Femtosecond Laser Pulse Irradiation Research Articles

Influence of film thickness and substrate roughness on the formation of laser induced periodic surface structures in poly(ethylene terephthalate) films deposited over gold substrates

A study of the formation of Laser Induced Periodic Surface Structures (LIPSS) using near-infrared femtosecond pulsed laser irradiation on poly(ethylene terephthalate) (PET) films deposited over gold substrates has been carried out. We report the influence of the gold substrate roughness and the PET film thickness on LIPSS formation and analyze it in terms of the features of the electric field distribution obtained by computer simulations using COMSOLTM. We obtain LIPSS with periods close to the irradiation wavelength as long as the aforementioned substrate and film parameters remain below certain threshold values, in particular for polymer thicknesses below 200 nm and substrate roughness of few nm. However, experiments show the impossibility of LIPSS formation for rough substrates as well as thick films above these threshold values. In our numerical simulations, we notice the generation of Surface Plasmon Polariton (SPP) in the film-substrate interface that gives rise to a periodical field pattern on the surface of the thin film. This periodicity is broken for a certain level of substrate roughness or film thickness. Moreover, the evolution of the period of the SPP as the substrate roughness and film thickness change for given laser parameters is qualitatively in good agreement with the experimental LIPSS period (below but close to the irradiation laser wavelength). In conclusion, the experimental findings are explained by the formation and behavior of SPP in the thin film-substrate interface. On these grounds, we propose that, for our case of study, this SPP formation and the subsequent inhomogeneous rise in temperature induced by the periodic field on the surface of the sample is the leading mechanism contributing to LIPSS formation.

Ultrafast Temporal-Spatial Dynamics of Phase Transition in N-Doped Ge2Sb2Te5 Film Induced by Femtosecond Laser Pulse Irradiation.

Element-doped phase change material (PCM) could improve the performances, e.g., better thermal stability, higher electrical resistance, and faster crystallization speed; thus, the influence of the doping element needs to be further investigated. In this paper, a femtosecond laser, which could realize the ultrafast phase transition rate of PCM between amorphization and crystallization, was used to explore the properties of nitrogen-doped Ge2Sb2Te5 (GST), and a bond effect was proposed. The pure GST and different nitrogen contents of doped GST films were investigated by femtosecond laser pulse excitation through a pump-probe shadowgraph imaging technique. The results showed that the element-doped films could change photon absorption because of the increase in free carriers. This caused the faster rate of reflectivity to change in the irradiated area by the laser beam as the more nitrogen doped. When the nitrogen content increased, the crystallization evolution became harder because it enhanced the bond effect, which suppressed crystalline grain growth and improved the thermal stability. Based on the analysis in the paper, the desired performances of PCMs, e.g., ultrafast dynamics, crystallization evolution, and thermal stability, could be controlled according to the demands by modifying the bond effect.

Electrical detection and current control of all-optical magnetization switching in GdFeCo ferrimagnetic alloy thin film

We experimentally demonstrated electrical detection of all-optical magnetization switching (AOS) induced by a single femtosecond laser pulse irradiation by measuring alternate rapid changes in anomalous Hall voltage and magneto-optic image pulse by pulse in a Hall-cross shape ferrimagnetic GdFeCo alloy thin film. We also demonstrated that the amplitude of the change in anomalous Hall voltage depended on the position of the AOS-created magnetic domain on the Hall cross. Furthermore, the AOS-created magnetic domains were stable against subsequent current applications in the Hall cross circuit, whereas reversed magnetic domains were not created when the laser pulse was irradiated with a high current. We found that the cooperative effect among magnetism, light, and electric current was assumed to have effects on the absence of the AOS. Combining the AOS phenomenon and electrical measurement/control techniques can realize ultrafast, deterministic, and distinguishable applications.

Au Quantum Clusters and Plasmonic Quantum Nanoparticles Synthesized under Femtosecond-Pulse Laser Irradiation in Aqueous Solution and in ZIF-8 for Catalytic Reduction of 4-Nitrophenol

Au Quantum Clusters and Plasmonic Quantum Nanoparticles Synthesized under Femtosecond-Pulse Laser Irradiation in Aqueous Solution and in ZIF-8 for Catalytic Reduction of 4-Nitrophenol

Optical and Thermal Behavior of Germanium Thin Films under Femtosecond Laser Irradiation.

In this study, we theoretically investigate the response of a germanium thin film under femtosecond pulsed laser irradiation. Electron and lattice temperatures, as well as material-specific optical properties such as dielectric function and reflectivity, were calculated during the irradiation using an extended two-temperature model coupled with the carrier density rate equation and the Drude model. Melting and ablation fluence thresholds were also predicted, resulting in 0.14 J cm-2 and 0.35 J cm-2, respectively. An ultrafast change in both optical and thermal properties was detected upon laser irradiation. Results also indicate that thermal melting occurs after germanium takes on a metallic character during irradiation, and that the impact ionization process may have a critical role in the laser-induced thermal effect. Therefore, we suggest that the origin of the thermal modification of germanium surface under femtosecond laser irradiation is mostly due the impact ionization process and that its effect becomes more important when increasing the laser fluence.

Ultrafast Dynamics of Different Phase States Ge2Sb2Te5 Film Induced by a Femtosecond Laser Pulse Irradiation.

A femtosecond laser could realize a high transition rate of the phase change material (PCM), and the properties of the amorphous and the crystalline Ge2Sb2Te5 (GST) induced by a femtosecond laser were studied, which was one of the candidates among the PCMs. However, the characteristics of the intermediate phase states in reversible phase transitions were also important and helpful to explore the mechanisms of the phase transitions. In this paper, the ultrafast dynamics of amorphous, crystalline face-centered-cubic (FCC), and hexagonal-close-packed (HCP) states were investigated using a femtosecond laser pulse excitation through a reflective-type pump–probe technique, obtained by annealing at certain temperatures, and verified using X-ray diffraction (XRD) and the Raman spectrum. It was found that as the annealing temperature increased, the electron of the GST films could be excited more easily, while the ablation threshold decreased. Due to annealing, the structure of bonding was changed for different phase states, which resulted in the decrease in the band gap of the films. In addition, it was hard for the intermediate state films to transit to the amorphous structure state via the femtosecond laser, and the crystallization would be enhanced, while the crystalline HCP structures of GST could be directly and easily changed to the amorphous state by a pulse, which resulted from the non-thermal phase change caused by the excited electron.

Investigation of structural transformation and residual stress under single femtosecond laser pulse irradiation of 4H–SiC

Silicon carbide (SiC) is an attractive semiconductor material for devices that operate under extreme conditions. However, its high hardness and chemical stability hinder micromachining. Femtosecond laser has been proposed extensively as an effective micromachining tool for SiC. However, the fundamental mechanisms of laser-material interaction during femtosecond laser irradiation process remain elusive. This paper presents a comprehensive study of the structural transformation and residual stress induced by irradiating a 4H–SiC target with a single-pulse femtosecond laser. The energy dependence of the structural characteristics at the spot center and the spatial distribution across the laser spot were determined using optical microscopy and micro-Raman spectroscopy. The effect of the laser fluence on the residual stress was discussed. The obtained results showed that no structural changes occurred at low energy, whereas bond breaking occurred among crystalline SiC (c-SiC) at high energy. A central disk with no structural transformation and no residual stress was formed for the fluence of 22.2 J/cm 2 , owing to phase explosion-induced spluttering. Meanwhile, an inhomogeneous distribution of the structural transformation was generated from the spot centre to the edge. Based on this, threshold fluences for modification and structural transformation were proposed and calculated. The fundamental mechanisms for different laser-fluence regimes are discussed, and some suggestions for improving the surface quality are put forward. This study provides deep insights into the laser-material interaction mechanisms and is beneficial for optimising the utilisation of femtosecond laser for 4H–SiC micromachining. • Structural characteristics and residual stress at the spot centre depend on laser energy. • Spatial distribution of structural transformation from the spot centre to the edge is inhomogeneous. • Threshold fluences for modification and structural transformation are proposed and calculated. • The fundamental mechanisms of laser-material interaction for different laser-fluence regimes are analysed. • The residual stress in central disk region vanishes for the laser fluence of 22.2 J/cm 2 , owing to explosive boiling.

Ultrafast Modulation of the Molten Metal Surface Tension under Femtosecond Laser Irradiation

We predict ultrafast modulation of the pure molten metal surface stress fields under the irradiation of the single femtosecond laser pulse through the two-temperature model molecular-dynamics simulations. High-resolution and precision calculations are used to resolve the ultrafast laser-induced anisotropic relaxations of the pressure components on the time-scale comparable to the intrinsic liquid density relaxation time. The magnitudes of the dynamic surface tensions are found being modulated sharply within picoseconds after the irradiation, due to the development of the nanometer scale non-hydrostatic regime behind the exterior atomic layer of the liquid surfaces. The reported novel regulation mechanism of the liquid surface stress field and the dynamic surface tension hints at levitating the manipulation of liquid surfaces, such as ultrafast steering the surface directional transport and patterning.

High efficiency laser-driven proton sources using 3D-printed micro-structure

Fine structured targets are promising in enhancing laser-driven proton acceleration for various applications. Here, we apply 3D-printed microwire-array (MWA) structure to boost the energy conversion efficiency from laser to proton beam. Under irradiation of high contrast femtosecond laser pulse, the MWA target generates over 1.2 × 1012 protons (>1 MeV) with cut-off energies extending to 25 MeV, corresponding to top-end of 8.7% energy conversion efficiency. When comparing to flat foils the efficiency is enhanced by three times, while the cut-off energy is increased by 32%. We find the dependence of proton energy/conversion-efficiency on the spacing of the MWA. The experimental trend is well reproduced by hydrodynamic and Particle-In-Cell simulations, which reveal the modulation of pre-plasma profile induced by laser diffraction within the fine structures. Our work validates the use of 3D-printed micro-structures to produce high efficiency laser-driven particle sources and pointed out the effect in optimizing the experimental conditions.

Generation of micro/nano hybrid surface structures on copper by femtosecond pulsed laser irradiation

The delamination of copper lead frames from epoxy molding compounds (EMC) is a severe problem for microelectronic devices, as it leads to reduced heat dissipation or circuit breakage. The micro/nanoscale surface structuring of copper is a promising method to improve the copper–EMC interfacial adhesion. In this study, the generation of micro/nano hybrid structures on copper surfaces through femtosecond pulsed laser irradiation is proposed to improve interfacial adhesion. The micro/nano hybrid structures were realized by generating nanoscale laser-induced periodic surface structures (LIPSS) on microscale parallel grooves. Several types of hybrid surface structures were generated by changing the laser polarization direction, fluence, and scanning speed. At a specific aspect ratio of microgrooves, a latticed structure was generated on the sides of microgrooves by combining LIPSS formation and direct laser interference patterning. This study provides an efficient method for the micro/nanoscale hybrid surface structure formation for interfacial adhesion improvement between copper and EMC.

Monte Carlo simulation of ultrafast nonequilibrium spin and charge transport in iron

Spin transport and spin dynamics after femtosecond laser pulse irradiation of iron (Fe) are studied using a kinetic Monte Carlo model. This model simulates spin dependent dynamics by taking into account two interaction processes during nonequilibrium: elastic electron–lattice scattering, where only the direction of the excited electrons changes, and inelastic electron–electron scattering processes, where secondary electrons are generated. An analysis of the spin dependent particle kinetics inside the material shows that a smaller elastic scattering time leads to a larger spatial spread of electrons in the material, whereas generation of secondary electrons extends the time span for superdiffusive transport and increases the spin current density.

Damage characteristics of pulse compression grating irradiated by a nanosecond laser

Multilayer dielectric gratings (MLDGs) have been widely used for pulse compression in chirped pulse amplification technology, and encounter amplified nanosecond (ns), picosecond, or femtosecond laser pulse irradiation. Damage behavior in the ns regime is statistically significant; however, only the 1-on-1 test method was employed in previous studies to identify the damage precursors. Here, we adopted a raster scan procedure with mass test samplings to comprehensively evaluate the damage characteristics of MLDGs. The damage experiment was conducted at 1064 nm with a pulse width of 8 ns. The laser-induced damage thresholds (LIDTs) for the MLDGs were shown to be approximately 30% lower than those of multilayer dielectric films (MLDFs). The normalized electric field intensity |E |2 (EFI) enhancement caused by the surface-relief grating structure and incomplete grating cleaning contributed to this LIDT reduction. Three discrete damage-initiation morphologies near the LIDT were found: nodular ejection, nano absorbing defect damage, and plasma scalding. In addition to the nodular defect damage that usually occurs in the fundamental frequency high reflectors, the strong absorption of nano defects and the poor interfacial quality make the interface nano absorbing defects of the MLDG also easily triggered. The interface differences between the MLDG and MLDF should be related to multiple annealing processes during MLDG fabrication. The plasma scalding behaves as a color change and is only involved at the surface of the grating pillar. The slight dependence of damage morphology on the EFI peak was first observed.

Femtosecond pulse laser fabrication of zinc oxide quantum clusters/nanoparticles and their electrical, optical, and chemical characteristics

Despite various studies on the preparation of different types and sizes of ZnO, the synthesis of quantum clusters of bare metal oxide has rarely been reported. The research goals of this study were to create clusters/nanoparticles using femtosecond laser irradiation to increase the electrical, optical, and chemical functionalities of ZnO. Femtosecond pulse laser irradiation deposition technology was used here to produce ZnO from a precursor in water (pH = 5.5) and aqueous alkaline solution (pH = 10.2). These products were named ZnO(F5.5) and ZnO(F10.2), respectively. In this procedure, Zn ions react with hydroxyl radicals (OH*) produced by the decomposition of water molecules, and Zn(OH*)2 is dehydrated by femtosecond laser energy to create ZnO. The spherical particle size of ZnO(F5.5) after 1–30 min irradiation was found to be small (1–7 nm) compared to that of ZnO(F10.2) spheres (10–13 nm). Furthermore, ZnO(F5.5) shows a larger band gap (5.3–5.6 eV), a longer electron life time (40.4 ms), and a higher emission intensity (483 a.u.) compared to ZnO(F10.2). For the photodegradation of harmful pollutants, ZnO(F5.5) prepared at 1 min of laser irradiation reduces formaldehyde by 98.5% under UV light irradiation for 15 min. However, ZnO(F10.2) and other larger ZnO particles with various shapes require a longer time for formaldehyde conversion. These results confirm that an ultrasmall ZnO nanoparticle (1 nm in size) can be called a quantum cluster and has better electrical, optical, and photocatalyst characteristics. In particular, efficient photocatalytic reactions may be used to study the ecological and environmental impacts of ZnO quantum cluster.

Time-domain measurement of coherent transition radiation using a photoconductive antenna with micro-structured electrodes

The longitudinal electric field profile of coherent transition radiation (CTR), which is a radially polarized terahertz (THz) pulse from an electron bunch, was measured using a large-aperture photoconductive antenna (PCA) with a diameter of 8.8 mm and micro-structured concentric electrodes to detect THz light pulses. Photo-induced charge carriers were generated on the PCA by the irradiation of femtosecond laser pulses on the electrode plane. Pulsed light irradiation enabled time-resolved measurement of the THz light pulses, and time-domain measurement of CTR was conducted by measuring the electric-field-induced current output from the PCA by sweeping the timing of the laser irradiation. The frequency spectra obtained from the detected longitudinal electric field profiles of the CTR showed a reasonable response signal from the low-frequency region as low as 0.1 THz. In the present demonstration, the large-aperture PCA-detected THz wave from the CTR was from 0.1 to at least 1 THz. THz generation using the PCA was also performed and measured using a Michelson interferometer with a helium-cooled bolometer. Comparison of the THz pulse of the CTR detected using the PCA and that generated using the PCA revealed that time-domain measurement of the THz pulse using the PCA was limited by the response time of the charge carriers in the semiconductor. Enlargement of the aperture of the PCA enhanced the detectable frequency region toward a lower frequency.

Multi-Angular Colorimetric Responses of Uni- and Omni-Directional Femtosecond Laser-Induced Periodic Surface Structures on Metals.

We investigated the colorimetric behaviors of metal surfaces with unidirectional low-spatial-frequency laser-induced periodic surface structures (UD-LSFLs) and omnidirectional LSFLs (OD-LSFLs) fabricated using femtosecond laser pulse irradiation. With the CIE standard illuminant D65, incident at −45°, we show that UD-LSFLs on metals transform polished metals to gonio-apparent materials with a unique behavior of colorimetric responses, depending on both the detection and rotation angles, whereas OD-LSFLs have nearly uniform gonio-apparent colors at each detection angle, regardless of their rotation. These colorimetric behaviors can be observed not only at the angles of diffraction but also near the angle of reflection, and we find that the power redistribution due to Rayleigh anomalies also plays an important role in the colorimetric responses of UD- and OD-LSFLs, in addition to diffraction.

The Influence of UV Femtosecond Laser Pulses on Bacterial DNA Structure, as Proved by Fourier Transform Infrared (FT‐IR) Spectroscopy

AbstractIn this work, FT‐IR absorbance spectra of genomic DNAs isolated from four strains of Escherichia coli (A, B, C and D) and two strains of Salmonella (E and F), respectively, have been analyzed in both untreated and UV femtosecond laser pulses irradiated samples, to investigate their screening characteristics and their structural radiotolerance at 256 nm. Spectroscopic band assignments and structural aspects are discussed in the 600–1800 cm−1 and 3000–4000 cm−1 spectral ranges. DNA structural changes at the level of nucleic acids backbone, sugar, PO2− group, nitrogenous base ring and deoxycytidine (C2=O2) have been observed. Consequently, modifications in DNA conformations have been found. Bacterial strain dependent femtosecond laser pulses irradiation effects on DNA structure were observed. Based on FT‐IR biomarkers novel early diagnosis and screening tools for routine use in health care and life sciences can be developed. Furthermore, cancer research could benefit from ultra‐fast lasers technology. Besides, analyzing the interaction mechanisms of DNA with femtosecond laser pulses is highly appreciated for understanding the pathways of irradiation damaged DNA response in living cells.

Microscale Surface Patterning of Zirconia by Femtosecond Pulsed Laser Irradiation

Microscale Surface Patterning of Zirconia by Femtosecond Pulsed Laser Irradiation

Copper circuits fabricated on flexible polymer substrates by a high repetition rate femtosecond laser-induced selective local reduction of copper oxide nanoparticles.

In this work, copper circuits were fabricated on flexible polyimide (PI) substrates by high repetition rate femtosecond laser-induced selective local reduction of copper oxide nanoparticles (CuO NPs). The effects of laser pulse energy and laser scanning velocity on the quality of the copper circuit were studied. By optimizing laser processing parameters, we prepared a Cu circuit of a line width of 5.5 µm and an electrical resistivity of 130.9 µΩ·cm. The Cu/O atomic ratio of the Cu circuit reaches ∼10.6 and the proportion of Cu is 91.42%. We then studied the formation mechanism of the copper circuit by simulating the temperature field under the irradiation of high repetition rate femtosecond laser pulses. The results show that the thermochemical reduction reaction induced by the high repetition rate femtosecond laser reduces CuO NPs into Cu NPs. Under the thermal effect of the high repetition rate femtosecond laser, Cu NPs agglomerate and grow to form a uniform and continuous Cu circuit.

Fourier two-temperature model to describe ultrafast laser pulses interaction with metals: A novel mathematical technique

Ultrafast thermal phenomena during femtosecond laser pulse heating of solids, first of all of metals, were analytically developed using a new mathematical method based upon the two-temperature model via the Fourier approach. A straightforward but powerful mathematical formalism is proposed to evaluate the spatial and temporal profiles of electron temperature under the irradiation with a single Gaussian femtoseconds laser pulse. An illustrative example was selected: a semi-infinite Gold target under femtosecond laser pulse irradiation. The proposed model can be used to guide and design experimental activities, i.e. parameters optimization before conducting real-time experiments of ultrashort laser pulses - metals interaction.

Numerical simulation of the period of surface micro-protrusions generated on titanium and stainless steel targets by femtosecond laser irradiation

This article reports our simulation results on a period of surface micro-protrusions, which are generated on titanium and stainless steel 304 target surfaces by femtosecond pulsed laser irradiation. The period of the generated micro-protrusions for varying laser fluence level has been estimated using an approach derived from the linear hydrodynamic Kuramoto Sivashinsky model. Some of the parameters, needed for calculating the period of surface micro-protrusions, have been estimated by numerically solving one-dimensional heat equations. Temperature evolution inside the target upon irradiation with a femtosecond laser pulse has been simulated using the two temperature model until the time electron and lattice subsystems attain thermal equilibrium. Thereafter, temperature evolution in the target has been simulated by defining a single temperature of the target at every position and time. We have validated our theoretical model by comparing simulated variation of period of surface micro-protrusions with incident laser fluence, ablation depth per pulse, and time required for thermalization between electrons and lattice subsystems with the reported experimental data for titanium target. Subsequently, the validated model has been used to simulate the period of surface micro-protrusions which are generated on the stainless steel 304 target via femtosecond laser irradiation.