High-power Ultrashort Laser Pulses Research Articles

Gaining a New Technological Readiness Level for Laser-Structured Electrodes in High-Capacity Lithium-Ion Pouch Cells

For the first time, the laser structuring of large-footprint electrodes with a loading of 4 mAh cm−2 has been validated in a relevant environment, including subsequent multi-layer stack cell assembly and electrochemical characterization of the resulting high-capacity lithium-ion pouch cell prototypes, i.e., a technological readiness level of 6 has been achieved for the 3D battery concept. The structuring was performed using a high-power ultrashort-pulsed laser, resulting in well-defined line structures in electrodes without damaging the current collector, and without melting or altering the battery active materials. For cells containing structured electrodes, higher charge and discharge capacities were measured for C-rates >1C compared to reference cells based on unstructured electrodes. In addition, cells with structured electrodes showed a three-fold increase in cycle lifetime at a C-rate of 1C compared to those with reference electrodes.

Simple single-shot complete spatiotemporal intensity and phase measurement of an arbitrary ultrashort pulse using coherent modulation imaging.

We propose a simple single-shot spatiotemporal measurement technique called coherent modulation imaging for the spatio-spectrum (CMISS), which reconstructs the full three-dimensional high-resolution characteristics of ultrashort pulses based on frequency-space division and coherent modulation imaging. We demonstrated it experimentally by measuring the spatiotemporal amplitude and phase of a single pulse with a spatial resolution of 44 µm and a phase accuracy of 0.04 rad. CMISS has good potential for high-power ultrashort-pulse laser facilities and can measure even spatiotemporally complicated pulses with important applications.

Multiscale model for ultrashort pulsed parallel laser structuring—part II. The macroscale model

The increasing pixel density in displays demands high quality in the production of fine metal masks (FMMs). The production process of FMMs boils down to structuring tiny holes in thin metal sheets or foils. The manufacturing requirements of FMMs are high precision in terms of the hole geometry to let enough light escape from each diode and high productivity to produce the required amount. To achieve both objectives, high power ultrashort pulse (USP) lasers can be utilized. Because USP lasers fall short of the productivity requirements, they are combined with multibeam scanners. During production, the multibeam scanners deposit a lot of heat in the metal foil, which can ultimately yield temperature-induced distortions. To understand and finally avoid such distortions, a process simulation is sought. In a preceding study, the structuring of a single hole (the microscale) was investigated, but due to the large differences in the time and spatial scales involved, it was not feasible to simulate the production of the whole part (the macroscale). Within this treatise, a multiscale approach that takes into account the necessary information from the microscale to describe temperature-induced distortions on the macroscale is described. This approach targets laser ablation processes with pulse durations ranging from picoseconds up to nanoseconds provided the ablation is not melt-driven. First, a representative volume element (RVE) is generated from the results of the microscale model. Then, this RVE is utilized in the thermo-elastic structural mechanics simulation on the macroscale. The multiscale model is validated numerically against a hole-resolved computation, which shows good agreement. Naturally, the simulation is highly dependent on the microscale model, which in turn depends on the material properties. To handle material changes well, an experimental calibration has to be performed. This calibration is not part of this treatise, but it will be described in a future publication. In addition to the calibration process, the validation with experiments will be conducted in future research. Additionally, the authors envision the automation of the whole process, resulting in a first-time-right approach for the development of FMMs. Finally, the procedure might be extended to the requirements of other filtration purposes.

Micromachining of Alumina Using a High-Power Ultrashort-Pulsed Laser.

We report on a comprehensive study of laser ablation and micromachining of alumina using a high-power 1030 nm ultrashort-pulsed laser. By varying laser power up to 150 W, pulse duration between 900 fs and 10 ps, repetition rates between 200 kHz and 800 kHz), spatial pulse overlap between 70% and 80% and a layer-wise rotation of the scan direction, the ablation efficiency, ablation rate and surface roughness are determined and discussed with respect to an efficient and optimized process strategy. As a result, the combination of a high pulse repetition rate of 800 kHz and the longest evaluated pulse duration of 10 ps leads to the highest ablation efficiency of 0.76 mm/(W*min). However, the highest ablation rate of up to 57 mm/min is achieved at a smaller repetition rate of 200 kHz and the shortest evaluated pulse duration of 900 fs. The surface roughness is predominantly affected by the applied laser fluence. The application of a high repetition rate leads to a small surface roughness Ra below 2 m even for the usage of 150 W laser power. By an interlayer rotation of the scan path, optimization of the ablation characteristics can be achieved, while an interlayer rotation of 90 leads to increasing the ablation rate, the application of a rotation angle of 11 minimizes the surface roughness. The evaluation by scanning electron microscopy shows the formation of thin melt films on the surface but also reveals a minimized heat affected zone for the in-depth modification. Overall, the results of this study pave the way for high-power ultrashort-pulsed lasers to efficient, high-quality micromachining of ceramics.

On the effect of stimulated raman scattering on THz radiation in the laser plasma interactions

In present study, two-dimensional Particle-In-Cell (PIC) simulations are used to investigate the interaction of ultrashort high-power laser pulses with an under-dense plasma medium. The results of the numerical simulations are followed by some mathematical procedures to obtain the terahertz electric field and the growth rates of the Stimulated Raman Forward (RFS) and Backward Scattering (RBS). The influences of incident laser pulse intensity and initial plasma density are examined on RFS and RBS instabilities and their consequent effects on the characteristics of THz radiation generation. It is seen that at the lower initial plasma densities i.e. 0.0001nc, the larger laser intensities will lead to more broadband and intense THz signals. Where nc=1.74 × 1027m−3 is known as critical plasma density corresponding to the laser pulse frequency. On the other hand, at the higher plasma densities, i.e., 0.0026nc, due to the instabilities growth, the effects of higher laser pulse intensities on the THz emission are insignificant. While for all the considered plasma densities and incident laser intensities, RBS inevitably affects the THz generation, RFS starts to grow for the initial plasma densities of n0=0.0026nc at the incident laser intensities higher than 1 × 1015Wcm−2. Hence, a higher bound is established on the pump intensity which is based on the RFS growth initiation. Thus, to achieve a stable THz signal for the initial plasma density of n0=0.0026nc (4.5 × 1018 cm−3), the incident laser intensity needs to be less than 1 × 1016Wcm−2.

Air-Lasing: High-Resolution Spectroscopy for Atmospheric Remote Sensing

Air-lasing is a cavityless coherent radiation generated in free space from air constituents as the gain medium, featuring significant advantages such as high collimation, high coherence and high intensity. Benefited from the long-range filamentation of high-power ultrashort laser pulses propagating in air, the air-laser can be induced remotely which provides an ideal light source for atmospheric remote sensing and chemical specie-resolved detection. Thanks to the coherent atomic/molecular excitation process accompanying the generation of air laser, remote sensing based on air-laser has high spectral resolution and high detection sensitivity, which is recently proved to be a powerful tool for important applications such as trace molecule detection, greenhouse gas monitoring and industrial pollutant detection. In this short review, the physical mechanism of air laser is briefly introduced, and various applications of air laser remote sensing are reviewed emphatically, and the future research is prospected.

Air-lasing high-resolution spectroscopy for atmospheric remote sensing

Air-lasing is a cavityless coherent radiation generated in free space from air constituents as gain medium, featuring high collimation, high coherence, and high intensity. Benefited from the long-range filamentation of high-power ultrashort laser pulses propagating in air, the air-lasing can be induced remotely, providing an ideal light source for atmospheric remote sensing and chemical species-resolved detection. Owing to the coherent atomic/molecular excitation process accompanied with the generation of air laser, remote sensing based on air-lasing has high spectral resolution and high detection sensitivity, which recently proved to be a powerful tool for important applications such as in trace molecule detection, greenhouse gas monitoring and industrial pollutant detection. In this short review, the physical mechanism of air lasing is briefly introduced, and various applications of air laser remote sensing are reviewed emphatically, and the future research is prospected.

Phototoxicity induced in living HeLa cells by focused femtosecond laser pulses: a data-driven approach.

Nonlinear optical microscopy is a powerful label-free imaging technology, providing biochemical and structural information in living cells and tissues. A possible drawback is photodamage induced by high-power ultrashort laser pulses. Here we present an experimental study on thousands of HeLa cells, to characterize the damage induced by focused femtosecond near-infrared laser pulses as a function of laser power, scanning speed and exposure time, in both wide-field and point-scanning illumination configurations. Our data-driven approach offers an interpretation of the underlying damage mechanisms and provides a predictive model that estimates its probability and extension and a safety limit for the working conditions in nonlinear optical microscopy. In particular, we demonstrate that cells can withstand high temperatures for a short amount of time, while they die if exposed for longer times to mild temperatures. It is thus better to illuminate the samples with high irradiances: thanks to the nonlinear imaging mechanism, much stronger signals will be generated, enabling fast imaging and thus avoiding sample photodamage.

Thermal imaging of high power ultrashort pulse laser ablation of alumina towards temperature optimized micro machining strategies

The application of pulsed laser systems with pulse durations in the pico- and femtosecond regime for material processing is commonly associated with a cold ablation. Due to the minimized interaction-time between the ultrashort laser pulses and the material, this statement is almost valid as long as no heat accumulation effect appears. With the increasing demand of high productivity processes, the average power of ultrashort pulsed laser systems increases above 100 W, which leads, however, to increased thermal effects during laser processing. This is especially important for laser processing of technical ceramics like alumina. Large temperatures gradients, which locally occur during laser processing using high average power could lead to thermal modifications and cracks in the material. In this study, we present a process-optimization method for high power laser ablation of alumina based on thermal imaging. The use of a 2D IR camera enables the estimation of the temperature distribution during the laser processing. We investigate the influence of laser power up to 80 W, pulse duration between 900 fs and 10 ps and processing duration on the resulting material temperature. Beside the material temperature we evaluate the material removal rate and the resulting surface quality.

Passively mode-locked fiber laser based on graphene covered single-mode fiber with inner short waveguides

A saturable absorber based on graphene film covered single-mode fiber with inner short waveguides is proposed and demonstrated for passively mode-locked laser operation. The output laser pulses produced have the center wavelength of 1556.9 nm and pulse width of 830.5 fs at the fundamental repetition rate of 18.7 MHz. Such a saturable absorber is highly robust as the graphene film is covered on the surface of single-mode fiber, instead on tapered fiber or side-polished fiber. Moreover, compared with the saturable absorber based on clamping the graphene film between the two fiber connectors in a fiber adaptor, our device provides a large interaction surface area, thus effectively avoiding the thermal damage and ensuring a long time and stable system operation. The device is attractive for high power ultra-short laser pulse generation.

Generation of IR radiation in the interaction of an ultrashort laser pulse with a gas jet

We consider the generation of IR radiation during the interaction of a high-power ultrashort laser pulse with a gas jet using numerical simulation by the particle-in-cell method. The laser pulse parameters correspond to the capabilities of the PEARL sub-petawatt laser facility in Nizhny Novgorod (Russia) when using the compression after compressor approach (CafCA). It is demonstrated that about 1 % of the energy can be converted into radiation in the wavelength range of 5 – 10 μm.

Ultra-broadband supercontinuum covering a spectrum from visible to mid-infrared generated by high-power and ultrashort noise-like pulses.

To facilitate a fiber-based supercontinuum generation system, single-mode fibers with different cutoff wavelengths are introduced to serve as shortpass filters to replace conventional reflective or transmissive filters. Meanwhile, an ytterbium-doped fiber amplifier is adopted to amplify the filtrated pulses, scaling their average power to the watt level up to 4.33 W. Through this approach, ultrashort high-power laser pulses of 1.56 µm and 1.06 µm wavelengths, which are commonly used in optical communications and industrial applications, can be generated by this single system. Furthermore, it is found that the noise-like pulses still maintain their temporal features, even after they undergo multiple optical processes including amplification, supercontinuum generation, and filtration. After that, the generated pulses at 1.06 µm were launched into a photonic crystal fiber to generate a supercontinuum of 1.85 W covering a spectral range from 560 nm in the visible region to 3.5 µm in the mid-infrared region. This is one of the widest records of spectrum in broadband supercontinuum generation.

Multi beam microprocessing for printing and embossing applications with high power ultrashort pulsed lasers

Abstract Micro structuring of surfaces is of great interest for various applications, e.g. for the tooling industry, the printing industry and for consumer goods. In suitable mass production applications, such as injection molding or roll-to-roll processing for various markets, the final product could be equipped with new properties, such as hydrophilic behavior, adjustable gloss level, soft-touch behavior, light management properties etc. To generate functionalities at reasonable cost, embossing dies can be augmented with additional micro/nano-scale structure using laser ablation technologies. Despite the availability of ultrashort pulsed (USP) high power lasers (up to several hundred watts), it is still a challenge to structure large areas, as required on embossing rolls, in an acceptable processing time for industrial production. In terms of industrial implementation, direct digital transfer is a limiting factor for ultrahigh resolution. Shorter machining times by further increasing spot or workpiece motion are limited. Enlarging the ablation diameter, and thus the tool diameter, delivers a higher ablation rate with the comparable ablation quality, but entails a reduction in resolution. While maintaining the achieved state-of-the-art performance, upscaling of single modulated lasers provides a less demanding way to increase productivity. In the processing of steel surfaces, an increase in material removal can also be achieved by using pulse burst. In this work, the parallel process of single modulated multi laser sources is compared with a laser source split by diffractive optical elements (DOE) for applications in a cylinder micro patterning system. A newly developed highly compact ps laser with repetition rates up to 8 MHz and an average power of 300 or 500 W was divided into 8 or 16 parallel beamlets by a DOE. The ablation rate of each approach was investigated by typical microstructures on copper surfaces. At surface speeds of 10 m/s and a resolution of 5080 dpi, an ablation rate of up to 27 mm³/min was achieved. Different functional surface geometries were realized on an embossing roll as master, which is used for replication of the structures in roll-to-roll processes. Functional structures, such as friction reduction, improved soft touch or light guiding elements on large surfaces are demonstrated.

Energy limit for linear-to-nonlinear femtosecond laser pulse focusing in air

Propagation of a tightly focused high-power ultrashort laser pulse in an optical medium is usually substantially influenced by the medium optical nonlinearity that can noticeably affect the laser pulse parameters around the nonlinear focus and lead to unavoidable and often undesirable spatial distortions of the focal waist. We present the results of our experimental study and numerical simulations on a femtosecond Ti:Sapphire laser pulse propagation in air under different spatial focusing. We concentrated our study on spectral-angular and spatial pulse transformations under different focusing regimes - from linear to nonlinear one, when pulse filamentation occurs. For the first time to the best of our knowledge, we found the laser pulse numerical apertures range – namely, from NA = 2·10-3 to 5 10-3(for the laser pulse energy of 1 mJ), where the laser pulse distortions both in frequency-angular spectrum and pulse spatial shape are minimal. By means of the numerical simulations, we found the threshold pulse energy and peak power in a wide range of focusing conditions, within which a transition between the linear and strongly nonlinear laser pulse focusing in air takes place. This energy limit is shown to decrease with pulse numerical aperture enhancement. Our findings identify the laser pulse numerical apertures and energes adequate for getting a maximum laser intensity with a good beam quality around the focal point suitable for various laser micropatterning and micromachining technologies.

Spectral-angular patterns and energy threshold for linear-to-nonlinear femtosecond laser pulse focusing in air

Focused propagation of a high-power ultrashort laser pulse is strongly influenced by optical medium nonlinearity that can significantly affect pulse parameters in the focusing region as compared to the linear focusing regime. We present the results of our experimental study and numerical simulations of frequency-angular patterns and laser plasma distribution of a gigawatt-power femtosecond Ti-sapphire laser pulse propagating in air under various focusing conditions. For the first time to the best of our knowledge, we found experimentally and confirm theoretically the range of pulse numerical aperture (NA), from approximately NA = 2 ⋅ 10−3 to 5 ⋅ 10−3, when both frequency and angular spectra broadening during supercritical laser pulse focusing in air is minimal. We attribute this NA-range to the transition region between quasilinear and nonlinear pulse focusing regimes. Additionally, the energy threshold for different pulse focusing regimes in the wide range of focusing conditions is obtained for the first time through the numerical simulations. This threshold is demonstrated to decrease with pulse NA increase.

Kerr-lens mode-locked Yb:LuAG ceramic thin-disk laser.

A Kerr-lens mode-locked (KLM) thin-disk laser with Yb:LuAG ceramic was demonstrated. Yb:LuAG ceramic is an attractive material for high-power lasers due to its high thermal conductivity and large emission cross section. The highest output power of 17 W with a pulse duration of 130 fs was achieved. Moreover, the pulse duration of 88 fs was also obtained with a high-Q factor cavity. To the best of our knowledge, this is the first demonstration of a KLM thin-disk laser based on Yb:LuAG, including both ceramic and single crystal. The results show the usefulness of ceramic thin disks for high-power ultrashort pulse laser sources.

An in-band diode-end-pumped high-power and high-efficiency ultrashort pulse Nd:YVO4 bulk laser mode-locked by a frequency doubling LBO crystal

Nonlinear mirror (NLM) mode locking has also been verified to be an efficient technology for high-power ultrashort pulse laser generation. In this paper, we report on high-power and high-efficiency NLM mode-locked Nd:YVO4 laser with a 880-nm diode laser for in-band pumping, which has very rarely been investigated in the past. The achieved maximum average output power reached 6.76 W at an absorbed power of 13.82 W corresponding to a slope efficiency of 54.5%. Thanks to the used type I LBO crystal with small group velocity mismatch, the achieved pulse duration was as short as 6.5 ps. The results represent one of the best performances that ever achieved for NLM mode-locked Nd:YVO4 laser. NLM mode locking could be very promising for high-power ultrashort pulse generation in visible and middle infrared wavelengths, at which SESAMs are still not mature and the average output powers of ultrafast laser oscillators are also still limited to great extent.

Research progress of ultrabright <i>γ</i>-ray radiation and electron-positron pair production driven by extremely intense laser fields

The advent of high-power ultra-short ultra-intense laser pulses opens up the new frontiers of relativistic nonlinear optics, high-field physics, laser-driven inertial confined fusion, etc. In recent years, with the construction of high power laser facilities at a multi-petawatt (PW) level and above, the interaction between laser and matter enters into a new realm of high field physics, where extremely rich nonlinear physics is involved. In addition to classical nonlinear physics involving wave-particle interactions, relativistic effects, and ponderomotive force effects, the quantum electrodynamic (QED) effects occur, such as radiation reaction force, electron-positron pair production, strong γ-ray radiation, QED cascades, and vacuum polarization. This paper presents a brief overview of electron-positron pair creation and bright γ-ray emission driven by the extremely intense laser fields.

Fabrication and evaluation of negative axicons for ultrashort pulsed laser applications

We report on the fabrication and evaluation of a sharp tip negative axicon paving the way for applications in high-power ultrashort pulsed laser systems. The negative axicon is manufactured by applying a two-step all laser-based process chain consisting of ultrashort pulsed laser ablation and CO2 laser polishing finishing the component in less than 5 minutes. The finalized negative axicon reveals a surface roughness of 18 nm, fulfilling optical quality. Two measurement setups, including the ultrashort pulsed laser itself, are used to evaluate the formation of Bessel beams in detail. By applying a focusing lens behind the negative axicon, well-developed Bessel beams are generated while their lengths depend on the distance between the negative axicon and the lens. Furthermore, the diameter of the Bessel beams increase strongly with the propagation distance. By adding a second focusing lens, Bessel beams are generated at its focal position, being almost invariant of its position. Hence, the typical Bessel beam intensity distribution is observed over an entire moving range of this second lens of 300 mm. While these Bessel beams show superior quality in terms of sharp peaks with homogeneous concentric rings, only minor deviations in intensity and diameter are observed over the moving range.

Picosecond laser machining of tungsten carbide

Hard materials such as tungsten carbide (WC) are extensively used in cutting tools in high-value manufacturing, and the machining of these materials with sufficient speed and quality is essential to exploit their full potential. Over the last two decades, short (nanosecond (ns)) and ultra-short (picosecond (ps); femtosecond (fs)) pulse laser machining has been evaluated by various researchers and proposed as an alternative to the current state-of-the-art machining techniques for advanced materials like WC, which include mechanical grinding and electrical discharge machining. However, most of the established/existing research on this topic is based on low power lasers, which may not be adopted in industrial production environments due to its low material removal rate. This paper presents the results of a fundamental study, on using a 300 W picosecond laser for the deep machining of tungsten carbide. The influence of various laser parameters on the geometric precision and quality (surface and sub-surface) of the ablated area was analysed, and the ablation mechanism is discussed in detail. Laser pulse frequency and scanning speed have minimal effect on ablation rate at high power levels. The surface roughness of the ablated area increases with the ablation depth. At optimal conditions, no significant thermal defects such as a recast layer, micro crack or heat affected zone were observed, even at a high average power of 300 W. The material removal rate (MRR) seems to be proportional to the average power of the laser, and a removal rate of around 45 mm3 per minute can be achieved at 300 W power level. Edge wall taper appears to be a significant issue that needs to be resolved to enable industrial exploitation of high power ultra-short pulse lasers.