Multi-pulse Laser Research Articles

Dynamics of cavitation bubbles generated by multi-pulse laser irradiation of a solid target in water

The effect of the second and later pulses on the expansion dynamics of the cavitation bubble produced by multi-pulse microchip laser irradiation of a Cu target in water has been investigated. We clarified the bubble dynamics by taking shadowgraph images and measuring the bubble radius as a function of time. Shock waves were also measured to investigate the explosive expansion of the bubble. As a result, the second and later pulses did not cause an explosive expansion, and the ablation of the target by these pulses was rather mild, although they had a certain contribution to the expansion of the bubble. The energies given to the bubble expansion from the first pulse and also from the second pulse were estimated by comparing the experimental results with the calculation based on the Rayleigh model.

Principles of laser drilling with adapted control over the parameters of repeated laser pulses

Principles of laser drilling of deep holes with adapted control over the parameters of the multipulse laser irradiation are determined. In good agreement with earlier experimental results, the possibilities for increasing the depth and productivity of the laser treatment of deep cavities via controlled modulation of the Q-factor and active synchronization of the modes of a high-power single-mode YAG:Nd laser are substantiated theoretically.

Structure and optical properties of carbon films obtained by multipulse pulsed laser deposition

We have obtained carbon thin films on silicon and glass substrates with multipulse pulsed laser irradiation of graphite under vacuum (p ≈ 2.6 Pa) using a high-frequency series of nanosecond laser pulses (τ = 85 ns, λ = 1060 nm) with pulse repetition frequency f ≈ 10–20 kHz and laser power density q ≈ 15–40 MW/cm2. We established the optimal laser power density and laser pulse repetition frequency for obtaining amorphous nanostructured diamond-like films.

Multipulse laser damage in potassium titanyl phosphate: statistical interpretation of measurements and the damage initiation mechanism

Multipulse laser-induced damage is an important topic for many applications of nonlinear crystals. We studied multipulse damage in X-cut KTiOPO4. Using a 6-ns Nd:YAG laser with a weakly focused beam, a fatigue phenomenon was observed. We addressed whether this phenomenon necessarily implies material modifications. Two possible models were tested, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant. The first model, pulse energy fluctuations and depointing, increases the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number; however, this effect is too small to explain the observed fatigue. The second model assumes a constant single-shot damage probability p1, so a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement was found between the 2000-on-1 volume damage data and this statistical multipulse model. Additionally, the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are transient, the presented data explain the experimental results without supposing material modifications.

Comparison of the thermal effects of the transport of a short-pulse laser and a multi-pulse laser through a participating medium

Temporal signals resulting from the boundaries of a participating medium subjected to a short-pulse laser source are investigated. Cases of both a single as well as a pulse-train consisting of multiple pulses are considered. Temporal variation of the incident laser pulse can be a step or a Gaussian function of time. Temporal evolutions of transmittance and reflectance signals, and incident radiation at various locations in the planar medium are studied for a wide range of the values of the extinction coefficient. The problem is analyzed using both the finite volume method and the discrete ordinates method. In all cases, results of both the methods are compared. An excellent agreement has been found.

High pulse energy, high repetition picosecond chirped-multi-pulse regenerative amplifier laser

We successfully presented a multi-pulse picosecond laser with passively mode-locking, grating-stretching, regenerative amplifier and grating compression technologies. Firstly, 8.5ps pulses with a repetition rate of 143MHz and a maximum average output power of 160mW were obtained by a semiconductor saturable absorption mirror (SESAM). Secondly, we got the pulses width stretched to 99.9ps with 90mW and single grating. Thirdly, a pulse sequence with each of five pulses per group was obtained by using a multi-pulse regeneration amplifier system, from which output energy was about 28mJ at the repetition rate of 1kHz and 112.1ps in single pulse width. Finally, using a grating compressor we acquired these pulses compressed to a pulse width of 28ps and 14mJ in each group at a repetition rate of 1kHz.

Analysis and discrimination of soldering tin samples by collinear multi-pulse laser induced breakdown spectrometry, supported by inductively coupled plasma optical emission and mass spectrometry

An assessment of the accuracy and precision of collinear multi-pulse laser induced breakdown spectrometry (MP-LIBS) analysis of soldering tin samples for their lead and tin content was performed. Quantitative results from inductively coupled plasma (ICP) atomic emission spectrometry carried out on dissolved samples were used as reference. For this latter purpose, a novel acid dissolution method was also developed that is equally suitable for leaded and lead-free solder compositions. It was found that the useful concentration range of time- and space‐integrated MP-LIBS lead and tin signals is significantly broader than for single‐pulse LIBS. Analytically useful calibration plots were obtained for up to 80w/w% concentration for two resonance Sn lines and up to 100w/w% concentration for three resonance Pb lines. The accuracy and precision of MP-LIBS analysis in this application was found to be 1–5%. This is sufficient for technical (semiquantitative) purposes and also provides the analytical benefits of LIBS: fast, in-situ analysis potentially also applicable to soldering joints. We also demonstrated that single-shot MP-LIBS spectra can be used for the discrimination of soldering alloys of the same nominal composition based on the trace contaminants present in the alloys. Supporting analytical information was obtained by solution sampling ICP mass spectrometry. The discrimination was tested by the use of the linear correlation, sum of squared deviation and overlapping integral functions, and it was found that the most suitable mathematic approach is the use of overlapping integrals.

Plasma formation by high-repetition rate pulsed periodic laser irradiation of metals in air at atmospheric and reduced pressures

High-speed spectroscopy with spatial and temporal resolution is used to determine the basic plasma parameters and to examine the shift from an air plasma to an erosion plasma during multipulse laser irradiation of various metals at repetition rates ranging from 5 to 50 kHz with irradiances q > 2∙108 W/cm2. The formation of a distinctive surface plasma structure is observed which (a) facilitates the expulsion of the shielding layer of "air" plasma produced during the first of a series of laser pulses by vapor from the target and (b) inhibits the propagation of a metal vapor plasma front into the air during the subsequent pulses.

Incubation effect and its influence on laser patterning of ITO thin film

Results are presented on the surface damage thresholds of ITO thin films induced by single- and multi-pulse laser irradiation at a pulse duration of 10 ps and a wavelength of 1064 nm. For multi-pulse ablation the incubation effect results in a reduction of the damage threshold, especially apparent at low pulse numbers and very small film thicknesses. The incubation effect attributes to the accumulation of defect sites and/or the storage of thermal stress-strain energy induced by the incident laser pulses. An incubation coefficient of S=0.82 has been obtained which is independent on the film thickness in the range of 10–100 nm. In practical applications, the incubation effect determines the laser patterning structure of ITO films while increasing the pulse overlapping rate. The width of the patterned line can be predicted by the proposed model involving the laser fluence, the overlapping rate and the incubation coefficient.

Effects of the amorphous layer on laser-induced subwavelength structures on carbon allotropes

By micro-Raman spectroscopy, we show that the structured surfaces of highly oriented pyrolytic graphite and diamond induced by 800 nm, 125 fs or 532 nm, 30 ps laser pulses are capped by thin amorphous carbon layers. Based on the results, we propose that for multiphoton ablation the thin amorphous layer with a reduced bandgap can facilitate surface ionization, raise free electron density, bring on plasmonic effects, and thus promote the growth of subwavelength structures. Therefore, concerning multipulse laser ablation of wide bandgap materials, we should take into account the effects of the superficial amorphous layer produced by preceding pulses instead of the intrinsic surface.

Effect of 0.3-Millisecond Multi-Pulsed 1064-nm Nd:YAG Laser in Patients with Postherpetic Neuralgia

Postherpetic neuralgia (PHN) is the most common complication following acute varicella zoster virus infection. PHN is associated with chronic severe pain and is resistant to conservative management treatments. The purpose of this study was to evaluate the effect of 0.3-millisecond multi-pulsed 1064-nm Nd:YAG laser treatment on PHN. Five subjects were treated with 2 - 3 sessions at 2-week interval. After the treatments, reduced visual analog scale (VAS) scores were noted in all patients. Treatments showed no adverse or intolerant effects and all patients felt warmth and comfort during the therapy. We first reported treating PHN patients using 0.3-millisecond multi-pulsed 1064-nm laser Nd:YAG. The results showed remarkable improvements in pain. This laser treatment could be an alternative choice for PHN patients with intractable neuralgia.

Characterization of regular periodic surface structure by multi-pulse laser irradiation of a Zinc target

The circular-like sidewall and trench around the periphery of the crater are obtained on Zinc metal surface subjected to fewer than 200 cumulative pulses laser ablation. These patterns can be attributed to the higher secondary heating by regular plasma spherical pressure. The 600 pulses laser ablation, however, results in the formation of undesirable bulk wrinkles of super-heated liquid at the side of the trench. These features may be applied in micro-manufacturing of high localizability and selectivity using laser zone texturing.

Carrier-envelope phase locking of multi-pulse lasers with an intra-cavity Mach-Zehnder interferometer

We propose the use of an intra-cavity Mach Zehnder interferometer (MZI), for increasing the repetition rate at which carrier-envelope phase-locked pulses are generated in passively mode-locked fiber lasers. The attractive feature of the proposed scheme is that light escaping through the open output ports of the MZI can be used as a monitor signal feeding a servo loop that allows multiple pulses to co-exist in the cavity, while rigidly controlling their separation. The proposed scheme enables in principle a significant increase in the pulse-rate with no deterioration in the properties of the generated pulses.

Picosecond-laser-induced structural modifications in the bulk of single-crystal diamond

Arrays of through laser-graphitized microstructures have been fabricated in type IIa single-crystal 1.2-mm-thick diamond plates by multipulse laser irradiation with 10-ps pulses at λ=532 nm wavelength. Raman and photoluminescence (PL) spectroscopy studies of the bulk microstructures have evidenced the diamond transformation to amorphous carbon and graphitic phases and the formation of radiation defects pronounced in the PL spectra as the self-interstitial related center, the 3H center, at 504 nm. It is found that the ultrafast-laser-induced structural modifications in the bulk of single-crystal diamond plates occur along {111} planes, known as the planes of the lowest cleavage energy and strength in diamond.

The Transient Temperature and Thermal Stresses on Metal Material Irradiated by Multi-Pulse Short Power

Based on the non-Fourier law and thermo-elastic theory, the non-Fourier expressions of the temperature field and the thermal stress field of metal material under multi-pulse laser irradiation were deduced. Taken stainless metal and two pulse width (ω=4ps,4fs) as an example, The effects that distributions and variations of transient temperature and thermal stress are influenced by laser pulse width are studied. The results show that the magnitude and frequency of transient temperature and thermal stress are seriously affected by the laser pulse width. More narrower the laser pulse width, faster and higher the local temperature rise; as well as the thermal stresses.

Bimodal size-distribution function in a nanoparticle ensemble formed during the laser irradiation of the surfaces of solids

A defect-deformational (DD) model of the formation of a nanoparticle ensemble on the surface of solids under multipulse laser ablation is developed. The DD theory of bimodal nanoparticle size distribution is developed, which expresses size distribution via the bimodal growth rate of DD surface gratings generated by laser irradiation. The obtained size-distribution function corresponds to the experimental data.

Laser treatment of nanocomposite Ni/Ti multilayer thin films in air

Interaction of a Nd:YAG laser, operating at 1064nm wavelength and pulse duration of 150ps, with Ni/Ti multilayer thin films was studied. Nanocomposite structure of five (Ni/Ti) bilayers deposited by DC ion sputtering on Si(100) wafers to a total thickness of ca. 180nm was treated with a laser pulse energy of about 55mJ without focusing (fluence 1Jcm−2). Multi-pulse laser irradiation was done at an incidence angle of 22° in air. The composition and surface morphology were monitored by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and by atomic force microscopy (AFM). The main part of the absorbed laser energy was rapidly transformed into heat, producing intensive modifications of composition and morphology on the target surface. Besides surface oxidation, the results show an increase in surface roughness, formation of a regularly rippled surface topography and microcracking. Depth profile AES analysis revealed that laser annealing induced intermixing between the components of individual Ni and Ti layers with indications of possible formation of NiTi intermetallic compounds. An interesting finding is the surface segregation of Ti, although the top layer was Ni, prior to laser irradiation.

Deep Ablation and Depth Profiling by Laser-Induced Breakdown Spectroscopy (LIBS) Employing Multi-Pulse Laser Excitation: Application to Galvanized Steel

The potential of a multi-pulse (MP) laser excitation scheme for deep stratigraphy of electrolytically galvanized steel using laser-induced breakdown spectrometry (LIBS) has been evaluated. For this purpose, a commercial electro-optically (EO) Q-switched Nd:YAG laser was employed, where by reducing the delay between the Q-switch opening and the flash lamp, a train of pulses (up to 11) separated by approximately 7.40 μs was generated during one lamp flashing. Plasma emission after each individual laser pulse of the MP sequence was detected by a spectrograph equipped with an intensified charge-coupled device (iCCD) detector. With MP excitation, the ablation efficiency was increased ten-fold on iron sample and 22.5-fold on zinc material with respect to dual-pulse or single-pulse excitation. The LIBS signal generated by MP excitation shows an analogous enhancement. Although the total energy per shot delivered to samples was only 60 mJ, it was possible using LIBS to measure the sample stratigraphy up to depths of 90 μm on zinc-coated steel sheets. A satisfactory agreement between the Zn thickness determined by the MP-LIBS system and data from the manufacturer has also been obtained.

Upgraded multipulse laser and multipoint Thomson scattering diagnostics on EAST

Recently a new Thomson scattering diagnostic system was upgraded in EAST tokamak experiment using a multipulse Nd:YAG (neodymium-yttrium aluminium garnet) laser and a multipoint observation volumes. This diagnostic uses a new optical laser alignment technique that was made to determine accurately the laser position, and a new lens collection system that enables the measurement of wider plasma's object. A composite control system made we can get the results in several seconds. Furthermore, a new data processing method was adopted for much exact results.

Chemical and structural modifications of laser treated WTi surfaces at different ambient conditions

In this work we have studied the influence of laser modification on the composition and structure of tungsten titanium (WTi) thin films, deposited on n-type (100) silicon wafers. After deposition, the samples were multi-pulse laser irradiated in a nitrogen, oxygen, and helium ambient. The composition of the WTi/Si sample was determined by Elastic Recoil Detection Analysis (ERDA). Surface morphology was monitored by Atomic Force Microscopy (AFM). In the experiment, typical laser output parameters were: wavelength 1064 nm, pulse duration 150 ps, and laser pulse energy 30 mJ. Surface concentrations of W and Ti, as well as the concentration of gas components nitrogen and oxygen were determinated before and after the action of laser radiation in different ambient conditions. The contents of W and Ti decreased after irradiation due to adsorbed gases from the surrounding atmosphere. After surface irradiation in the inert ambient (He), the concentrations of the components were not significantly changed. In other cases, oxygen was the dominant component at the surface, probably due to the high affinity of thin film components. Also, the morphological changes occurred at the surface of WTi, as an increase in the surface roughness and formation of the granular structures are a result of laser-induced surface oxidation and recrystallization.