This PDF file contains the front matter associated with SPIE Proceedings Volume 11677, including the Title Page, Copyright information, and Table of Contents.

Welcome and Introduction to SPIE Photonics West LASE conference 11677: Laser 3D Manufacturing VIII

LASE Hot Topics presentation on Controlling 3D printed metal properties using tailored laser beams

Christophe Moser is Associate Professor of Optics and Section Director in the Microengineering department at EPFL. He obtained his PhD at the California Institute of Technology in optical information processing in 2000. He co-founded and was the CEO of Ondax Inc (acquired by Coherent Inc.), Monrovia California for 10 years before joining EPFL in 2010. His current interests are ultra-compact endoscopic optical imaging through multimode fibers, retinal imaging, additive manufacturing via volumetric 3D printing with light. He is also the co-founder of Composyt light lab in the field of head worn displays in 2014 (acquired by Intel Corp), Earlysight and Readily3D. He is the author and co-author of 70 peer reviewed publications and 50 patents.

This keynote presentation was recorded for SPIE Photonics West LASE, 2021.

In this talk I will review recent developments in understanding and controlling light-matter interaction and material response associated with laser powder bed fusion additive manufacturing. As part of the critical assessment of the physics of the process, validated hydrodynamic finite element model simulations have proven to be extremely valuable and can be used to inform rapid solidification microstructural models. I will also discuss new approaches to process optimization that have emerged from our modeling efforts which can improve material properties and part performance. Prepared by LLNL under Contract DE-AC52-07NA27344.

Keynote presentation, SPIE Photonics West LASE, 2021: Advanced design for additive manufacturing

3D printing technology has been considered disruptive solution for next generation manufacturing. We will review various modality of this process from laser DMLM, E-beam, Binder-jet to high speed Light DMLS. Comparison of different technologies with pros and cons will be discussed based on actual industrial application. Future needs in part quality and throughput/productivity will play a critical role to drive the direction of next gen 3D printing technology.

Keynote presentation, SPIE Photonics West LASE, 2021. Controlling and evaluating microstructural evolution in AM

Conventional microscale 3D printing techniques mostly rely on the raster scanning method, which needs constant changing of printer head/light beam/substrate directions to print a solid structure. Therefore, throughput is a longstanding bottleneck and it is more challenging to print microfeatures in large areas. This study demonstrates the possibility of 3D printing microfeatures on a fast-spinning disc. A Blu-ray drives based high-throughput 3D printer (BRIGHT^3D) is developed to demonstrate the spin printing on disc concept and evaluate the highest linear printing speed. The BRIGHT^3D integrates two Blu-ray drives that are synchronized by a customized controller. The printing substrate is a standard Blu-ray RW disc spun by a spindle motor. Both drives utilize the same optical pick-up unit (OPU), which equips a voice coil motor (VCM) for the disc wobbling compensation. The bottom OPU detects the disc wobbling and feeds the VCM control signal back to top OPU for maintaining laser (405 nm, 658 microwatts) focused on the spinning substrate disc. The BRIGHT^3D can directly spin-coat (up to 6,440 rpm) commercial photopolymers with a controllable thickness on top of the substrate disc. The top OPU laser was switched with a frequency of 1~500 kHz (duty cycle: 80 %) for the preliminary spinning 3D printing evaluation. Microfeatures can be cured by the BRIGHT^3D while the disc is spinning at a speed of 265 rpm, which has a linear speed from inner diameter, 20 mm, to the outer diameter, 58.5 mm, of 0.55~1.63 meters per second. After removing the photopolymer residues by 75% ethanol, various microscale features on the disc can be seen and measured by scanning electron microscopy. Microscale lines (height/width: 1.43/8.25 microns) and dots (length: 5.97 microns) were successfully printed on the disc. The BRIGHT^3D is aiming for multiple layer printing on the disc to realize sophisticated features of high-throughput 3D printing in the near future.

Multiphoton polymerization has become a standard for 3D nanofabrication for its superior resolution and processing steps. However, it is often considered limited in its throughput capabilities, resulting in a desire for a more scalable process. An improvement to the speed of 3D nanoprinting via multiphoton polymerization can be achieved through a projection-based method instead of the traditionally serial printing process. Here a printing process using a digital micro-mirror device and spatiotemporal focusing scheme demonstrates a rapid, continuous 3D printing process.

Multi-photon laser-lithography has become a versatile technology for writing complex 3D structures in high-resolution. Most efforts are focusing on single material printing. To realize multi-material printing, two main steps have to be carried out consecutively: developing the previous material and introducing the next material to the print side. So far, this is done ex-situ, and alignment errors are unavoidable. In this work, a fast in situ material exchange is demonstrated. The designed setup can effectively replace the print material in the optical focus, enabling the writing of detailed 3D structures while conserving efficiency and precision.

At Photonics West 2020, we reported progress on rapid multi-focus multi-photon 3D laser printing, enabling printing rates of up to ten million voxels/s at voxel sizes of about 400 nm (mean of lateral and axial extent). Meanwhile, we have refined and improved this setup. Here, we present three-dimensional chiral mechanical metamaterials with tailorable characteristic length as a scientific application. The characteristic length is as large as 10 unit cells. The largest 3D printed samples have a side length of 27 unit cells, corresponding to 118,098 unit cells total. Our experimental findings are in good qualitative agreement with modeling.

We present a novel two-photon 3D printing approach based on a dedicated resist chamber in which we apply a quasi-static electric field with variable orientation and amplitude during the 3D printing process. This allows aligning the director of not yet polymerized liquid-crystal elastomer resist. After two-photon exposure, the alignment is “frozen” in the polymerized voxel. For the next voxel, the electric field vector can be changed, etc. In this manner, we can 3D print hetero-microstructures of liquid-crystal elastomers. We envision applications under ambient conditions where mechanical actuation is induced by temperature variation or by focused light.

Recently, the necessary average total laser power for rapid multi-photon multi-focus applications has exceeded the Watt level. To enable even faster 3D printing, we have developed new sensitive photoresists by adding co-initiators to commonly used standard systems. We yet exceed these advances in sensitivity using a novel photoresist system based on a modified benzylidene ketone photoinitiator, making it a very attractive system for advanced rapid 3D laser nanoprinting. To compare our original results to more than 70 different published systems from the literature, we define a photoresist-sensitivity figure-of-merit, enabling a fair comparison to measurements taken under vastly different conditions.

In recent years, Laser Powder Bed Fusion (LPBF) has become an industrially established manufacturing technique due to the possibility to manufacture complex part geometries without additional tools. State-of-the-art LPBF machines feature a combination of (multiple) fiber lasers and galvanometer scanners due to their high dynamic and excellent focusability, leading to significant investment costs. Highly efficient high-power diode lasers (HP-DL) could present an alternative for L-PBF machines. However, the lower beam quality (BPP > 8 mm · mrad) and spectral width (920 – 1050 nm) of commercial HP-DL results in chromatic aberrations and reduced focusability, thus making modifications to LPBF machines necessary. Current approaches to address these challenges rely on a combination of fixed focusing optics with a gantry-based positioning system resulting in less dynamic laser positioning and thus reduced system productivity and part quality. In this study an optical system, featuring a standard galvanometer scanner and a color corrected f-theta lens, is developed and integrated into a LPBF lab machine. The resulting machine configuration is combined with a fiber coupled HP-DL and used for the manufacturing of test specimen out of stainless steel AISI 316L. The manufactured parts using this approach are analyzed in terms of surface roughness, detail resolution and part density as well as build-up rate and compared with state-of-the-art LPBF-machines.

This presentation was recorded for SPIE Photonics West LASE, 2021.

Additive manufacturing (AM) of pure copper using laser assisted powder bed fusion (LPBF) at a wavelength of 1070 nm is demonstrated. The rapid drop of absorption at the melting point of pure copper hinders the identification of an appropriate processing window. With a combination of an extensive parameter study and a comprehensive analysis using a multiscale numerical model of the LPBF process different processing windows extending over a wide range of incident laser powers were verified. The model allows to investigate the sensitivity of single parameters, and thereby gives the opportunity to carefully adjust the strategy of the AM process.

A pure titanium (Ti) plate was fabricated by selective laser melting (SLM) in a vacuum. Ti is attractive materials for medical, aerospace, and automotive applications, because they have properties of biocompatibility, high corrosion and erosion resistance, and mechanical resistance, but forming complicated structures is challenging due to difficulties working the material. Although some studies have reported to form pores in SLMed samples due to spattering generated by laser irradiation, and there are few reports that investigate the details of the correlation between spatter and pores. Spatter results in an insufficient input energy to the powder bed because the laser is absorbed by the spatter particles. Consequently, technology to suppress the amount of spatter for SLM processes is needed. In this study, the quantitatively evaluation method for the amount of spatter was developed with high speed video camera while the SLM processes.

Laser Metal Deposition (LMD) is a high deposition rate metal Additive Manufacturing process. Its applications are basically repair, cladding and manufacturing. The two most commonly used LMD processes are powder-based (LMD-p) and wire-based (LMD-w). Despite the fact that wire-based LMD is more material efficient, process stability is a major concern. By adding a modulated laser beam to the continuous process beam, a change of the melt pool geometry and increased energy absorption are observed. This relation shows great potential to increase process stability. In this contribution, the positive effect of the dual laser-beam use on LMD-w processes is demonstrated. To understand the cause-effect relation, the workpiece temperature field was investigated by optical backscatter reflectometry (OBR). The results were then correlated to simultaneously performed IR camera measurements of the workpiece’s upper surface. By better understanding the thermal phenomena in dual-beam LMD, research can improve process temperature control. This leads to a new perspective for the LMD-w manufacturing process in many industry sectors such as mobility, energy and engineering.

Contrary to structural and material complexities found in nature, man-made manufacturing technologies and associated materials remain relatively simple. Despite continued technological advances in additive manufacturing, current methods remain limited in their capabilities. We report a new technology coined as Hybrid Laser Printing (HLP) that is capable of shaping hydrogel materials into 3D multiscale, multi-material, and functional constructs. Using several proof-of-concept studies, we present that HLP can print 3D structures that are either (i) technically challenging to print, and/or (ii) extremely time consuming to manufacture, and/or (iii) not possible with current technologies.

Additive manufacturing (AM) and printing concepts have been employed in various fields, including electronics and functional device manufacturing industries. However, to date, the ability to print multifunctional materials and devices have been limited with the current printing technologies. Here we present a new printing concept for additive nanomanufacturing (ANM) of multifunctional material and structures on various substrates. In this method, we show that a stream of pure nanoparticles can be laser-generated in real-time at room temperature and at atmospheric pressure. These nanoparticles are then directed toward a printer nozzle and laser-sintered in-situ to form crystals with desired patterns and structures. Currently, with our ANM systems, we have achieved printing various materials (TiO2, BTO, ITO) on different substrates, including SiO2, PDMS, and paper. We believe this new ANM concept would bring excitement into the field of printing functional structures devices.

Direct laser writing with ultrafast lasers surpasses limitations of conventional fabrication techniques that have low resolution, require multiple post-processing steps or are restricted to fabrication in 2D. The first part of the talk discusses fabrication of 3D metal-dielectric nanocomposite structures of tunable dimensions ranging from hundreds of nanometers to micrometers. By directly reducing metal ions with femtosecond pulses, direct laser written high-quality single-crystals, polymer-matrix embedded diffraction gratings are fabricated. The next part discusses methods of direct laser writing graphene structures. Graphene patterns written with ultrafast lasers show higher conductivity, indicating that limited thermal processes can help achieve better quality direct-laser-written graphene patterns.

Architected materials present an opportunity to overcome the limited ability of brittle piezoelectric ceramics to strain under electromechanical load. In the absence of a commercially available resin containing piezoelectric nanoparticles, this work seeks to investigate the printability and thermal processability of a prepared piezoelectric particle loaded slurry using laser stereolithography. This was accomplished by comparing the cure depth, in-plane resolution, and the dimensional accuracy achieved with a piezoelectric slurry prepared with barium titanate, to a commercially available silica and alumina-based suspension. The study of thermal processability revealed the dimensional sensitivity of fabricated open architectures to sintering temperature and duration. The prepared piezoceramic slurry, containing barium titanate, was successfully polymerized using laser-stereolithography and its cure depth exhibited a similar response to exposure duration and fluence level as the commercial slurries. The in-plane resolution of the barium titanate-based slurry was unexpectedly high, and may be due to the opacity of the piezoelectric particles. Open architectures with millimeter sized features were successfully fabricated using laser stereolithography. Dimensional accuracy was highest for the alumina-based material system, and the inverse relationship between cured depth and in-plane resolution was reflected in the dimensions of the silica-based parts. Open architected structures further withstood thermal processing. During thermal processing, there was a greater reduction in height for all parts due to higher in-plane concentrations of ceramic particles, relative to the z-direction. Both an increase in sintering temperature and duration resulted in a uniform change of 1%, respectively, in part dimensions. Data collected in this work is to be used as a benchmark to inform formulation requirements, laser stereolithography parameter settings and thermal processing procedures for a custom ceramic slurry containing piezoelectric nanoparticles.

We present a visible light interference lithography technique that utilizes a 2x2 cm metasurface mask to enable fabrication of 8x8 cm continuous and homogenous nano-architected materials. Patterns are resolved both in commercial 20-60 um films of SU-8 and >20 um films of custom glycidyl methacrylate-derived negative-tone photoresists. The combination of our metasurface-enabled large-scale 3D patterning technique with customizable photoresist chemistry provides a new pathway for scalable production of architected materials with nanometer feature resolution and advanced functional properties. Impact experiments using Laser-Induced Particle Impact Testing (LIPIT) were conducted to probe mechanical response and material homogeneity.

Additive Manufacturing of glass opens up new possibilities for the design and integration of optical components. By varying the shape and size of optical elements, optical systems specifically adapted to various applications can be fabricated cost-effectively. The Laser Glass Deposition (LGD) process uses a CO₂ laser with a wavelength of 10.6 μm to locally generate temperatures above 2000 °C in fused silica fibers. This enables the Additive Manufacturing and Rapid Prototyping of glass by melting and then layer-by-layer deposition of fibers. However, these high temperatures can result in very high residual stress in the material. The development of a coaxial LGD process aims for a more uniform heating of the glass fiber during the printing process in order to enable a direction-independent process and to reduce the residual stresses within the printed components. In this work, a novel concept for the coaxial LGD process and its successful experimental application is presented. Further, a numerical simulation model is developed to describe the temperature distribution in the glass fiber during the coaxial LGD process. Based on experimental results and on the numerical simulation, the potentials and challenges of the coaxial LGD process are discussed.

We demonstrate the fabrication of optical elements on the millimeter scale by stitching-free 3D printing via two-photon polymerization. Previous limitations are overcome by the use of a large writing field objective as well as a novel high transparency resist. The printed optical components are free of stitching defects due to a single step exposure and exhibit an unpreceded glass-like appearance due to the low absorption of the resist material throughout the entire visible wavelength range. We print aspherical focusing lenses, characterize and optimize their shape fidelity, and find their optical performance close to the simulated optimum, demonstrating the superior performance of our fabrication. For comparison with commercially available glass lenses we also fabricate spherical half-ball lenses of different sizes. The imaging quality of the lenses is very similar, underpinning the powerfulness of our fabrication strategy.

Fused silica glass is a commonly used high-performance material in scientific and industrial applications, due to the exceptional optical, mechanical and thermal properties. However, its production can be challenging and expensive due to the high processing temperatures required, in both manufacturing and geometrical structuring. In this work we have studied additive manufacturing of transparent fused silica glass using the laser cladding process. Here a CO₂-laser is used to locally melt the glass, while injecting a stream of glass powder into the hot-zone. A challenge specifically addressed in this work is the shadowing effect, i.e., when the injected powder interacts with the laser beam resulting in non-stable heating dynamics, and partial sintering of powder prior to reaching the substrate surface. To reduce these effects, we have studied the use of sub-micron sized glass powders in order to minimize the laser beam interactions, both absorption and scattering. Using fumed silica powder injected via a single, off-axis nozzle, combined with additional powder cone shaping gas, transparent silica glass has been fabricated with an achieved deposition efficiency of up to 30 %. Typical, single deposition tracks have a width of approximately 850 μm with single layer heights of up to 150 μm.

There are several techniques for 3D printing glass by sequentially fusing molten tracks. We investigate a process feeding cool glass filament into a CO2 laser to provide local heating. Unlike most crystalline materials, glasses retain significant viscosity when molten. In filament-fed laser heated processing the feed exerts a significant stress on the laser heated region which strongly influences on final track geometry. This introduces challenges but also allows the creation of fully dense glass volumes and free-standing structures. The stress field on the molten region is controlled by using pneumatics and orienting the feed in the moving deposition coordinate system.

Direct laser writing (DLW) based on the femtosecond (fs) pulse-induced light-matter interaction expanded considerably during the last decades. The key advantage of using fs lasers for DLW is the possibility to exploit various nonlinear light-matter interaction regimes as well as control the thermal aspect of the process. This work is dedicated to exploring the capabilities of expanding DLW in several possible biomedical application areas where fs lasers could yield a very attractive, high throughput solution. Namely, we will be discussing how hybrid additive-subtractive DLW can be exploited for the high-throughput fabrication of integrated microfluidic systems. Furthermore, a mechanical flexible scaffold will be presented. Finally, a possibility to produce very high precision metalized 3D structures by using pre-existing high-throughput multi-photon polymerization capabilities will be shown. In all cases, attention will be placed on the unique capabilities of fs-lasers in DLW as well as practical considerations of the processes and their up-scaling.

The glass was proven to be a material of choice in many science and engineering fields. While various glass processing techniques exist, most of them are very complicated or completely unable to produce 3D high-fidelity (~μm) structures out of glass. It limits the adoption of 3D glass structures in many areas. One of the most promising technology to produce 3D glass structures is selective laser etching (SLE). Potentially, many types of glasses and crystals can be processed in this way. Nevertheless, this process is not exploited widely. The problems lie in the complex nature of light-matter interactions needed to induce modifications and challenges in optimizing the technique for true 3D fabrication. Laser parameters, translation velocity, etching properties are all important factors that cannot be disregarded. Overall, while the premise of SLE is simple, so far realization was proven to be rather complicated. Thus, this work aims to discover ways to simplify the production methodology of SLE while still maintaining sufficiently low surface roughness of a few hundred of nanometers and the possibility to acquire true 3D structures. A compromise between various fabrication parameters is found which allows meeting these criteria. Non-intuitive dependencies on scanning parameters are obtained which shows that not only radiation and material interaction are important, but also scanning techniques themselves change inscribed modifications. Optimized techniques are then used to manufacture complex high surface quality structures such as microfluidic systems and assembly-free movable micromechanical structures proving that this optimization can be used for functional device fabrication.

Nonimaging optical concentrators have applications spanning from solar concentration to optical wireless, as well as collimation for LEDs. Using femtosecond laser irradiation followed by chemical etching (FLICE), the fabrication of millimeter-length concentrators from fused silica substrates with output diameters on the order of a hundred microns is demonstrated. The design of dielectric total internal reflection concentrators (DTIRC) is discussed, and ray trace simulations are provided that compare linear taper (LT) and CPC theta1-theta2 concentrator designs for two use cases: 1) as the primary concentrator and 2) as a secondary concentrator. When used with a spherical lens, an overall geometric concentration ratio of 1300 with 1.1 degree half acceptance angle can be achieved, per simulations. Compared to a ball lens with diameter around 1.5mm, widely used for coupling, a 3x larger acceptance angle can be achieved with a DTIRC of similar height.

Additive manufacturing (AM) and rapid prototyping process (RPP) have revolutionized the production of 3D objects in the last few decades. RPP has considerably increased the rate of production and the possibility of manufacturing prototypes in the fields of electrical, optical, and mechanical engineering. The manufacturing of optical prototypes including spherical, aspheric, and special kinds of lenses and lens arrays has reformed the fabrication of optical components. In this paper, specifically designed lens array prototypes for application in visible light communication (VLC) are introduced. These lens array prototypes are manufactured using the stereolithography apparatus (SLA) process. These lens arrays are designed to achieve optimal transmission of the light beam for VLC systems. One of the prototypes from the lens arrays contains primarily four spherical lenses and one thicker convex lens and the other contains one fresnel lens as a substitute for thicker convex lens. These lens arrays are further post-processed to achieve the required transparency. These lens array prototypes are tested using laser and LEDs. The ON-OFF keying modulated light beam was transmitted through the lens array at the sender side and focused on the photo-receiver using another lens array at the receiver side which is 200 cm apart. After evaluating these lens prototypes, it can be concluded that with appropriate post-processing and high-resolution stereolithography based manufacturing, a low data rate VLC link can be formed.

It is challenging for stereolithography systems to print submicron features without two-photon lasers. For the first time, we implement an HD-DVD optical pickup unit (OPU) for building a customized stereolithography 3D printer. The OPU equips a 405 nm single-photon laser and an objective lens with a numerical aperture of 0.65. This has a focal laser spot diameter of 430 nm (1/e²) and can thereby, achieve submicron scale features photopolymerization. Moreover, the OPU embeds astigmatic optical path and voice coil motor which can be used for closed-loop printing alignment and this increases printing stability significantly. The OPU 3D printing system integrates an XYZ linear stage, providing nanoscale positioning resolution and macroscale printing area (c.a. 50 X 50 X 25 mm). A commercial photo-resin is utilized for the assessment of the system performance. The OPU printer crosslinks structures ranging from tens of microns down to submicron scale by tuning the printing parameters (laser intensity, printing speed, and photo-resin thickness). After optimization of the system, the OPU printer achieved the highest printing resolution of 210 nm which is beyond conventional stereolithography systems. Furthermore, several microstructures have been printed for verifying multiple layer printing performance. In conclusion, the mass-produced, low-cost and compact size OPU can not only dramatically simplify the stereolithography 3D printer design, but also achieve submicron printing performance.

In order to solve the problem of composite detection for laser welded sample, a laser opto-ultrasonic dual (LOUD) detection, which combined with laser-induced breakdown spectroscopy and laser ultrasonic technology, was used to obtain both elemental composition and defect information simultaneously. In this study, we gathered the atomic emission spectroscopy and ultrasonic time-domain signals from the laser welded sample to analysis the composition and defect information. On this basis, a 40 × 15 mm² LOUD detection scanning was finished to get a visual mapping of silicon element distribution and defect C-scan detection for 2-mm internal hole, respectively. The results showed that the relative errors of element and defect detection with conventional method were 6.00 % and 3.41 %, respectively. We demonstrate that LOUD detection has great potential for laser 3D manufacturing, which makes the detection more efficient.