High Average Laser Research Articles

Model for designing process strategies in ultrafast laser micromachining at high average powers

The great potential to scale the productivity of laser micromachining processes offered by the development of ultrafast lasers with average powers exceeding 1 kW comes at the cost of new physical and technical challenges. The large number of adjustable parameters, the different physical process limits, and the various possible optimization strategies complicate the design of suitable laser micromachining processes that enable high quality and high throughput.In this contribution, a comprehensive model is proposed that helps to identify the optimization strategies and process parameters required for the scaling of laser micromachining to high throughput at high average laser power without exceeding the process limits to ensure high surface quality.The model was experimentally verified on the example of laser micromachining of pockets in metals and silicon using an ultrafast laser with an average power of 1.01 kW, a pulse duration of 600 fs, different pulse energies, pulse bursts, and beam diameters. As a result, high material removal rates exceeding 120 mm3/min were achieved for silicon, stainless steel, copper, and aluminum, which exceed previously achieved removal rates by one to two orders of magnitude. The machined surfaces exhibit high quality as confirmed by the low measured roughness of about 1 µm.

Self-Shielding of X-ray Emission from Ultrafast Laser Processing Due to Geometrical Changes of the Interaction Zone.

Materials processing with ultrashort laser pulses is one of the most important approaches when it comes to machining with very high accuracy. High pulse repetition rates and high average laser power can be used to attain high productivity. By tightly focusing the laser beam, the irradiances on the workpiece can exceed 1013 W/cm2, and thus cause usually unwanted X-ray emission. Pulsed laser processing of micro holes exhibits two typical features: a gradual increase in the irradiated surface within the hole and, with this, a decrease in the local irradiance. This and the shielding by the surrounding material diminishes the amount of ionizing radiation emitted from the process; therefore, both effects lead to a reduction in the potential X-ray exposure of an operator or any nearby person. The present study was performed to quantify this self-shielding of the X-ray emission from laser-drilled micro holes. Percussion drilling in standard air atmosphere was investigated using a laser with a wavelength of 800 nm a pulse duration of 1 ps, a repetition rate of 1 kHz, and with irradiances of up to 1.1·1014 W/cm. The X-ray emission was measured by means of a spectrometer. In addition to the experimental results, we present a model to predict the expected X-ray emission at different angles to the surface. These calculations are based on raytracing simulations to obtain the local irradiance, from which the local X-ray emission inside the holes can be calculated. It was found that the X-ray exposure measured in the surroundings strongly depends on the geometry of the hole and the measuring direction, as predicted by the theoretical model.

Numerical study of a convective cooling strategy for increasing safe power levels in two-photon brain imaging.

Two-photon excitation fluorescence microscopy has become an effective tool for tracking neural activity in the brain at high resolutions thanks to its intrinsic optical sectioning and deep penetration capabilities. However, advanced two-photon microscopy modalities enabling high-speed and/or deep-tissue imaging necessitate high average laser powers, thus increasing the susceptibility of tissue heating due to out-of-focus absorption. Despite cooling the cranial window by maintaining the objective at a fixed temperature, average laser powers exceeding 100-200 mW have been shown to exhibit the potential for altering physiological responses of the brain. This paper proposes an enhanced cooling technique for inducing a laminar flow to the objective immersion layer while implementing duty cycles. Through a numerical study, we analyze the efficacy of heat dissipation of the proposed method and compare it with that of the conventional, fixed-temperature objective cooling technique. The results show that improved cooling could be achieved by choosing appropriate flow rates and physiologically relevant immersion cooling temperatures, potentially increasing safe laser power levels by up to three times (3×). The proposed active cooling method can provide an opportunity for faster scan speeds and enhanced signals in nonlinear deep brain imaging.

High energy and average power solid-state UV laser for industrial applications

High energy and average power solid-state UV laser involves several challenges to manage degradation due to UV radiations. Recent results achieved in this direction are presented.

Simultaneous micromachining with beam splitting of ultrashort laser pulses at high average power for the productive fabrication of microchannels

Laser micromachining with ultrashort laser pulses is a promising approach for the generation of small geometries in the micro-meter scale, such as microchannels for passive fluid transport. The ablation of small structures with high average laser power still poses a challenge when high surface quality is required. In this work, beam splitting with a diffractive optical element (DOE) was used to divide the laser beam of a high-power ultrafast laser into an array of 1x15 spots on the workpiece. Fast scanning of up to 20 m/s perpendicular to the orientation of the array using a Galvanometer-scanner allowed for simultaneous micromachining of multiple microchannels with high surface quality on cemented tungsten carbide. The high average laser power of 160 W in combination with the adapted processing strategy enabled the simultaneous and productive fabrication of 15 microchannels with a low surface roughness of Sa < 1.5 µm along the channel wall.

Sensitive and high laser damage threshold substrates for surface-enhanced Raman scattering based on gold and silver nanoparticles.

Surface-enhanced Raman scattering (SERS) is a sensitive and fast technique for sensing applications such as chemical trace analysis. However, a successful, high-throughput practical implementation necessitates the availability of simple-to-use and economical SERS substrates. In this work, we present a robust, reproducible, flexible and yet cost-effective SERS substrate suited for the sensitive detection of analytes at near-infrared (NIR) excitation wavelengths. The fabrication is based on a simple dropcast deposition of silver or gold nanomaterials on an aluminium foil support, making the design suitable for mass production. The fabricated SERS substrates can withstand very high average Raman laser power of up to 400mW in the NIR wavelength range while maintaining a linear signal response of the analyte. This enables a combined high signal enhancement potential provided by (i) the field enhancement via the localized surface plasmon resonance introduced by the noble metal nanomaterials and (ii) additional enhancement proportional to an increase of the applicable Raman laser power without causing the thermal decomposition of the analyte. The application of the SERS substrates for the trace detection of melamine and rhodamine 6G is demonstrated, which shows limits of detection smaller than 0.1ppm and analytical enhancement factors on the order of 104 as compared to bare aluminium foil.

Picosecond pulsed 532nm laser system for roughening and secondary electron yield reduction of inner surfaces of up to 15m long tubes.

Laser-induced surface structuring is a promising method to suppress electron mulitpacting in the vacuum pipes of particle accelerators. Electrons are scattered inside the rough surface structure, resulting in a low Secondary Electron Yield (SEY) of the material. However, laser processing of internal pipe surfaces with a large aspect ratio is technologically challenging in terms of laser beam guidance and focusing. We present a 532nm ultrashort-pulse laser setup to process the inner parts of 15m long beam vacuum tubes of the Large Hadron Collider (LHC). Picosecond pulses at a repetition rate of 200kHz are guided through an optical fiber toward an inchworm robot traveling inside the beam pipe. The system was installed, characterized, and tested for reliability. First surface treatments achieved the required scan precision. Cu2O-dominated nano-features were observed when processing at high average laser power (5 W) and slow scanning speed (5mms-1) in nitrogen flow, and the maximum SEY of copper was decreased from 2.1 to 0.7.

Pulse-on-Demand Operation for Precise High-Speed UV Laser Microstructuring

Laser microstructuring has been studied extensively in the last decades due to its versatile, contactless processing and outstanding precision and structure quality on a wide range of materials. A limitation of the approach has been identified in the utilization of high average laser powers, with scanner movement fundamentally limited by laws of inertia. In this work, we apply a nanosecond UV laser working in an intrinsic pulse-on-demand mode, ensuring maximal utilization of the fastest commercially available galvanometric scanners at scanning speeds from 0 to 20 m/s. The effects of high-frequency pulse-on-demand operation were analyzed in terms of processing speeds, ablation efficiency, resulting surface quality, repeatability, and precision of the approach. Additionally, laser pulse duration was varied in single-digit nanosecond pulse durations and applied to high throughput microstructuring. We studied the effects of scanning speed on pulse-on-demand operation, single- and multipass laser percussion drilling performance, surface structuring of sensitive materials, and ablation efficiency for pulse durations in the range of 1–4 ns. We confirmed the pulse-on-demand operation suitability for microstructuring for a range of frequencies from below 1 kHz to 1.0 MHz with 5 ns timing precision and identified the scanners as the limiting factor even at full utilization. The ablation efficiency was improved with longer pulse durations, but structure quality degraded.

Heating influence on hierarchical structures fabricated by direct laser interference patterning

The combination of direct laser interference patterning (DLIP) with laser-induced periodic surface structures (LIPSS) enables the fabrication of functional surfaces reported for a wide spectrum of materials. The process throughput is usually increased by applying higher average laser powers. However, this causes heat accumulation impacting the roughness and shape of produced surface patterns. Consequently, the effect of substrate temperature on the topography of fabricated features requires detailed investigations. In this study, steel surfaces were structured with line-like patterns by ps-DLIP at 532 nm. To investigate the influence of substrate temperature on the resulting topography, a heating plate was used to adjust the temperature. Heating to 250 ^{circ }C led to a significant reduction of the produced structure depths, from 2.33 to 1.06 µm. The reduction is associated with the appearance of a different LIPSS type, depending on the grain orientation of the substrates and laser-induced superficial oxidation. This study revealed a strong effect of substrate temperature, which is also to be expected when heat accumulation effects arise from processing surfaces at high average laser power.

Direct-write formation of integrated bottom contacts to laser-induced graphene-like carbon

We report a simple, scalable two-step method for direct-write laser fabrication of 3D, porous graphene-like carbon electrodes from polyimide films with integrated contact plugs to underlying metal layers (Au or Ni). Irradiation at high average CO2 laser power (30 W) and low scan speed (∼18 mm s)−1 leads to formation of ‘keyhole’ contact plugs through local ablation of polyimide (initial thickness 17 μm) and graphitization of the plug perimeter wall. Top-surface laser-induced graphene (LIG) electrodes are then formed and connected to the plug by raster patterning at lower laser power (3.7 W) and higher scan speed (200 mm s)−1. Sheet resistance data (71 ± 15 Ω sq.)−1 indicates formation of high-quality surface LIG, consistent with Raman data which yield sharp first- and second-order peaks. We have also demonstrated that high-quality LIG requires a minimum initial polyimide thickness. Capacitance data measured between surface LIG electrodes and the buried metal film indicate a polyimide layer of thickness ∼7 μm remaining following laser processing. By contrast, laser graphitization of polyimide of initial thickness ∼8 μm yielded devices with large sheet resistance (>1 kΩ sq.)−1. Raman data also indicated significant disorder. Plug contact resistance values were calculated from analysis of transfer line measurement data for single- and multi-plug test structures. Contacts to buried nickel layers yielded lower plug resistances (1-plug: 158 ± 7 Ω , 4-plug: 31 ± 14 Ω) compared to contacts to buried gold (1-plug: 346 ± 37 Ω , 4-plug: 52 ± 3 Ω). Further reductions are expected for multi-plug structures with increased areal density. Proof-of-concept mm-scale LIG electrochemical devices with local contact plugs yielded rapid electron transfer kinetics (rate constant k 0 ∼ 0.017 cm s−1), comparable to values measured for exposed Au films (k 0 ∼0.023 cm s)−1. Our results highlight the potential for integration of LIG-based sensor electrodes with semiconductor or roll-to-roll manufacturing.

Index-leveling for forced-flow turbulent face-cooling of laser amplifiers.

Direct laser slab face-cooling by a fluid crossing the main and pump laser beams is an important method to reach high average laser powers. However, the flow regime is usually maintained at low Reynolds numbers to prevent the onset of turbulence features in the flow that would degrade the wavefront quality. We show here how bringing the fluid temperature to the thermo-optical null point, close to the water/ice transition in the case of water, allows one to mitigate the optical consequences of hydrodynamic instabilities, by bleaching optically the temperature inhomogeneities within the flow. This optical process, dubbed index-leveling, opens the door to a highly efficient forced-flow, weakly turbulent face-cooling regime that should be instrumental to boost the kilowatt capabilities of next-generation high-power lasers.

Fundamentals of Scanning Surface Structuring by Ultrashort Laser Pulses: From Electron Diffusion to Final Morphology

Industrial use of ultrashort pulse surface structuring would significantly increase by effective utilization of the average laser powers available currently. However, the unexplained degradation of surfaces processed with numerous pulses at high average laser power makes this difficult. Based on a systematic experimental study, the structure formation underlying such surface degradation is investigated. Furthermore, a hierarchical structural formation model that bridges the gap between laser‐induced periodic surface structures and surface degradation is presented. Contrary to expectations based on previous research, less structure formation on titanium was observed for higher laser fluences. As a possible reason, enhanced electron diffusion with increasing intensity is investigated within the framework of the two‐temperature model. Our findings provide a deeper understanding of the microscopic mechanisms involved in surface structuring with ultrashort pulses.

Efficient Water-Assisted Glass Cutting with 355 nm Picosecond Laser Pulses.

In this study, the cutting of borosilicate glass plates in ambient air and water with a 355 nm wavelength picosecond laser was carried out. Low (2.1–2.75 W) and high (15.5 W) average laser power cutting regimes were studied. Thorough attention was paid to the effect of the hatch distance on the cutting quality and characteristic strength of glass strips cut in both environments. At optimal cutting parameters, ablation efficiency and cutting rates were the highest but cut sidewalls were covered with periodically recurring ridges. Transition to smaller hatch values improved the cut sidewall quality by suppressing the ridge formation, but negatively affected the ablation efficiency and overall strength of glass strips. Glass strips cut in water in the low-laser-power regime had the highest characteristic strength of 117.6 and 107.3 MPa for the front and back sides, respectively. Cutting in a high-laser-power regime was only carried out in water. At 15.5 W, the ablation efficiency and effective cutting speed per incident laser power increased by 16% and 22%, respectively, compared with cutting in water in a low-laser-power regime.

Experimental Investigation on the High Average Power ns Mid-Infrared Laser Output at 3.8 μm Through Difference Frequency Generation

Experimental Investigation on the High Average Power ns Mid-Infrared Laser Output at 3.8 μm Through Difference Frequency Generation

Fabrication of micro-pillar with high aspect ratio on monocrystalline diamond by galvanometer-assisted femtosecond laser milling

• The GFM trajectory was modeled and found beneficial for promoting aspect ratio. • The aspect ratio of diamond micro-pillar reached 42.25 fabricated by optimized GFM. • Graphitization level of lateral surface by GFM was slight and non-linearly changed. Diamond micro/nano-pillar is widely applied in quantum transportation and nanomechanics, but it is very difficult to be machined by conventional mechanical methods, duo to its feature of high hardness, wear-proof and extremely small scale. Thus, a novel strategy of galvanometer-assisted femtosecond laser milling (GFM) was proposed to overcome the difficulty of fabricating the micro-pillar with high aspect ratio on monocrystalline diamond. Through numeric calculation, it is found that GFM trajectory possesses the special distribution consisting of separate pulse-concentrated regions and expanded flat-distributed region, which was then experimentally proved beneficial for improving the energy consumption and highly promoting the material removal rate. Moreover, the GFM was found to be much easier to generate micro-pillar, whereas in the case of conventional non-galvanometer milling (NGM) micro-pillar underwent severe degeneration or height loss under various average laser powers. Through optimizing the machining parameters, the micro-pillar was successfully prepared with an aspect ratio of as high as 42.25 by GFM. A 5 × 5 micro-pillar array of monocrystalline diamond was also fabricated with consistent diameter, height and aspect ratio. With Raman spectroscopy, the graphitization induced by GFM was slight, though it was unavoidably introduced with relatively high average laser power. Besides, the graphitizing level on the lateral face of the machined micro-pillar was found to non-linearly change with different machining parameters, because the material removal mode underwent a transformation from graphitization dominated to sublimation dominated with the increasing of applied energy density.

228‐nm quadrupled quasi‐three‐level Nd:GdVO4 laser for ultraviolet resonance Raman spectroscopy of explosives and biological molecules

AbstractWe describe a new compact diode‐pumped solid‐state frequency quadrupled quasi‐three‐level neodymium‐doped gadolinium vanadate (Nd:GdVO4) laser that generates ~50 mW of 228‐nm quasi‐continuous wave light as ns pulses at a tunable kilohertz repetition rate. We developed two generations of this laser. The first generation has a high duty cycle and a tunable repetition rate. The second generation is optimized for maximum output power. We utilize these new lasers to measure ultraviolet resonance Raman (UVRR) spectra of many important chromophores that absorb in deep ultraviolet (UV). We demonstrate the utility of this excitation by measuring the 228‐nm absolute differential Raman cross sections of explosives, peptides, aromatic amino acids, and DNA/RNA nucleotides. Deep UV excitation at 228 nm occurs within the π → π* electronic transitions of these molecules. The 228‐nm resonance excitation enhances the Raman intensities of vibrations of NOx groups, peptide bonds, aromatic amino acid side chains, and DNA/RNA nucleotides. The measured 228‐nm UVRR cross sections of these molecules are 103–104 fold greater than those excited in the visible spectral region. These new lasers should be of great interest for UVRR spectroscopy and for other applications that benefit from compact, high average power deep UV laser light sources with low peak powers.

Smooth Surfaces by Pulsed Laser Processing with Bursts

AbstractThe aim in ultrashort‐pulsed materials processing is generally to keep interactions between the effects of the individual laser pulses to a minimum in order to minimize quality‐reducing effects and to avoid exceeding process‐specific thresholds. A contrasting approach aims at exploiting the interaction of consecutively applied laser pulses with the respective material response. In this case, it is possible to use particularly high average laser powers as well as bursts that feature time intervals of only a few nanoseconds.

Experimental study of the effects of femtosecond laser power on the induced-plasma geometry, supercontinuum white light generation and conical emission in air

This paper presents the experimental investigation of the changes in the geometrical shape of femtosecond laser-induced plasma in air under different laser power, and its effects on supercontinuum white light generation and conical emission. When a femtosecond laser is focused into a tiny spot in air, optical breakdown of air molecules occurs and this leads to the generation of plasma filament whose geometrical size and shape depend on laser power. This process is then followed by two light-emitting processes, namely supercontinuum white light generation and conical emission, both of which scatter light that reveals the characteristics of the plasma filament. Our experiment shows that the laser-induced plasma becomes thinner and longer at high average laser power but appears thick and round at lower laser power. At higher laser power, conical emission which scatters laser light in the forward direction dominates the scattering process while at lower laser power, it is the scattering of supercontinuum white light in all directions that plays a bigger role. The intricate rainbow-like pattern formed on a white screen located far away in the forward direction reveals sophisticated nonlinear optical processes that take place in conical emission which slowly diminishes as the laser power is gradually reduced.

High-throughput micromachining with ultrashort pulsed lasers and multiple spots

A one step functionalization of large surfaces and layers could be enabled by high power ultrashort pulsed lasers. Despite today's availability of high power ultrashort pulsed lasers (up to several hundred watts), it is still a challenge to structure large surface areas, which is required on embossing rollers, within an acceptable processing time for industrial roll-to-roll production. Furthermore, fluencies near the ablation threshold are necessary to provide the highest depth resolution with minor side effects. So, it is a challenge to convert the increasing high average laser power into a high processing speed by maintaining the well-known quality of ultrashort pulsed lasers constant. One approach is the combination of a high-speed application and several parallel ablating laser spots. In this contribution, a newly developed high compact picosecond laser with high pulse repetition rates and an average power of 500 W was distributed into 8 and 16 parallel beamlets by a diffractive optical element. The power was controlled by single acousto-optical modulators per beamlet. Different functional surface geometries have been realized on an embossing roller as a master, which is used for the replication of the structures in a roll-to-roll or a roll-to-plate process. Feature sizes from 2 μm to 10 μm in areas of 2 m2 could be processed. With this approach, functional structures such as reduction in friction, improved soft touch, or light guiding elements can be generated on large surfaces within short processing times.

Design and comparison of superconducting rf gun cavities and beam dynamics for linear electron accelerators

Abstract Designs and simulations of 1.4, 1.5, 1.6, and 1.8-cell SRF gun cavities are studied at a linear accelerator gun system and they are compared with each other. We obtain high power beam from high energy electron beam produced at a photocathode with high quantum efficiency, high average current laser with high brightness, and high accelerating gradient. Beam energy of the whole accelerator system is effected mainly by the optimized RF power usage inside cells, the RF power interaction with particle beams , and the accelerated beam obtained by using SRF-gun cells. Thus, the optimized beam parameters, the RF parameters, and the accelerator gun cavity parameters that depend on each cavity geometries of SRF gun are obtained and presented in this paper. Additionally, the energy values of the electrons with optimized emittance at the gun exit are achieved and shown. We will conclude our paper by giving the gun output energies specifying which elliptical cavity-cell geometry is more appropriate to push the beam further in consistent conditions. For the cavity electromagnetic fields and the geometric designs of the cavities and the solenoid, we have utilized 2D Superfish/Poisson and 3D Computer Simulation Technology Programs. Also beam dynamics studies are included for only 1.6-cell SRF gun cavity which has the highest quality factor in order to figure out how the beam behaves along the beamline at the gun system. Transverse emittance ( π mm mrad) and beam energy ( ∼ 3.5 MeV) at the exit of the gun system are obtained as expected.