Ultrashort Laser Pulses Research Articles

Femtosecond Laser Direct Writing of Flexible Electronic Devices: A Mini Review.

By virtue of its narrow pulse width and high peak power, the femtosecond pulsed laser can achieve high-precision material modification, material additive or subtractive, and other forms of processing. With additional good material adaptability and process compatibility, femtosecond laser-induced application has achieved significant progress in flexible electronics in recent years. These advancements in the femtosecond laser fabrication of flexible electronic devices are comprehensively summarized here. This review first briefly introduces the physical mechanism and characteristics of the femtosecond laser fabrication of various electronic microdevices. It then focuses on effective methods of improving processing efficiency, resolution, and size. It further highlights the typical progress of applications, including flexible energy storage devices, nanogenerators, flexible sensors, and detectors, etc. Finally, it discusses the development tendency of ultrashort pulse laser processing. This review should facilitate the precision manufacturing of flexible electronics using a femtosecond laser.

The impact of laser lift-off with sub-ps pulses on the electrical and optical properties of InGaN/GaN light-emitting diodes

Laser lift-off (LLO) is an important step in the processing chain of nitride-based light-emitting diodes (LEDs), as it enables the transfer of LEDs from the growth substrate to a more suitable carrier. A distinctive feature of LLO with ultrashort pulses is the ability to use either above- or below-bandgap radiation, since nonlinear absorption becomes relevant for ultrashort pulses. This study addresses the differences in the absorption scheme for below- and above-bandgap radiation and investigates the electrical and optical properties of InGaN/GaN LEDs before and after LLO with 347 and 520 nm laser light via current–voltage and power- as well as temperature-dependent photoluminescence measurements. LLO could be successfully realized with both wavelengths. The threshold fluence required for LLO is about a factor of two larger for 520 nm compared to that for 347 nm. Furthermore, an increase in leakage current by several orders of magnitude and a significant decrease in efficiency with laser fluence are observed for below-bandgap radiation. In contrast, leakage current hardly increases and efficiency is less dependent on the laser fluence for samples lifted with 347 nm. This degradation is ascribed to the absorption of laser light in the active region, which facilitates a modification of the local defect landscape. The effect is more severe for below-bandgap radiation, as more laser light penetrates deep into the structure and reaches the active region. Ultimately, we show that LEDs lifted with ultrashort laser pulses can exhibit good quality, making ultrashort pulse LLO a viable alternative to conventional LLO with nanosecond pulses.

Mechanical Abrasion of Laser‐Machined Highly Hydrophobic Stainless Steel Surface Depending on Surface Topography

Stainless steel (SST) is an important material for a variety of applications including construction, food, and medical. Highly hydrophobic wetting properties enhance the surface properties of SST to support processes such as self‐cleaning . However, applications also require long‐term stability of such properties against chemical and mechanical influences from the environment or technical processes. Therefore, the reduction of highly hydrophobicity of chemically modified, laser‐textured SST surfaces is investigated in relation to abrasive wear using hierarchical structures, micro‐/nanotextured surface, and support structures that shield the highly hydrophobic pattern. Surface textures comprising ridges, grooves (size: 50–500 μm; depth: up to 100 μm), and a nanostructured grooves bottom are machined by infrared ultrashort pulse laser ablation into SST and are subsequently chemically modified by a self‐assembled monolayer of a fluorinated, phosphonic acid‐modified alkane. Abrasive wear tests of these surfaces show decreasing water contact angles with increasing wear of the modified surface of the support structures. However, there is good stability of the highly hydrophobic properties due to the protection of modified areas at the groove bottom. The proposed wetting model for such designed functionalized laser textures shows possibilities for further optimization of such robust highly hydrophobic surfaces and adaptation to specific applications.

Effects of pulsed laser ablation on magnetic losses of GO electrical steels along various excitation directions

Surface laser treatment is an effective way of improving magnetic properties of Grain-Oriented Electrical Steels (GOES). However, these enhancements generally only concern their easy magnetization axis aligned with their Rolling Direction (RD). In this work, the effects of an ablation laser process (implemented with advanced UltraShort Pulse Lasers (USPL)) on the magnetic properties of GOES are investigated for various excitation configurations. Both conventional and non-conventional laser patterns are also used and compared. Experimental results show that the ablation process can improve performances of GOES not only along the RD but also along other excitation directions providing an adaptation of laser patterns when the excitation direction changes.

Towards spatiotemporal-resolved spectroscopic measurement of freezing desalination using femtosecond laser filaments

Freezing desalination can produce freshwater from seawater or industrial wastewater by ice crystallization with low energy consumption and less corrosion, and is promising for alleviating current stress on the world's water supply. However, the salt rejection dynamics during the freezing process is less understood due to the lack of real-time, high spatiotemporal-resolved detection methods. Here we present a proof-of-principle, quantitative, and simultaneous multi-salt detection approach for the measurement of salt rejection dynamics in salt-contaminated water at the ppm level using an ultrashort-pulsed femtosecond laser propagating in the filamentation regime. We show that this approach of filament-induced plasma spectroscopy possesses the abilities for both monitoring the change of salt concentration in the remaining unfrozen water during the freezing process and imaging the spatial concentration distributions of metal contaminants on the resultant ice surface. With this approach, we find that the initial salt concentration in water plays an essential role in the efficiency of the freezing desalination process, which shows a metal-species-dependent characteristic. Our results pave the way towards real-time, online, and high spatiotemporal-resolved detection of salt rejection dynamics of freezing desalination by ultrashort-pulsed lasers.

Laser Engineering Nanocarbon Phases within Diamond for Science and Electronics.

Diamond, as the densest allotrope of carbon, displays a range of exemplary material properties that are attractive from a device perspective. Despite diamond displaying high carbon-carbon bond strength, ultrashort (femtosecond) pulse laser radiation can provide sufficient energy for highly localized internal breakdown of the diamond lattice. The less-dense carbon structures generated on lattice breakdown are subject to significant pressure from the surrounding diamond matrix, leading to highly unusual formation conditions. By tailoring the laser dose delivered to the diamond, it is shown that it is possible to create continuously modified internal tracks with varying electrical conduction properties. In addition to the widely reported conducting tracks, conditions leading to semiconducting and insulating written tracks have been identified. High-resolution transmission electron microscopy (HRTEM) is used to visualize the structural transformations taking place and provide insight into the different conduction regimes. The HRTEM reveals a highly diverse range of nanocarbon structures are generated by the laser irradiation, including many signatures for different so-called diaphite complexes, which have been seen in meteorite samples and seem to mediate the laser-induced breakdown of the diamond. This work offers insight into possible formation methods for the diamond and related nanocarbon phases found in meteorites.

Controlling the photoelectron holography with shaped intense ultrashort laser pulses

We perform a theoretical investigation on the control of photoelectron holography with shaped intense ultrashort laser pulses. By adjusting the π phase-flip position in the frequency domain, the shape of the time-dependent envelope of the input laser electric field changes from Gaussian to trapezoidal and finally to double-peak pulse. The photoelectron momentum spectra for different pulse shapes are calculated by both three-dimensional time-dependent Schrödinger equation (TDSE) method and Quantum-trajectory Monte Carlo (QTMC) model, and good agreement is achieved between results calculated by the two methods. Interference pattern in photoelectron holography exhibits a strong dependence on the shape of the electric field. Analysis shows that adjacent and nonadjacent intercycle interference as well as intracycle interference are all effectively regulated by the shaped pulse.

Generation mechanism and temporal–spatial evolution of electron excitation induced by an ultrashort pulse laser in zirconia ceramic

Femtosecond lasers have been applied in the machining of zirconia (ZrO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ extrm{ZrO}}_2}$$\\end{document}) ceramics because of their ultrashort pulse duration and high peak power. However, the high-precision micromachining of zirconia remains challenging owing to an unclear understanding of the ultrafast laser–material interaction mechanisms. In this study, the transient processing phenomenon induced by a femtosecond laser pulse, namely electron excitation (filament), occurring on a picosecond to nanosecond timescale, was directly observed and quantitatively investigated inside ZrO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ extrm{ZrO}}_2}$$\\end{document}. A pump-probe imaging method comprising a focusing probe beam integrated with a high-speed camera was used. The evolution process, type variation, and related interaction mechanisms of the filament were revealed under different parameters, including laser pulse energy, sample position, and pulse duration. Finally, the generation mechanism of multiple filaments was clarified and successfully elucidated. This study reveals the ultrafast laser–material interaction mechanisms and the high-precision processing in the laser drilling of zirconia ceramics.

Parametric investigation of ultrashort pulsed laser surface texturing on aluminium alloy 7075 for hydrophobicity enhancement

Hydrophobicity plays a pivotal role in mitigating surface fouling, corrosion, and icing in critical marine and aerospace environments. By employing ultrafast laser texturing, the characteristic properties of a material’s surface can be modified. This work investigates the potential of an advanced ultrafast laser texturing manufacturing process to enhance the hydrophobicity of aluminium alloy 7075. The surface properties were characterized using goniometry, 3D profilometry, SEM, and XPS analysis. The findings from this study show that the laser process parameters play a crucial role in the manufacturing of the required surface structures. Numerical optimization with response surface optimization was conducted to maximize the contact angle on these surfaces. The maximum water contact angle achieved was 142º, with an average height roughness (Sa) of 0.87 ± 0.075 µm, maximum height roughness (Sz) of 19.4 ± 2.12 µm, and texture aspect ratio of 0.042. This sample was manufactured with the process parameters of 3W laser power, 0.08 mm hatch distance, and a 3 mm/s scan speed. This study highlights the importance of laser process parameters in the manufacturing of the required surface structures and presents a parametric modeling approach that can be used to optimize the laser process parameters to obtain a specific surface morphology and hydrophobicity.

Influence of Laser Process Factors on Laser‐Induced X‐Ray Emission in High‐Repetition Laser Processes

Lasers have established themselves as a versatile tool that generates innovations in a wide range of fields, from automotive to surface engineering, medical applications, etc. A current research field for lasers is surface texturing in biomimetic, which aims to mimic nature‐inspired functionalities and mechanisms, for example, to tailor and control wetting, adhesion/friction, or optical properties of technical surfaces. Therefore, ultrashort pulse lasers are often the right choice to alter surface structures at microscopic scale. Moreover, the ongoing trend toward high‐average power lasers reaching “kilowatt class” levels even for ultrashort pulse lasers seems to be beneficial to increase throughput of this powerful laser technology for industrial production. However, several recent studies are reported on harmful X‐ray emissions arising from high‐intense ultrashort laser pulses which must be considered as a secondary laser beam hazard in risk assessment. Shining light on this topic, this article reports on a set of experiments investigating X‐rays emission induced by ultrashort pulse laser processing of different laser‐pretreated stainless steel sheets. The X‐ray measurements show the effect of roughness and scan number on the laser‐induced X‐ray emission dose. In addition, the dependency of X‐ray emission on the scan direction relatively to the laser beam polarization state is discussed.

Automated, real-time material detection during ultrashort pulsed laser machining using laser-induced breakdown spectroscopy, for process tuning, end-pointing, and segmentation.

The rapid, high-resolution material processing offered by ultrashort pulsed lasers enables a wide range of micro and nanomachining applications in a variety of disciplines. Complex laser processing jobs conducted on composite samples, require an awareness of the material type that is interacting with laser both for adjustment of the lasering process and for endpointing. This calls for real-time detection of the materials. Several methods such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-Ray spectroscopy (EDS) can be used for material characterization. However, these methods often need interruption of the machining process to transfer the sample to another instrument for inspection. Such interruption significantly increases the required time and effort for the machining task, acting as a prohibitive factor for many laser machining applications. Laser induced breakdown spectroscopy (LIBS) is a powerful technique that can be used for material characterization, by analyzing a signal that is generated upon the interaction of laser with matter, and thus, it can be considered as a strong candidate for developing an in-situ characterization method. In this work, we propose a method that uses LIBS in a feedback loop system for real time detection and decision making for adjustment of the lasering process on-the-fly. Further, use of LIBS for automated material segmentation, in the 3D image resulting from consecutive lasering and imaging steps, is showcased.

LGYSB:Nd─High-Performance Lasing in the Near-Infrared Region.

Three incongruent melting LawNdxGdyYzSc4-w-x-y-z(BO3)4 (LGYSB:Nd) crystals with different Y concentrations (z = 0.15, 0.05, and 0.025) have been grown by the Czochralski method for the first time. The LGYSB:Nd-type crystals exhibit an acentric structure similar to that of the natural mineral huntite CaMg3(CO3)4, with space group R32. The composition of the LGYSB:Nd_3 (z = 0.025) crystal grown from the starting melt composition La0.628Gd0.547Y0.025Nd0.05Sc2.75(BO3)4 was measured by inductively coupled plasma mass spectrometry method, and it was found to be La0.6794Gd0.4105Y0.0178Nd0.0381Sc2.8542(BO3)4. The transversal spatial distribution of the refractive index in the LGYSB:Nd_3 crystal was investigated. Third-order nonlinear optical susceptibility χ(3) of the LGYSB:Nd_3 crystal was determined from third-harmonic generation experiments with ultrashort (fs) laser pulses. The optical transmission spectrum was measured in the range of 200-2000 nm. The absorption cross-section at 808 nm in σ-polarization was determined to be 1.18 × 10-19 cm2 for the LGYSB:Nd_3 crystal. The 10K absorption spectra revealed that the Nd3+ ions occupy only La3+ cationic sites in the LGYSB host matrix. The emission cross-section at 1064 nm in σ-polarization was determined to be σem(σ) = 1.74 × 10-19 cm2 for the LGYSB:Nd_3 crystal. The fluorescence lifetime was found to be τ = 115 μs for all of the LGYSB:Nd crystals. The LGYSB:Nd_3 laser was operated at an emission wavelength of 1062 nm with very high slope efficiencies of ηsa = 0.74 and ηsa = 0.64 in quasi-cw and cw regimes, respectively.

Spatiotemporal imaging and manipulation of surface plasmons

Abstract Surface plasmon polaritons (SPPs) are travelling surface waves that have shown promise for applications in nanophotonics as they provide a direct route toward photon-mediated electrical signal generation – a central paradigm for speeding up and scaling down photonic elements. SPP waves have also generated fundamental interest due to their high-field strength and sub-wavelength confinement, properties that have enabled the surface-enhanced Raman effect. Over the last decade, photoemission electron microscopy (PEEM) has emerged as a pioneering technique for imaging surface electric fields through ultrashort laser pulse mediated electron emission and has therefore become an indispensable tool for characterizing plasmonic phenomena at interfaces in a variety of materials. PEEM offers nanometer spatial resolution and femtosecond temporal resolution, allowing SPPs to be prepared, monitored, and manipulated on the nanometer-femtosecond scale. Through a brief review of recent reports, we aim to introduce PEEM-based SPP imaging and manipulation modalities and highlight their utility in the context of emerging nanoscale and quantum materials science advancements.

Zero-Point Energy Density at the Origin of the Vacuum Permittivity and Photon Propagation Time Fluctuation

We give a vacuum description with zero-point density for virtual fluctuations. One of the goals is to explain the origin of the vacuum permittivity and permeability and to calculate their values. In particular, we improve on existing calculations by avoiding assumptions on the volume occupied by virtual fluctuations. We propose testing of the models that assume a finite lifetime of virtual fluctuation. If during its propagation, the photon is stochastically trapped and released by virtual pairs, the propagation velocity may fluctuate. The propagation time fluctuation is estimated for several existing models. The obtained values are measurable with available technologies involving ultra-short laser pulses, and some of the models are already in conflict with the existing astronomical observations. The phase velocity is not affected significantly, which is consistent with the interferometric measurements.

UV-ultrashort pulsed laser ablation of fused silica

The authors report on ultraviolet ultrashort pulsed laser ablation of fused silica and compare the achievable micromachining results to those obtained by using the fundamental emission wavelength in infrared. Ablation in ultraviolet reveals a stable efficiency for increasing fluences, whereas using an infrared beam exhibits a decreasing trend of the ablation efficiency at higher and increasing fluences. In addition, a significant improvement in the surface quality is found by using an ultraviolet wavelength in a fluence range up to 20 J/cm2 compared to infrared, e.g., revealing an Ra of down to 0.45 μm on using the ultraviolet wavelength compared to Ra = 0.56 μm on using infrared at fluences up 15 J/cm2. Moreover, taking advantage of the high available pulse energy, the authors compare the achievable ablation efficiency and surface roughness using a conventionally focused ultraviolet beam and a defocused ultraviolet beam, finding that the defocused ultraviolet beam possesses a processing quality comparable to that of the focused beam. Finally, the authors exemplify the potential of ultraviolet ultrashort pulsed laser ablation by using a Tesla mixer for microfluidic integration of fused silica.

Transient ablation topography of a thin chromium film after ultrashort pulsed laser irradiation in the spallation and phase explosion regime

Time-resolved imaging pump–probe reflectometry is an established technique to characterize the dynamic processes occurring during the material ablation upon ultrashort pulsed laser irradiation. In the phase explosion regime, however, the presence of an expanding liquid–vapor mixture on top of the ablated material makes an unambiguous determination of the transient ablation topography almost impossible, especially when only front-side pump–probe reflectometry is used. To circumvent this limitation, the front-side reflectometry was complemented by rear-side reflectometry, where the probe radiation does not interact with the liquid–vapor mixture, and the underneath ablation kinetics of the material becomes observable. To interpret the measured front- and rear-side reflectances, an optical multilayer model of the ablation process was developed. Its applicability is demonstrated on a thin chromium film that was ablated by a single pump pulse. The optical model replicates the Newton rings measured during spallation and phase explosion very well, which allows the reconstruction of the transient ablation topography. The combination of the front- and rear-side reflectometry with modeling offers a valuable tool for studying the ablation processes in thin metal films, because it overcomes the challenges of the phase explosion regime, and thus enhances the understanding of material dynamics upon laser irradiation.

Repetition Frequency Control of a Mid-Infrared Ultrashort Pulse Laser

In this study, a method for controlling the repetition frequency of a mid-infrared ultrashort pulse laser with a central wavelength of 2.8 µm is developed. A ring cavity that is insensitive to the polarization state of the laser light emitted from the fiber end was constructed to stabilize the oscillation of the mid-infrared ultrashort pulse laser. More oscillation conditions for the ultrashort pulse laser based on nonlinear polarization rotation are found than the conventional method. To confirm that the pulse oscillation is mode-locked, ultrashort pulse oscillation was confirmed by an autocorrelator. The pulse repetition frequency of this robust ultrashort pulse laser was controlled. The control method was based on the phase-locked loop (PLL) control. A wedge window was inserted into the cavity and mounted on a linear stage driven by a piezoelectric transducer. By driving the piezoelectric transducer, the position of the wedge window changed, and the resulting optical path length also changed. The repetition frequency was controlled based on this principle. Optical path length control by the wedge window and temperature control provides an Allan deviation of approximately 1 mHz.

Understanding factors affecting the process efficiency, quality and carbon emission in laser drilling of CFRP composite via tailored sequence multiple-ring scanning

Carbon fibre reinforced polymer (CFRP) is increasingly being used in the aerospace, marine, and land transport due to its lightweight, corrosion resistance, and high strength. However, hole drilling during assembly remains challenging due to extensive tool wear and delamination. Laser processing, typically used for drilling and machining, often produces heat effects or is low in efficiency if ultrashort pulse lasers are used. Here, we report an optimised result for a tailored sequence multiple-ring scanning laser drilling method that significantly improves the process efficiency (up to 800 %) by using a continuous wave high power fibre laser operating in pulse mode. Factors such as ring numbers, ring spacing (hatch distance), laser power, and pulse frequency are investigated using statistical design of experiments, and their interactions and effects on process efficiency, carbon emissions, heat-affected zone size, and hole taper are evaluated. The work shows that process efficiency can be significantly increased as the number of rings increases. This is counter-intuitive and would be useful for industrial applications. Using nitrogen as the assist gas results in significantly lower unit process carbon emissions than using argon. The mechanisms behind this efficiency improvement are discussed.

Ionization of hydrogen atom driven by ultrashort intense laser pulses: study in momentum space of phase-dependent effects

Ionization of hydrogen atom driven by ultrashort intense laser pulses: study in momentum space of phase-dependent effects

Enhancing second harmonic generation by Q-boosting lossless cavities beyond the time bandwidth limit

Abstract Nanostructures proved to be versatile platforms to control the electromagnetic field at subwavelength scale. Indeed, high-quality-factors nanocavities have been used to boost and control nonlinear frequency generation by increasing the light–matter interaction. However, nonlinear processes are triggered by high-intensities, which are provided by ultrashort laser pulses with large bandwidth, which cannot be fully exploited in such devices. Time-varying optical systems allow one to overcome the time-bandwidth limit by modulating the cavity external coupling. Here we present a general treatment, based on coupled mode theory, to describe second harmonic generation in a doubly resonant cavity for which the quality-factor at the fundamental frequency is modulated in time. We identify the initial quality factor maximizing second harmonic efficiency when performing Q-boosting and we predict a theoretical energy conversion efficiency close to unity. Our results have direct impact on the design of next generation time-dependent metasurfaces to boost nonlinear frequency conversion of ultrashort laser pulses.