An all-fiber high-power linearly polarized chirped pulse amplification (CPA) system is experimentally demonstrated. Through stretching the pulse duration to a full width of approximately 2 ns with two cascaded chirped fiber Bragg gratings (CFBGs), a maximum average output power of 612 W is achieved from a high-gain Yb-doped fiber that has a core diameter of 20 μm with a slope efficiency of approximately 68% at the repetition rate of 80 MHz. At the maximum output power, the polarization degree is 92.5% and the M2 factor of the output beam quality is approximately 1.29; the slight performance degradations are attributed to the thermal effects in the main amplifier. By optimizing the B-integral of the amplifier and finely adjusting the higher-order dispersion of one of the CFBGs, the pulse width is compressed to 863 fs at the highest power with a compression efficiency of 72%, corresponding to a maximum compressed average power of 440.6 W, single pulse energy of 5.5 μJ and peak power of about 4.67 MW. To the best of our knowledge, this is the highest average power of a femtosecond laser directly generated from an all-fiber linearly polarized CPA system.

SwissFEL is a compact, high-brilliance, soft and hard X-ray free electron laser (FEL) facility that started user operation in 2019. The facility is composed of two parallel beam lines seeded by a common linear accelerator (LINAC), and a two-bunch photo-injector. For the injector, an innovative dual-photocathode laser scheme has been developed based on state-of-the-art ytterbium femtosecond laser systems. In this paper, we describe the performance of the SwissFEL photocathode drive lasers (PCDLs), the pulse-shaping capabilities as well as the versatility of the systems, which allow many different modes of operation of SwissFEL. The full control over the SwissFEL electron bunch properties via the unique architecture of the PCDLs will enable in the future the advent of more-advanced FEL modes; these modes include, but are not restricted to, the generation of single or trains of sub-femtosecond FEL pulses, multi-color FEL and finally, the generation of fully coherent X-ray pulses via laser-based seeding.

A novel method to generate shock waves in small tubes is demonstrated. A femtosecond laser is applied to generate an optical breakdown an aluminum film as target. Due to the sudden appearance of this non-equilibrium state of the target, a shock wave is induced. The shock wave is further driven by the expanding high-pressure plasma (up to 10 Mbar), which serves as a quasi-piston, until the plasma recombines. The shock wave then propagates further into a glass capillary (different square capillaries with hydraulic diameter D down to 50 µm are applied). Shock wave propagation is investigated by laser interferometry. Although the plasma is an unsteady driver, due to the geometrical confinement of the capillaries, rather strong micro shocks can still propagate as far as 35 times D. In addition to the experiments, the initial conditions of this novel method are investigated by hydrocode simulations using MULTI-fs.

A study of ASP 30 steel surface modification with high intensity Ti:sapphire laser, operating at 804 nm wavelength and pulse duration of 60 fs, in vacuum ambient, is presented. ASP 30 steel surface variations were studied at laser intensities of 1014 and 1013 W/cm2. The steel target specific surface changes and phenomena observed are: (i) Creation of craters at 1014 W/cm2 intensity; (ii) formation of periodic surface structures only at the reduced intensity of 1013 W/cm2; (iii) chemical surface changes registered only at higher laser intensity, and (iv) occurrence of plasma in front of the surface, including its emission in X-ray region. It can be concluded from this study that the reported laser intensities can effectively be applied for ASP 30 steel surface modification. Careful choosing of laser intensity and pulse count can lead to precise superficial material removal, for example laser intensity ~1013 W/cm2 and low pulse count can lead to ultra-precise surface processing. Generally, femtosecond laser surface modification of ASP 30 steel is non-contact and very rapid compared with traditional modification methods.

We demonstrate that the surface topography of a sample can be reconstructed from electron backscatter diffraction (EBSD) patterns collected with a commercial EBSD system. This technique combines the location of the maximum background intensity with a correction from Monte Carlo simulations to determine the local surface normals at each point in an EBSD scan. A surface height map is then reconstructed from the local surface normals. In this study, a Ni sample was machined with a femtosecond laser, which causes the formation of a laser-induced periodic surface structure (LIPSS). The topography of the LIPSS was analyzed using atomic force microscopy (AFM) and reconstructions from EBSD patterns collected at 5 and 20 kV. The LIPSS consisted of a combination of low frequency waviness due to curtaining and high frequency ridges. The morphology of the reconstructed low frequency waviness and high frequency ridges matched the AFM data. The reconstruction technique does not require any modification to existing EBSD systems and so can be particularly useful for measuring topography and its evolution during in situ experiments.

The interaction of femtosecond laser with initially cold solid metallic targets (Al, Au, Cu, Mo, Ni) was investigated in a wide range of laser intensity with focus on the laser energy absorption efficiency. Our developed simulation code (FEMTO-2D) is based on two-temperature model in two-dimensional configuration, where the temperature-dependent optical and thermodynamic properties of the target material were considered. The role of the collisional processes in the ultrashort pulse laser–matter interaction has been carefully analyzed throughout the process of material transition from the cold solid state into the dense plasma state during the pulse. We have compared the simulation predictions of the laser pulse absorption with temperature-dependent reflectivity and optical penetration depth to the case of constant optical parameters. By considering the effect of the temporal and spatial (radial) distribution of the laser intensity on the light absorption efficiency, we obtained a good agreement between the simulated results and available experimental data. The appropriate model for temperature-dependent optical parameters defining the laser absorption efficiency will allow more accurate simulation of the target thermal response in the applications where it is critical, such as prediction of the material damage threshold, laser ablation threshold, and the ablation profile.

The emission of X-rays from solid tin targets irradiated by low-energy (few mJ) femtosecond laser pulses propagated through air plasma sparks is investigated. The aim is that to better understand the X-ray emission mechanism and to show the possibility to produce proper radiation for spectroscopic and imaging applications with a table-top laser system. The utilization of a controlled ultrashort prepulse is found necessary to optimize the conversion efficiency of laser energy into Lα radiation. The optimum contrast between the main pulse and the controlled prepulse is found about 102. A correlation between the laser contrast value and the laser near-infrared spectra at the exit of the plasma spark is observed.

The ultrafast dynamic process in semiconductor Ge irradiated by the femtosecond laser pulses is numerically simulated on the basis of van Driel system. It is found that with the increase of depth, the carrier density and lattice temperature decrease, while the carrier temperature first increases and then drops. The laser fluence has a great influence on the ultrafast dynamical process in Ge. As the laser fluence remains a constant value, though the overall evolution of the carrier density and lattice temperature is almost independent of pulse duration and laser intensity, increasing the laser intensity will be more effective than increasing the pulse duration in the generation of carriers. Irradiating the Ge sample by the femtosecond double pulses, the ultrafast dynamical process of semiconductor can be affected by the temporal interval between the double pulses.

Our experiments show that external focusing and initial laser energy strongly influences filament generated by the femtosecond Ti–sapphire laser in air. The experimental measurements show the filament length can be extended both by increasing the laser energy and focal length of focusing lens. On the other hand, the plasma fluorescence emission can be enhanced by increasing the laser energy with fixed focal length or decreasing the focal length. In addition, the collapse distance measured experimentally are larger than the calculated ones owing to the group-velocity-dispersion effect. In addition, we find that the line widths of the spectral lines from $\text{N}_{2}$ is independent of filament positions, laser energies and external focusing.

Models and experiments were developed to study femtosecond laser ablation of silicon using 800 nm, 40 fs pulses with fluences ranging from 0.5 to 35 J/cm2. At low fluences, ablation was found to occur due to bubble formation and splashing within the melt layer. At higher fluences, it was found that the ablation depth exceeded the melt layer depth due to shockwave ablation. The variation in ion flux and ion velocity was also studied both experimentally and theoretically. It was found that the variation in ion flux is mainly dependent on the variation in the average charge state, with only a small variation in the total number of ions above $\sim \!\!1.5\; \,{\rm J/c}{{\rm m}^2}$. Comparisons between the theoretical and experimental ion flux showed that higher charge state ions received greater portion of the laser energy compared with lower charge state ions.

A double layer a-CN/TiAlN coating deposited on ASP30 steel substrate was irradiated by femtosecond laser and surface modification effects were observed. Moderate laser intensities used were in the range of 1014–1013 W/cm2, while the total thickness of double layer coating was 4.8 µm (a-CN = 0.6 and TiAlN = 4.2 µm). Laser-induced changes of the surface showed dependence on laser intensity and number of accumulated pulses. Irradiations at the highest intensity resulted in preservation of one or both layers up to 10 pulses, while at lower intensity (1013 W/cm2) a-CN layer is removed after several pulses and TiAlN is preserved up to 50 pulses. Evaluated damage threshold of the target was 0.49 J/cm2. Lower laser intensity irradiation produced periodic surface structures (LIPSS) over the entire irradiated spot with periodicity of ~700 nm, almost in agreement with the laser wavelength used. Irradiations carried out at the highest laser intensity (1014 W/cm2) and laser pulse count of ≥50 resulted in the creation of crater like damages with depth up to 20 µm. Craters were conically shaped, implying intensive processes which took place at the surface. Generation of LIPSS as well as craters can be of great interest for contemporary technologies.

In this work we reported low and high repetition frequency femtosecond laser-induced modifications of tungsten-based thin film. The tungsten-titanium (WTi) thin film, thickness of 190 nm, was deposited by sputtering on single crystal Si (100) wafer. Irradiations were performed in air by linearly polarized and focused femtosecond laser beams with following parameters: (1) pulse duration 160 fs, wavelength 800 nm, laser repetition frequency (LRF) 75 MHz — high LRF, and (2) duration 40 fs, wavelength 800 nm, LRF of 1 kHz — low LRF. The results of femtosecond lasers processing of the WTi thin film revealed laser induced periodical surface structures (LIPSS) in the case of low LRF regime. LIPSSs were formed with different periodicity and different orientation to the laser polarization at the surface: micro-scale LIPSSs with orientation perpendicular to the laser polarization and nano-scale LIPSSs parallel and perpendicular to the laser polarization. After processing of the WTi/Si system in high LRF regime ablation and nano-particles formation were registered.

We present a bottom-up fabrication route to fabricate solid-solution Rh–Pd–Pt ternary alloy nanoparticles (NPs), with well-controlled compositions through femtosecond laser irradiation of a mixed solution of metallic ions, without any reducing agents and complicated processes. The structure of fabricated NPs was crystalline, and formation of solid-solution alloy formation was confirmed by electron and x-ray diffraction measurements. The crystalline nature of alloy NPs was also confirmed through high-resolution transmission electron microscope measurement. According to energy dispersive spectroscopy analysis, elemental composition of an individual NP was almost the same as the initial feeding ratio of ions in the mixed solutions. The electronic state of the element in alloy NPs was confirmed to be pure metal by XPS measurement. The structural studies of Rh–Pd–Pt NPs suggested that the demonstrated technique opens up a new dimension for the fabrication of NPs, which has well-controlled properties for practical use in various fields.

Non-equilibrium heat transfer described by a two-step temperature model was developed to study the thermal evolution through target materials irradiated by femtosecond laser pulse. Two-dimensional heat transfer equations were solved numerically. The temperature dependent thermo-physical properties of the electron and the lattice are considered in the model. The Gaussian spatial and temporal distribution of the heat and temperature of the electron and the lattice is presented. The effect of reflectivity, electron — lattice coupling factor, and the spot size was studied using copper targets.

The response of titanium surface irradiated with high intensity (1013 – 1015 W/cm2) Ti:sapphire laser was studied in vacuum. Most of the reported investigations were conducted with nano- to femtosecond lasers in gas atmospheres while the studies of titanium surface interacting with femtosecond laser in vacuum are scarce. The laser employed in our experiment was operating at 800 nm wavelength and pulse duration of 60 fs in single pulse regime. The observed surface changes and phenomena are (1) creation of craters, (2) formation of periodic surface structures at the reduced intensity, and (3) occurrence of plasma in front the target. Since microstructuring of titanium is very interesting in many areas (industry, medicine), it can be concluded from this study that the reported laser intensities can effectively be applied for micromachining of the titanium surface (increasing the roughness, formation of parallel periodic surface structures etc.).

The interaction of near infrared femtosecond laser pulses with a Cu based alloy (brass) in ambient air at atmospheric pressure and under different laser conditions was investigated. The effects of laser fluence and number of pulses on surface morphology and ablation rate were studied using scanning electron microscopy (SEM) and optical microscopy. Ablation rates were found to rapidly increase from 83 to 604 nm/pulse in the fluence range 1.14–12.21 J/cm2. At fluence >12.21 J/cm2, ablation rates increased slowly to a maximum (607 nm/pulse at 19.14 J/cm2), and then decreased at fluence higher than 20.47 J/cm2 to 564 nm/pulse at 24.89 J/cm2. Large amounts of ablated material in a form of agglomerated fine particles were observed around the ablation craters as the number of laser pulses and fluence increased. The study of surface morphology shows reduced thermal effects with femtosecond laser ablation in comparison to nanosecond laser ablation at low fluence.

For a real, meaningful pump-probe experiment with attosecond temporal resolution, an intense isolated attosecond pulse is in demand. For that purpose we report the generation of an intense isolated attosecond pulse, especially in X-ray region using a current-enhanced self-amplified spontaneous emission in a free electron laser (FEL). We use a few cycle laser pulse to manipulate the electron-bunch inside a two-period planar wiggler. In our study, we employ the electron beam parameters of Pohang Accelerator Laboratory (PAL)-XFEL. The RF phase effect of accelerator columns on the longitudinal energy distribution profile and current profile of electron-bunch is also studied, aiming that these results can be experimentally realized in PAL-XFEL. We show indeed that the manipulation of electron-energy bunch profile may lead to the generation of an isolated attosecond hard X-ray pulse: 150 attosecond radiation pulse at 0.1 nm wavelength can be generated.

In recent years, biomaterial investigators have increasingly focused their research on hydrogels and their capability to be fabricated into tissue engineering scaffolds. Although several fabrication methods have been used to produce hydrogel scaffolds, those methods are unable to routinely produce three-dimensional submicron and nanoscale scaffolds with precise control of the geometry, a crucial factor necessitated by the recent developments in the field of tissue engineering. Femtosecond laser-induced two-photon polymerization is a promising technique that fulfills these requirements. In our work, we used a femtosecond laser to fabricate three-dimensional submicron-scale scaffolds with poly(ethylene glycol) (PEG). The modulus, dimensions, and shape of the scaffold can be readily adjusted by changing both the laser parameters and the molecular weight of the PEG prepolymer. With the femtosecond laser, we also fabricated two-dimensional topographical patterns, which have important applications in basic biological research. To improve the throughput of femtosecond laser fabrication, we integrated the femtosecond direct-write process with a nano-imprint process by which the femtosecond laser is used to produce nano-patterned molds. We then carried out nanoimprinting to transfer the nanofeatures in the mold to the hydrogel in a massively parallel fashion.

The interaction of medium sized Argon clusters (30 Å) with high-intensity femtosecond laser pulses (806 nm, 8 × 1016 W/cm2) of durations ranging from 10 fs to 120 fs have been studied using a three-dimensional relativistic time dependent molecular dynamic approach. The dynamics of cluster expansion is explained in terms of temporal evolution of electron population in the cluster and snapshots of particle positions at various times. The effects of inter-cluster distance on ionization dynamics are presented. It is observed that the collisional ionization increases with decreasing inter-cluster distance. The effect of pulse duration on laser energy absorption is also studied. For a laser pulse of gaussian time profile, there exists an optimum pulse duration for maximum absorption. No such optimum exists for a nearly flat top (super-gaussian) laser pulse. Results indicate the existence of resonance absorption inside the cluster. It is also observed that the high energy component of ion emission from cluster is anisotropic, showing a preferential direction of emission along laser polarization while the low energy ions emerge almost isotropically.

An improved two-temperature model to describe femtosecond laser ablation of metal target was presented. The temperature-dependent heat capacity and thermal conductivity of the electron, as well as electron temperature-dependent absorption coefficient and absorptivity are all considered in this two-temperature model. The tailored two-temperature model is solved using a finite difference method for copper target. The time-dependence of lattice and electron temperature of the surface for different laser fluence are performed, respectively. The temperature distribution of the electron and lattice along with space and time for a certain laser fluence is also presented. Moreover, the variation of ablation rate per pulse with laser fluence is obtained. The satisfactory agreement between our numerical results and experimental data indicates that the temperature dependence of heat capacity, thermal conductivity, absorption coefficient and absorptivity in femtosecond laser ablation of metal target must not be neglected. The present model will be helpful for the further experimental investigation of application of the femtosecond laser.