Ultrafast Laser Beam Research Articles

High-throughput homogenization of a quasi-Gaussian ultrafast laser beam using a combined refractive beam shaper and spatial light modulator

High-throughput homogenization of a quasi-Gaussian ultrafast laser beam using a combined refractive beam shaper and spatial light modulator

Ultrafast laser-induced topochemistry on metallic glass surfaces

Manufacturing multifunctional nanocomposite materials and engineered surface nanopatterns involves a strategic blend of topography, crystal structures, and chemistry. Here, we report the controllable formation of crystalline nanoparticles and intermetallic compounds on thin films of metallic glasses (Zr50Cu50, Ti50Cu50, and Zr67Ag33) irradiated by ultrafast laser beams. Mapping the structural modification of the photoexcited and subsequently heated alloys reveals previously neglected chemical reactions with air, offering a direct solution for incorporating nanoparticles into an amorphous oxide matrix and broadening the range of laser-induced surface self-organization features. Our findings are attributed to the occurrence and enrichment of oxygen surface contamination that reacts with selected elements of the metallic glasses. Additionally, the growth of the crystalline phase from undercooled liquid may originate from the dissolution of oxides. Finally, our results establish that the combination of crystalline nanoparticles on amorphous periodic patterns can be universally obtained in a wide range of binary systems of irradiated metallic glasses.

Continuous beam scanning by transverse mode-locking in a Pound-Drever-Hall stabilized resonator

Mode-locking, i.e., the phase-locked superposition of resonator modes, enables ultrafast laser beam dynamics. Here, high-speed beam scanning at 79.6 MHz is achieved by generating sets of phase-locked transverse resonator modes. Such transverse mode-locked (TML) beams are obtained via conversion in an empty, optical resonator from incident longitudinal mode-locked beams. In this conversion scheme the modal power and phase distributions can be controlled to alter the dynamics of the emerging scanning beams. It is shown, that the Pound-Drever-Hall stabilization scheme is applicable to compound TML resonances and it is used to maintain resonance between the incident beam’s spectral components and the transverse mode resonances. This way, TML states are reliably generated and continuously emitted.

Femtosecond laser machining of the novel superhydrophobic microstructure for the oil-water separation

Bionic micro/nano structures with superhydrophobicity is extensively used in oil-water separation. However, the fabrication of such tiny structures involves intricate processes, often requiring additional steps such as chemical or mechanical post-processing. With the development of the laser machining, it is feasible to fabricate structures in micro/nano scale via the ablation of ultrafast laser beam. Inspired by the function of the funnel, a novel through-hole structure was designed and manufactured by femtosecond laser in high dimensional precision. The copper film with the funnel-like through-hole structure exhibited remarkable hydrophobic properties, with a water contact angle of 160 ± 2° and a sliding angle ≤ 5°, facilitating the high-quality and high-efficiency separation of water-oil/organic liquids, compared with the conventional through-hole structures, such as circular or rectangular structures. Furthermore, the prepared film demonstrated exceptional capabilities of anti-freezing, anti-pollution, environmental stability, and versatility, examined by enduring treatments involving acidic liquids, immersion in hot solutions, freezing, and scratching. In sum, this work offers innovative concepts and valuable insights on the design and fabrication of functional structures with ultrafast laser machining.

Quantitative Mapping of Transient Thermodynamic States in Ultrafast Laser Nanostructuring of Quartz

Understanding material structural reaction to light is of utmost importance to advance processing resolution in ultrafast laser volume structuring into the nanoscale. Selective thermodynamic pathways are required to quench energy transport in the most rapid manner and to confine the process to nanometer lengths, bypassing optical resolution. Quantifying material dynamics under confinement, with in situ access to transient local temperature and density parameters, thus becomes key in understanding the process. We report in situ reconstruction of thermodynamic states over the entire matter relaxation path in bulk α -quartz irradiated by ultrafast nondiffractive laser beams using time-resolved qualitative and quantitative optical phase microscopy. Thermooptic dynamics indicate rapid spatially confined crystalline-to-amorphous transition to a hot dense fused silica form. Densification exceeds 20% and the matrix temperature rises to more than 2,000 K in the first nanosecond. This structural state relaxes in hundreds of nanoseconds. The dispersion and time design of the optical beam to picosecond durations increases the spatial confinement and triggers an extreme nanostructuring process based on nanocavitation that occurs within the amorphizing material, where the low-viscosity phase lowers the mechanical requirements for the process. Processing feature scales of less than a tenth of the optical wavelength are obtained in the volume. This allows for structural and morphological nanoscale material features under 3D confinement that can engineer optical materials.

Transition acceleration of electrons and reconnection of magnetic fields driven by two-beam relativistic laser pulses traversing a NCD plasma slab target

The self-generation and reconnection of magnetic fields and accompanying electron acceleration were studied when two ultrafast intense laser beams propagate synchronously through a thin plasma slab target with homogeneous near-critical density (NCD). The results show that the magnetic reconnection occurs behind the target and the electrons undergo three important accelerating stages, ponderomotive force acceleration in the plasma channels, transition (or transboundary) acceleration from the plasma medium to the vacuum, and acceleration by the reconnecting fields. In the reconnection process the hot electrons are compelled to be reverse-accelerated (generating refluxes) and consequently the double annular magnetic tube fields in channels are collapsed into one large-scale annular magnetic tube structure, in which a significant pinching effect is revealed. Meanwhile, two peculiar enclosed “field-free” zones were shown, which is suggested to be a combined effect of magnetic annihilation (MA) and longitudinal quasi-static electric field generated by separation of positive and negative charges.

Application of ultrafast laser beam shaping in micro-optical elements

The manufacturing and application of micro-optical elements are constantly evolving toward miniaturization, integration, and intelligence and have important applications in holographic displays, optical imaging, laser processing, information processing, and other fields. Ultrafast lasers, with their ultrashort pulse width, extremely high peak power, high processing resolution, small thermal influence zone, and nondestructive material processing advantages, have become an important processing method for preparing micro-optical elements. However, the laser output from the laser usually has a Gaussian distribution, with limitations in spatial and temporal energy and shape distribution, making it difficult to meet the requirements of processing efficiency and quality, which poses new challenges to ultrafast laser manufacturing technology. Therefore, by shaping the ultrafast laser beam and regulating nonlinear optical effects, the optimization and adjustment of the beam shape can be achieved, thus improving the quality and efficiency of micro-optical element processing. Ultrafast laser beam shaping technology provides a new method for the manufacture of micro-optical elements. This article first introduces the commonly used manufacturing methods for micro-optical elements. Second, from the perspective of the temporal domain, spatial domain, and spatiotemporal domain, the basic principles, methods, and existing problems of ultrafast laser beam shaping are summarized. Then, the application of these shaping technologies in the preparation of micro-optical elements is elaborated. Finally, the challenges and future development prospects of ultrafast laser beam shaping technology are discussed.

Transient heat transfer analysis of a cracked strip irradiated by ultrafast Gaussian laser beam using dual-phase-lag theory

With the rapid development of ultrafast laser technology, the effect of non-Fourier heat conduction on the transient process has received considerable attention. In the present article, the non-Fourier dual-phase-lag heat conduction theory is adopted to analyze the transient thermal process of a cracked strip subjected to ultrafast laser heating. The originality of this work lies in considerations of Gaussian type distribution of the laser beam and the insulated crack's disturbance to the heat flow. Fourier transform and Laplace transform are utilized to reduce the complex boundary-value problems to the Cauchy-type singular integral equation, which is then solved numerically by the Lobatto-Chebyshev technique. With the aid of the numerical inversion of Laplace transform, the transient temperature characteristics of the cracked strip are displayed graphically to illustrate the effects of the two non-Fourier thermal lags, laser incident position, and the Gaussian type heating source.

Armored Nanocones Engraved by Selective Laser Doping Enhanced Plasma Etching for Robust Supertransmissivity.

Optical antireflection surfaces equipped with subwavelength nanocone arrays are commonly used to reach broadband supertransmissivity but are limited by the lack of wear resistance. We design and manufacture a structured surface with robust antireflection structures (R-ARS) composed of substrate-engraved nanocone arrays with micro-grid-shaped walls as protective armor. An ultrafast laser beam is used to selectively ablate and dope the metal from the deposited film into the subsurface of optical substrates to strengthen self-assembled nanoparticles formed during plasma etching as masks for nanocones. The untreated microscale metal grids serve as etching masks for the remaining protective armor. The geometrical features of nanocones and spatial distribution of protective armor with a proper duty cycle are theoretically optimized for improvement in both transmissivity and mechanical robustness. We demonstrate armored dense engraved nanocone arrays (with tip diameters of ∼50 nm and heights of ∼0.8 μm) on visible fused silica and infrared semi-insulating SiC with protective micro-square-grid armor. The average transmittances are improved from 93% to over 97% (on 0.4-1.2 μm) for double-face-structured fused silica, and from 60 to 65% (on 3-5 μm) for single-face-structured SiC, with few reductions of fused silica after 150 cycles of severe abrasion (under a pressure of 5.34 MPa) proving the excellent mechanical robust performance of R-ARS.

Nano-spheroid formation on YAG surfaces induced by single ultrafast Bessel laser pulses

We report on single pulse ultrafast Bessel laser beam processing of YAG ceramic surfaces as a method for producing contamination-free submicron particles and size adjustable surface hemisphere structures of different curvature signs. The micro-machining process was performed in both air and liquid environments, leading to a variety of surface structures depending on the processing parameters. Particularly, a transformation of the surface structure morphology from micro-hole profiles to hemisphere extrusions was observed. The size of the hemisphere structure is found to be highly sensitive to laser parameters, such as pulse energy, pulse duration and beam focusing position. Through careful analyses of the influence of the laser pulse parameters, a precise regulation of the lateral diameter and height of the hemisphere structure was achieved. Large area hemisphere arrays with low standard deviation in size were fabricated. The detachment of the emerging structures and subsequent particle deposition can be observed in liquid environments when the height-diameter aspect ratio of the hemisphere exceeds a factor of 0.65. The Mechanisms for the formation and detaching of the hemisphere structure are discussed with cross-sectional and morphology imaging via Scanning Electron Microscopy and Atomic Force Microscopy. As a preliminary step towards submicron particle generation in liquid environments, the observation of surface hemispheres has interest in exploring the initial mechanisms of particles formation under laser ablation in liquids. The presented method allows for the fabrication of contamination-free and size adjustable YAG submicron convex structures which have potential applications in integrated optics, biotechnology and other advanced processing techniques.

Iterative wavefront optimization of ultrafast laser beams carrying orbital angular momentum.

Structured intense laser beams offer degrees of freedom that are highly attractive for high-field science applications. However, the performance of high-power laser beams in these applications is often hindered by deviations from the desired spatiotemporal profile. This study reports the wavefront optimization of ultrafast Laguerre-Gaussian beams through the synergy of adaptive optics and genetic algorithm-guided feedback. The results indicate that the intensity fluctuations along the perimeter of the target ring-shaped profile can be reduced up to ∼15%. Furthermore, the radius of the ring beam profile can be tailored to a certain extent by establishing threshold fitting criteria. The versatility of this approach is experimentally demonstrated in conjunction with different focusing geometries.

Polarization of Femtosecond Laser for Titanium Alloy Nanopatterning Influences Osteoblastic Differentiation.

Ultrashort pulse lasers have significant advantages over conventional continuous wave and long pulse lasers for the texturing of metallic surfaces, especially for nanoscale surface structure patterning. Furthermore, ultrafast laser beam polarization allows for the precise control of the spatial alignment of nanotextures imprinted on titanium-based implant surfaces. In this article, we report the biological effect of beam polarization on human mesenchymal stem cell differentiation. We created, on polished titanium-6aluminum-4vanadium (Ti-6Al-4V) plates, a laser-induced periodic surface structure (LIPSS) using linear or azimuthal polarization of infrared beams to generate linear or radial LIPSS, respectively. The main difference between the two surfaces was the microstructural anisotropy of the linear LIPSS and the isotropy of the radial LIPSS. At 7 d post seeding, cells on the radial LIPSS surface showed the highest extracellular fibronectin production. At 14 days, qRT-PCR showed on the same surface an increase in osteogenesis-related genes, such as alkaline phosphatase and osterix. At 21 d, mineralization clusters indicative of final osteoinduction were more abundant on the radial LIPSS. Taken together, we identified that creating more isotropic than linear surfaces enhances cell differentiation, resulting in an improved osseointegration. Thus, the fine tuning of ultrashort pulse lasers may be a promising new route for the functionalization of medical implants.

Positioning of diffuse metallic target with a point diffractive interferometer

Proton acceleration obtained by focusing an ultraintense ultrafast laser beam presents the technological and metrological challenge of correctly placing diffuse metallic target at the focus of the laser beam. In this work we present the use of the Point diffractive interferometer for solving this problem. We studied the accuracy and precision of the system at repositioning the metallic target after displacing and horizontally tilting the target out of its reference position. We achieved an accuracy at repositioning the target at its reference position of 1.50 µm and 6 arcmin with a precision of 1.40 µm and 5.7 arcmin. Our work shows the high accuracy provided by a system as simple as the Point Diffraction interferometer even at positioning surfaces with diffuse reflection.

Super-efficient drilling of metals with ultrafast non diffractive laser beams

A highly efficient drilling process is found in non-transparent metallic materials enabled by the use of non-diffractive ultrafast Bessel beams. Applied for deep drilling through a 200 μm-thick steel plate, the Bessel beam demonstrates twofold higher drilling efficiency compared to a Gaussian beam of similar fluence and spot size. Notwithstanding that surface ablation occurs with the same efficiency for both beams, the drilling booster results from a self-replication and reconstruction of the beam along the axis, driven by internal reflections within the crater at quasi-grazing incidence, bypassing potential obstacles. The mechanism is the consequence of an oblique wavevectors geometry with low angular dispersion and generates a propagation length beyond the projection range allowed by the geometry of the channel. With only the main lobe being selected by the channel entrance, side-wall reflection determines the refolding of the lobe on the axis, enhancing and replicating the beam multiple times inside the channel. The process is critically assisted by the reduction of particle shielding enabled by the intrinsic self-healing of the Bessel beam. Thus the drilling process is sustained in a way which is uniquely different from that of the conventional Gaussian beam, the latter being damped within its Rayleigh range. These mechanisms are supported and quantified by Finite Difference Time Domain calculations of the beam propagation. The results show key advantages for the quest towards efficient laser drilling and fabrication processes.

Linearity and Nonlinearity in Hollow-Core Antiresonant Fiber Sensors in the Terahertz Regime

In nonlinear optics, a major challenge is to maximize the interaction between the light from laser sources and low-density media such as gases. An ultrafast laser beam can be focused to form a highly intense spot over a small concentrated area. The ultrafast laser beam has a short pulse width of less than one picosecond and high peak power in the beam profile (Fig. 1). The beam profile of the laser source describes the energy density and distribution of light, and the beam profile of a laser source is usually affected during the propagation and collimation of the beam. An efficient nonlinear optical sensor also requires high peak power at low energy (low average power) over short duration of laser pulse, a high beam profile and long interaction length. These requirements of a nonlinear optical sensor with low attenuation constant can be achieved in hollow-core photonic crystal fiber (HC-PCF). Gas-filled HC-PCF exhibits optical nonlinearity for an ultrashort temporal and spectral broadening of NIR pulses [1], [2]. The nonlinearity in HC-PCF can be achieved by tuning the gas pressure. Gas-filled HC-PCF nonlinear media are low-cost, replenishable, reconfigurable and exhibit sharp spectral lines [3]. Linear and nonlinear responses are also found in other types of optical fibers.

Spectral vector beams for high-speed spectroscopic measurements

Structuring light in multiple degrees of freedom has become a powerful approach to create complex states of light for fundamental studies and applications. Here, we investigate the light field of an ultrafast laser beam with a wavelength-dependent polarization state, which we term a spectral vector beam. We present a simple technique to generate and tune such structured beams and demonstrate their spectroscopic capabilities. By measuring only the polarization state using fast photodetectors, it is possible to track pulse-to-pulse changes in the frequency spectrum caused by, e.g., narrowband transmission or absorption. In our experiments, we reach readout rates of around 6 MHz, which is limited by our technical ability to modulate the spectrum and can in principle reach GHz readout rates. In simulations we extend the spectral range to more than 1000 nm by using a supercontinuum light source, thereby paving the way to various applications requiring high-speed spectroscopic measurements.

LIPSS manufacturing with regularity control through laser wavefront curvature

Laser-Induced Periodic Surface Structures (LIPSS) manufacturing is a convenient laser direct-writing technique for the fabrication of nanostructures with adaptable characteristics on the surface of virtually any material. In this paper, we study the influence of 1D laser wavefront curvature on nanoripples spatial regularity, by irradiating stainless steel with a line-focused ultrafast laser beam emitting 120 fs pulses at a wavelength of 800 nm and with 1 kHz repetition rate. We find high correlation between the spatial regularity of the fabricated nanostructures and the wavefront characteristics of the laser beam, with higher regularity being found with quasi-plane-wave illumination. Our results provide insight regarding the control of LIPSS regularity, which is essential for industrial applications involving the LIPSS generation technique.

超快激光光束整形原理与方法及其在功能性微结构制造中的应用

超快激光光束整形原理与方法及其在功能性微结构制造中的应用

High Performance Electrode Architectures By Laser Structuring of Silicon-Graphite Anodes and Ultra-Thick Film NMC-Type Cathode Materials

Laser structuring of battery materials such as electrodes, current collectors, and separator materials becomes more and more a versatile tool for a flexible designing of battery performance with regard to high rate capability, reduced impedance and diffusion over-potential, enhanced battery lifetime, and a tremendous improvement of electrode wetting with liquid electrolyte. Especially structuring of thick film composite electrodes with film thickness up to 300 µm is of great interest for combining high energy and high power applications. Laser ablation by using ultrafast laser radiation ensures a high aspect ratio structuring without damaging or modifying the material properties of the active material, inactive compounds, and current collector. Thermal induced material modification and laser-induced material splatter on top of the electrode surfaces have been so far a major problem by using conventional nanosecond lasers. However, in recent years high power OEM-type ultrafast laser beam sources became available for industrial production. Those lasers provide powers of several 100 W up to the kW-regime with repetition rates of several tenfold MHz, which makes high-speed parallel material processing feasible. Finally, after introducing high-speed nanosecond laser cutting of electrodes almost one decade ago, we forecast that with regard to recent technical approaches quite soon high-speed electrode ultrafast laser structuring will entering the pilot line level. For this purpose, at KIT, parallel processing of cathode and anode materials with large foot print area is being developed in a roll-to-roll (R2R) environment in order to meet the process speed requirements of cell production. Furthermore, we are developing thick film electrode concepts for anodes made of graphite and silicon-graphite and cathodes based on Lithium-Nickel-Manganese-Cobalt-Oxide (NMC) with varying nickel content and water-based slurries. Electrode thicknesses in the range of 100-300 µm were synthesized for subsequent laser processing, cell assembly, and electrochemical characterization such as galvanostatic measurements, galvanostatic intermittent titration technique, cyclic voltammetry, and electrochemical impedance spectroscopy. Large area laser structuring in R2R environment for generation of three-dimensional (3D) electrode architectures with increased active surface area, enhanced mechanical integrity for high energy materials such as Si-graphite anodes and an overall boost in electrochemical performances on pouch cell level will be presented. Laser-induced breakdown spectroscopy (LIBS) was applied for elemental mapping of entire electrodes in order to characterize binder migration in ultra-thick film electrodes, and in post-mortem studies to evaluate degradation processes in unstructured, damaged, and structured thick film electrodes with large footprint (pouch cell level). Post-mortem studies show that significant chemical modifications across from electrodes with macroscopic surface defects could be detected while laser structured electrodes provide less inhomogeneity in lithium distribution in comparison to cells with unstructured electrodes. Furthermore, an inhomogeneity in mechanical load on cells could be identified as a main reason for cell degradation. LIBS is presented as a perfect tool to rapidly characterize and identify starting points for cell degradation on coin cell and pouch cell level. In summary, ultrafast laser structuring of electrodes provides a new manufacturing tool for next generation battery production to overcome current limitations in electrode design and cell performances.

Insight into diffusive and convective processes affecting gold nanoparticles microclustering by multiphoton photoreduction

Multi-photon direct laser writing (MP-DLW) in polymeric matrices doped with a tetrachloroauric acid (HAuCl4) water solution allows creating clusters of gold nanoparticles (GNPs) inside the focus figure of a tightly focused ultrafast laser beam. The key physical phenomena involved in the process are analyzed, with the aim to assess the limits and potential of this promising technology. Multi Photon Absorption (MPA) triggers the photo-reduction of AuCl4– ions and the consequent creation of GNPs in the spotted volume. Thermal electronic decays lead to a local abrupt increase of the temperature, which influences the morphology of the created structures. At the same time, two different effects take place, related to the dehydration of the polymeric matrix, and the concentration gradient of the gold precursor upon localized photoreduction. Given their different timescales, these phenomena allow for controlling the GNPs density and size dispersity when a given energy dose is delivered in multiple exposures, tuning the delay between consecutive laser exposures. A simple yet effective experiment to estimate the temperature distribution at the micron-scale is also proposed.