Multiple Femtosecond Laser Research Articles

Formation and modulation of multi-pulse femtosecond laser-induced periodic surface structures by electromagnetic and partial differential equation simulations.

In order to demonstrate the formation of laser-induced periodic surface structures (LIPSS), simulations were performed to investigate the effect of multiple femtosecond laser pulses with different laser energy densities on a Ti6Al4V surface. In this work, a set of partial differential equations calculating the electron and lattice temperature variations, followed by coupling with an electric field, is used to analyze the evolution of the periodic surface structure induced by the interaction of the femtosecond laser with the material. As the number of pulses increases, the surface structure of the material changes from none to produce LIPSS structure and from low spatial frequency LIPSS (LSFL) structure to high spatial frequency LIPSS (HSFL) structure. In order to compare the results, single-point laser scanning ablation experiments were carried out at femtosecond laser energy. The experimental results are consistent with the simulation results.

Functional Shape-Morphing Microarchitectures Fabricated by Dynamic Holographically Shifted Femtosecond Multifoci.

Functional microdevices based on responsive hydrogel show great promise in targeted delivery and biomedical analysis. Among state-of-the-art techniques for manufacturing hydrogel-based microarchitectures, femtosecond laser two-photon polymerization distinguishes itself by high designability and precision, but the point-by-point writing scheme requires mechanical apparatuses to support focus scanning. In this work, by predesigning holograms combined with lens phase modulation, multiple femtosecond laser spots are holographically generated and shifted for prototyping of three-dimensional shape-morphing structures without any moving equipment in the construction process. The microcage array is rapidly fabricated for high-performance target capturing enabled by switching environmental pH. Moreover, the built scaffolds can serve as arrayed analytical platforms for observing cell behaviors in normal or changeable living spaces or revealing the anticancer effects of loaded drugs. The proposed approach opens a new path for facile and flexible manufacturing of hydrogel-based functional microstructures with great versatility in micro-object manipulation.

Mechanisms driving self-organization phenomena in random plasmonic metasurfaces under multipulse femtosecond laser exposure: a multitime scale study

Abstract Laser-induced transformations of plasmonic metasurfaces pave the way for controlling their anisotropic optical response with a micrometric resolution over large surfaces. Understanding the transient state of matter is crucial to optimize laser processing and reach specific optical properties. This article proposes an experimental and numerical study to follow and explain the diverse irreversible transformations encountered by a random plasmonic metasurface submitted to multiple femtosecond laser pulses at a high repetition rate. A pump-probe spectroscopic imaging setup records pulse after pulse, and with a nanosecond time resolution, the polarized transmission spectra of the plasmonic metasurface, submitted to 50,000 ultrashort laser pulses at 75 kHz. The measurements reveal different regimes, occurring in different ranges of accumulated pulse numbers, where successive self-organized embedded periodic nanostructures with very different periods are observed by post-mortem electron microscopy characterizations. Analyses are carried out; thanks to laser-induced temperature rise simulations and calculations of the mode effective indices that can be guided in the structure. The overall study provides a detailed insight into successive mechanisms leading to shape transformation and self-organization in the system, their respective predominance as a function of the laser-induced temperature relative to the melting temperature of metallic nanoparticles and their kinetics. The article also demonstrates the dependence of the self-organized period on the guided-mode effective index, which approaches a resonance due to system transformation. Such anisotropic plasmonic metasurfaces have a great potential for security printing or data storage, and better understanding their formation opens the way to smart optimization of their properties.

Realization of ∼10 nm Features on Semiconductor Surfaces via Femtosecond Laser Direct Patterning in Far Field and in Ambient Air.

Direct fabrication of ∼10 nm features by optical means in far field and in ambient air on semiconductor surfaces is significant for next-generation advances nanomanufacturing. We report here a new method that enables the direct formation of 12 nm (λ/66) features on silicon surfaces. It is processed in far field and in ambient air via the irradiation of orthogonally polarized double femtosecond laser beams. The coupling of orthogonally polarized double femtosecond laser beams and the incubation effect due to multiple femtosecond laser pulses irradiation under high repetition rate enable the 12 nm nanostructures creation parallel to the scanning direction, regardless of scanning path.

Engineering optical anisotropy in nonlinear crystals with ultrafast light

Photonic technology is widely based on anisotropic (and) nonlinear materials, which allow light modulation and parametric light conversion. Because the number of naturally occurring crystals is limited, there is a growing demand for artificial metamaterials with optical properties specifically tailored to a given application. Here, we utilize the top-down method to synthesize sub-wavelength periodic nanostructures inside a uniaxial optically nonlinear crystal (lithium niobate, LiNbO3) by irradiating it with multiple femtosecond laser pulses. By superimposing form-birefringence associated with the light-induced nanostructures onto natural birefringence of the host crystal we create macroscopic domains of a biaxial metamaterial embedded into otherwise uniaxial medium.

Nanostructure patterning of C-Sb2Te3 by maskless thermal lithography using femtosecond laser pulses

Thermal lithography is an alternative technique that can fabricate patterns beyond the diffraction limit. In thermal lithography, it is hard to obtain morphology on demand due to the difficulty in controlling thermal diffusion caused by the long laser pulses. To address this issue, enhanced thermal lithography with multiple femtosecond laser pulse exposure is proposed in this paper. C-Sb2Te3 (CST), a fine-tunable phase change material, is used as a negative heat-mode resist for the first time. The resolution and steepness of the microstructure prepared by this method are obviously enhanced compared with the conventional way. The height of the wet-etched microstructures can be preciously adjusted by tuning the exposure dose of femtosecond laser pulses. Microstructure and thermal field analysis results reveal the origin of the high resolution and fine-tuning. CST-based phase-change memory cell arrays were fabricated with this method to confirm the feasibility and advantage. This work provides an effective way to achieve enhanced thermal lithography in the manufacturing of micro/nanostructure-based electronic and photonic devices.

Ultrafast temporal-spatial dynamics of amorphous-to-crystalline phase transition in Ge2Sb2Te5 thin film triggered by multiple femtosecond laser pulses irradiation

Studies have shown that the crystallization phase state of Ge2Sb2Te5 (GST) can be reversibly modulated by femtosecond (fs) laser multiple pulses, which have excellent applications in reconfigurable multi-level operation fields. In this study, the temporal-spatial crystalline evolution dynamics of amorphous GST film is investigated during two fs laser pulses excitation through a pump-probe shadowgraph imaging technique. A quasi-amorphous phase state, which is different from that in the initial as-deposited amorphous GST, is emerged through the first fs laser pulse excitation with a pulse energy lower than crystallization threshold. The experimental results reveal that a crystallization enhancement effect can be induced through the second pulse excitation based on this quasi-amorphous surface structure. The stimulative cluster generated in the quasi-amorphous reduces the amorphous-to-crystalline phase transition threshold for the second fs laser pulse irradiation. The spatially-resolved phase-transition threshold extension effect in a horizontal direction is proposed with the increasing pulse number to summarize the mechanism of the crystallization enhancement effect. The specific-grain-appearance (coarse grains and fine grains representing different phase transition approach) distributed area induced by single and double fs laser pulses irradiation are experimentally demonstrated corresponding to threshold extension theory.

Monitoring of Evolving Laser Induced Periodic Surface Structures

Laser induced periodic surface structures (LIPSS) are generated on titanium and silicon nitride surfaces by multiple femtosecond laser pulses. An optical imaging system is used to observe the backscattered light during the patterning process. A characteristic fringe pattern in the backscattered light is observed and evidences the surface modification. Experiments are complemented by finite difference time domain numerical simulations which clearly show that the periodic surface modulation leads to characteristic modulations in the coherently scattered light field. It is proposed that these characteristic fringe pattern can be used as a very fast and low-cost monitor of LIPSS formation formation during the manufacturing process.

Amplification and regulation of periodic nanostructures in multipulse ultrashort laser-induced surface evolution by electromagnetic-hydrodynamic simulations

The formation of periodic structures in ultrafast laser-irradiated surfaces implies dynamic coupling between the incoming light and the light-driven material. To capture the mutual influence and feedback between light and evolving surface topographies, we investigate numerically the evolution of metal surfaces irradiated by multiple femtosecond laser pulses of sub-, near-, and slightly above-threshold ablation fluence. The multiphysical model combines Maxwell equations and thermohydrodynamic approach based on electron-ion heat transfer and compressible Navier-Stokes equations and allows us to account for interpulse feedback on the resulting surface topographies. First pulses of the subthreshold energy lead to material swelling, nanocavitation few tens of nanometers below the surface, and, as a result, nanoroughness formation on the initially flat surface. Further pulses contribute to the development of periodic surface structures, enhanced absorption, and increased removal rate. Cavitation in the tips of ripples is found to play a crucial role in modification and regulation of surface topography for sub- and near-threshold ablation fluences. At higher laser pulse energy, thermal ablation is mostly involved in surface modification, and the ablation rate per power reaches its maximum at three to five times the ablation threshold fluence, resulting from the optimal heat penetration depth for laser ablation. The numerical results offer a better understanding of the surface topography modifications upon multipulse femtosecond laser irradiation.

Periodic surface structures on dielectrics upon femtosecond laser pulses irradiation.

The formation of laser-induced periodic surface structures (LIPSS) on two different dielectrics of K9 glass and fused silica upon irradiation in ambient conditions and in vacuum with multiple femtosecond (fs) laser pulse sequences at different pulse durations (35fs, 260 fs, and 500 fs) was studied experimentally. Three types of LIPSS, so-called high-spatial-frequency LIPSS (HSFL), low-spatial-frequency LIPSS (LSFL), and supra-wavelength periodic surface structures (SWPSS) with different spatial periods and orientations were identified. The appearance was characterized with respect to the experimental parameters of laser fluence and number of laser pulses per spot. The crater morphologies - including nanoripples, periodic microgrooves, quasiperiodic microspikes, and central smooth zone - were observed by scanning electron microscope (SEM). The supra-wavelength structures exhibit periodicities, which are markedly, even multiple times, higher than the laser excitation wavelength. The SWPSS were formed with a broader range of laser fluences, upon the longer laser pulse durations (260 fs and 500 fs) and/or on the lower band-gap dielectrics (K9 glass), due to the deeper effective light penetration depths and thicker viscous surface layers formation. The HSFL were observed on the higher band-gap dielectrics (fused silica) and within a certain narrow laser parameter window. The formation mechanisms of LIPSS were also discussed.

Consolidating multiple femtosecond lasers in coupled curved plasma capillaries

The ability to guide high-energy femtosecond scale lasers while augmenting their energy is crucial in future laser based TeV particle accelerators where the laser energy depletion is the major setback. We propose, analyze, and experimentally demonstrate consolidating multiple femtosecond pulse lasers in coupled curved capillaries. We demonstrate a proof of principle scheme of coupled curved capillaries where two femtosecond laser pulses are combined. Furthermore, we show simulations that demonstrate that high-intensity short pulse lasers can be guided within a small curvature radius capillary.

Multiscale Investigation of Femtosecond Laser Pulses Processing Aluminum in Burst Mode

ABSTRACTMegahertz is the highest femtosecond laser repetition rate that the state-of-the art technology can achieve. In this article, a single femtosecond laser pulse is burst into multiple femtosecond laser pulses to process aluminum. The temporal gap between two consecutive burst pulses is 2 picoseconds, which is much shorter than the temporal gap between two consecutive pulses at the repetition rate of megahertz. By taking the thermophysical scenarios of femtosecond laser induced of electron thermalization, electron heat conduction, electron–phonon-coupled heat transfer and atomic motion into account, a multiscale framework integrating ab initio quantum mechanical calculation, molecular dynamics and two-temperature model are constructed. The effect of femtosecond laser pulse number on the incubation phenomenon is studied. Comparing with the single pulse-processing aluminum film, the femtosecond laser in burst mode leads to smaller thermal stress, which is favorable to reduce the thermal mechanical damage of the material beneath the laser-irradiated surface. Appreciable differences among the simulation results by using electron thermophysical parameters from ab initio quantum mechanical calculation and those from experimental measurement, empirical estimation and calculation are found, indicating the essentials to precisely model the electron thermal response subject to femtosecond laser excitation.

Time-resolved shadowgraphs and morphology analyses of aluminum ablation with multiple femtosecond laser pulses**Project supported by the Science and Technology Development Fund Planning Project for the Universities of Tianjin, China (Grant No. 20140902), the Natural Science Foundation of Tianjin City, China (Grant No. 16JCQNJC01900), the National Natural Science Foundation of China (Grant Nos. 51376136 and 61474082), and the Science and Technology

Aluminum ablation by multiple femtosecond laser pulses is investigated via time-resolved shadowgraphs and scanning electron microscope (SEM) images of the ablation spot. The spatial distribution of the ejected material and the radius of the shock wave generated during the ablation are found to vary with the increase in the number of pulses. In the initial two pulses, nearly concentric and semicircular stripes within the shock wave front are observed, unlike in subsequent pulses. Ablation by multiple femtosecond pulses exhibits different characteristics compared with the case induced by single femtosecond pulse because of the changes to the aluminum target surface induced by the preceding pulses.

Dynamics of laser-induced defects by multiple femtosecond pulses in potassium dihydrogen phosphate crystals

The dynamics of the formation and annihilation of point defects in potassium dihydrogen phosphate crystals induced by multiple femtosecond laser pulses at 800 nm and 266 nm wavelengths is addressed. It is investigated through time-resolved photo-luminescence (PL) experiments in the [4.3 eV; 6.3 eV] spectral region for sample temperatures ranging from 20 K to 300 K. For both laser excitation wavelengths, a 5.1 eV energy PL band is found, for which amplitude depends on the temperature, and exhibiting a particular behavior around the Curie temperature (123 K). The low-temperature PL band characteristics (energy, width, kinetics of PL decay) suggest that it is due to the radiative annihilation of self-trapped excitons. Above the Curie temperature, the PL band is centered around the same previous energy but exhibits a different width, suggesting radiative annihilation of self-trapped holes next to the [HPO4]− group. During the laser irradiation at 1 kHz repetition rate of a low-temperature sample, the PL signal at 5.1 eV decreases as a function of the irradiation time on a timescale going from a few seconds to a few minutes, depending on both the laser wavelength and the intensity. This behavior is attributed to the formation and annihilation of mobile hydrogen defects. A model based on rate equations provides the temporal evolution of the defects’ density, allowing us to reproduce and interpret the observed evolution of the PL signal. This model also provides timescales for the defects’ density evolution with respect to the laser wavelength and intensity.

Orientation of ripples induced by ultrafast laser pulses on copper in different liquids

Formation of laser-induced periodic surface structures (LIPSS or ripples) was studied on a metallic surface of polished copper by using irradiation with multiple femtosecond laser pulses in different environmental conditions (air, water, ethanol and methanol). Uniform LIPSS have been achieved by controlling the peak fluence and the overlapping rate. Ripples in both orientations, perpendicular and parallel to laser polarization, were observed in all liquids simultaneously. The orientation of these ripples in the center of the ablated line was changing with the incident light intensity. For low intensities the orientation of the ripples is perpendicular to the laser polarization, whereas for high intensities it turns parallel to it without considerable changes in the period. Multi-directional LIPSS formation was also observed for moderate peak fluence in liquid environments.

Nanometer-resolved chemical analyses of femtosecond laser-induced periodic surface structures on titanium

The chemical characteristics of two different types of laser-induced periodic surface structures (LIPSS), so-called high and low spatial frequency LIPSS (HSFL and LSFL), formed upon irradiation of titanium surfaces by multiple femtosecond laser pulses in air (30 fs, 790 nm, 1 kHz), are analyzed by various optical and electron beam based surface analytical techniques, including micro-Raman spectroscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. The latter method was employed in a high-resolution mode being capable of spatially resolving even the smallest HSFL structures featuring spatial periods below 100 nm. In combination with an ion sputtering technique, depths-resolved chemical information of superficial oxidation processes was obtained, revealing characteristic differences between the two different types of LIPSS. Our results indicate that a few tens of nanometer shallow HSFL are formed on top of a ∼150 nm thick graded superficial oxide layer ...

Surface Morphology Evolution Induced by Multiple Femtosecond Laser Ablation on 4H-SiC Substrate and Its Application to CMP

4H silicon carbide (SiC) substrates were processed by multiple femtosecond (fs) laser ablation. Its surface morphologies changed dramatically at different scanning pitches, different scanning velocities, different repetition rates and near-threshold fluence. The evolution of surface morphologies and the uniformity of rippled structures were respectively investigated and discussed. The overlapped zones of adjacent pulses were focused in discussion. In the case of high repetition rate, periodic ripples with average spatial periodicities ranging from 145 nm to 196 nm were induced; whereas in the case of lower repetition rate, coarse ripples (roughly 440 nm) and fine ripples (about 117 nm) were simultaneously formed. Large amount of agglomeration was fabricated in the case of scanning pitch 0.05 μm and scanning velocity ≤ 5 mm/s. At near-threshold fluence, 4H-SiC surface was partly ablated. When the rippled structures induced in Transverse Irradiation Mode were further ablated in Cross Scanning Mode, horizontal ripples changed into vertical ripples. X-ray diffraction and Raman spectra were utilized for surface analysis. Confirmatory experiment was carried out, demonstrating that chemical mechanical polishing (CMP) process could be promoted by fs laser ablation.

Dense ionization and subsequent non-homogeneous radical-mediated chemistry of femtosecond laser-induced low density plasma in aqueous solutions: synthesis of colloidal gold.

The "cold" low density plasma channels generated by the filamentation of powerful femtosecond laser pulses in aqueous solutions constitute a source of dense ionization. Here, we probed the radiation-assisted chemistry of water triggered by laser ionization via the radical-mediated synthesis of nanoparticles in gold chloride aqueous solutions. We showed that the formation of colloidal gold originates from the reduction of trivalent ionic gold initially present in solution by the reactive radicals (e.g. hydrated electrons) produced upon the photolysis of water. We analyzed both the reaction kinetics of the laser-induced hydrated electrons and the growth kinetics of the gold nanoparticles. Introduction of radical scavengers into the solutions and different initial concentrations of gold chloride provided information about the radical-mediated chemistry. The dense ionization results in the second order cross-recombination of the photolysis primary byproducts. Competition with recombination imposes the non-homogeneous interaction of reactive radicals with solute present in irradiated aqueous solutions. Such a laser-induced non-homogeneous chemistry suggests similarities with the radiation chemistry of water exposed to conventional densely ionizing radiation (high dose rate, high linear energy transfer).

Modification of narrow ablating capillaries under the influence of multiple femtosecond laser pulses

Powerful femtosecond laser pulses that propagate through narrow ablating capillaries cause modification of capillary walls, which is studied experimentally and theoretically. At low intensities, the laser-induced periodic surface structures and porous coating composed of sub-micron particles appear on the walls. At higher intensities, the surface is covered by deposited droplets of the size up to 10 μm. In both cases, the ablated material forms a solid plug that completely blocks the capillary after several hundreds or thousands of pulses. The suggested theoretical model indicates that the plug formation is a universal effect. It must take place in any narrow tube subject to ablation under the action of short laser pulses.

Analysis on the damage threshold of MgO:LiNbO3 crystals under multiple femtosecond laser pulses

An improved theoretical model of the interaction between multiple femtosecond laser pulses and MgO:LiNbO3 crystals with different doping concentrations has been established based on the classical two-temperature model. The evolutions of electron and lattice temperature with the duration, the repetition frequency and the numbers of multiple femtosecond laser pulses in MgO:LiNbO3 crystals have been simulated numerically by the Crank-Nicholson implicit finite-difference method. Furthermore, the variations of the damage threshold of MgO:LiNbO3 crystals with the parameters of multiple femtosecond laser pulses at different doping concentrations, as well as the influence of doping concentration on damage threshold have also been analyzed. The results show that, the damage threshold of MgO:LiNbO3 crystals increases with the increasing of the duration of the femtosecond laser pulse. The damage threshold of MgO:LiNbO3 crystals first decreases with the increasing of the numbers and the pulse repetition frequency of the laser pulses and then tends to be a constant. The damage threshold of a small amount of MgO-doped LiNbO3 crystals is higher than that of undoped LiNbO3 crystals. Consequently, the resist damage capability of LiNbO3 crystals can be enhanced by doping appropriate MgO in many practical applications.