Laser Micromachining Research Articles

Development of a multi-sensor system for in-situ process monitoring of femtosecond laser micromachining

Development of a multi-sensor system for in-situ process monitoring of femtosecond laser micromachining

Geometrical modeling of ultrashort pulse laser ablation with redeposition and analysis of the influence of spot size and shape

Ultrashort-pulse lasers are an established technology for micromachining and have a great future. A major challenge remains the search for suitable laser parameters and a scanning strategy. To date, this has been achieved mainly by conducting experiments and measuring the resulting surfaces. The use of modeling methods for this purpose is not yet practical for industrial processing. In this work, the recently introduced geometric modeling method for pulsed laser ablation is applied to ultrashort laser ablation and extended to consider the power dependence of the process footprint and the pointing stability during ablation. This work highlights this modeling method to cover a wide range of process parameters in a time-saving manner and to simulate laser micromachining in real time. Furthermore, the influence of the processing height compared to the focal height on oxygen-free copper and steel 1.4034 was investigated. For both the ablation depth is decreased while simultaneously increasing the ablation width. For oxygen-free copper, the ablated volume is insensitive to changes in the processing height, which is not the case for steel 1.4034. The results also show the influence of the pulse repetition rate on the change in the shape of material removal and its influence on process stability.

Flow analysis and fabrication of micro scale controlled surfaces by ultrashort pulse laser for microfluidic device applications

Microfluidics finds a wide range of industrial applications mainly in fields such as biomedical, clinical, electrical, and thermal engineering. Microchannels are the building blocks of most microfluidic devices. Thus, it becomes evident to understand the properties and the effect of the surfaces on the outcomes of devices. In this study, the effect of the shape of microchannels and the roughness elements on the flow is evaluated. Microchannels with three cross-sections and roughness elements of three different geometries are considered. The effect of varying Reynolds numbers on the flow parameters such as Friction factor, Nusselt number, Poiseuille number, pressure drop and surface temperature at the base of the substrate is studied. It is observed that rough microchannels and rectangular roughness elements showed a higher Nusselt number than smooth microchannels. The rough microchannels with triangular roughness elements showed higher friction factors. The surface temperature is higher for the smooth microchannels with triangular roughness elements. Furthermore, to demonstrate the manufacturability of microfluidic channels with controlled surfaces and to validate the preliminary results of the numerical simulations, the ultrashort pulse laser micromachining technique is used to fabricate the microchannels with structures on Polymethyl Methacrylate (PMMA) material.

Elastic Modulus Measurement at High Temperatures for Miniature Ceramic Samples Using Laser Micro-Machining and Thermal Mechanical Analyzer.

In this paper, we demonstrate a method of measuring the flexural elastic modulus of ceramics at an intermediate (~millimeter) scale at high temperatures. We used a picosecond laser to precisely cut microbeams from the location of interest in a bulk ceramic. They had a cross-section of approximately 100 μm × 300 μm and a length of ~1 cm. They were then tested in a thermal mechanical analyzer at room temperature, 500 °C, 800 °C, and 1100 °C using the four-point flexural testing method. We compared the elastic moduli of high-purity Al2O3 and AlN measured by our method with the reported values in the literature and found that the difference was less than 5% for both materials. This paper provides a new and accurate method of characterizing the high-temperature elastic modulus of miniature samples extracted from representative/selected areas of bulk materials.

Effect of the Surface Peak-Valley Features on Droplet Impact Dynamics under Leidenfrost Temperature.

This study explores the kinetic behavior of droplets impacting microtextured surfaces under a Leidenfrost temperature, employing high-speed photography and picosecond laser micromachining techniques. The investigation focuses on two types of microtextured surfaces with totally different surface peak-valley features: a negatively skewed surface with micropit arrays (Ssk < 0) and a positively skewed surface with micropillar arrays (Ssk > 0). The results indicate that both microtextured surfaces contribute to a higher Leidenfrost temperature compared with the original smooth surface, which is consistent with previous studies. However, it is worth noting that the Leidenfrost points of the micropit and micropillar surfaces showed opposite trends with the microtexture area occupancy. Specifically, the Leidenfrost temperature on micropit surfaces increases with greater micropit area occupancy, while it decreases on micropillar surfaces under similar conditions, which is mainly attributed to the differential impact of area occupancy on droplet heat transfer efficiency. When the microtexture area occupancy is 50%, it is worth noting that the micropit and micropillar surfaces have nearly same roughness (Sa), but the Leidenfrost temperature was notably higher on the micropit surface with negative skewness (Ssk < 0), which was related to differences in vapor flow dynamics. We further find that the Weber number (We) significantly influences the Leidenfrost point, with the droplet impact wall behavior going through the states of film bounce back, ejecting tiny droplets and bounce back, and ultimately droplet breakup as the We increases. The dynamic Leidenfrost point was found to be generally higher than the static point and increases with the We. Finally, we compare the cooling efficiency of these surfaces, and it is found that the micropit surfaces with a negative skewness exhibit superior heat dissipation performance under the same conditions, which proved that the negatively skewed surface may have great potential in high-density heat dissipation technology.

Inscription of Complex Stent Profiles on the Polished Surfaces of NiTiCo Shape Memory Alloys by Fiber Lasers: Examining the Impact of Diffuse Reflectance

Drug eluting stents (DES) have become essential to alleviate stenosis with minimally invasive procedures. Nickel-titanium (NiTi) based shape memory alloys (SMAs) are the most suitable materials for self-expanding stents due to their good stability and superior thermo-mechanical properties. This work explains the inscription of complex DES profiles by fiber laser micro-cutting (FLMC) on Nickel-Titanium-Cobalt (NiTiCo) SMA surfaces polished with various grits (1000P, 1500P, 2000P, 2500P, 3000P, and cloth polished) of abrasive sheets. The impact of the SMA surface diffuse reflectance on laser micromachining characteristics was investigated. The Fuzzy PROMETHEE II multi-criteria decision-making (FP II-MCDM) technique was employed to find the best-polished surface that provides the most desired responses (kerf quality, surface roughness, and depth of penetration) for clinical applications. The cloth-polished surface responds better in laser machining without compromising the kerf quality and surface roughness. A significant variation of 18% in kerf quality was found between the 3000P and the cloth-polished surfaces. This indicates that the surface diffuse reflectance has maximum impact when the surfaces are highly polished. For applications where surface roughness and kerf width are not a criterion, then polished surfaces between 1500P and 2000P give the maximum machining rate.

Self-healing Bessel-Gaussian beam generation based on multimode interference effect

Two typed Bessel-Gaussian beams with good non-diffraction and self-healing properties have been generated by using tapered hollow tubes with double layers. The evolution process of light propagation in the tapered hollow tube cavity is studied theoretically. The theoretically analysis and simulation results show that the Bessel-Gaussian beam generation is due to the multimode interference. Two typed Bessel-Gaussian beams are generated by changing the inner layer thickness of such hollow tubes, which both experiences autofocusing at the output port and evolve into Bessel-Gaussian beams. The effect of single and multiple silica particles on their self-healing performance has also been analyzed, which has a good property for ultrafast laser micromachining.

Femtosecond laser processing of amorphous silicon films

Femtosecond laser micromachining is a pivotal approach within the dynamic field of laser surface structuring, offering unparalleled precision for materials processing. By exploiting the unique nonlinear light-matter interactions, this technique allows the creation of functional micro- and nanostructures with exceptional accuracy and reproducibility. Of particular interest is its ability to process diverse materials, including glasses, polymers, metals, and semiconductors, with minimal thermal impact and collateral damage. Moreover, femtosecond laser micromachining eliminates the need for traditional lithographic methods, enabling intricate structure fabrication under ambient conditions, thereby revolutionizing microfabrication techniques. This study demonstrates the micromachining using femtosecond lasers at 800 nm wavelength and explores the critical role of process parameters, such as laser pulse energy and the number of pulses in determining the damage threshold fluence, especially considering incubation effects in multi-pulse scenarios. Furthermore, it explores the potential of femtosecond laser micromachining for controlled crystallization in hydrogenated amorphous silicon (a-Si:H), unleashing enhanced electrical and optical properties. With its versatility, precision, and potential for diverse applications, femtosecond laser micromachining holds promise for driving advancements in electronic and photonic devices, particularly in high-efficiency photovoltaics, integrated photonics, and novel optoelectronic technologies.

Optical fiber integrated WGM cylindrical cavity resonator.

Whispering gallery mode (WGM) resonators are usually discrete optical devices, which have integration difficulties with an optical fiber system. Here we present a new, to the best of our knowledge, type of optical fiber whispering gallery mode resonator based on a cylindrical cavity, which is located in the multimode fiber core and fabricated by femtosecond laser micromachining together with fast hydrofluoric acid etching techniques. When light traveling in the fiber core is tangent to the cylindrical cavity wall, it is coupled into the cavity and circulates along the cavity wall to excite WGM resonance before being coupled out to the same tangential path and continuing propagation in the fiber core. The device is fully integrated into the optical fiber, simple in fabrication, convenient in operation, low in cost, and has a good quality factor (Q) of 1.06 × 104. The device enriches the family of WGM resonators and is expected to have promising applications in photonics.

Optical Fiber Probe with Integrated Micro-Optical Filter for Raman and Surface-Enhanced Raman Scattering Sensing.

Optical fiber Raman and surface-enhanced Raman scattering (SERS) probes hold great promise for in vivo biosensing and in situ monitoring of hostile environments. However, the silica Raman scattering background generated within the optical fiber increases in proportion to the length of the fiber, and it can swamp the signal from the target analyte. While filtering can be applied at the distal end of the fiber, the use of bulk optical elements has limited probe miniaturization to a diameter of 600 µm, which in turn limits the potential applications. To overcome this limitation, femtosecond laser micromachining was used to fabricate a prototype micro-optical filter, which was directly integrated on the tip of a 125 µm diameter double-clad fiber (DCF) probe. The outer surface of the microfilter was further modified with a nanostructured, SERS-active, plasmonic film that was used to demonstrate proof-of-concept performance with thiophenol as a test analyte. With further optimization of the associated spectroscopic system, this ultra-compact microprobe shows great promise for Raman and SERS optical fiber sensing.

Nanosecond laser micromachining of graphene nanoplatelets reinforced ZK60 matrix composites

AbstractIn this paper, a nanosecond laser was used to etch the surface of graphene nanoplatelets reinforced ZK60 (GNPs/ZK60) matrix composites, and the effects of different scanning speeds, pulse repetition frequency and laser power on the etched morphology of the machined surface were investigated. The experimental results show that the etched groove width and heat affected zone width of GNPs/ZK60 composites are significantly increased compared with ZK60 material due to the influence of graphene, and the growth rate of the difference in heat affected zone width between GNPs/ZK60 composites and ZK60 materials is most significantly affected by the pulse repetition frequency. In addition, the dross height increases with the increase of laser power, while the scaly dross structure becomes increasingly clear.

Revealing Subsurface Damage Morphology and Patterns in areal Ultrashort Pulse Laser Machining of Glass

Material removal rates as well as surface and subsurface quality are key aspects for the industrial application of ultrashort pulse (USP) laser machining. However, revealing so-called subsurface damage (SSD) is challenging. The presented study visualizes and quantifies subsurface damage patterns in areal USP laser ablation of fused silica (FS) and glass N-BK7 (BK). For the first time, using high-resolution optical coherence tomography (OCT) as non-destructive and three-dimensional (3D) evaluation method, SSD morphologies of areal laser machining induced damages are analysed. Influences of laser wavelength, beam geometry and processed material are investigated. Discovered differences of damage morphologies and depth in FS and BK point out the relevance of selecting suitable process parameters. Based on the evaluation of volumetric OCT data, the authors were able to quantify damage morphologies using the surface texture ratio as well as power spectral density functions. One important finding for the quantification and comparability of damage depths in USP laser processing is the influence of applicable evaluation thresholds. In comparison to area thresholds of 0.001% being applicable to OCT measurements, more lenient thresholds of e.g. 1% commonly applied in destructive SSD measurement methods in average result in a reduction of measured damage depths by a factor of ~ 2. This potentially leads to an underestimation of damage depths depending on methods on thresholds used. The presented measurement and evaluation methods as well as gained process insights are important assets for the future optimization of low-damage USP laser micromachining of brittle materials. Moreover, the general applicability and relevance of OCT-based morphological damage analysis in laser material processing is shown.

Flexible Thermoelectric Wearable Architecture for Wireless Continuous Physiological Monitoring.

Continuous monitoring of physiological signals from the human body is critical in health monitoring, disease diagnosis, and therapeutics. Despite the needs, the existing wearable medical devices rely on either bulky wired systems or battery-powered devices needing frequent recharging. Here, we introduce a wearable, self-powered, thermoelectric flexible system architecture for wireless portable monitoring of physiological signals without recharging batteries. This system harvests an exceptionally high open circuit voltage of 175-180 mV from the human body, powering the wireless wearable bioelectronics to detect electrophysiological signals on the skin continuously. The thermoelectric system shows long-term stability in performance for 7 days with stable power management. Integrating screen printing, laser micromachining, and soft packaging technologies enables a multilayered, soft, wearable device to be mounted on any body part. The demonstration of the self-sustainable wearable system for detecting electromyograms and electrocardiograms captures the potential of the platform technology to offer various opportunities for continuous monitoring of biosignals, remote health monitoring, and automated disease diagnosis.

Investigating the Effects of Lubricant Infusion Methods on Polymer SLIPS.

Slippery lubricant infused porous surfaces (SLIPSs) are promising bioinspired surfaces with self-healing and droplet wetting properties, among many others, that are desirable due to their range of applications. Recently, there have been many developments in the SLIPS field regarding the creation of textured surfaces and lubricant selection. However, there is a lack of knowledge regarding the method of lubricant infusion. In this study, we aim to fill this void by investigating different infusion methods that impose external forces on the lubricant. We developed our SLIPS by hot embossing nanostructures onto polypropylene by using molds that were laser micromachined. These textured surfaces were then infused with silicone oil using three different infusion methods: ultrasonication, vacuum, and hydrostatic pressure. We analyzed the wettability and slipperiness of the SLIPS by evaluating the critical tilt angle and comparing the sliding velocities of water droplets on each sample at a tilt angle of 20°. Additionally, the durability of the SLIPS was tested by dropping 50 successive water drops onto the samples and evaluating the droplet-surface interactions throughout. The sonicated infusion method yielded SLIPS that performed the best with a contact angle hysteresis of 13°, a critical tilt angle of 18.3°, a sliding velocity of 1.66 mm/s, and the least accumulation of droplets over time with use. These values are greatly improved when compared to the control sample where lubricant was simply dripped on, which resulted in a contact angle hysteresis of 20°, a critical tilt angle of 26.3°, and a sliding velocity of 0.23 mm/s. The sonicated and drip infusion methods were also compared with different materials (stainless steel) and different textures (microstructures). It was found that the improvement in slipperiness using the sonicated infusion method is prominent for nanoscale textures on both stainless steel and polypropylene. In this study, we discuss the challenges with oil depletion in SLIPS (cloaking and wetting ridges) and with the selection of contact angle measurement methods. While further investigation as to why certain applied forces during infusion yield better SLIPS is warranted, these forces greatly affect the outcome. This work suggests that researchers should consider using sonication or other methods of lubricant infusion that apply external forces as infusion techniques to yield better SLIPS on the nanoscale.

Bidirectional motion of a planar fabricated piezoelectric motor based on unimorph arms

A fabrication methodology for piezoelectric motors based on multiple unimorph arms at the mesoscale is proposed. The combination of laser micromachining and lamination steps allows for a stator with a diameter of 9 mm and a thickness of 0.267 mm to be batch manufactured. This design is investigated via a finite element analysis where it is shown that the contact condition between the stator unimorph arms and rotor is dependent on the input drive frequency. The fabricated stator is then characterized experimentally where it is shown that a shift in the sinusoidal drive frequency from 3220 Hz to 3900 Hz results in a change to the rotational direction of the rotor from the positive to the negative direction. The torque of the motor is evaluated numerically to demonstrate the performance of the mesoscale piezoelectric motor in both rotational directions.

Laser Machining at High ∼PW/cm2 Intensity and High Throughput

Laser machining by ultra-short (sub-ps) pulses at high intensity offers high precision, high throughput in terms of area or volume per unit time, and flexibility to adapt processing protocols to different materials on the same workpiece. Here, we consider the challenge of optimization for high throughput: how to use the maximum available laser power and larger focal spots for larger ablation volumes by implementing a fast scan. This implies the use of high-intensity pulses approaching ∼PW/cm2 at the threshold where tunneling ionization starts to contribute to overall ionization. A custom laser micromachining setup was developed and built to enable high speed, large-area processing, and easy system reconfiguration for different tasks. The main components include the laser, stages, scanners, control system, and software. Machining of metals such as Cu, Al, or stainless steel and fused silica surfaces at high fluence and high exposure doses at high scan speeds up to 3 m/s were tested for the fluence scaling of ablation volume, which was found to be linear. The largest material removal rate was 10 mm3/min for Cu and 20 mm3/min for Al at the maximum power 80 W (25 J/cm2 per pulse). Modified surfaces are color-classified for their appearance, which is dependent on surface roughness and chemical modification. Such color-coding can be used as a feedback parameter for industrial process control.

Observation of Boyer-Wolf Gaussian modes

Stable laser resonators support three fundamental families of transverse modes: the Hermite, Laguerre, and Ince Gaussian modes. These modes are crucial for understanding complex resonators, beam propagation, and structured light. We experimentally observe a new family of fundamental laser modes in stable resonators: Boyer-Wolf Gaussian modes. By studying the isomorphism between laser cavities and quadratic Hamiltonians, we design a laser resonator equivalent to a quantum two-dimensional anisotropic harmonic oscillator with a 2:1 frequency ratio. The generated Boyer-Wolf Gaussian modes exhibit a parabolic structure and show remarkable agreement with our theoretical predictions. These modes are also eigenmodes of a 2:1 anisotropic gradient refractive index medium, suggesting their presence in any physical system with a 2:1 anisotropic quadratic potential. We identify a transition connecting Boyer-Wolf Gaussian modes to Weber nondiffractive parabolic beams. These new modes are foundational for structured light, and open exciting possibilities for applications in laser micromachining, particle micromanipulation, and optical communications.

Recent advances in materials and manufacturing of implantable devices for continuous health monitoring

Implantable devices are vital in healthcare, enabling continuous monitoring, early disease detection, informed decision-making, enhanced outcomes, cost reduction, and chronic condition management. These devices provide real-time data, allowing proactive healthcare interventions, and contribute to overall improvements in patient care and quality of life. The success of implantable devices relies on the careful selection of materials and manufacturing methods. Recent materials research and manufacturing advancements have yielded implantable devices with enhanced biocompatibility, reliability, and functionality, benefiting human healthcare. This paper provides a comprehensive overview of the latest developments in implantable medical devices, emphasizing the importance of material selection and manufacturing methods, including biocompatibility, self-healing capabilities, corrosion resistance, mechanical properties, and conductivity. It explores various manufacturing techniques such as microfabrication, 3D printing, laser micromachining, electrospinning, screen printing, inkjet printing, and nanofabrication. The paper also discusses challenges and limitations in the field, including biocompatibility concerns, privacy and data security issues, and regulatory hurdles for implantable devices.

Enhanced electrochemical ozone production via sp2 carbon content optimization in boron doped diamond electrodes using laser micromachining

The role and bonding arrangement of deliberately added sp2 carbon in maximising the current efficiency, output and longevity of boron doped diamond (BDD) electrodes for electrochemical dissolved ozone generation is elucidated. We show, using a zero-gap cell (ZGC) arrangement, how systematically increasing sp2 carbon results in increased ozone concentration and current efficiency. sp2 carbon addition is made using nanosecond pulse laser micromachining which converts BDD to sp2 carbon. Two ZGC geometries are investigated which both incorporate a Nafion membrane sandwiched between two BDD electrodes. Through-holes (perforations) are integrated into either the membrane or the BDD, the latter using laser micromachining which also converts the hole walls to sp2 carbon. Increasing the number of through-holes (or changing hole geometry), in the perforated BDD, increases the sp2 carbon content of the electrode (from 5 to 100 %). For the perforated Nafion membrane in contact with a planar BDD electrode, patterned laser micromachining of the BDD surface is used to control the sp2 carbon content of the electrode (from 4 to 100 %). For both electrodes, sp2 carbon content is significantly higher than is possible using diamond growth. sp2 carbon contents >40 % and >60 % for the planar and perforated BDD electrodes, respectively, are found to be particularly effective, allowing electrode designs to be proposed for optimised ZGC ozone generation. Notably, the sp2 carbon introduced via laser micromachining is shown to be extremely stable over 20 h (anode potential ∼ 10 V) in contrast to glassy carbon, which corrodes within 10 min. Whilst both are 100 % sp2 carbon, the laser-machined surface (after ozonolysis) is amorphous whereas the glassy carbon contains disorganized graphitic layers. This work also highlights the intriguing stability of amorphous sp2 carbon towards high oxidative potentials.

Polarization singularities for shaping of vector flat-top beams in utilization for high-power laser micromachining of various materials

Structured light is desired in the micromachining of various materials. As the latest ultrashort laser sources increase the average power by increasing the repetition rate, the efficiency of the process becomes crucial in laser micromachining. The standard Gaussian beam shape, because of its slow ascending/descending slope, is not the most efficient beam, neither in precise energy deposition nor in heat management. Flat-top beams are required for precise and even energy distribution and are used in laser ablation or thin-film removal. We look at a rather abstract concept of polarization singularities and investigate them to create a vector flat-top beam. For this purpose, we devise a beam-shaping method based on the volumetric birefringent nano gratings which allow the generation of high-energy polarization singularities. By tuning the input beam waist, we have generated three most common laser micromachining beam profiles (Gaussian, Flat-top, and ring- or doughnut-shaped). The resulting high-quality and high-energy polarization singularities allow us to systematically demonstrate their use in transparent and opaque material micromachining, as well as the usual application for flat-top beams of thin-film removal.