailoring the beam profile, we want to understand if it is possible to increase the process throughput. A comprehensive study has been carried out with a 35 W, 2MHz, femtosecond industrial laser. The impact of the beam intensity profile on texturing of metallic surface has been investigated. We systematically studied how the beam size (from 10 μm to few hundreds of microns) impacts the generation and evolution of Laser Induced Periodic Surface Structures (LIPSS) on Stainless Steel surface. SEM analysis has been carried out for all the generated morphologies.

The formation of a twisted Au microstructure was demonstrated by irradiating the Au thin film with a focused optical vortex laser pulse. The theoretically calculated torque generated by optical radiation force of optical vortex was confirmed to be consistent with the twist direction of the experimental results. The microstructures formed by changing the focal position were experimentally investigated. Through simulations, it was shown that the spherical wave of the focused beam may affect the distribution of the optical radiation force at the defocus positions.

Time and energy sharing is an established technique in high power cw laser applications. Especially in conjunction with modern ultrafast laser systems the combination with an energy sharing system allows to take full advantage of the laser system. By including these options into a flexible fiber beam delivery the spatial freedom can be included as well to offer new and unique freedom in designing application processes for demanding and advanced processes. We will discuss the special challenges which arise when designing a time and energy sharing system for ultrafast lasers in combination with a fiber beam delivery. The requirements for precision and security are substantially higher due to the high beam quality and energy density when guiding ultrafast laser sources and require special solutions when switching securely between different beam delivery paths. The realization of a time and energy sharing system and its integration with an ultrafast laser system will be presented.

Direct laser writing commonly uses high numerical objective oil lens and femtosecond lasers to realize high precision micro-nano fabrication. Here we report a simple maskless lithography system utilizing a controllably designed high NA planar diffractive lens based on binary amplitude modulation and a diode laser at 405nm wavelength to realize submicron far-field lithography. The design procedure is based on vectorial Rayleigh-Sommerfeld diffraction integrals and genetic algorithm realized by Matlab programming language. The planar diffractive lens reported here can be designed to produce a tightly focused spot (~300-800nm) with an ultra-long depth of focus(~4μm) at a focal length of 1mm.

Ultrafast beam delivery based on micro structured hollow-core fibers is now regularly used in industrial applications. It enables stable multi-axis applications with ultrashort pulses for highest precision and demanding materials. With the increase in readily available laser sources incorporating a second harmonic module and more material processes being investigated which benefit from shorter wavelength, the beam delivery has to be adapted to these new laser sources. Transmission in the green spectral range can now be transmitted with a flexible hollow-core fiber. Recent results transmitting high energy pulses from an SHG source will be shown. Additionally, as laser sources increase in available power, transmission and handling of ever increasing light pulses is of interest as well as polarization maintaining transmission of ultrashort pulses. We show recent results which include solutions for highest power delivery and operation with stable polarization at the output.

We compare the surface modification response of various bandgap materials to femtosecond laser pulses at non-conventional driving wavelengths, from UV to MIR. The wavelength dependence of the fluence ablation threshold and machining performances are discussed, making special emphasis on industrially-relevant aspects as machining resolution and repeatability. Firstly, we demonstrate that resolution is not dependent of the nonlinear nature of light absorption, leading to modification features that are perfect imprints of the beam profiles in all cases. Secondly, we present a simple and general model for aprioristic evaluation of machining repeatability and access reproducible features as small as 1/10 of the beam size with a very stable 200-fs laser source.

Biomimetic technical surfaces are very interesting for a wide field of possible applications in material science and engineering. For example, changing the wetting and deicing properties of components used in cold environmental conditions can help to reduce ice or snow aggregation, and thereby improve functionality and operational stability. In this study we investigate the correlation between wetting and deicing behavior of micro- and nanostructured stainless steel samples (1.4301). The samples were modified using a Ti:Sapphire femtosecond laser system with 800 nm central wavelength, a pulse duration of 30 fs and a repetition rate of 1 kHz. We generated two fundamentally different types of hydrophobic and superhydrophobic structures by varying the laser fluence and the number of applied pulses, thereby creating hierarchical structures in the micrometer regime and laser induced periodic surface structures (LIPSS) in the nanometer regime. The static water contact angle has been measured to quantify wetting properties of laser treated samples. To determine the ice adhesion shear stresses at the ice/steel-interface, cuvette encased ice columns were frozen onto the structured samples and sheared off by a push rod, while recording the forces. Several icing/deicing cycles have been carried out to investigate a possible decline in wetting behavior due to wear or other mechanisms. We could show, that surfaces with hierarchical microstructures and superhydrophobic wetting behavior will lose its ability to repel the applied water while freezing. Larger structures with higher surface roughness lead to increased ice adhesion shear stresses compared to the initial unstructured surface. LIPSS on the other hand might be not as hydrophobic, but showed lower ice adhesion in the range of the reference sample.

The introduction of laser-generated 3D capillary structures in electrodes of Li-ion cells enables a more homogenized electrolyte wetting, which helps avoiding warm aging and shortens cell storage time prior and after electrochemical formation. In this study, the roll-to-roll (R2R) laser patterning of graphite anode material with a high-power ultrafast laser source was investigated. The governing parameters of laser structuring and their impact on active mass loss, electrode architecture, and electrolyte wetting behavior were analyzed. The trade-off between the quality of the structures and the processing times for an optimized R2R-processing was assessed. The application of a diffractive beam-splitter was evaluated.

This presentation was recorded for SPIE Photonics West LASE 2021.

Due to the increasing power, contacting of high-power semiconductor components requires ever larger cross sections, which can no longer be handled by conventional joining processes such as ultrasonic wire and ribbon bonding. To generate larger cross sections than those that can be joined by an ultrasonic bonding process, an alternative joining method must be selected. Laser beam welding offers the possibility of contactless joining without additional external force. Due to the process, however, a minimum thickness of the lower joining partner is required, on the one hand to prevent a through-weld and on the other hand to reduce the thermal stress for sensitive layers below the lower joining partner. A joining process that can also be used for contacting thin metallizations is laser-based soldering. An advantage of this method is the lower thermal load on sensitive components. However, disadvantages are on the one hand a higher contact resistance at the joining point and on the other hand a poor solderability of some materials (e.g. stainless steel, aluminum) when using a soft soldering process. In the context of this work, first results from the development of a combined laser-based joining process of freely combinable welding and soldering technology are presented. The contacting on a substrate to be soldered is done by means of a tin-plated strip connector and the contacting on the metallized semiconductor is done as laser welding. In both cases, the contact material or the connector is designed as a tin-plated ribbon material. This provides the user with a process with which the connection can be carried out either as welding, as soldering or in a combination of welding and soldering.

Laser structuring with ultrashort laser pulses of electrodes for lithium ion batteries is a promising technology to further enhance their application for energy storage in electric vehicles. We investigated the influence of laser parameters, such as average power, pulse repetition frequency and pulse number when processing electrodes of different thicknesses. Using an ultrashort pulse laser, two graphite anodes with a thickness of 136 μm and 248 μm, were structured with a dot pattern and then analyzed, with respect to their shape and size of the created structure units. Whereas the electrodes individually show similar ablation behavior, the higher coating thickness of 248 μm compared to a coating thickness of 136 μm results in deeper hole structures with a lower ablation volume when processed with the same laser parameters. Thus, the objective of creating electrodes with a three-dimensional structure and reducing material loss can be achieved more efficiently with electrodes with a higher coating thickness.

Hybrid femtosecond laser processing, which consists of femtosecond laser assisted wet etching, selective metallization, and laser induced periodic surface structure (LIPSS) formation, has enabled fabricating three-dimensional microfluidic surface enhanced Raman scattering (SERS) chips for highly sensitive sensing. To investigate the dependence of laser wavelength on the period of LIPSS which strongly affects the sensitivity of SERS substrate, two different wavelengths (515 nm and 1030 nm) of femtosecond laser beams have been employed. We observed the morphology of nanoripple on the metal layers under different laser parameters to optimize laser parameters, resulting in fabrication of homogenous LIPSS. The nanoripple with narrower groove (~40 nm) fabricated by 515 nm femtosecond laser induced stronger Raman scattering to achieve the SERS analytical enhancement factor exceeding 1 × 10⁸. Furthermore, we introduced a novel method termed liquid-interface assisted SERS (LI-SERS) to realize extremely sensitive sensing, which achieved the detection limit of aM with analytical enhancement factor exceeding 1 × 10¹⁴ for R6G detection. We found the LI-SERS was able to locally aggregate the analyte molecules by Raman excitation laser irradiation at the interface of air and analyte solution containing the analytes in a microchannel. The aggregation forced the analyte molecules to enter into the “hot-spots” by Marangoni effect, which extraordinarily increased the SERS intensity. Furthermore, we employed LI-SERS to detect DNA bases which realized the DNA discrimination in the microfluidic channel by LI-SERS.

SiO₂ nanoporous films has been attracting attention as low-k dielectric constant insulating films. We have succeeded in SiO₂ nanoparticles with a particle size of a few nm and depositing a nanoporous film by pulsed laser deposition with controlling the ambient gas pressure. However, the details of the formation process of SiO₂ nanoparticles have not been clarified. In this study, we visualized the time-resolved nanoparticle distribution in the gas phase by laser imaging technique to clarify the nanoparticle formation process and to be helpful for optimizing the growth condition of the low-k nanoporous film.

Since recently researchers at UFOLAB Group, Bilkent University discovered a new regime of ultrafast laser material processing which they named ablation-cooled material removal that utilizes very high repetition rate pulses [1], ultrashort pulses at GHz repetition rates have been attracting a lot of attention with respect to industrial material processing. Ablation cooling here refers to the method which has been used for the atmospheric reentry of rockets since 1950’s where a sacrificial layer is used to protect the rockets from burning and as a term it was first used in a 1934 novel “Triplanetary,” by E.E. ‘Doc’ Smith. In this regime, cooling of the material takes place simultaneously with ablation under the bombardment of pulses repeated with a period short enough so that relatively limited heat diffusion can take place from the targeted area to the surroundings. The targeted material heats with each successive pulse up to the critical temperature required for the evaporative removal, and the ablation takes place removing a major portion of the thermal energy localized at the targeted spot.

High quality thin glasses are emerging materials that find vast areas of applications in screen technologies, VR, microfluidics and solar cells. Due to the density of the glass material and the miniaturization trend in devices, thinner glasses are needed in the industry. Hence, in high-tech applications glasses and devices thin glasses are mostly used after a tempering process to achieve durability needed by the application. Some of the applications and design geometries necessitate these glasses to be welded. But due to the balance between compressive strength and tension inside glass is very delicate, laser processing of tempered glass is risky and may cause the glass to shatter if this balance is disturbed during a laser processing. Here we report, to our knowledge, first tempered glass welding of chemically tempered glasses using fs pulsed laser. We developed a fs laser system with adjustable parameters and integrated with a spiral scanning system to keep the stress load at minimum to prevent the shattering of the tempered glass due to stress. We observed successful welded regions in mm² areas, by keeping the glasses transparent and intact.

Hydrogels have recently emerged as a promising material for broad range of biomedical applications including implantation, drug delivery and tissue engineering. New applications are driven by their unique physical and chemical properties making them chemically and mechanically biocompatible. In this report we assess non-invasive, nondestructive, and potentially high-throughput technique based on combined Brillouin / Raman spectroscopy and microscopy to interrogate hydrogels’ viscoelastic and chemical properties.

During the last decade the zeroth order Bessel-Gauss laser beam has found many uses in the transparent material processing. The high aspect ratio channels can be created that slice through various thin transparent materials and increase the efficiency of cutting. However, the generation of high-quality Bessel-Gauss beam remains a challenge due to imperfection of glass axicon manufacturing, i.e. rounded tip, not smooth surface etc. These imperfections generate intensity modulation along propagation axis or even modify transversal central core intensity distribution, that results in worsening of micro-machining quality. The diffractive optical element (DOE) is a great alternative that do not suffer from previously mentioned problems. In this study we show the possibility of generating high quality Bessel-type beams with geometric phase optical elements (GPOEs) (manufactured by Workshop of Photonics). These elements act as precise flat DOEs that have very high diffraction efficiency (>90%), high optical damage threshold and can be freely customized for specific needs. Therefore, with the use of high-power laser they can be applied to process transparent materials. In this work, controllable phase shifts are implemented in axicon phase masks to create unique and fanciful Bessel-type beams as well as asymmetric core beams for thin glass modification/cutting application. Using numerical simulations and experimental data we compare performance of GPOEs and demonstrate thin glass processing using powerful laser with reshaped intensity distribution by GPOE.

Direct Laser-based carbonization of commercial polymers, such as polyimide, is a promising alternative to printing conductive carbon electrodes on flexible substrates. These laser-induced nanocarbons (LINCs) are formed along surface patterns based on the irradiation of high intensity beams of CO2 lasers in a direct-write fashion. Recent efforts have demonstrated the fabrication of various functional devices like micro-supercapacitors, sensors and microfluidic devices directly on polyimide films. While LINCs have been observed to have a turbostratic carbon structure with different hierarchical porous and fibrous morphologies, the fundamental mechanisms underlying the formation of LINCs is still largely missing. Here, we elucidate the process-structure-property relationships that are needed in order to correlate the laser processing parameters to the resulting micro-scale and nanoscale morphology, as well as to the achieved electrode properties.