Incident Pulse Energy Research Articles

Optimizing the third harmonic generated from air plasma filaments pumped by femtosecond laser pulses

Third-harmonic (TH) generation inside air plasma filaments provides a simple method to convert near-infrared laser pulses in the UV range. Here, we report on the optimization of the third-harmonic generation during filamentation in the parameter space of focusing geometry, gas pressure, and incident laser pulse energy. It is observed that, in the nonlinear filamentation regime, the TH yield is largely dominated by the geometrical focusing condition and gas pressure. Using ambient air as the medium for filamentation, an optimal numerical aperture (NA) = 0.07 was found for a laser pulse with a peak power close to and above the critical power for self-focusing. For more loose focusing geometry (NA<0.047), reduction of the air pressure can improve the conversion efficiency. In this case, the spatial mode of the third harmonic was also found to be more uniform than in ambient air. With the scanning pinhole and crossing-filaments methods, it is revealed that in the situation of optimized focusing geometry (NA = 0.07) the TH accumulates constructively along the filament and exits at its end. In contrast, for the loose focusing geometry, the destructive interference effect of the TH generated at the leading and trailing edges of the filaments decreases the TH yield efficiency. This study provides a practical guidance for efficient conversion of near-infrared femtosecond pulses to the UV domain.

Simulation of femtosecond laser ablation sapphire based on free electron density

Femtosecond laser has become an effective method for processing super hard materials, due to the multiphoton ionization and avalanche ionization induced by its high peak power. A FEM model with Fokker-plank equation and Drude equation was established to study the space-time evolution of free electron density caused by the ionization. The study focuses on investigating the temporal and spatial evolution law of free electron density induced by femtosecond laser pulse and its influence on absorption coefficient, reflectivity and ablation depth. It was shown that there is a dramatic increase of sapphire surface absorption coefficient and reflectivity within picosecond, and femtosecond laser pulse energy is rapidly absorbed in a very small depth about 140 nm. When the single pulse energy exceeds a certain threshold (8 J/cm2), the predicated ablation depth is basically not affected by the incident laser pulse energy and remains unchanged, which are in agreement with the experimental data. The FEM model is applicable with advantages of relative simplicity and acceptable precision.

Characterization of hard X-ray sources produced via the interaction of relativistic femtosecond laser pulses with metallic targets

X-ray emission resulting from interactions of intense laser pulses with solid metal targets (Ni, Cu, Mo, Ag, and Sn) at 0.5 kHz repetition rate is measured using pulse energies of up to 12 mJ. A comparison of the conversion of laser pulse energy to total X-ray emission energy is made with respect to the previous measurements at lower energy (< 3 mJ). In the present experiments, the total bremsstrahlung conversion efficiency is found to increase by an order of magnitude for all targets as the energy in increased. The Kα line emission conversion efficiency also increases with incident pulse energy for all targets. In addition, the ratio between line and bremsstrahlung emission in the X-ray spectral region was significantly reduced at higher energies because of the large increase in bremsstrahlung. In general, the X-ray source size increases as the laser energy increased and the ellipticity of the X-ray source also increased in the laser polarization direction, with the effect becoming very pronounced at higher energies. Phase-contrast imaging of a nanospray emitter and a 3D printed plastic target was also performed using Cu and Mo targets.

Interferometric method for specific impulse determination

One of the main figures of merit in laser-ablative propulsion is the specific impulse, Isp, defined as the impulse per unit weight of fuel, and it is related to the exhaust velocity, ve, by the acceleration of gravity, Isp = ve / g. Being a key magnitude, Isp needs to be accurately determined. It is usually inferred from other measurable quantities: the impulse coupling coefficient, Cm, defined as the ratio of the target momentum produced to the incident laser pulse energy, and Q * , the laser energy consumed per unit weight of ablated target material. Thus, Isp is calculated as Isp = CmQ * / g. However, single pulse ablated mass leading to Q * is in the nanogram scale and cannot be directly measured by weighting the targets. So, mass loss measurements are performed by analyzing the volumes of the craters produced by a large number of laser pulses. These procedures lead to larger than desired uncertainties in the Isp values. On the other hand, more precise measurements of Isp can be carried out from the direct measurement of the exhaust velocity of the ejected particles by interferometric methods. In this work, a system based on a Nomarsky interferometer has been set up for the time-resolved diagnostic in the nanometric scale of laser ablation plumes. The performance of the implemented system was first validated by measuring the Isp produced by aluminum targets and solid propellants based on metal/salt mixtures. The Cm dependence on laser parameters and binary composition of these propellants have been determined in previous works with a torsion pendulum and a piezoelectric sensor. Once the interferometer performance is characterized, the Isp produced by solid propellants composed of metal (Zn) and metal oxides (ZnO) matrices doped with nanoparticles of different materials is determined and compared.

Spectral features of the Stokes part of supercontinuum generated by femtosecond light pulses in selected oxide crystals: A comparative study

The peculiarities of the Stokes part of supercontinuum (SC) generated by femtosecond light pulses at wavelength 800 nm in single crystals of Gd2SiO5(GSO), Ca4GdO(BO3)3(GCOB), Gd3Ga5O12(GGG), LiTaO3 (LTO) and LuVO4 (LVO) were investigated. Spectral bandwidth and intensity of SC were measured as a function of energy of incident 100 fs pulses employing a grating spectrograph coupled with an InGaAs detector and spatial characteristics of the beam inside crystal samples were monitored perpendicularly to the direction of propagation and recorded using an optical microscope coupled with a camera. It was found that spectral widths of the Stokes part of SC increase markedly with increasing energy of incident pulses for all crystals under study. For fixed focusing conditions the spectral widths of generated SC in GSO, GCOB and GGG wide band-gap crystals are relatively large with cut-off wavelengths close to 1500 nm. Bandwidths of SC generated in LVO and LTO crystals, characterized by band-gaps Eg inferior to three times incident photon energy, are markedly smaller with cut-off wavelengths of 1300 nm and 1150 nm, respectively. Increase of incident pulse energy affects SC spectra giving rise to plateau-like regions stretching to ca 1000 nm.

Residual heat generated during laser processing of CFRP with picosecond laser pulses

Abstract One of the major reasons for the formation of a heat-affected zone during laser processing of carbon fiber-reinforced plastics (CFRP) with repetitive picosecond (ps) laser pulses is heat accumulation. A fraction of every laser pulse is left as what we termed residual heat in the material also after the completed ablation process and leads to a gradual temperature increase in the processed workpiece. If the time between two consecutive pulses is too short to allow for a sufficient cooling of the material in the interaction zone, the resulting temperature can finally exceed a critical temperature and lead to the formation of a heat-affected zone. This accumulation effect depends on the amount of energy per laser pulse that is left in the material as residual heat. Which fraction of the incident pulse energy is left as residual heat in the workpiece depends on the laser and process parameters, the material properties, and the geometry of the interaction zone, but the influence of the individual quantities at the present state of knowledge is not known precisely due to the lack of comprehensive theoretical models. With the present study, we, therefore, experimentally determined the amount of residual heat by means of calorimetry. We investigated the dependence of the residual heat on the fluence, the pulse overlap, and the depth of laser-generated grooves in CRFP. As expected, the residual heat was found to increase with increasing groove depth. This increase occurs due to an indirect heating of the kerf walls by the ablation plasma and the change in the absorbed laser fluence caused by the altered geometry of the generated structures.

1200 nm pumped Tm3+:Lu2O3 ceramic lasers.

We report on an experimental demonstration of a 1200-nm pumped Tm3+:Lu2O3 ceramic laser. By using a gain-switched, tunable Cr4+:forsterite laser, the excitation spectrum was measured, with optimum pumping bands centered near 1198nm, 1204nm, and 1211nm. The highest slope efficiency of 21.5% was obtained at the pump wavelength of 1204nm. Comparative energy efficiency measurements performed near 1200-nm and 800-nm pumping further showed that nearly 40% improvement was obtained in slope efficiency measured with respect to the incident pump energy for 1200-nm pumping. A transition was further observed from single-wavelength operation at 2066nm to dual-wavelength operation near 2066nm and 1967nm for absorbed pump energies above 50μJ. In this regime, two consecutive output pulses were observed in the time domain. The shortest temporal duration of the first pulse was 1.1μs at the incident pulse energy of 105μJ. The duration and build-up time of the second pulse remained around 5.9μs and 18.5μs. We believe that the improved energy efficiency demonstrated for the 1.5% Tm3+:Lu2O3 ceramic with 1200-nm pumping can be used as an alternative scheme for the excitation of Tm3+:Lu2O3 ceramic lasers.

Filamentation and supercontinuum emission with flattened femtosecond laser beam by use of microlens array in fused silica

The high power supercontinuum from femtosecond filamentation has attracted great attention for recent years due to its various applications. In our previous researches, we have used microlens array to obtain filament-array in fused silica and to generate the high spectral power supercontinuum. To further improve the ability to generate the high power supercontinuum by using microlens array, in this work we adopt flattened femtosecond laser beam with a flat-top energy distribution to generate filament-array in fused silica and supercontinuum. By using a laser beam shaping system consisting of aspherical lenses, the Gaussian intensity distribution of initial femtosecond laser beam is converted into a flat-top distribution. The flattened laser beam is focused by a microlens array into a fused silica block, and consequently a filament array is formed in the block. Our experimental results show that compared with the filaments formed by a Gaussian laser beam, the filaments formed by the flattened beam have a uniform distribution and almost the same onset due to the initial uniform energy distribution across the section of the laser beam. Furthermore, the spectral stability of supercontinuum emission is used to evaluate the damage of the fused silica block. It is demonstrated that the flattened beam with a pulse energy of 1.9 mJ does not induce permanent damage to the fused silica block, while the Gaussian beam with a relatively low pulse energy of 1.46 mJ leads to the damage to the block. Therefore, a higher incident laser pulse energy is allowed in the case of flattened laser beam, and consequently stronger supercontinuum generation than in the case of the Gaussian laser beam can be expected. In our experiments, the relative spectral intensity of flattened beam generated supercontinuum in the visible range is about twice higher than that for the Gaussian beam case. The conversion efficiencies of the supercontinuum for the two kinds of laser beams are further analyzed. The conversion efficiencies are 49% and 55% for the cases of Gaussian and flattened beams respectively. In this work, we demonstrate the formation of filament array with uniform distribution in fused silica, and, as a proof of principle, we also demonstrate the high power supercontinuum generation with high conversion efficiency from the filamentation, by using flattened femtosecond laser beam as the incident laser and microlens array as the focusing element. This approach provides a way to obtain a high power femtosecond supercontinuum source which is of great importance in many applications such as some absorption spectroscopies based on coherent supercontinuum light.

Single-shot arrival timing diagnostics for a soft X-ray free-electron laser beamline at SACLA.

Arrival timing diagnostics performed at a soft X-ray free-electron laser (FEL) beamline of SACLA are described. Intense soft X-ray FEL pulses with one-dimensional focusing efficiently induce transient changes of optical reflectivity on the surface of GaAs. The arrival timing between soft X-ray FEL and optical laser pulses was successfully measured as a spatial position of the reflectivity change. The temporal resolution evaluated from the imaging system reaches ∼10 fs. This method requires only a small portion of the incident pulse energy, which enables the simultaneous operation of the arrival timing diagnostics and experiments by introducing a wavefront-splitting scheme.

Influence of temperature on supercontinuum generation induced by femtosecond laser filamentation in NaCl solution

Supercontinuum generation is an important nonlinear phenomenon that occurs during the femtosecond laser filamentation in transparent medium, and its potential and promising applications like remote sensing, biomedical imaging and generation of few-cycle femtosecond pulses, etc. have aroused a great deal of interest. With the extensive and thorough theoretical simulation and experimental research of the supercontinuum generation in air, the mechanism of the supercontinuum induced by femtosecond laser filament in gaseous medium has become clear. However, the femtosecond laser filament-induced supercontinuum in liquid is still an open question. In this work, by taking NaCl solution for example, we investigate the influence of solution temperature on the supercontinuum induced by the femtosecond laser filamentation in solution. It is found that when the laser pulse energy is relatively low (e.g. 20 and 50 J), the influence of solution temperature on supercontinuum generation can be neglected. In contrast, when the laser pulse energy is relatively high (e.g. 200 J), with the increase of solution temperature, the supercontinuum generation shows a suppression tendency. The water molecules in NaCl solution are photo-ionized due to the high intensity of femtosecond laser filament, generating a great deal of oxygen (O2), hydrogen (H2) and water vapor (H2O), and thus forming bubbles that float upwards. In the case of lower pulse energy, the multi-photon ionization rate is low, therefore, only a few bubbles are generated, and they are small in size, which hardly affects the supercontinuum generation. In the case of higher pulse energy, a large number of bubbles can be observed in the NaCl solution, and their sizes become increasingly large when the temperature of NaCl solution increases. The generation of bubbles leads to the reflection and refraction of light, which inevitably influences the spectral intensity. Furthermore, the components (e.g. O2, H2 and H2O) in the bubbles also absorb the supercontinuum, which further lowers the spectral intensity. This work reveals that the main factors leading to the supercontinuum suppression in solution can be attributed to the generation of bubbles during femtosecond laser filamentation and the scattering and absorption of light caused by water vapor in bubbles. When we detect the components in solution via the femtosecond laser filament-induced supercontiunum, the influence of tempera-ture can be effectively eliminated by adjusting the incident pulse energy. Moreover, in the case of high pulse energy, the supercontinuum generation can be controlled by adjusting the solution temperature. This study is conducible to the application of supercontinuum as well as its generation.

Fabrication of porous nanostructures for Raman signal amplification

Pulsed-laser dry printing of noble-metal microrings with a tunable internal porous structure, which can be revealed via an ion-beam etching post-procedure, was demonstrated. Average size of the pores inside the microrings were shown to be tuned in a wide range by varying the incident pulse energy and a nitrogen doping level controlled in the process of magnetron deposition of the gold film in the appropriate gaseous environment. The fabricated porous microrings were shown to provide many-fold near-field enhancement of incident electromagnetic fields, which was confirmed by mapping of the characteristic Raman band of a nanometer-thick covering layer of Rhodamine 6G molecules and supporting calculations. The proposed laser-printing/ion-beam etching approach is demonstrated to be a unique tool aimed at designing and fabricating multifunctional plasmonic structures and metasurfaces for spectroscopic bioidentification based on surface-enhanced Raman scattering and photoluminescence detection schemes.

Fabrication of porous microrings via laser printing and ion-beam post-etching

Pulsed-laser dry printing of noble-metal microrings with a tunable internal porous structure, which can be revealed via an ion-beam etching post-procedure, was demonstrated. The abundance and average size of the pores inside the microrings were shown to be tuned in a wide range by varying the incident pulse energy and a nitrogen doping level controlled in the process of magnetron deposition of the gold film in the appropriate gaseous environment. The fabricated porous microrings were shown to provide many-fold near-field enhancement of incident electromagnetic fields, which was confirmed by mapping of the characteristic Raman band of a nanometer-thick covering layer of Rhodamine 6G dye molecules and supporting finite-difference time-domain calculations. The proposed laser-printing/ion-beam etching approach is demonstrated to be a unique tool aimed at designing and fabricating multifunctional plasmonic structures and metasurfaces for spectroscopic bioidentification based on surface-enhanced infrared absorption, Raman scattering, and photoluminescence detection schemes.

Examination of nanosecond laser melting thresholds in refractory metals by shear wave acoustics

Nanosecond laser pulse-induced melting thresholds in refractory (Nb, Mo, Ta and W) metals are measured using detected laser-generated acoustic shear waves. Obtained melting threshold values were found to be scaled with corresponding melting point temperatures of investigated materials displaying dissimilar shearing behavior. The experiments were conducted with motorized control of the incident laser pulse energies with small and uniform energy increments to reach high measurement accuracy and real-time monitoring of the epicentral acoustic waveforms from the opposite side of irradiated sample plates. Measured results were found to be in good agreement with numerical finite element model solving coupled elastodynamic and thermal conduction governing equations on structured quadrilateral mesh. Solid-melt phase transition was handled by means of apparent heat capacity method. The onset of melting was attributed to vanished shear modulus and rapid radial molten pool propagation within laser-heated metal leading to preferential generation of transverse acoustic waves from sources surrounding the molten mass resulting in the delay of shear wave transit times. Developed laser-based technique aims for applications involving remote examination of rapid melting processes of materials present in harsh environment (e.g. spent nuclear fuels) with high spatio-temporal resolution.

Erratum to: Femtosecond pulse laser ablation of chromium: experimental results and two-temperature model simulations

In this work, the results of experimental and computational single- and multi-shot ablation threshold and the incubation effect of chromium metal sample, irradiated by ultrashort laser pulses, are presented. The experimental value of the ablation threshold is determined based on $$D^{2}$$ method by measuring the outer ablation crater diameters as a function of incident laser pulse energy using $$800 \, \mathrm{nm}, 30 \, \mathrm{fs},$$ laser pulses. The value of $$0.19 \pm 0.04 \, (\mathrm{J/cm}^{2} )$$ , is obtained for the single-shot ablation threshold fluence. The experimental results are compared with time-dependent heat flow calculations based on the two-temperature model and the effect of number and separation time of two consecutive laser pulses with the same total fluence is studied for the Cr target. Moreover, the role of pulse width and absorbed fluence in thermal equilibrium time between electrons and lattice is investigated in two-temperature model. The thermal equilibrium between electron and lattice is established after a few picoseconds for low fluences and after a few tens of picoseconds at higher fluences.

Propagation of a filamentary femtosecond laser beam with high intensities at an air-solid interface

The propagation of a filamentary laser beam at an air-glass surface is studied by setting the incident angle satisfying the total reflection condition. The images of the trajectory of the filamentary laser beam inside the sample and the output far-field spatial profiles are measured with varying incident laser pulse energies. Different from the general total reflection, a transmitted laser beam is detected along the propagation direction of the incident laser beam. The energy ratio of the transmitted laser beam depends on the pulse energies of the incident laser beam. The background energy reservoir surrounding the filament core can break the law of total reflection at the air-glass surface, resulting in the regeneration of the transmitted laser beam.

Predictable surface ablation of dielectrics with few-cycle laser pulse even beyond air ionization

We study surface ablation of dielectrics with single-shot few-cycle optical pulse (∼10 fs) in air, at intensities below and above the onset of air ionization. We perform 3D analysis and careful calibration of the fluence distribution at the laser focus, spanning from linear– to nonlinear– focusing regimes, enabling to thoroughly characterize the severe limitation of the fluence delivered onto the sample surface upon increase of incident pulse energy. Despite significant beam reshaping taking place at high fluence, we demonstrate that it is nevertheless possible to confidently predict the resulting crater profiles on fused silica surface, even in the regime of filamentation.

Kagome-fiber-based pulse compression of mid-infrared picosecond pulses from a Ho:YLF amplifier

Over the last decade, the development of ultrafast laser pulses in the mid-infrared (MIR) region has led to important breakthroughs in attosecond science and strong-field physics. However, as most such broadband MIR laser sources are near-IR pumped, the generation of high-intensity, long-wavelength MIR pulses is still a challenge, especially starting from picosecond pulses. Here we report, both experimentally and numerically, nonlinear pulse compression of sub-millijoule picosecond pulses down to sub-300 fs at 2050 nm wavelength in gas-filled Kagome-type hollow-core photonic crystal fibers for driving MIR optical parametric amplifiers. The pump laser is comprised of a compact fiber laser-seeded 2 μm chirped pulse amplification system based on a Ho:YLF crystal at 1 kHz repetition rate. Spectral broadening is studied for different experimental conditions with variations of gas pressure and incident pulse energies. The spectrally broadened 1.8 ps pulses with a Fourier-limited duration of 250 fs are compressed using an external prism-based compressor down to 285 fs and output energy of 125 μJ.

Temporal contrast improvement based on the self-diffraction process with a cylinder mirror

A cylinder reflective mirror was used to increase incident pulse energy to multi-millijoules in the research of temporal contrast enhancement based on a self-diffraction (SD) process. A 170 μJ first-order SD pulse with temporal contrast improved by 4 orders of magnitude was obtained when incident pulse energy was 5.1 mJ. In order to improve energy-conversion efficiency, a 67/33 beam splitter was used to replace the original 50/50 one. A 265 μJ first-order SD signal was achieved without damage of the glass plate at 3.7 mJ input pulse energy with an energy-conversion efficiency of about 7.1%. The generated SD pulse is expected to be used as the seed of high-contrast and high-power laser systems.

Morphology and structure of particles produced by femtosecond laser ablation of fused silica

The aim of the present work was to study the morphology and structure of the nanoparticles produced by femtosecond laser ablation of fused silica. Ultrashort laser pulses of 1030 nm wavelength and 550 fs duration were tightly focused by a high numerical aperture microscope objective at the surface of fused silica samples while scanning the sample in relation to the stationary laser beam. Laser tracks were created with pulse energies in the range 5–100 μJ, resulting in ablation debris of different morphologies. The debris were examined by scanning and transmission electron microscopy for their morphology and crystal structure in relation to the incident laser pulse energy. Ejected particles with sizes ranging from a few nanometers to a few microns were found. Their morphologies can be broadly classified into three categories: very fine round nanoparticles with diameters lower than 20 nm, nanoparticles with intermediate sizes between 50 and 200 nm, and big irregular particles with typical size between 0.5 and 1.5 μm. The fine nanoparticles of the first category are predominantly observed at higher pulse energies and tend to aggregate to form web-like and arborescent-like structures. The nanoparticles with intermediate sizes are observed for all pulse energies used and may appear isolated or aggregated in clusters. Finally, the larger irregular particles of the third category are observed for all energies and appear normally isolated.

Influence of femtosecond laser pulse energy on the surface reflection of black silicon in alkaline solution

A highly efficient approach for reducing the reflection of the black silicon surface is demonstrated, in which the black silicon is fabricated in alkaline solution via a femtosecond laser irradiation. The junglelike microstructures are formed on the surface of the black silicon. Compared to the polished silicon, the black silicon can significantly suppress the surface reflection. Throughout the region of visible light, the average minimum reflectance of the blackened surface is about 5.6%. Meanwhile, in order to get an optimal laser energy, we investigated the evolution on silicon surface as a function of incident pulse energy. Taking into account the height of junglelike microstructures and the reflectance of black silicon samples, the optimal laser energy is 1400 μJ. By choosing the right laser energy, it is possible to fabricate the highly absorptive black silicon. These results are of extreme importance in the control of surface morphology and the modification of material surface.