United States

GHz burst mode femtosecond (fs) laser processing that contains a series of fs laser pulse (intra-pulse) trains with a pulse interval of several hundred picoseconds, has been attracting considerable attention, since it can offer some distinct characteristics in material processing compared with the conventional fs laser irradiation scheme (single-pulse mode). Specifically, laser ablation with the GHz burst mode can be achieved higher-quality with enhanced ablation efficiency than that by the single-pulse mode. Most of the research using GHz burst mode to date has been focused on ablation of materials. The objective of this study is to examine the possibility of GHz burst mode fs laser processing for the fabrication of novel nanostructures, that is different from laser-induced periodic surface structures (LIPSS) by the conventional single-pulse mode.&lt;br&gt;
In the experiment, linearly-polarized fs laser pulses (central wavelength 1030 nm, pulse width 220 fs) were focused onto crystalline silicon and titanium surfaces. It’s well known that the single-pulse mode creates nanoripple structures on both substrates, whose direction is perpendicular to the laser polarization direction. &amp;nbsp;Interestingly, we found that GHz burst mode fs laser pulses can form unique two-dimensional LIPSS on both substrates with periodic structures perpendicular and parallel to the laser polarization direction. The formation mechanism of 2D LIPSS is discussed and their surface properties are characterized.

ICALEO 2023

Novel 2D Periodic Surface Structures Fabricated by Femtosecond Laser Pulses with GHz Burst Mode

two-dimensional (2d) periodic surface structures

laser-induced periodic surface structures (lipss)

ghz burst mode

femtosecond laser

GHz burst mode femtosecond (fs) laser processing that contains a series of fs laser pulse (intra-pulse) trains with a pulse interval of several hundred picoseconds, has been attracting considerable attention, since it can offer some distinct characteristics in material processing compared with the conventional fs laser irradiation scheme (single-pulse mode). Specifically, laser ablation with the GHz burst mode can be achieved higher-quality with enhanced ablation efficiency than that by the single-pulse mode. Most of the research using GHz burst mode to date has been focused on ablation of materials. The objective of this study is to examine the possibility of GHz burst mode fs laser processing for the fabrication of novel nanostructures, that is different from laser-induced periodic surface structures (LIPSS) by the conventional single-pulse mode. 
In the experiment, linearly-polarized fs laser pulses (central wavelength 1030 nm, pulse width 220 fs) were focused onto crystalline silicon and titanium surfaces. It’s well known that the single-pulse mode creates nanoripple structures on both substrates, whose direction is perpendicular to the laser polarization direction. &nbsp;Interestingly, we found that GHz burst mode fs laser pulses can form unique two-dimensional LIPSS on both substrates with periodic structures perpendicular and parallel to the laser polarization direction. The formation mechanism of 2D LIPSS is discussed and their surface properties are characterized.

technical paper

**General Chair Welcome**

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Welcome to ICALEO® 2023!
	
We were excited to host the world’s leading international conference on Lasers and Electro-Optics in Chicago. The Laser has been shown to have the potential to revolutionize solutions for major global challenges like energy usage and supply, global warming, and CO2 footprint. Even the fusion energy breakthrough is based on laser technology. The conference, with peer-reviewed scientific and technical presentations, focused on the newest results in Micro Applications, Macro Applications, Additive Manufacturing, Beam Shaping and Frontiers in Laser Applications. Additionally, the Battery-Manufacturing Track gave an overview of the process combined with the newest advances in laser application results. 

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Whether this is the first time you are hearing about ICALEO or you are a longtime attendee, you will find something of interest, new ideas, and an excellent opportunity to network with other laser specialists worldwide. 

**Klaus Löffler **  
ICALEO General Chair   
Managing Director, Precitec GmbH

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ICALEO® brings together the brightest minds, seasoned professionals, and pioneering researchers from around the world. With a laser-focused agenda, ICALEO offers an exceptional platform to delve into the cutting-edge advancements, diverse applications, and transformative research that define lasers and electro-optics.

Seurat Technologies has developed an optically-addressed light valve used for high repetition rate dynamic laser beam shaping for primary use in Metal Additive Manufacturing [1]. This patented Area Printing® process—analogous to a programmable stamp—delivers high-power laser pulses to a bed of metal powder that locally sinters and bonds to form fully dense metal parts across a wide range of industrial applications. 
Area Printing® enables physical and economic scaling for Additive Manufacturing, while maintaining high spatial resolution, capable of printing features beyond reach of conventional manufacturing; with greater efficiency and minimal spatter defects. 
In this presentation, we address the optoelectronic properties of the optically-addressed photoconducting insulator (semiconductor) and liquid crystals that control the dynamic beam shaping, in addition to the limitations and challenges met with the use of industrial power laser intensities. Specifically, we focus on the contrast ratio achieved between dark and bright fields, the resulting spatial resolution of the beam delivered to the print bed, and the temporal response of the processed beam, as it relates to the semiconductor photoexcited carrier dynamics and mid-bandgap states mediated relaxation pathways, and the related electrical properties of the beam shaping device. 
Parasitic absorption imposes further device-level constraints that we characterize temporally and spatially via multi-physics finite element analysis of the thermomechanical response when the device is exposed to kW or MW levels of laser power. Key focus is placed on semiconductor and thermal management challenging the performance capabilities in light valves used in high power Additive Manufacturing. 
[1]&nbsp; https://www.seurat.com/area-printing

Light Valve Technology for Rapid, High Resolution 3D Area Printing

Extreme High-Speed Directed Energy Deposition (EHLA) is a modified variant of the laser based Directed Energy Deposition (DED-LB) and is being applied as an efficient coating process for rotational symmetric components. Characteristic for EHLA processes are feed rates of up to 200 m/min which result in smaller weld bead deposition and thinner layer thicknesses compared to conventional DED-LB. When transferred to Additive Manufacturing (AM) this characteristic utilizes the potential of depositing thin-walled filigree structures at deposition rates which are comparable to typical DED-LB processes (EHLA3D). 
Due to the high feed rates typical EHLA handlings are lathe type machines which realize the relative movement by rotating the rotational symmetric component. During the recent years the transfer to AM processes were demonstrated on a tripod machine developed by ponticon GmbH in collaboration with Fraunhofer ILT. The results of this work were achieved with a modified CNC-type machine which is capable of feed rates of up to 30 m/min. 
In this work process parameters were developed for the deposition of thin-walled filigree structures with the Ni-based superalloy IN718. Single tracks with constant feed rates and variation of beam diameter and powder mass flow were deposited and analysed regarding the resulting weld bead dimension. Then process parameters were selected and transferred to the deposition of thin walls and guidelines of the parameter adaption towards thin-walled deposition were defined. Depending on the developed parameter sets wall thicknesses between 300 µm – 500 µm are achieved.

Process Development and Process Parameter Guidelines for Thin Wall Deposition with IN718 using Extreme High Speed Laser

Laser metal deposition (LMD) is a blown powder process used for the additive manufacturing of large as well as complex parts. The laser spot size is determined by the fiber optic cable and by the imaging ratio of the process optics. Spot sizes typically used for LMD can range between 200 µm up to several millimeters. Zoom optics are able to change the laser spot focus within seconds during the process. However industrial powder nozzles are still static in their powder spot size. Changing the powder spot size accordingly to the laser spot size could lead to a desirable union of time savings when printing large volumes while also generating fine near-net-shape features. 
To overcome the current limitations in the LMD process, this work examines an adaptive powder nozzle system. In this discrete coaxial layout of three single powder nozzles the individual powder nozzles can be adjusted closer and further from the process to dilate or shrink the powder focus. Different diameters of powder channels are examined. The resulting powder propagation behavior is characterized for different settings of the single powder nozzles. Single tracks are welded with different nozzle settings for a fine and coarse powder spot, while accordingly changing the laser spot size with a zoom optic. The laser power is closed-loop controlled by a 2-color pyrometer to achieve comparative process temperatures. The single tracks are evaluated with regard to their geometry. High-Speed imaging gives supplementary information on the welt track generation.

Adaptive Power Nozzle Setting for Enhanced Efficiency in Laser Metal Deposition

We report a series of in-situ monitoring experiments on the high-speed laser directed energy deposition (HSL-DED) with a discernible distance between the identified laser spot and melt pool boundary, termed melt pool lag, during the high-speed laser-substrate interaction. The experiments investigated the melt pool lag feature on both mild steel and 300M high-strength steel substrates at processing speeds from 5 to 20 m/min with and without powder delivery. The analysis of results indicates that the correlation between melt pool lag measurements and several deposit characteristics could shorten the parametric investigation cycle in HSL-DED. The melt pool lag measurement is positively proportional to the processing speed at a given incident laser power level. Further examinations also found that substrate material properties significantly impact melt pool lag for a given set of process conditions. The melt pool lag measurements obtained from the mild steel substrate was significantly higher than that on the 300M high-strength steel substrate. An analytical model on melt pool formation has been developed with temperature-dependent thermophysical data of substrate materials to estimate the melt pool lag in HSL-DED. The observation of melt pool lag feature and the proposed analytical model enhance the understanding of the melt pool dynamics in HSL-DED. They also provide insights to improve powder utilisation during HSL-DED deposition.

The Melt Pool Dynamics on Different Substrate Materials In High-Speed Laser Directed Energy Deposition Process

Automotive industry's shift towards low-emission electric vehicles and induce a huge demand for EV-batteries. Along the entire process chain of Li-battery production you will find a variety of potential new or already established laser applications which are crucial for the output of GW battery capacities. Within electrode manufacturing there is the already established laser cutting and notching with ns lasers but also new developments like laser-based drying processes by VCSEL or other diode emitters which are getting high attention. These technologies offer dedicated advantages in surface heating of coated electrode foils by IR-laser radiation resulting into a significant reduction of operating and energy costs and hence to a decrease of CO2 footprint. In cell assembly laser welding of demanding materials such as copper and aluminum are still essential for the entire battery system to secure technical functionality like electric conductivity, strength, and tightness. A common approach for internal connection of stacked foils combines both, a pre fixation of stacked foils by ultrasonic welding followed by laser welding to ensure highest electric conductivity. In addition, new laser-based production solution trends in cell assembly like e.g., media-tight welding of cover assembly in prismatic cell housing “Can-Cap” and busbar welding will be exemplified to meet highest quality and productiveness. 
Finally, in battery module and pack manufacturing laser and combined sheet metal manufacturing processes are key elements in smart factory solutions. Ground breaking new technologies like multi focus beam shapes open new horizons in e.g. gas tight welding of aluminum battery heat exchangers.

Laser Applications Trends in EV Battery Manufacturing

Laser beam welding is a very efficient process for joining copper pins in electric drives, which are referred to as hairpins. The occurrence of pores reduces the conducting cross-sectional area, which increases the electric resistance and further impairs the tensile strength of the joint. 
The present work compares different welding strategies regarding the formation of pores and spatters during welding cu-hairpins. The welding strategy was varied by different geometries of the scan contour, different beam shapes and an adaptive line energy. Different beam shapes were realized using a two-in-one beam delivery fiber and using a civan laser. In order to capture the formation of pores, the processing zone was observed by means of X-ray imaging at the IFSW in Stuttgart and at Petra III at Desy in Hamburg. The formation of spatters was observed using a high-speed camera. With the observation it was possible to separately determine the influence of scan contour and beam shape on the formation of pores and spatters. Finally, these findings were applied to identify beneficial welding strategies to reduce the formation of pores and spatters. Furthermore, tensile testing proves the increase of the tensile strength of the hairpins, which were welded with the optimized welding strategies.

High-Speed X-Ray Imaging of Pore and Spatter Formation During Welding of Copper Pins

Pulsed laser material processing with high repetition frequencies is able to initiate heat accumulation effects that can decrease processing quality even for ultrashort laser pulses. In order to gain deeper insights into these effects, a fast temperature measurement system was developed using infrared detectors and dedicated optics. The system evolved through years when responding to different challenges, like homogeneous nanosecond laser melting of metals and semiconductors or laser marking of stainless steel for corrosion resistance conservation. Appropriate calibration methods were developed for temperature evaluation from measured signal to be used in different application fields. Evaluation algorithms were invented for analyses of thermal characteristics from the signal. Hardware implementation of the analyses using field programmable gate array (FPGA) was prepared for online evaluation of heat accumulation in nanosecond time resolution during the laser micro-processing. The system was used for example for real-time analysis of thermal processes in laser surface texturing by nanosecond laser and polygon scanner for increasing adhesion of thermal spray coatings, LIPSS formation by high power multibeam picosecond laser system or processing of lithium-ion battery electrodes for enhancing charging speed. The presented diagnostics system can be applied for analysis, monitoring and control of processes in a wide range of pulsed laser applications, e.g. laser surface texturing, micromachining, polishing, drilling or 3D printing.

Infrared online diagnostics for pulsed laser micro-processing

This presentation showcases various methods for producing laser-induced micro and nano structures with controllable spatial features, scale, and shape. These structures can be directly fabricated on dielectric surfaces and thin metallic films to enable efficient light manipulation, polarization control, and perfect absorption. By selecting the appropriate structure and processing conditions, we achieve optimal combinations of super-transmissivity, anti-glare, and extreme wetting properties for variable dielectric materials and optical fiber tip use cases. The nanostructured glass surfaces exhibit a high lased damage threshold, even more, that double in particular wavelengths. In addition, we demonstrate a method for producing polarizing plates exclusively via direct laser nano-structuring. Our grating-type planar metasurfaces, consisting of periodically aligned infinite ridges on a uniform transparent substrate, can be used as wire grid polarizers for far or mid-infrared electromagnetic frequencies. We also show that perfect narrow-band absorption can be achieved with gap plasmons polarizing metasurface plates operating in reflection at IR and mid-IR via laser structuring, specifically, laser-induced periodic surface structures (LIPSS) formed on sub-micrometer metallic films. The polarizing efficiency along with the optical responce of these structures was validated through numerical calculations and experimental results. Our study demonstrates that exploiting nanostructure formation on sub-micron metal films and dielectric materials is a promising approach for sophisticated light manipulation and may pave the way toward laser-induced meta-optics.

Advanced Laser-Induced Micro & Nano Surface Structuring for Photonic Applications

Antibacterial surfaces are a huge concern for many applications and uses. To solve this, chemical coatings with limited lifetime are often used to inhibit bacterial attachment, like E.Coli and S.Aurus, bacteria commonly found in hospitals.Hospital Acquired Infections (HAI) affect 1 patient out of 217 in Canada [1], cost up to 15 billion $US every year in the US and Europe, and caused 250,000 deaths in 2020[2]. Many works [3][4][5] highlighted the potential of laser textured surfaces to reduce bacterial adhesion. This passive approach consists in femtosecond laser-induced submicron structures such as Laser Induced Periodic Surface Structures (LIPSS), which can be smaller than the bacteria size. Through this novel approach, it has been demonstrated that bacterial attachment and thus biofilm formation can be significantly reduced. On the other hand, superhydrophobic surfaces could be mimicked by laser texturing of macrostructures, offering antibiofouling behavior to the treated surface. 
Many laser parameters could be tweaked to tailor structures sizes and morphologies, offering mixtures of structures for multifunctional surfaces. In this work, different strategies to obtain biofouling and antibacterial laser textured surfaces for medical applications shall be presented. 
 
[1]https://www.canada.ca/en/public-health/services/publications/science-research-data/healthcare-associated-infection-rates-canadian-hospitals-infographic.html 
[2] World Health Organization. Report on the Burden of Endemic Health Care-Associated Infection Worldwide. 2011 
[3] Peter, Alexander, et al. "Direct laser interference patterning of stainless steel by ultrashort pulses for antibacterial surfaces." (2020) 
[4] Romoli, Luca, et al. "Influence of ns laser texturing of AISI 316L surfaces for reducing bacterial adhesion." (2020) 
[5] Lutey, A.H.A., Gemini, L., Romoli, L. et al. Towards Laser-Textured Antibacterial Surfaces.(2018).

Structures Tailoring of Laser Textured Stainless Steel for Antibiofouling and Antibacterial Applications

The ability to deliver high energy and high average power laser radiation through a flexible fiber is a key enabler for numerous processing applications. This provides better space management, direct delivery to the processing area and better serviceability. Silica fibers were successfully used to deliver signals in the near-infrared and visible spectra, but they suffer notably from high absorption in the UV region. Hollow-core fibers alleviates this issue as the mode of propagation minimally overlaps with the surrounding guiding microstructure. However, their power scaling was currently limited by two factors: beam quality/stability of available UV sources and power handling of the thin capillary microstructure, especially due to the limited control of the internal surface roughness. 
In this work we will present recent results achieved by the combination of a high beam quality UV nanosecond lasers and low-internal roughness hollow-core fibers to achieve efficient UV fiber delivery. After giving experimental characterizations of the improved hollow-core fiber, we will detail the characterization of the 343nm UV source, including beam stability measurements at full power and evolution of beam quality on a large range of parameters. Fiber delivery with power above 20W (150 µJ energy) and transmission above 90% will be presented. We will also present preliminary results achieved at 266 nm with output power of 20.5mW (20.5 µJ) and transmission efficiency above 90%.

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Novel 2D Periodic Surface Structures Fabricated by Femtosecond Laser Pulses with GHz Burst Mode

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Light Valve Technology for Rapid, High Resolution 3D Area Printing