Ultrashort Pulse Laser System Research Articles

Influence of Laser Treatment of Ti6Al4V on the Behavior of Biological Cells.

This article explores the enhancement of material surface properties of Ti6Al4V, potentially applicable to dental implants, through ultra-short pulse laser systems. This study investigates potential connections between surface wettability and biocompatibility, addressing the challenge of improving variability in material properties with specific laser treatment. Several designed microstructures were manufactured using a picosecond laser system. After that, the wettability of these structures was measured using the sessile drop method. The basic behavior and growth activity of biological cells (MG-63 cell line) on treated surfaces were also analyzed. While the conducted tests did not conclusively establish correlations between wettability and biocompatibility, the results indicated that laser treatment of Ti6Al4V could effectively enlarge the active surface to better biological cell colonization and adhesion and provide a focused moving orientation. This outcome suggests the potential application of laser treatment in producing special dental implants to mitigate the issues during and following implantation.

Laser assisted Powder Bed Fusion of pure Molybdenum using ultrashort laser pulses

Refractory metals are used in applications requiring extreme high temperature resistance, e.g. jet propulsion systems in aerospace applications or for controlling nuclear reactions. However, the traditional manufacturing of these metals is demanding and expensive, especially if fine structures are required. Here, we report on the laser assisted powder bed fusion of pure Molybdenum. The high melting point and thermal conductivity together with high susceptibility to solidification fractures of Molybdenum make it difficult to identify a suitable parameter window for the laser based melting process. Thus, we apply an ultrashort pulse laser system operating at a center wavelength of 1030 nm and a repetition rate of 32 MHz with a pulse width of 270 fs for the additive processing. Using the additional parameter field provided by the pulsed laser source we are able to fabricate thin-walled structures well below 100 µm as well as volumetric parts with a relative density close to 100 %.

(Invited) Wafer Singulation - Laser Processing Combined: From Past to Future Hybrid Bonding

Wafer singulation in the semiconductor industry has transformed from single step, straight forward diamond blade cutting of the product wafer into a process flow which may involve several different steps in different orders [1]. Nowadays, lasers play a central role in these schemes, involving ablation-based dicing or grooving, cold-ablation regimes and sub-surface material modification methods.Over the years, laser singulation and grooving has proven to provide solutions for different substrate materials and front- or backside layers [2]. Obviously, central topics for a production environment are the reliability and cost. Initial limitations on the application of the technique, such as a lowered material die strength, a large heat affect zone (HAZ) alongside the kerf, sidewall contamination and high burr along the surface edge of the cut have been partially or totally overcome. For example, for RFIC devices based on GaAs, multibeam laser singulation was the enabler for thinner substrates and higher production yields, and the elimination of process steps when using full metal wafer backside without etched streets cutting costs. In the production flow, it has been combined with wet etch processing steps to increase material die strength and remove burr. Similarly, such advantages can be exploited when combining multibeam laser processing of Si-based substrates with remote plasma etching schemes [3].To tackle the challenges of today and tomorrow in terms of wafer stack build and advanced packaging technologies, we are looking at innovative approaches and similar combinations of process steps. The introduction of ultra-short pulsed (USP) laser systems for wafer grooving has shown an important qualitative step forward. Due to the different temporal regime compared to nanosecond pulsed lasers, a different material removal mechanism is active. This results in higher die strengths, lower burr and better edge quality, matching the singulation requirements of different semiconductor market segments. For die-to-wafer hybrid bonding processing, burr free die edges and high die strength are essential. Here, the qualitative edge of USP multibeam laser grooving in removal of metals, polymers and dielectrics from the dicing street make a combination with plasma dicing a strong proposition. In plasma dicing, different plasma etch steps can be applied for the full or partial singulation of the wafer [4]. Typically, the bulk silicon is removed with the so-called Bosch-process. As mask removal and opening of the dicing street are essential to this, it implies that pairing USP grooving and plasma dicing can provide the decisive advantage for future advanced packaging approaches.The latest results in a range of applications of laser-based singulation of wafers will be presented with or without the combination of other process steps.Keywords: Laser singulation, semiconductor, etching, plasma dicing, hybrid bonding. [1] W.-S. Lei, A. Kumar, and R. Yalamanchili, Die singulation technologies for advanced packaging: A critical review, Journal of Vacuum Science and Technology B, vol. 30, no. 4, p. 040801, 2012.[2] M. R. Mark, A review of laser ablation and dicing of Si wafers, Precision Engineering, 73 377–408, 2022.[3] R. Evertsen, N. Beckers, S. Wang and R. Van der Stam, Remote Plasma Etching of Backend Semiconductor Materials for Reliable Packaging, Solid State Phenomena , 1662-9779, Vol. 314, 312-317.[4] R. Barnett; O. Ansell and D. Thomas, Considerations and benefits of plasma etch based wafer dicing, Electronics, Packaging Technology Conference (EPTC 2013), 2013 IEEE 15th 569–574.

Modular Direct Laser Writing setup for high precision nanostructuring

Abstract The increasing demand for micro- and nanofabrication and in parallel the increasing requirements on feature size and resolution is leading to an enormous growth in the field of multi-photon three-dimensional fabrication. To enable new and diverse investigations in this field and to enable high precision for nanofabrication on large areas, a high precision positioning system is combined with an ultra-short pulse laser system. The aim is a modular setup with constant adherence to the Abbe-comparator principle in order to achieve systematic improvements in the area of Direct Laser Writing. For a high-quality identification of the microstructures a measurement tool based on atomic force microscopy is used. To enable the fabrication of continuous micro- and nanostructures on large area, an extremely high positioning precision is used, where no further stitching methods are necessary. Therefore as base of the Direct Laser Writing system the nanopositioning and nanomeasuring machine (NMM-1) is used, which was developed at Technische Universität Ilmenau together with SIOS Meßtechnik GmbH, with a positioning volume of 25 mm × 25 mm × 5 mm and a positioning resolution in the sub-nanometer range. First investigations already confirmed that microfabrication with a Femtosecond Laser and the NMM-1 could be realized and showed the possibility of further developments in the field of Direct Laser Writing. Now the modular structure as a research platform is designed in such a way that the various extensions and measurement setups for large-scale investigations can always be implemented in a metrologically traceable manner. The presented work shows the development of a modular functional setup of an exposure system and NMM-1, which enables micro- and nanofabrication and an improvement in the structure size over large areas.

A simple intra-beam alignment setup for tiled grating assembly based laser pulse compressor of high energy ultrashort pulse laser systems

A simple intra-beam alignment setup for tiled grating assembly based laser pulse compressor of high energy ultrashort pulse laser systems

Advanced micro- & nanostructuring for enhanced biocompatibility of stainless steel by multi-beam and beamshaping technology

Biocompatibility is one of the key issues for implants, especially in the case of stainless steel with medium to low biocompatibility, which may lead to a lack of osseointegration and consequently to implant failure or rejection. To precisely control preferential cell growth sites and, consequently, the biocompatibility of prosthetic devices, two types of surfaces were analyzed, containing periodic nanogrooves laser induced periodic surface structure (LIPSS) and square-shaped micropillars. For the fast and efficient production of these surfaces, the unique combination of high energy ultrashort pulsed laser system with multi-beam and beamshaping technology was applied, resulting in increased productivity by 526% for micropillars and 14 570% for LIPSS compared to single beam methods. In vitro analysis revealed that micro and nanostructured surfaces provide a better environment for cell attachment and proliferation compared to untreated ones, showing an increase of up to 496% in the number of cells compared to the reference. Moreover, the combination of LIPSS and micropillars resulted in a precise cell orientation along the periodic microgroove pattern. The combination of these results demonstrates the possibility of mass production of functionalized implants with control over cell organization and growth. Thus, reducing the risk of implant failure due to low biocompatibility.

Anesthetic-, irrigation- and pain-free dentistry? The case for a femtosecond laser enabled intraoral robotic device

By leveraging ultrashort pulse laser and micro-electromechanical systems (MEMS) technologies, we are developing a miniaturized intraoral dental robotic device that clamps onto teeth, is remotely controlled, and equipped with a focusing and scanning system to perform efficient, fast, and ultra-precise laser treatments of teeth and dental restorative materials. The device will be supported by a real-time monitoring system for visualization and diagnostic analysis with appropriate digital controls. It will liberate dentists from repetitive manual operations, physical strain and proximity to the patient's oro-pharyngal area that potentially contains infectious agents. The technology will provide patients with high-accuracy, minimally invasive and pain-free treatment. Unlike conventional lasers, femtosecond lasers can ablate all materials without generating heat, thus negating the need for water irrigation, allowing for a clear field of view, and lowering cross-infection hazards. Additionally, dentists can check, analyze, and perform precise cutting of tooth structure with automatic correction, reducing human error. Performing early-stage diagnosis and intervention remotely will be possible through units installed at schools, rural health centers and aged care facilities. Not only can the combination of femtosecond lasers, robotics and MEMS provide practical solutions to dentistry's enduring issues by allowing more precise, efficient, and predictable treatment, but it will also lead to improving the overall access to oral healthcare for communities at large.

Upscaling of the Laser Structuring of Lithium-Ion Battery Electrodes - Process Development and Electrochemical Properties As a Function of Design Patterns

Structuring of electrodes for lithium-ion batteries by using ultrashort pulsed laser ablation is a quite promising approach to significantly increase the electrochemical performance of lithium-ion batteries. The main impact of such 3D electrodes is due to shortening of lithium-ion diffusion pathways and improving the electrolyte wetting of composite electrodes The latter is reducing the electrolyte filling process time and saving the need for warm aging in the manufacturing process. The 3D electrode concept, in combination with an increase in electrode layer thickness, facilitates an increase in energy and power density of batteries at the same time. To implement the laser structuring process in industrial cell manufacturing, an adaptation of the ultrafast laser technology to the coating speed has to be established.In the presented study, the use of an ultrashort pulsed laser system of high power up to 300 W and high repetition rate in the MHz regime was evaluated regarding processing speed for patterning of graphite and silicon/graphite anodes. For composite graphite anodes (film thickness > 76 µm) with an areal capacity of about 2.5 – 4.0 mAh/cm2 and a thick-film silicon/graphite anode with 5 wt.% silicon content (film thickness 80 µm, areal capacity 4.3 mAh/cm2) the laser and process parameters and processing speed were evaluated and optimized. Furthermore, different structure patterns including hole, grid and line pattern, were studied regarding upscaling in roll-to-roll processing. The different types of patterns were evaluated with respect to accessible electrochemical properties. Prior to cell assembly and during post-mortem studies, the quality of structured electrodes was examined by profilometry and scanning electron microscopy. Debris formation and a thermal impact to active material and current collector could be avoided. To evaluate the electrochemical properties such as high rate capability, cell impedance, and cell lifetime, coin cells and pouch cells were manufactured. For this purpose, NMC 622 cathodes were prepared as the respective counter-electrodes and a cell balancing in the range of 1.1-1.2 was ensured.

Ultrafast cold-brewing of coffee by picosecond-pulsed laser extraction

Coffee is typically brewed by extracting roasted and milled beans with hot water, but alternative methods such as cold brewing became increasingly popular over the past years. Cold-brewed coffee is attributed to health benefits, fewer acids, and bitter substances. But the preparation of cold brew typically needs several hours or even days. To create a cold-brew coffee within a few minutes, we present an approach in which an ultrashort-pulsed laser system is applied at the brewing entity without heating the powder suspension in water, efficiently extracting caffeine and aromatic substances from the powder. Already 3 min irradiation at room temperature leads to a caffeine concentration of 25 mg caffeine per 100 ml, comparable to the concentrations achieved by traditional hot brewing methods but comes without heating the suspension. Furthermore, the liquid phase’s alkaloid content, analyzed by reversed-phase liquid chromatography coupled to high-resolution mass spectrometry, is dominated by caffeine and trigonelline and is comparable to traditional cold-brewed coffee rather than hot-brewed coffee. Furthermore, analyzing the head-space of the prepared coffee variants, using in-tube extraction dynamic head-space followed by gas chromatography coupled to mass spectrometry, gives evidence that the lack of heating leads to the preservation of more (semi-)volatile substances like pyridine, which provide cold-brew coffee its unique taste. This pioneering study may give the impetus to investigate further the possibility of cold-brewing coffee, accelerated by more than one order of magnitude, using ultrafast laser systems.

Influence of spatiotemporal coupling distortion on evaluation of pulse-duration-charactrization and focused intensity of ultra-fast and ultra-intensity laser

In a large-scale ultra-fast and ultra-intensity laser system, with the increase of spectral bandwidth and beam aperture, the spatiotemporal coupling distortion will become more and more significant. This effect will not only degrade the beam quality and reduce the focusing intensity of the laser, but also invalidate the conventional evaluation method for laser far-field parameters. A pair of beam-expanding lenses, which may bring spatiotemporal coupling distortion to an ultrashort laser pulse, is taken as an example. And the influence of spatiotemporal coupling distortion on laser parameter measurement is analyzed in detail. It shows that in an ultrashort pulse laser system, an ordinary beam-expanding lens-pair can reduce the far-field peak intensity dramatically, and the actual pulse duration in the far field is more than 10 times longer than that measured at the near field by a single-shot autocorrelator. In this case, the focusing intensity estimated by using the measured value of near-field pulse width will be one order of magnitude bigger than the real value. It is expected that the results will be helpful in the optimal design of a laser system, the accurate characterization of an ultrafast laser pulse and relevant physical experiments.

LIPSS-based functional surfaces produced by multi-beam nanostructuring with 2601 beams and real-time thermal processes measurement

A unique combination of the ultrashort high-energy pulsed laser system with exceptional beam quality and a novel Diffractive Optical Element (DOE) enables simultaneous production of 2601 spots organized in the square-shaped 1 × 1 mm matrix in less than 0.01 ms. By adjusting the laser and processing parameters each spot can contain Laser Induced Periodic Surface Structures (LIPSS, ripples), including high-spatial frequency LIPSS (HFSL) and low-spatial frequency LIPSS (LSFL). DOE placed before galvanometric scanner allows easy integration and stitching of the pattern over larger areas. In addition, the LIPSS formation was monitored for the first time using fast infrared radiometry for verification of real-time quality control possibilities. During the LIPSS fabrication, solidification plateaus were observed after each laser pulse, which enables process control by monitoring heat accumulation or plateau length using a new signal derivation approach. Analysis of solidification plateaus after each laser pulse enabled dynamic calibration of the measurement. Heat accumulation temperatures from 200 to 1000 °C were observed from measurement and compared to the theoretical model. The temperature measurements revealed interesting changes in the physics of the laser ablation process. Moreover, the highest throughput on the area of 40 × 40 mm reached 1910 cm2/min, which is the highest demonstrated throughput of LIPSS nanostructuring, to the best of our knowledge. Thus, showing great potential for the efficient production of LIPSS-based functional surfaces which can be used to improve surface mechanical, biological or optical properties.

Thermal imaging of high power ultrashort pulse laser ablation of alumina towards temperature optimized micro machining strategies

The application of pulsed laser systems with pulse durations in the pico- and femtosecond regime for material processing is commonly associated with a cold ablation. Due to the minimized interaction-time between the ultrashort laser pulses and the material, this statement is almost valid as long as no heat accumulation effect appears. With the increasing demand of high productivity processes, the average power of ultrashort pulsed laser systems increases above 100 W, which leads, however, to increased thermal effects during laser processing. This is especially important for laser processing of technical ceramics like alumina. Large temperatures gradients, which locally occur during laser processing using high average power could lead to thermal modifications and cracks in the material. In this study, we present a process-optimization method for high power laser ablation of alumina based on thermal imaging. The use of a 2D IR camera enables the estimation of the temperature distribution during the laser processing. We investigate the influence of laser power up to 80 W, pulse duration between 900 fs and 10 ps and processing duration on the resulting material temperature. Beside the material temperature we evaluate the material removal rate and the resulting surface quality.

Study on X-ray Emission Using Ultrashort Pulsed Lasers in Materials Processing.

The interaction of ultrashort pulsed laser radiation with intensities of 1013 W cm−2 and above with materials often results in an unexpected high X-ray photon flux. It has been shown so far, on the one hand, that X-ray photon emissions increase proportionally with higher laser power and the accumulated X-ray dose rates can cause serious health risks for the laser operators. On the other hand, there is clear evidence that little variations of the operational conditions can considerably affect the spectral X-ray photon flux and X-ray emissions dose. In order to enhance the knowledge in this field, four ultrashort pulse laser systems for providing different complementary beam characteristics were employed in this study on laser-induced X-ray emissions, including peak intensities between 8 × 1012 W∙cm−2 < I0 < 5.2 × 1016 W∙cm−2, up to 72.2 W average laser power as well as burst/bi-burst processing mode. By the example of AISI 304 stainless steel, it was verified that X-ray emission dose rates as high as > 45 mSv h−1 can be produced when low-intensity ultrashort pulses irradiate at a small 1 µm intra-line pulse distance during laser beam scanning and megahertz pulse repetition frequencies. For burst and bi-burst pulses, the second intra-burst pulse was found to significantly enhance the X-ray emission potentially induced by laser pulse and plasma interaction.

Sub-Picosecond Micromachining of Monocrystalline Silicon for Solar Cell Manufacturing

In this study a prototype sub-picosecond laser was investigated for cutting and scribing of silicon wafers. The Yb:KYW laser used for this investigation, unlike ultrashort systems used previously, generates pulses of 650 fs, i.e., between the pico and femtosecond range. The laser was placed in a micromachining setup, involving a galvo scanner and a telecentric lens. A study of the influence of the processing parameters on the crater width, depth, and quality of machining was carried out. The optimal parameters were found to be 343 nm, 200 kHz, 7 mm/s, and 15 pattern repetitions. The experiments were performed using samples of a silicon wafer of 210-µm thickness. The experimental results show that the sub-picosecond laser can be a promising and competitive tool for solar cell micromachining. In comparison to the commercially available ultrashort pulse laser systems, we find the sub-picosecond laser to be a more cost efficient and reliable source, than a femtosecond one. In addition, the prototype Yb:KYW design offers some unique parameters, such as repetition rate in the range of 100–400 kHz, UV wavelength or obtainable laser fluence close to the silicon ablation thresholds.

Characterization of Accumulated B-Integral of Regenerative Amplifier Based CPA Systems

We report on a new approach to measure the accumulated B-integral in the regenerative and multipass amplifier stages of ultrashort-pulse high-power laser systems by B-integral-induced coupling between delayed test post-pulses and the main pulse. A numerical model for such non-linear pulse coupling is presented and compared to data taken at the high-power laser Draco with self-referenced spectral interferometry (SRSI). The dependence of the B-integral accumulated in the regenerative amplifier on its operation mode enables optimization strategies for extracted energy vs. collected B-integral. The technique presented here can, in principle, be applied to characterize any type of ultrashort pulse laser system and is essential for pre-pulse reduction.

Fabrication and evaluation of negative axicons for ultrashort pulsed laser applications

We report on the fabrication and evaluation of a sharp tip negative axicon paving the way for applications in high-power ultrashort pulsed laser systems. The negative axicon is manufactured by applying a two-step all laser-based process chain consisting of ultrashort pulsed laser ablation and CO2 laser polishing finishing the component in less than 5 minutes. The finalized negative axicon reveals a surface roughness of 18 nm, fulfilling optical quality. Two measurement setups, including the ultrashort pulsed laser itself, are used to evaluate the formation of Bessel beams in detail. By applying a focusing lens behind the negative axicon, well-developed Bessel beams are generated while their lengths depend on the distance between the negative axicon and the lens. Furthermore, the diameter of the Bessel beams increase strongly with the propagation distance. By adding a second focusing lens, Bessel beams are generated at its focal position, being almost invariant of its position. Hence, the typical Bessel beam intensity distribution is observed over an entire moving range of this second lens of 300 mm. While these Bessel beams show superior quality in terms of sharp peaks with homogeneous concentric rings, only minor deviations in intensity and diameter are observed over the moving range.

A compact, highly stable spectral shearing interferometer to accurately reconstruct ultrafast laser fields

This paper presents a compact design for spectral shearing interferometry to reconstruct an ultrafast electric-field (SPIDER) with a high signal-to-noise ratio (SNR) and stability. Compared experimentally with the traditional SPIDER based on a Michelson interferometer (MI), this design can improve the SNR by a factor of 3.7 in a 25 dB BW region. Using the material dispersions of 6 mm BK7 plates as a target, the phase errors between the measured and theoretical values are less than 2.6 mrad, which is improved by a factor of ~4 from 10.3 mrad measured with the MI-based version. Even for a 1 mm fused silica plate, the measured phase errors are less than 2.1 mrad from the calculated values. Using this design for a 3-day test of our ultrashort pulse laser system, the variation in the output pulse width from the first to the third day is only 1.5%. We attribute this excellent performance to the common-path and all-transmission design of the test pulse pair, giving the design superior capacity against external disturbances. Our design can also work with the two-step phase-shift (TSPS) method to improve its measurement of the plulses with complex temporal/spectral structures.

Study on nonthermal–thermal processing boundary in drilling of ceramics using ultrashort pulse laser system with variable parameters over a wide range

Quality and throughput of laser material processing are influenced by multiple laser parameters as well as material properties. Since material degradation due to laser processing is generally related to heat-affected zone, quick optimization and realization of nonthermal process is one of the most important issues for higher-grade laser processing. In this work, we developed various parameter-controlled ultrashort pulse laser system that can control various laser parameters over a wide range, such as pulse duration over three orders of magnitude. Using this laser system (wavelength: 1033 nm, pulse duration: 0.4 to 400 ps, repetition: up to 1 MHz), we have investigated nonthermal–thermal processing boundary in drilling of ceramics: aluminum nitride (AlN) and yttria-stabilized zirconia (YSZ), with high and low thermal conductivity, respectively. As a result, the AlN ceramic exhibited strong dependence of ablation rate on fluence: low-, mid- and high-fluence regimes appeared with different logarithmic correlations. Further, the nonthermal–thermal process boundaries appeared, depending on the pulse duration. On the other hand, the ablation behavior of YSZ was much different from AlN; the ablation rate did not show distinct three regimes. Further, there were no nonthermal process windows for the YSZ. Therefore, the nonthermal–thermal process boundaries were governed by the laser parameters like pulse duration as well as the material thermal conductivity.

Nonlinear Spectrum Compression of Negatively Chirped Picosecond Pulses in Dispersion-Shifted Telecom Fiber

We investigated both experimentally and numerically self-phase modulation-induced spectrum compression dynamics of negatively chirped picosecond Gaussian pulses in the low-loss dispersion-shifted telecom fiber near its zero dispersion wavelength (ZDW). It was experimentally observed linearly interpolated monotonous increase in spectrum compression ratio (SCR) with negative chirp growth of the incident pulse in the range from −5.9 to −91.9. According to numerical simulations, despite its small values in the vicinity of ZDW, fiber dispersion limits both SCR growth and spectrum compression quality expressed in terms of time-bandwidth product value and energy confinement ratio at larger chirp values (|C| > 100), in a combined action with self-phase modulation (SPM). Having optimized fiber length and pulse energy, we achieved record 49.2-fold spectrum compression of 38.3-ps, 2.08 nJ negatively chirped Gaussian pulses with compressed spectrum FWHM of 0.23 nm and a50% energy confinement at 1560 nm wavelength, leading to a13.3-times spectral brightness magnification. We believe that results obtained can be promising for ultra-short pulse laser systems development with enhanced spectral brightness.

Discovery of Several New Families of Saturable Absorbers for Ultrashort Pulsed Laser Systems

Saturable Absorber (SA) is a key element of any passive mode-locked laser system to provide ultrashort laser system. So far various materials have been proposed that could be used for this purpose. However, the field is still looking for new ways to make the fabrication process easier and cost-effective. Another challenge in testing mode-locked laser systems using various SA samples is the lack of knowledge in preparing these by laser physicists given this is outside their remit of expertise. In this study, we have proposed a novel method to produce these SAs from plastic materials and glycol. Our new method relies upon increase in thickness up to a value where the modulation depth is enough to give stable ultrashort pulses. Although we have shown this method for four materials; similar approach could be applied to any material. This will open the door of unlimited families of SAs that could be easily prepared and applied without any prior knowledge in material sciences.