Selective triggering of phase change in dielectrics by femtosecond pulse trains based on electron dynamics control

In this study we experimentally reveal that the phase change mechanism can be selectively triggered by shaping femtosecond pulse trains based on electron dynamics control (EDC), including manipulation of excitations, ionizations, densities, and temperatures of electrons. By designing the pulse energy distribution to adjust the absorptions, excitations, ionizations, and recombinations of electrons, the dominant phase change mechanism experiences transition from nonthermal to thermal process. This phenomenon is observed in quadruple, triple, and double pulses per train ablation of fused silica separately. This opens up possibilities for controlling phase change mechanisms by EDC, which is of great significance in laser processing of dielectrics and fabrication of integrated nano- and micro-optical devices.     

First-principles electron dynamics control simulation of diamond under femtosecond laser pulse train irradiation

A real-time and real-space time-dependent density functional is applied to simulate the nonlinear electron-photon interactions during shaped femtosecond laser pulse train ablation of diamond. Effects of the key pulse train parameters such as the pulse separation, spatial/temporal pulse energy distribution and pulse number per train on the electron excitation and energy absorption are discussed. The calculations show that photon-electron interactions and transient localized electron dynamics can be controlled including photon absorption, electron excitation, electron density, and free electron distribution by the ultrafast laser pulse train.     

Ultrafast electron dynamics on the silicon surface excited by an intense femtosecond laser pulse

The electron dynamics on the silicon surface during the pump ultrashort infrared laser pulse is studied by time-resolved optical microscopy and electron-emission measurements. It is found that the optical response of the material under the conditions where a dense electron-hole plasma is formed is determined by the renormalization of the band spectrum of the material rather than by intraband transitions of photoexcited carriers. Nonlinear Auger recombination in the plasma enhanced by the plasma-induced renormalization of the band gap and accompanied by the generation of hot charge carriers stimulates intense prompt emission of such carriers from the surface of the photoexcited material, whose work function decreases owing to the large plasma-induced renormalization of the energies of higher conduction bands.     

Ultrafast dynamics control on ablation of Cu using shaped femtosecond pulse trains

The ablation processes of Cu film are investigated using temporal shaped femtosecond pulse trains. The depth is modulated by changing the number and interval of the sub-pulses. The underlying ultrafast dynamic processes are discussed based on plasma shielding and electron multiple heating mechanisms. When the sub-pulse interval is less than 0.4 ps electron multiple heating is the dominant mechanism, while the plasma shielding dominates the subsequent ablation processes when the sub-pulse interval is larger than 0.4 ps. The curve of depth obtained by three pulse trains shows more significant oscillation as the function of sub-pulse interval under the low-fluence. We propose that the oscillation of depth is due to the coherent phonon oscillation excited by the pulse train. The study provides a basis for giving insight into the ultrafast dynamics for improving micromachining and nano-fabrications using shaped femtosecond pulse trains.     

Ultra-fast laser absorption and ablation dynamics in wide-band-gap dielectrics

The highly nonlinear laser–matter interaction conditions produced by high-intensity amplified ultra-fast laser pulses have proven to be beneficial in the processing of normally transparent wide-band-gap dielectric materials. This article presents experimental studies of the ultra-fast laser absorption process in three wide-band-gap dielectrics: fused silica, calcium fluoride, and sapphire. Time-resolved measurements of the probe transmissivity and reflectivity show both the formation of dense free-electron plasma at the surface due to nonlinear absorption of the laser pulses and rapid structural damage on the order of a few picoseconds. Pump–probe data with intense pump and probe pulses was also correlated to atomic force microscopy measurements of the ablated volume. It was observed that the material removal peaked near zero delay between the pulses and decreased within a temporal separation of about 1 ps. PACS 52.38.Mf; 78.47.+p; 79.20.Ds     

Simulation of rippled structure adjustments based on localized transient electron dynamics control by femtosecond laser pulse trains

This study investigates the effects of pulse energy distributions on subwavelength ripple structures (the ablation shapes and subwavelength ripples) using the plasma model with the consideration of laser particle–wave duality. In the case studies, the laser pulse (800 nm, 50 fs) trains consist of double pulses within a train with the energy ratios of 1:2, 1:1, and 2:1. Localized transient electron densities, material optical properties, and surface plasmon generation are strongly affected by the energy distributions. Hence, the adjustment of the ablation shape and subwavelength ripples can be achieved based on localized transient electron dynamics control during femtosecond laser pulse train processing of dielectrics. The simulation results show that better, more uniform structures, in terms of ablation shapes and subwavelength ripples, can be easily formed at a lower fluence or subpulse energy ratio of 1:1 with a fixed fluence. It is also found that pulse trains at a 1:1 energy ratio are preferred for drilling high-aspect-ratio microholes or microchannels.     

Accuracy control of three-dimensional Nd:YAG laser shaping by ablation

Improving the dimensional accuracy along the optical axis without decreasing the materials removing rate is a key issue in three-dimensional laser shaping. This paper presents a concept for performing three-dimensional laser shaping by directly using machining laser as the photo source of the non-contacting measuring device. Due to the high power measuring photo source and a 1.06 μm bandpass filter, the interference caused by the emission light of ablated surface can be effectively avoided, the delay time is not needed to be inserted between the laser pulse and the measurement. So the measurement will not decrease the material removal rate and productivity. By using this system, the shaping accuracy of 30 μm can be achieved at the removing rate of about 4.0×10⁻² mm³/sec for Si₃N₄ ceramic, both are much better than the results obtained before.     

Size distribution control of metal nanoparticles using femtosecond laser pulse train: a molecular dynamics simulation

Microscopic mechanisms and optimization of metal nanoparticle size distribution control using femtosecond laser pulse trains are studied by molecular dynamics simulations combined with the two-temperature model. Various pulse train designs, including subpulse numbers, separations, and energy distributions are compared, which demonstrate that the minimal mean nanoparticle sizes are achieved at the maximal subpulse numbers with uniform energy distributions. Femtosecond laser pulse trains significantly alter the film thermodynamical properties, adjust the film phase change mechanisms, and hence control the nanoparticle size distributions. As subpulse numbers and separations increase, alternation of film thermodynamical properties suppresses phase explosion, favors critical point phase separation, and significantly reduces mean nanoparticle size distributions. Correspondingly, the relative ratio of two phase change mechanisms causes two distinct nanoparticle size control regimes, where phase explosion leads to strong nanoparticle size control, and increasing ratio of critical point phase separation leads to gentle nanoparticles size control.     

Electron dynamics and prompt ablation of aluminum surface excited by intense femtosecond laser pulse

Thin aluminum film homogeneously heated by intense IR femtosecond laser pulses exhibits on the excitation timescale consequent fluence-dependent rise and drop of the IR-pump self-reflectivity, followed by its final saturation at higher fluences F > 0.3 J/cm². This prompt optical dynamics correlates with the initial monotonic increase in the accompanying laser-induced electron emission, which is succeeded by its non-linear (three-photon) increase for F > 0.3 J/cm². The underlying electronic dynamics is related to the initial saturation of IR resonant interband transitions in this material, followed by its strong instantaneous electronic heating via intraband transitions during the pump pulse resulting in thermionic emission. Above the threshold fluence of 0.3 J/cm², the surface electronic heating is balanced during the pump pulse by simultaneous cooling via intense plasma removal (prompt ablation). The relationship between the deposited volume energy density in the film and its prompt electronic temperature derived from the self-reflection measurements using a Drude model, demonstrates a kind of electron “liquid–vapor” phase transition, driven by strong cubic optical non-linearity of the photo-excited aluminum.     

High-throughput rear-surface drilling of microchannels in glass based on electron dynamics control using femtosecond pulse trains

This study proposes a rear-surface ablation enhancement approach to fabricate high-aspect-ratio microchannels by temporally shaping femtosecond laser pulse trains. In the case study of K9 glass, enhancements of up to a 56 times higher material removal rate and a three times greater maximum drilling depth are obtained by the proposed method, as compared with conventional femtosecond laser drilling at the same processing parameters. The improvements are due to the changes of photon-electron interactions by shaping femtosecond pulse train, which can effectively adjust the photon absorption and localized transient material properties by changing electron dynamics such as free electron densities.     

Picosecond control of coherent magnetisation dynamics in permalloy thin films by picosecond magnetic field pulse shaping

We have measured the response of a 20×10 μm, 8 nm thin NiFe (80:20) permalloy film due to excitation by short in-plane magnetic field pulses. We will show that using a two-pulse-technique a complete control of the precessional motion of the magnetisation can be achieved on picosecond timescales. Furthermore, we will present numerical calculations which show that a complete suppression of magnetisation ringing after switching can only be realised by a cascade of short field pulses.     

Remaining noise-induced spatiotemporal dynamics in a free electron laser stabilized by feedback control

We investigate the efficiency of the pulsed regime stabilization by feedback control in a free electron laser (FEL) in oscillator configuration. We show that a hidden dynamics remains in the controlled FEL. Though not visible in the evolution of the pulse envelope, this dynamics affects the internal pulse shape evolution, and thus the FEL spectrum and coherence properties. The experimental results obtained on the UVSOR FEL are interpreted in the framework of a field master equation model. The stochastic nature of spontaneous emission is taken into account, and appears as an essential ingredient since the dynamics of the controlled structures is governed by noise like the other stable regimes of the FEL.     

Laser ablation of copper based alloys by single and double pulse laser induced breakdown spectroscopy

Single and double pulse laser induced breakdown spectroscopy was applied to the analysis of copper based alloys samples, in order to investigate the material ablation process under two different experimental conditions. An Nd:YAG laser, emitting at the fundamental wavelength (1064 nm) for a fixed value of total energy with the same pulse width, has been used for both set-up configurations. Certified samples of copper based alloys were examined in order to investigate the effect of their different compositions on the ablation process efficiency. The craters produced by the laser on the samples surfaces were measured by a contact profilometer in order to evaluate the mass of ablated material. In this respect the differences in behaviour of the single and the double pulse ablation for Laser Induced Breakdown Spectroscopy (LIBS), respectively, have been considered. The dependence, under the same experimental conditions, of the LIBS background emission of the different ablated masses of the material has also been investigated, by looking for normalization algorithms for quantitative data reduction.The behaviour of some peculiar intensity ratios, suitable for building calibration curves, has been studied in order to evaluate the trend in fractionated evaporation on going from the single pulse to the double pulse technique. PACS 52.70.-m; 52.50.Jm; 42.62.Fi     

In situ size measurement of Si nanoparticles and formation dynamics after laser ablation

We have developed a technique to detect Si nanoparticles selectively and to measure size in situ. Applying the technique, we have investigated formation process of Si nanoparticles after pulsed laser ablation of Si targets in Ar gas. Time-resolved photoluminescence (PL) spectroscopy revealed that PL only from Si nanoparticles is observed below 2.4 eV while PL from Si nanoparticles as well as defects in SiO₂ is observed above 2.4 eV. Therefore, Si nanoparticles can be detected selectively by excitation light with a photon energy below 2.4 eV. It is found that the onset of the PL from Si nanoparticles is delayed by approximately 0.3 ms from that of the defects and smaller Si nanoparticles. A size can be estimated by a band gap, which is roughly equal to the lowest photon energy at which Si nanoparticles can be excited. Thus, we estimated the sizes of growing Si nanoparticles. PACS 61.46.+w; 78.66.w; 07.60.Yi     

Ultrafast semiconductor-metal phase transition in vanadium dioxide induced by a femtosecond laser pulse

A model is proposed for a photoinduced Peierls-type semiconductor-metal phase transition that makes it possible to determine the time dependence of the bandgap width in the electronic spectrum of vanadium dioxide subjected to a light field and the dependence of the time at which a photoinduced semiconductor-metal phase transition occurs on the laser pulse duration. The theoretical results obtained are consistent with experimental data on the illumination of a VO₂ film with an intense laser pulse.     

Ultrafast dynamics of three types of simultaneous shockwaves and filament attenuation in femtosecond laser multi-pulse ablation of PMMA

Cylindrical shockwaves inside polymethyl methacrylate(PMMA) generated simultaneously with two hemispherical shockwaves induced by a femtosecond Gaussian beam laser were investigated using an ultrafast pump–probe imaging technique. The evolutions of these three shockwaves with probe delay and incident pulse number have been systematically analyzed. The plasma intensity and filament length in the center of cylindrical shockwave both decayed with pulse number. Moreover, the self-focused filament moved downstream towards the output surface with an increased pulse number. The experimental results and mechanism illustrated that energy deposition was suppressed by a degraded nonlinear effect due to a pre-ablated structure in multi-pulse irradiation.