Double Femtosecond Laser Pulses Research Articles

Single-crystal silicon ablation with temporally delayed femtosecond laser double-pulse trains

Abstract A double-pulse femtosecond laser is used to process single-crystal silicon. Modulating the delay time was discovered to increase the ablation depth and improve the morphology of the ablated surface. The hole fabricated by a dual-pulse with a 200 ps interval is 24.4% deeper than that created by a single pulse of the same energy. Moreover, utilizing a dual pulse with an interval ranging from 100 to 1000 ps produces a considerably smoother ablation area as compared to the single pulse. The effect of the sub-pulse energy ratio of the double-pulse femtosecond laser on the size and morphology of the ablated area was also investigated. As the sub-pulse energy ratio decreases from 3:1 to 1:3, the size of the ablation area initially decreases and then increases, while the size of the ablation area is minimized when the sub-pulse ratio is 1:1, enabling precise control over the machining size. As the energy of the second sub-pulse increases, the ablation area becomes smoother due to the plasma heating of the double-pulse femtosecond laser.

Ultrafast dynamics and internal processing mechanism of silica glass under double-pulse femtosecond laser irradiation

Femtosecond laser-induced plasma filaments have potential for various applications including attosecond physics, spectroscopy, and microprocessing. However, the use of plasma filaments to generate high-aspect-ratio internal modifications remains low-efficiency. Here, we experimentally demonstrated high-efficiency internal processing using plasma filaments induced by a double-pulse femtosecond laser. The processing mechanism was revealed through an investigation of the ultrafast dynamics of plasma filaments in experiments and simulations. We found that the excitation region of the first pulse (P1) exerted a temporal effect on the propagation and absorption of the second pulse (P2) due to the evolution of excited electrons, thus resulting in different processing characterizations. At a smaller inter-pulse delay (IPD), electrons and self-trapped excitons induced by P1 improved the absorption of P2 in the shallow region. Consequently, the main excitation regions of P1 and P2 were separated, resulting in a lower density of energy deposition and weak modifications. Whereas, at a larger IPD, P2 penetrated a deeper region with the relaxation of electrons and excitons induced by P1, leading to a better overlap of excitation regions between P2 and P1, thus improving the density of energy deposition and achieving efficient microprocessing. Besides, at an infinite IPD, P2 behaved like P1, but no modification was obtained owing to the complete energy diffusion of P1. Therefore, controlling the electron dynamic and energy diffusion contributes to the improvement of modification efficiency. Furthermore, the distribution of electron densities on the cross section was estimated to precisely analyze the microprocessing. These results are expected to aid in a better understanding of the interaction mechanism between dielectrics and intense ultrafast lasers and be useful for microprocessing applications.

Atomistic-Continuum Study of an Ultrafast Melting Process Controlled by a Femtosecond Laser-Pulse Train.

In femtosecond laser fabrication, the laser-pulse train shows great promise in improving processing efficiency, quality, and precision. This research investigates the influence of pulse number, pulse interval, and pulse energy ratio on the lateral and longitudinal ultrafast melting process using an experiment and the molecular dynamics coupling two-temperature model (MD-TTM model), which incorporates temperature-dependent thermophysical parameters. The comparison of experimental and simulation results under single and double pulses proves the reliability of the MD-TTM model and indicates that as the pulse number increases, the melting threshold at the edge region of the laser spot decreases, resulting in a larger diameter of the melting region in the 2D lateral melting results. Using the same model, the lateral melting results of five pulses are simulated. Moreover, the longitudinal melting results are also predicted, and an increasing pulse number leads to a greater early-stage melting depth in the melting process. In the case of double femtosecond laser pulses, the pulse interval and pulse energy ratio also affect the early-stage melting depth, with the best enhancement observed with a 2 ps interval and a 3:7 energy ratio. However, pulse number, pulse energy ratio, and pulse interval do not affect the final melting depth with the same total energies. The findings mean that the phenomena of melting region can be flexibly manipulated through the laser-pulse train, which is expected to be applied to improve the structural precision and boundary quality.

Ablation depth enhancement on a copper surface using a dual-color double-pulse femtosecond laser.

We investigate the femtosecond laser ablation of copper with a dual-color double-pulse femtosecond laser at the wavelengths of 515 nm and 1030 nm. By properly choosing the energy of the 515 nm pulse, the optical properties such as surface reflectivity and absorption coefficient on copper surface can be modified to increase the absorption of the subsequent 1030 nm pulse. The ablation depth of dual-color double-pulse laser is at least 50% higher than the total ablation depth of both the 515 nm and 1030 nm pulses, provided that the inter-pulse delay of the double-pulse laser is within the electron-phonon coupling time. The ablation depth enhancement on a copper surface using a dual-color double-pulse femtosecond laser is of significant interest for scientific research and industrial application.

Efficient surface processing of high surface quality with two-color double-pulse femtosecond laser ablation

We investigate the femtosecond laser ablation of 304 stainless steel (AISI 304), aiming to achieve a high ablation rate, while maintaining low surface roughness. We observe that the surface ablation with 1030 nm-wavelength laser pulses shows a higher ablation rate than that with the 515 nm-wavelength pulses, whereas the surface roughness obtained with the 1030 nm pulses is significantly higher as compared to that obtained with the ablation of 515 nm pulses. We also show that by the use of the two-color double-pulses at 1030 and 515 nm wavelengths, a much higher ablation rate has been achieved with a surface roughness comparable to that obtained with the ablation of 515 nm pulses.

Nanoscale ultra-high-resolution non-destructive detection by correlated double femtosecond laser pulses

Nanoscale ultra-high-resolution non-destructive detection by correlated double femtosecond laser pulses

Micro drilling of PMMA with double-pulse femtosecond laser

Micro holes drilling in polymers have a variety of applications in industry and research. Especially, the fabrication of micro hole with high aspect ratio is a key technology for advanced applications. In this work, we report an improvement in drilling on poly-methyl methacrylate (PMMA) by using double-pulse femtosecond laser. Compared to single pulse, the achievable depth by double pulses can be increased by 50% when the pulse delay is 100 fs and gradually decreases with delay increasing. The depth evolution with the increasing of pulse number is investigated. When the micro hole depth reaches a certain value, the processing speed of double pulse is less restricted than single pulse. Furthermore, the relationship between the depth improvement and electrons decay together with absorption increase is studied.

Thermo-mechanical coupling induced Near-threshold double pulses femtosecond laser ablation enhancement in titanium

Near-threshold femtosecond laser surface treatment of titanium produces functional nanostructures, which has received much attention in applications and scientific researches. However, researches on the mechanism of near-threshold temporally-shaped femtosecond laser ablation of titanium are scarce, because the difficulty of investigating the modification mechanism of surface properties by sub-pulses below the ablation threshold. In this study, the double-pulse femtosecond laser ablation with various pulse separation was investigated. We found that the maximum ablation area appeared around 1 ps pulse separation at different fluences, and the ablation enhancement weakened with the increase of the fluence. The ultrafast pump–probe microscopy technique and ultrafast thermoelasticity model were used to investigate the mechanism. The result of pump–probe microscopy technique indicated that the enhancement of the area was caused by the decrease of transient reflectivity in the early stages which facilitated the subsequent sub-pulse energy absorption. The theoretical results indicate that the maximum ablation enhancement occurred at 1 ps, which is due to the combination of thermal and mechanical effects. The maximum of long axis diameter of periodic structure also appeared at 1 ps in multi-point processing. These findings have significant implications for the modulation of titanium surface nanostructures and providing theoretical support for processing.

Enhancement of ablation and ultrafast electron dynamics observation of nickel-based superalloy under double-pulse ultrashort laser irradiation

Temporally shaped femtosecond laser machining is a flexible and effective method to improve the efficiency and quality of cooling film holes. This study investigated the ablation of nickel-based superalloy by double-pulse femtosecond laser with different pulse-separations and fluences. Compared with single-pulse ablation, approximate 1.5 times enhancement of ablation area was obtained in double-pulse ablation with about 2 ps pulse-separation. By varying the pulse-separations, the ablation area can be tuned, and at the same time, the ablation depth can be kept for little fluctuation. An improved two-temperature model and time-resolved transient reflectivity technique were used for analyzing the ablation mechanisms. We found that more energy deposition can happen from electron system to lattice system for double-pulse ablation, which makes ablation area increase. However, mechanical relaxation started at around 2 ps, which could be suppressed by the pressure wave induced by the second sub-pulse, and finally achieved the maximum ablation area at about 2 ps pulse-separation. Besides, laser-induced subwavelength periodic surface structures were observed under irradiation of multiple pulses. The findings may aid in understanding the ablation mechanism between nickel-based superalloy and femtosecond laser, as well as in optimizing the processing of cooling film holes.

Hologram imaging quality improvement by ionization controlling based on the self-trapped excitons with double-pulse femtosecond laser

Abstract Holograms hidden inside transparent materials are important for information encryption storage because of their advantages of secrecy, and could completely avoid information loss caused by surface wear. Inside the transparent material, the modified filaments array was need for hologram fabrication to change the optical phase or amplitude of incident laser, which is sensitive to the change of refractive index. Then the uniformity of modified filaments inside transparent materials is highly required. In this study, by tuning the interval time of the double-pulse processing, holograms with improved imaging quality were fabricated by double-pulse femtosecond laser and the effect and mechanism of self-trapped excitons (STEs) on the ablation have been systematically studied. The imaging quality of the hologram fabricated with double-pulse laser was superior to that of the one fabricated with the single-pulse laser and 350 fs was verified to be the best time interval for double-pulse processing. The evolution of the electrons dynamics was investigated by using the pump-probe technology. With the double-pulse time interval increasing, the residual electrons, excitons, STEs, and defects caused by the first sub-pulse would become dominated sequentially. The results demonstrated the controllability of STEs and quality improvement of final structures by double-pulse femtosecond laser processing.

Subwavelength Quasi-Periodic Array for Infrared Antireflection.

Infrared antireflection of a zinc sulfide (ZnS) surface is important to improve performance of infrared detector systems. In this paper, double-pulse femtosecond laser micro-machining is proposed to fabricate a subwavelength quasi-periodic array (SQA) on ZnS substrate for infrared antireflection. The SQA consisting of approximately 30 million holes within a 2 × 2 cm2 area is uniformly formed in a short time. The double-pulse beam can effectively suppress the surface plasma shielding effect, resulting in obtaining a larger array depth. Further, the SQA depth is tunable by changing pulse energy and pulse delay, and can be used to readily regulate the infrared transmittance spectra as well as hydrophobicity. Additionally, the optical field intensity distributions of the SQA simulated by the rigorous coupled-wave analysis method indicate the modulation effect by the array depth. Finally, the infrared imaging quality captured through an infrared window embedded SQA is evaluated by a self-built infrared detection system.

Influence of inter-pulse delay on CN molecular emission from femtosecond-nanosecond double-pulse laser-induced breakdown spectroscopy of PMMA

An experimental study on molecular emission in laser-induced breakdown spectroscopy (LIBS) was performed using an integrated femtosecond (fs) and nanosecond (ns) double-pulse (DP) laser scheme at the different inter-pulse delay between the fs and ns pulses. This work mainly included the influence of the DP inter-pulse delay on the spectral emission intensity and signal-to-background ratio (SBR). The ablated sample was polymethyl methacrylate (PMMA), and the measured molecular band was CN (Δv = 0) in the B2Σ+ − X2Σ+ transition. The results showed that the negative (ns + fs scheme) and positive (fs + ns scheme) inter-pulse delays were not much different in terms of spectral emission. The SBR excited by the fs + ns DP was higher than that excited by the ns + fs DP, and the maximum value of the SBR in the fs + ns DP-LIBS scheme was about two times that in the ns + fs DP-LIBS scheme. The higher SBR was suitable for spectral analysis, and was easy to reduce the influence of continuous emission on spectral analysis. Also, the vibration temperature of the CN molecule as a function of the inter-pulse delay between the two laser pulses was discussed. This discovery fully demonstrates that the fs + ns DP-LIBS is of great significance for detecting molecular emissions in LIBS.

Uniformity evaluation of laser-induced periodic surface structures formed by two-color double-pulse femtosecond laser irradiation.

The Perpendicular Period and Phase Scanning (P3S) method can evaluate the uniformity of a laser-induced periodic surface structure (LIPSS). P3S assesses the uniformity of LIPSS using the standard deviation of the peak period and the average of the phase difference in the direction perpendicular to LIPSS. The P3S method demonstrates that LIPSS formed by two-color double-pulse irradiation is reduced to a quarter of the period dispersion, and the average phase difference of LIPSS is also reduced compared to the single-pulse irradiation. In addition, a 3D electromagnetic particle-in-cell simulation was performed to evaluate the possibility of an improved uniformity of LIPSS. The results confirm that the two-color double-pulse irradiation produces a uniform LIPSS and validates the effectiveness of the P3S method to assess the uniformity of LIPSS.

Efficiency enhancement by transient electron dynamic control in shaped femtosecond laser fabrication of metals

Ultrafast laser pulse train processing has been widely used to improve the efficiency and quality of nonmetallic material processing. However, it is commonly believed that the split of one pulse into sub-pulses for metallic material will suppress the ablation with near-threshold laser fluence. In this article, a high-efficiency metal processing by femtosecond laser double pulses train with different energy ratio is performed in the aspects of theory simulation and experiment. In theory, a Femto-picosecond and Nano-micrometre multiscale framework combining photoelectric effect and electron-phonon-coupled heat transfer proved that the pulse train can effectively control the electron properties, so as to enhance processing efficiency. Theory-guided experiments indicate that the ablation crater reaches maximum ablation depth with a sub-pulse energy ratio optimized pulse train, which is 40%–100% deeper than that of single pulse processing. This method can be used for femtosecond laser processing of various metal materials with high efficiency and high quality, and is helpful to solve the manufacturing bottleneck challenges in the fields of aviation, mechanics, electronics and materials engineering.

Alternate morphology evolution of bulge structures on thin gold films induced by internal stress distribution adjusted by femtosecond laser double-pulse

In this study, the influence of femtosecond double-pulses in inducing bulge structures on thin gold films were experimentally examined. The experimental results indicate that the morphology of bulge structures evolved alternately with an increase in the time delay between two subpulses under the condition that the total pulse energy remained the same. This phenomenon is different from that observed in previous studies in which femtosecond double-pulses were used for processing metal materials. Through the combination of the two-temperature model and the generalized thermoelastic theory, the stress distribution inside the gold film was calculated under different time delays. Moreover, the aforementioned experimental phenomenon was explained using the superposition principle of the stress waves generated by two subpulses. The infrared reflection spectra of gold bulge arrays processed under different time delays were also investigated. This research presents a strategy for obtaining detailed understanding regarding the physical mechanism of the interaction between femtosecond double-pulses and thin metal films. It also presents a strategy for optimizing the parameters of pulse trains to adjust the stress distribution to achieve the shape evolution of the formed structures.

Micro drilling of PMMA with double-pulse femtosecond laser

Micro drilling of PMMA with double-pulse femtosecond laser

Filamentation of Double Femtosecond Laser Pulses in Fused Silica

Filamentation of Double Femtosecond Laser Pulses in Fused Silica

Numerical simulation of thermal processes occurring in materials under the action of femtosecond laser pulses

In this work, a numerical study of the solutions of the parabolic and hyperbolic equations of heat conduction with the same physical parameters is carried out and a comparative analysis of the results obtained is carried out. The mathematical formulation of the problem is discussed. The action of the laser is taken into account through the source function, which was chosen as a double femtosecond laser pulse. In the hyperbolic equation, in contrast to the parabolic one, there is an additional parameter that characterizes the relaxation time of the heat flux. In addition, the source of the hyperbolic equation contains an additional term - the derivative of the power density of the source of the parabolic equation. This means that the temperature of the sample is influenced not only by the power density of the source, but also by the rate of its change. The profiles of the sample temperature at different times and its dynamics at different target depths are shown. The calculations were carried out for different time delays between pulses and for different relaxation parameters.

Throughput Improvement in Femtosecond Laser Ablation of Nickel by Double Pulses.

In this study, femtosecond laser double pulses were tested to improve their nickel ablation efficiency. The experimental results indicated that compared with single pulses, double pulses with different delay times generated craters with larger diameters and depths. The results obtained for three sets of double pulses with different energy ratios indicated that double pulses with an energy ratio of 1:9 had the highest ablation efficiency, followed by those with energy ratios of 2:8 and 5:5. The double pulses with the aforementioned three energy ratios achieved the maximum ablation efficiency when the delay time was 3–4 ps. Compared with single pulses, double pulses with an energy ratio of 1:9 generated craters with an up to 34% greater depth and up to 14% larger diameter. In addition, an interference effect was observed with a double pulse delay time of 0 ps, which has seldom been reported in the literature. The double pulses were simulated using the two-temperature model. The simulation results indicated that double pulses with an energy ratio of 1:9 with a delay time of 4 ps can perform the strongest ablation. These simulation results are in line with the experimental results.

Nanopatterning of silicon via the near-field enhancement effect upon double-pulse femtosecond laser exposure.

Utilizing the near-field enhancement effect of a polystyrene microsphere, direct ablation of nanohole arrays by a temporal-shaping femtosecond (fs) laser pulse is presented. The nanohole arrays, which are circular, regular, and free of cracks, were processed without extra post-processing, and their average diameter decreased gradually, as the double-pulse delay increased until 2500 fs. The simulated results by a plasma model and finite difference time domain solution demonstrate that the size decrease of the structure is attributed to the increase of the ablation threshold of silicon. Through fs laser near-field fabrication, the FWHM of nanoholes can be reduced to approximately 50 nm (λ/16) and even to 23 nm when using the second harmonic laser at a wavelength of 400 nm.