Ultrashort Laser Pulses Research Articles

Soliton interaction in a MXene-based mode-locked fiber laser.

MXenes are a class of two-dimensional layered structure ternary metal carbide or/and nitride materials. Recently, the MXene V2CTx has demonstrated excellent long-term stability, strong saturable absorption, and fast optical-switching capability, used to generate Q-switched and ultrashort pulsed lasers. However, bound-state fiber lasers based on V2CTx have not been reported yet. In this study, V2CTx is combined with a D-shaped fiber to form a saturable absorber device, whose modulation depth is measured to be 1.6%. By inserting the saturable absorber into an Er-doped fiber laser, bound states with different soliton separation and munbers are successfully obtained. Additionally, bound states with a compound soliton structure, such as the (2 + 2)- and (2 + 1)-type, are also realized. Our findings show that V2CTx can be developed as an efficient ultrafast photonics candidate to further understand the complex nonlinear dynamics of bound-state pulses in fiber lasers.

Transient responses of dielectrics during ultrashort pulsed laser irradiation using a generalized thermoelastic model considering temperature-dependent thermophysical properties

A novel model involving electron dynamics, electron-lattice interactions, and thermoelastic excitation for dielectrics exposed to ultrashort pulsed laser is proposed. The thermoelastic equations are modified based on the G-L generalized thermoelasticity and consider the temperature-dependent thermophysical properties. An axisymmetric problem of ultrashort pulsed laser irradiation on fused silica is presented. The fully-coupled equations are solved simultaneously using the finite element method to obtain the transient electron density, electron temperature, lattice temperature, and stresses. The evolution of transient responses and the effects of laser intensity, pulse duration, and temperature-dependent thermophysical properties are analyzed in detail. It was found that there are kinks in the distribution of transient responses, which is attributed to the rapid change of reflectivity. The axial stress has wave-like characteristics. The electron temperature is slightly lower than the lattice temperature after the thermal equilibrium is achieved. Maybe the temperature-dependence of lattice thermal conductivity could be neglected to reduce computational expenses.

Evaluation of an ultrashort pulsed laser robot system for flexible and large-area micromachining

We report for the first time on the realization, characterization and application of an ultrashort pulsed laser robot system for flexible and large-area 2D and 3D laser micromachining with 6 articulated axes. To characterize the dynamic positioning of the laser beam during and after axes movement, CMOS image sensors were integrated into the beam path. A method introduced for the alignment of the optical axes allows a reduction of the deviations in laser beam positioning to less than 141.8 ± 92.9 μm within a 110° rotation range of axis 4. In addition, a high laser beam positioning repeatability of less than 102.2 μm is demonstrated over a total period of 14 h for a movement of axis 5 within a range of 0° to 90°. Initial laser cutting, laser structuring and laser marking applications on automotive dashboards and glass substrates are presented for flexible and large area 2D and 3D manufacturing. By applying a special laser cutting strategy for processing AF 32 eco thin glass, high cutting quality is achieved with a taper of up to 96.3% without the generation of cracks, demonstrating the innovative potential of the high-precision laser robot system. Nonetheless, different identified inherent influences of each axis 1–5 during robot axis movement demand for an innovative beam stabilization concept to achieve high precision in laser beam positioning.

Ultrafast laser texturing to improve wettability of polyimide (Kapton) films

Aim of this study is investigating the possibility of increasing the wettability of polyimide Kapton thin films by laser texturing. The laser textured surfaces were characterized through the water contact angle measurement and the surface tension evaluation; morphological and topological analyses were performed by scanning electron microscopy and atomic force microscope, respectively. As main outcome, it was found that laser texturing increases the wettability with respect to the untreated material with a decrease of contact angle of more than 20 %. In addition, the ultrashort pulse laser induced, on the material, LIPSS structures oriented in the direction of the polarization plane.

Reconstruction of Femtosecond Laser Pulses from FROG Traces by Convolutional Neural Networks

We report on the reconstruction of ultrashort laser pulses from computer-simulated and experimental second harmonic generation-frequency resolved optical gating (SHG-FROG) spectrograms. In order to retrieve the spectral amplitude and phase we use a convolutional neural network trained on simulated SHG-FROG spectrograms and the corresponding spectral-domain fields employed as labels for the network, which is a complex field encompassing the full information about the amplitude and phase. Our results show excellent retrieval capabilities of the neural network in case of the simulated pulses. Although trained only on computer generated data, the method shows promising results regarding experimentally measured pulses.

Picosecond laser structuring of graphite anodes—Ablation characteristics and process scaling

Laser structuring of graphite anodes substantially improves the electrochemical performance of lithium-ion batteries by facilitating lithium-ion diffusion through the electrode coatings. However, laser structuring is not yet established in industrial battery production due to limited knowledge of its ablation behavior and a low processing rate. This publication addresses these issues with a combination of experimental and theoretical approaches. In a comprehensive process study with picosecond pulsed laser radiation, the influence of various laser parameters on the obtained structure geometries, i.e., the hole diameters and depths, was examined. Wavelengths of 532 and 355 nm combined with pulse bursts and fluences of approximately 10 J cm−2 eventuated in favorable hole geometries with a high aspect ratio. Compared to singlebeam laser structuring, a nearly tenfold reduction in the processing time was achieved by beam splitting with a diffractive optical element without compromising structure geometries or mechanical electrode integrity. The experimental findings were used to model the scalability of electrode laser structuring, revealing the significant influence of the hole pattern and distance on the potential processing rate. Ultrashort pulsed laser powers in the kilowatt regime were found to be necessary to laser-structure electrodes at industrial processing rates resulting in estimated costs of roughly 1.96 $/kWh. The findings support the industrialization of laser electrode structuring for commercial lithium-ion battery production.

Numerical Aperture-Dependent Spatial Scaling of Plasma Channels in HPHT Diamond

The investigation of plasma channels induced by focused ultra-short 1030-nm laser pulses in bulk of synthetic High Pressure High Temperature (HPHT) diamond revealed strong dependence of their spatial parameters on the used numerical aperture of the lens (NA = 0.15–0.45). It was shown that at weak focusing conditions it is possible to significantly increase the length of the plasma channel with a slight increase in pulse power, while tight focusing allows one to obtain more compact structures in the same range of used powers. Such a dependence paves the way to new possibilities in 3D processing of transparent dielectrics, allowing one, for example, to vary the spatial parameters of modified regions without changing the setup, but only by controlling the lens aperture, which seems very promising for industrial applications.

Thermal Lattice Field during Ultra-Short Laser Pulse Irradiation of Metal Targets: A Fokker–Planck Analytical Model

The ultrafast fs laser pulse heating of thin metal films is studied for the first time using the two-temperature model on the basis of the Fokker–Planck formalism. The incident laser radiation is multi-modal, while the electron temperature is described during the first 2 fs. The predictions are intended for use by experimentalists in optoelectronics, photonics, laser processing, electronics, and bio- and nanomedicine. The crucial role of the nano-sized spatial dimensions of the metal sample is highlighted. A significant result of this study is the interdependence between the target’s size, the phonon/lattice characteristics, and the coefficient β (the quotient of non-diffusive phenomena), which varies between zero (pure diffusive case) and one (pure non-diffusive case).

Thermal-Mechanical Effect and Removal Mechanism of Ti-6Al-4V During Laser-Assisted Grinding

The low density and high corrosion resistance of titanium alloy make it a material with various applications in the aerospace industry. However, because of its high specific strength and poor thermal conductivity, there are problems such as high cutting force, poor surface integrity, and high cutting temperature during conventional machining. As an advanced processing method with high efficiency and low damage, laser-assisted machining can improve the machinability of titanium alloy. In this study, a picosecond pulse laser-assisted scratching (PPLAS) method considering both the temperature-dependent material properties and ultrashort pulse laser’s characteristics is first proposed. Then, the effects of laser power, scratching depth, and scratching speed on the distribution of stress and temperature field are investigated by simulation. Next, PPLAS experiments are conducted to verify the correctness of the simulation and reveal the removal behavior at various combinations of laser power and scratching depths. Finally, combined with simulated and experimental results, the removal mechanism under the two machining methods is illustrated. Compared with conventional scratching (CS), the tangential grinding force is reduced by more than 60% and the material removal degree is up to 0.948 during PPLAS, while the material removal is still primarily in the form of plastic removal. Grinding debris in CS takes the form of stacked flakes with a “fish scale” surface, whereas it takes the form of broken serrations in PPLAS. This research can provide important guidance for titanium alloy grinding with high surface quality and low surface damage.

Single-shot, high-repetition rate carrier-envelope-phase detection of ultrashort laser pulses.

We propose a single-shot, high-repetition rate measurement scheme of the carrier-envelope phase offset of ultrashort laser pulses. The spectral fringes resulting from f-2f nonlinear interferometry, encoding the carrier-envelope-phase, are evaluated completely optically via an optical Fourier transform. For demonstration, the carrier-envelope-phase of a 200 kHz, few-cycle optical parametric chirped-pulse amplification (OPCPA) laser system was measured employing an interferometer as a periodic optical filter. The proposed method shows excellent agreement with simultaneous measurement of the spectral fringes by a fast line-scan camera.

Development and comparison of algorithms for beam stabilization in ultrashort pulsed laser equipped on a six-axis robot

We demonstrate an innovative beam stabilization concept consisting of complementary metal-oxide semiconductor cameras and piezo actuators for a six-axis articulated ultrashort pulsed laser robot system. The beam stabilization system is fixed on robot axes 4 and 5. Moving robot axis 5 in an angular range between 0° and 90°, the laser beam position coupled to the actual robot position is monitored by two cameras integrated on robot axis 5 and used for laser beam characterization and model generation. A mathematical description and models generated with machine learning methods, namely, linear regression and neural network, are compared for predicting the beam position drift as a function of robot axis motion, where the neural network model shows a low prediction tolerance of about 7 pixels. In addition, a stand-alone time-triggered beam correction algorithm is developed and implemented on the system, which shows an excellent correction performance for large beam position drifts (below 500 pixels).

Ultra-short pulsed laser processing of single crystalline diamonds for tooling applications

This study investigates the feasibility of using ultra-short pulsed (USP) lasers to fabricate single crystalline diamond (SCD) tools. SCD has exceptional mechanical, tribological, and thermal properties and offers excellent performance in the precision machining of hard and brittle materials over polycrystalline diamond and diamond-coated tools. However, the anisotropic nature of the SCD makes it difficult for laser machining because the material shows susceptibility to cracking, defect growth, and breakout depending on its crystallographic orientation. Anisotropy needs to be considered while optimizing the geometry of the tool to minimize wear and improve tool performance. An advanced four-axis laser machining approach with optimized laser parameters and temporal beam shaping is used to mitigate challenges related to defect growth and orientation dependence, leading to the production of high-quality single cutting-edge SCD tools. Cylindrical diamonds and diamond crystals with top surface planes {100} and {111} are used in the study. The occurrence of defects in the diamond when laser machined and their dependence on the crystallographic orientation along the circumference of the diamond is thoroughly investigated via SEM, electron backscatter diffraction, and light microscopy images. Finally, the laser-manufactured SCD tools are tested by turning fully sintered zirconia ceramics (3Y-TZP-A). USP laser machining of SCD is demonstrated to be a viable alternative to traditional manufacturing methods for producing high-quality SCD tools with unique properties and performance. The results further emphasize the importance of understanding the crystallographic orientation dependence when laser machining crystalline materials like diamonds.

High repetition ultrafast laser ablation of graphite and silicon/graphite composite electrodes for lithium-ion batteries

Laser structuring can be applied to composite electrodes of lithium-ion cells to enhance wetting and to facilitate the usage of thick-film electrodes by reducing the lithium-ion diffusion overpotential and the tortuosity of the electrodes or the usage of electrodes containing silicon, where additional porosity is required to compensate the volume expansion during lithium de-/insertion. To integrate the additional laser processing step in the well-established electrode manufacturing route, the laser processing speed must be significantly increased to match with the belt speed, which is dependent on the electrode thickness and the type of manufacturing route. Upscaling can be realized by increasing the average laser power, laser intensity, and/or laser repetition rate. Here, an ultrashort pulsed laser source with an average power of 300 W and a pulse duration of 600 fs was applied. For the first time, the presented research provides detailed laser ablation processing data for thick-film composite anodes associated with high repetition rates ranging from 4.9 to 48.8 MHz. The patterning results are compared depending on the widths, depths, aspect ratios, the total appearance regarding debris and cracks, and the volume ablation rate. In high repetition rate laser patterning, the subsequent laser pulses interact with the material vapor plasma generated by the previous laser pulses, resulting in lower ablation depths and higher ablation widths. The increase in laser peak intensity leads to higher achievable ablation depths. Processing strategies are identified for two different ablation scenarios focusing on the pouch cells of a Volkswagen ID.3 and the Tesla 4680 cell.

The ultrashort pulse laser ablation model of silicon based on the generalized thermoelastic theory with spatio-temporal nonlocal effect

The present work is devoted to establishing the ultrashort pulse laser ablation generalized thermoelastic model with spatio-temporal nonlocal effect and investigating the transient responses of the process of silicon ablated by the picosecond pulse laser. The research is divided into three cases. 1) The solid silicon before the target surface melting is studied, and the transient responses of the final state of this case are taken as the initial conditions of the Case Two. 2) The liquid silicon after the target surface melting is researched, and the heat flux taken away by vaporization is considered in the boundary condition. 3) The solid silicon after the target surface melting is discussed, and the moving solid-liquid interface is regarded as the coordinate origin. The coupled governing equations containing spatial nonlocal parameter, time delay factor, and kernel function are established and solved by the Laplace transform together with its numerical inversion. The melting time and depth of target are emphatically discussed. The temperature, displacement and stress with different laser intensity, spatial nonlocal parameter, time delay factor, kernel function as well as time are obtained and illustrated graphically.

X-ray generation by fs-laser processing of biological material.

The use of ultrashort pulse lasers in medical treatments is increasing and is already an essential tool, particularly in the treatment of eyes, bones and skin. One of the main advantages of laser treatment is that it is fast and minimally invasive. Due to the interaction of ultrashort laser pulses with matter, X-rays can be generated during the laser ablation process. This is important not only for the safety of the patient, but also for the practitioner to ensure that the legally permissible dose is not exceeded. Although our results do not raise safety concerns for existing clinical applications, they might impact future developments at higher peak powers. In order to provide guidance to laser users in the medical field, this paper examines the X-ray emission spectra and dose of several biological materials and describes their dependence on the laser pulse energy.

Coupled Cluster Simulation of Impulsive Stimulated X-ray Raman Scattering.

Time-dependent equation-of-motion coupled cluster (TD-EOM-CC) is used to simulate impulsive stimulated X-ray Raman scattering (ISXRS) of ultrashort laser pulses by neon, carbon monoxide, pyrrole, and p-aminophenol. The TD-EOM-CC equations are expressed in the basis of field-free EOM-CC states, where the calculation of the core-excited states is simplified through the use of the core-valence separation (CVS) approximation. The transfer of electronic population from the ground state to the core- and valence-excited states is calculated for different numbers of included core- and valence-excited states, as well as for electric field pulses with different polarizations and carrier frequencies. The results indicate that Gaussian pulses can transfer significant electronic populations to the valence states through the Raman process. The sensitivity of this population transfer to the model parameters is analyzed. The time-dependent electronic density for p-aminophenol is also showcased, supporting the interpretation that ISXRS involves localized core excitations and can be used to rapidly generate valence wavepackets.

Coupling dynamics of dissipative localized structures: From polarized vector solitons to soliton molecules

Dissipative solitons are well known as representative nonlinear localized wave packets resulting from the composite balance of dispersion, nonlinearity, gain and loss in optical cavities. Since the last three decades, dissipative solitons keep as one of the most active research topics in the framework of nonlinear physics, not only for fundamental researches but also for various industrial applications. From one hand, due to the rich nonlinear dynamics, dissipative solitons have been considered as an ideal testbed to study the pumping-dissipation-bifurcation complexities. On the other hand, the emergences of dissipative solitons are tightly linked with the emission of ultrashort laser pulses, as well as the formation of broadband optical frequency combs. Compared with single-soliton counterpart, the multi-component dissipative solitons have exhibited even more fascinating properties and much wider range of applications, therefore the coupling dynamics of dissipative solitons is the blooming and emergent research area in the last decade. From this point of view, this review article covers the latest developments of compound dissipative solitons, not only in passive Kerr resonators, but also in mode-locked laser cavities. Ranging from polarized vector solitons to soliton molecules, the rising degree of freedom of these multi-component localized structures is boosting the discovery of novel nonlinear dynamics and underpin the various photonic applications in light science.

Excitonic Effect Drives Ultrafast Transition in Two-Dimensional Transition Metal Dichalcogenides.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are ideal platforms for exploring excitonic physics because of the tightly bound excitons. In this work, we observed the onset of band-edge exciton formation in monolayer MoS2 (WS2) and bilayer MoS2-WS2 by measuring the transient optical response upon excitation with ultrashort laser pulses. In addition to wavelength dependence on excitation under nonresonant excitation, we found that the onset of band-edge exciton formation in monolayer MoS2 (WS2) pumped in the exciton state is significantly faster than that with pumping in the nonexciton state, which could be attributed to the effective transition between exciton states induced by the excitonic effect. Besides, the onset of band-edge exciton formation in van der Waals heterostructures is similar to that for monolayer TMDCs regardless of charge transfer at the interface. Our work contributes to a better understanding of exciton dynamics in 2D TMDCs, providing a solid basis of the rational design of the 2D optoelectronic applications based on TMDCs.

Scaling up picosecond laser ablation of a LATGP-type glass-ceramic solid electrolyte for all-solid-state battery production

All-solid-state batteries are an intensively studied technology expected to complement conventional lithium-ion batteries in the near future. Research into fundamental production processes is crucial for enabling economic, industrial-scale all-solid-state battery production. Laser radiation is a production tool suited for contour cutting and surface structuring of inorganic solid electrolyte separators. In the scope of this work, the laser ablation of the commercial glass-ceramic solid electrolyte lithium aluminum germanium titanium phosphate (LAGTP) using ultrashort-pulsed laser radiation at different wavelengths, fluences, and pulse repetition rates is studied. Ablation thresholds, efficiencies, and rates were determined by manufacturing percussion drillings, trenches, and cavities. Geometry dependencies, such as the ablation rate decay with advancing depth during laser cutting, necessitated an application-oriented determination of ablation metrics. The surface qualities of the solid electrolyte layers were evaluated after laser ablation by scanning electron microscopy and surface roughness measurements. A wavelength of 532 nm was found to be beneficial compared to shorter and longer wavelengths in terms of productivity and ablation quality, reaching volume ablation efficiencies up to 36 μm3∙μJ−1 and surface roughnesses Sa below 500 nm. The determined metrics are discussed for ablation cutting and structuring of solid electrolyte layers in all-solid-state battery production, considering throughput and surface quality. The results provide insight into the ablation behavior of glass-ceramic materials and support the application of ultrashort-pulsed laser ablation in next-generation battery production.

Research on the control method of the structural stability of the precision tiled-grating

In a chirped pulse amplification device, tiled grating is a key component for compressing ultrashort pulse laser. The control system of the grating tiling device is essential for maintaining the stability of the array grating. In this study, a proportional-integral-derivative algorithm based on the latch compensation method and four-point center difference method is proposed for a truss-type 2 × 1 grating tiling device. Numerical analyses show that this method avoids the saturation loss in the stability control process of the array grating, effectively suppresses noise interference, and ensures the stability of the array grating. The prototype experiment shows that the use of this control algorithm can significantly improve the convergence speed and stability of the grating tiling device. Specifically, compared with a mechanism without a control algorithm, the time to reach the reference position for the first time is reduced by 39.1%, and the stability of the grating device is improved by 58.5%.