Ns Laser Research Articles

Fractal titanium oxide under inverse 10-ns laser deposition in air and water

This paper presents the preparation of different kinds of titanium oxide fractal structures on the surface of titanium by inverse pulsed laser deposition (IPLD) in air and water. In air, two-dimensional fractal structures are obtained with a low pulse energy. However, their branches units are aggregated and nanoscale branches disappear due to the high substrate temperature, causing the low fractal dimension of structure. When a higher laser energy is applied, the preformed deposited material forms a porous film, which reduces heat transfer from substrate. Therefore, three-dimensional and one-dimensional fractal structures with nanoscale branches on the topside of the film can be obtained. Then the desired two-dimensional fractal structures with nano-branches are obtained in water due to the water-induced rapid cooling of substrate temperature and plasma shock wave-induced particle’s expansion along the surface of substrate. Meanwhile, the asymmetry of fractal structure units analyzed by diffusion limited aggregation (DLA) model is caused by the difference of the distance between the initial deposited particles. In addition, when the pulse energy goes up to 111 mJ, the branches of two-dimensional fractal structure units are also aggregated and form isolated particles. The idea about modification of substrate temperature and water can guide the preparation of the desired titanium oxide fractal structures in pulsed laser deposition (PLD), which is also applicable to other materials.

Laser-induced damage threshold of silicon under combined millisecond and nanosecond laser irradiation

The laser–silicon interaction process was investigated with the superposed radiation of two pulsed Nd:YAG lasers. A pulse duration of 1 millisecond (ms) was superposed by 7 nanosecond (ns) pulses, creating a combined pulse laser (CPL). The time-resolved surface temperature of silicon was measured by an infrared radiation pyrometer. The melting thresholds of silicon were attained for a single ms laser and a CPL by infrared radiometry and time-resolved reflectance. The concept of threshold boundary was proposed, and a fitted curve of threshold boundary was obtained. An axisymmetric model was established for laser heating of silicon. The transient temperature fields were obtained for single ms laser and CPL irradiation using finite element analysis. The numerical results were validated experimentally, and an obvious decrease in melting threshold was found under CPL irradiation. That is attributed to pre-heating by the ms laser and the surface damage caused by the ns laser.

Radiative cooling induced plasma collapse observed in laser irradiation of a CH-tamped gold micro-disk

Time-resolved x-ray self-emission imaging was used to study the dynamic evolution of a laser-produced gold plasma tamped by plastic (CH), and a significant plasma collapse was observed during the laser irradiation. The plasma collapse, a kind of transverse contraction, has been ascribed to the radial compression caused by the different radiative cooling rates and thus different pressures between the central high-Z gold plasma and the surrounding low-Z CH plasma, and this has been reproduced by numerical simulations using the two-dimensional radiation-hydrodynamics code Multi2D. The experimental results represent an observation of the radiative cooling induced plasma jet within a 1 ns laser pulse duration, much more quickly than those reported previously. In addition, our experiment design may offer a method to study the radiative cooling rates of high-Z plasmas. The measured cooling rate is a factor of 2 higher than the theoretical result [Post et al., At. Data Nucl. Data Tables 20, 397 (1977)], but is within the stated calculational uncertainty.

Laser welding of fused silica glass with sapphire using a non- stoichiometric, fresnoitic Ba2TiSi2O8·3 SiO2 thin film as an absorber

Laser welding of dissimilar materials is challenging, due to their difference in coefficients of thermal expansion (CTE). In this work, fused silica-to-sapphire joints were achieved by employment of a ns laser focused in the intermediate Si-enriched fresnoitic glass thin film sealant. The microstructure of the bonded interphase was analyzed down to the nanometer scale and related to the laser parameters used. The crystallization of fresnoite in the glass sealant upon laser process leads to an intense blue emission intensity under UV excitation. This crystallization is favored in the interphase with the silica glass substrate, rather than in the border with the sapphire. The formation of SiO2 particles was confirmed, as well. The bond quality was evaluated by scanning acoustic microscopy (SAM). The substrates remain bonded even after heat treatment at 100°C for 30min, despite the large CTE difference between both substrates.

Direct Creation of Biopatterns via a Combination of Laser-Based Techniques and Click Chemistry

In this paper, we present the immobilization of thiol-modified aptamers on alkyne- or alkene-terminated silicon nitride surfaces by laser-induced click chemistry reactions. The aptamers are printed onto the surface by laser-induced forward transfer (LIFT), which also induces the covalent bonding of the aptamers by thiol-ene or thiol-yne reactions that occur upon UV irradiation of the thiol-modified aptamers with ns laser pulses. This combination of LIFT and laser-induced click chemistry allows the creation of high-resolution patterns without the need for masks. Whereas the click chemistry already takes place during the printing process (single-step process) by the laser pulse used for the printing process, further irradiation of the LIFT-printed aptamers by laser pulses (two-step process) leads to a further increase in the immobilization efficiency.

Polyyne formation by ns and fs laser induced breakdown in hydrocarbon gas flow

Flowing gases of hydrocarbon molecules were irradiated by tightly focused ns and fs laser, and the products were captured in hexane solution. In the case of ns laser irradiation, short polyynes up to C12H2 were produced efficiently from all the hydrocarbons, propane, hexane, octane, benzene, and toluene. The yields vary significantly among the products from different target molecules, extending beyond the effects of difference in the vapor pressures. With an aid of the visible emission spectra of the focused spot, polyyne forming reaction is discussed. In the case of fs laser irradiation of hexane, polyynes were produced efficiently much more than the ns-laser case, whereas for the toluene target, polyynes were not produced. Such a variation in the polyyne yield suggests there would be more suitable molecules for formation of polyynes, or any other carbon-rich materials, by gas-phase laser irradiation method.

On Design and Tribological Behaviour of Laser Textured Surfaces

The paper reports an investigation into the functional response of textured surfaces with different designs that incorporated arrays of micro-dimples and grooves (40μm diameter/width and 15μm depth for both patterns) produced on tungsten carbide (WC) blocks by employing nanosecond (ns) and femtosecond (fs) lasers. In particular, the tribological performance of the textured WC blocks against stainless steel (SS316L) counterbody was evaluated in terms of friction and wear under dry condition compared to an untextured specimen. Friction tests were carried out on a reciprocating sliding tester while unidirectional ball-on-disc method was utilised to assess wear on the mating surfaces. The untextured surface exhibited a continuous rise in the friction coefficient from 0.15 to 0.5 from the start of the cycle to the end while the specimens textured with ns and fs lasers reached steady-state condition after 100 and 200 cycles with values between 0.35-0.45 and 0.3-0.4, respectively. Energy dispersive spectroscopy following wear tests showed a pronounced material transfer from the balls to the textured surfaces with stainless steel filling up some of the dimple and groove cavities; however, the reverse phenomenon was not apparent. Additionally, texturing with the fs laser exhibited formation of nano-ripples/structures in the produced dimples and grooves that can be further studied for creating nano-textured cutting tools or surfaces with super-hydrophobic/anti-ice properties.

Diagnostics of laser-produced plasmas based on the analysis of intensity ratios of He-like ions X-ray emission

In this paper, we detail the diagnostic technique used to infer the spatially resolved electron temperatures and densities in experiments dedicated to investigate the generation of magnetically collimated plasma jets. It is shown that the relative intensities of the resonance transitions in emitting He-like ions can be used to measure the temperature in such recombining plasmas. The intensities of these transitions are sensitive to the plasma density in the range of 1016–1020 cm−3 and to plasma temperature ranges from 10 to 100 eV for ions with a nuclear charge Zn ∼ 10. We show how detailed calculations of the emissivity of F VIII ions allow to determine the parameters of the plasma jets that were created using ELFIE ns laser facility (Ecole Polytechnique, France). The diagnostic and analysis technique detailed here can be applied in a broader context than the one of this study, i.e., to diagnose any recombining plasma containing He-like fluorine ions.

Study of Structure–Third-Order Susceptibility Relation of Indandione Derivatives

By using the Z-scan method we studied the third-order nonlinear optical parameters of several aminobenziliden-1,3-indandione (ABI) derivatives that have previously been shown to own second-order nonlinear optical properties. Measurements were carried out using two 1064 nm Nd:YAG lasers with picosecond (ps) and nanosecond (ns) pulse widths, respectively. When ns laser was employed in the Z-scan setup, a strong thermal lensing took place resulting in severe overestimation of optical Kerr coefficients. Due to this reason the ps laser was employed to evaluate correct magnitude of Kerr effect. For investigated organic molecules, experimental results show that two-photon absorption at 1064 nm is in correlation with molar extinction coefficient at 532 nm. Here we demonstrate good linear correlation between values of second-order molecular hyperpolarizability determined experimentally and those obtained by quantum chemical modeling.

Signal enhancement of neutral He emission lines by fast electron bombardment of laser-induced He plasma

A time-resolved spectroscopic study is performed on the enhancement signals of He gas plasma emission using nanosecond (ns) and picosecond (ps) lasers in an orthogonal configuration. The ns laser is used for the He gas plasma generation and the ps laser is employed for the ejection of fast electrons from a metal target, which serves to excite subsequently the He atoms in the plasma. The study is focused on the most dominant He I 587.6 nm and He I 667.8 nm emission lines suggested to be responsible for the He-assisted excitation (HAE) mechanism. The time-dependent intensity enhancements induced by the fast electrons generated with a series of delayed ps laser ablations are deduced from the intensity time profiles of both He emission lines. The results clearly lead to the conclusion that the metastable excited triplet He atoms are actually the species overwhelmingly produced during the recombination process in the ns laser-induced He gas plasma. These metastable He atoms are believed to serve as the major energy source for the delayed excitation of analyte atoms in ns laser-induced breakdown spectroscopy (LIBS) using He ambient gas.

A facile synthesis of graphene oxide–ZnS/ZnO nanocomposites and observations of thermal quenching of visible photoluminescence emission and nonlinear optical properties

Here we have reported a facile synthesis of graphene oxide–ZnS/ZnO nanocomposite and the temperature dependent photoluminescence (PL) emissions in the synthesized materials, which are scarcely been available in the literature. In the present work PL emission in GO and its composites with ZnS and ZnO semiconductor quantum dots (QDs) have been measured at variable temperatures in 283–353K temperature region. From the measured results it has been found that quenching of PL emission has been taken place in the composite sample and it has been proposed that as the temperature is increased, the excited electrons in the localized states formed by the sp2 clusters in GO can migrate to the nearby sp3 defects states, thereby the intensity of PL emission is reduced. Nonlinear Optical (NLO) properties as well as the optical limiting (OL) properties has also been studied by using an indigenously developed Z-scan technique with a 10ns laser pulse at 1064nm laser radiation. Two photon absorptions (2PA) behavior have been found to be the dominant mechanism in the synthesized samples. A suitable energy level scheme has been proposed to explain the observed PL emission behavior as well as the 2PA mechanism. The present report will open up a lot of prospects for synthesizing GO-semiconductor nanocomposites with semiconductor materials as well as for potential applications in future luminescent devices.

Cracks growth behaviors of commercial pure titanium under nanosecond laser irradiation for formation of nanostructure-covered microstructures (with sub-5-μm)

This study reported on the formation of sub-5-μm microstructures covered on titanium by cracks growth under 10-ns laser radiation at the wavelength of 532nm and its induced light modification for production of nanostructures. The electric field intensity and laser power density absorbed by commercial pure titanium were computed to investigate the self-trapping introduced by cracks and the effect of surface morphology on laser propagation characteristics. It is found that nanostructures can form at the surface with the curvature radius below 20μm. Meanwhile, variable laser fluences were applied to explore the evolution of cracks on commercial pure titanium with or without melt as spot overlap number increased. Experimental study was first performed at the peak laser fluence of 1.063J/cm2 to investigate the microstructures induced only by cracks growth. The results demonstrated that angular microstructures with size between 1.68μm and 4.74μm was obtained and no nanostructure covered. Then, at the peak laser fluence of 2.126J/cm2, there were some nanostructures covered on the melt-induced curved microstructured surface. However, surface molten material submerged in the most of cracks at the spot overlap number of 744, where the old cracks disappeared. The results indicated that there was too much molten material and melting time at the peak laser fluence of 2.126J/cm2, which was not suitable for obtainment of perfect micro-nano structures. On this basis, peak laser fluence was reduced down to 1.595J/cm2 and the sharp sub–5μm microstructures with nanostructures covered was obtained at spot overlap number of 3720.

Crystallization of Ge2Sb2Te5 thin films by nano- and femtosecond single laser pulse irradiation.

The amorphous to crystalline phase transformation of Ge2Sb2Te5 (GST) films by UV nanosecond (ns) and femtosecond (fs) single laser pulse irradiation at the same wavelength is compared. Detailed structural information about the phase transformation is collected by x-ray diffraction and high resolution transmission electron microscopy (TEM). The threshold fluences to induce crystallization are determined for both pulse lengths. A large difference between ns and fs pulse irradiation was found regarding the grain size distribution and morphology of the crystallized films. For fs single pulse irradiated GST thin films, columnar grains with a diameter of 20 to 60 nm were obtained as evidenced by cross-sectional TEM analysis. The local atomic arrangement was investigated by high-resolution Cs-corrected scanning TEM. Neither tetrahedral nor off-octahedral positions of Ge-atoms could be observed in the largely defect-free grains. A high optical reflectivity contrast (~25%) between amorphous and completely crystallized GST films was achieved by fs laser irradiation induced at fluences between 13 and 16 mJ/cm2 and by ns laser irradiation induced at fluences between 67 and 130 mJ/cm2. Finally, the fluence dependent increase of the reflectivity is discussed in terms of each photon involved into the crystallization process for ns and fs pulses, respectively.

Corrections for variable plasma parameters in laser induced breakdown spectroscopy: Application on archeological samples

The final scope of this work was to determine the elemental composition of different types of decorative layers present on ancient ceramic fragments through depth profiling by laser induced breakdown spectroscopy (LIBS). The measurements were performed by a stand-off LIBS system at distance of 10.5m, by employing ns laser pulses at 1064nm and an Echelle spectrometer. The detected plume intensity strongly differs from one sample/coating to another and changes importantly also in repeated measurements on the almost homogeneous bulk materials. Furthermore, the plasma intensity and its parameters widely change during the depth profiling, as evident from the ratio of here monitored Fe I and Fe II spectral lines. Averaging the line intensities over six repeated measurements, also on the bulk material and for a selected consecutive shot number, produces the errors up to 60% around the mean value and this makes impossible to compare composition of the ceramic body with its decorative layers. To overcome this problem, we developed a theoretically supported procedure for the spectral line corrections in presence of variable plasma parameters, which considers the relative changes among a sufficiently large data set. This method allowed improving the measurement precision up to five times, obtaining a flat response during the depth profiling, and measuring composition of the surface layers. The correction factors are specific for one analytical line of the considered element. The proposed procedure could be universally applied for increasing the LIBS precision in repeated samplings or during the depth profiling, without time consuming calculations of the plasma temperature and the electron density, which also suffer from large measurement errors.

Droplet assisted laser micromachining of hard ceramics

Hard ceramic materials like tungsten carbide (WC) are extensively used in high value manufacturing, and micromachining of these materials with sufficient quality is essential to exploit its full potential. A new micro-machining technique called droplet assisted laser micromachining (DALM) was proposed and demonstrated as an alternative to the existing nanosecond (ns) dry pulse laser ablation (PLA). DALM involves injecting liquid micro-droplets at specific frequency during the nanosecond laser micromachining to create impulse shock pressure inside the laser irradiation zone. The impulse shock pressure is generated due to the explosive vaporisation of the droplet, during its interaction with high temperature laser irradiation zone. In this paper, the DALM uses a nanosecond pulsed Nd:YAG laser to machine tungsten carbide substrate. The results suggest that the impulse shock pressure generated during the DALM process can transform the melt ejection mechanism of the ns laser micromachining process. The change in ejection mechanism results in a 75% increase in material removal rate and 71% reduction in the spatter redeposited compared to conventional dry ns laser micromachining.

Laser thermal effect on silicon nitride ceramic based on thermo-chemical reaction with temperature-dependent thermo-physical parameters

In this study, a two-dimensional thermo-chemical reaction model with temperature-dependent thermo-physical parameters on Si3N4 with 10ns laser was developed to investigate the ablated size, volume and surface morphology after single pulse. For model parameters, thermal conductivity and heat capacity of β-Si3N4 were obtained from first-principles calculations. Thermal-chemical reaction rate was fitted by collision theory, and then, reaction element length was deduced using the relationship between reaction rate and temperature distribution. Furthermore, plasma absorption related to energy loss was approximated as a function of electron concentration in Si3N4. It turned out that theoretical ablated volume and radius increased and then remained constant with increasing laser energy, and the maximum ablated depth was not in the center of the ablated zone. Moreover, the surface maximum temperature of Si3N4 was verified to be above 3000K within pulse duration, and it was much higher than its thermal decomposition temperature of 1800K, which indicated that Si3N4 was not ablated directly above the thermal decomposition temperature. Meanwhile, the single pulse ablation of Si3N4 was performed at different powers using a TEM00 10ns pulse Nd:YAG laser to validate the model. The model showed a satisfactory consistence between the experimental data and numerical predictions, presenting a new modeling technology that may significantly increase the accuracy of the predicated results for laser ablation of materials undergoing thermo-chemical reactions.

Single Laser Shot Spin State Switching of {FeII(pz)[Pt(CN)4]} Inside Thermal Hysteresis Studied by X-ray Diffraction

We study by x-ray diffraction and optical microscopy the photoswitching, initiated by a single ns laser pulse, of the spin-crossover {Fe II (pz)[Pt(CN) 4 ]} material inside its thermal hysteresis. The single-crystal study shows that a complete conversion from low spin to high spin states can be reached with a single laser shot. Partial conversions obtained with weaker laser energy gives rise to HS domains. Our results indicate that the non-linear response to light excitation is mainly driven by the temperature jump of the crystal following laser excitation.

Study of the integrated fluence threshold condition for the formation of β-Bi2O3 on Bi thin films by using ns laser pulses

The formation of β-Bi2O3 through laser irradiation of a bismuth (Bi) thin film is reported. The bismuth thin films were irradiated in atmospheric air using Nd:YAG laser pulses of 7 ns duration and 1064nm wavelength. A set of irradiations was done on the samples varying the total irradiation time (i. e. the number of pulses) for a fix per pulse laser fluence of 25mJ/cm2. The laser processed regions were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and microRaman spectroscopy (mRS). OM results show that the laser modified cross section on the film is smaller than the laser beam cross section, which means a thermally confined interaction; SEM micrographs reveled the formation of submicron sized particles as a result of the multi-pulse laser irradiation; using microRaman spectroscopy characterization we were able to determine the formation of the β-Bi2O3 crystalline phase within the laser irradiated spot on the sample.

Third order nonlinear optical properties of organometal halide perovskite by means of the Z-scan technique

The nonlinear optical response of CH3NH3PbBr3 perovskites is investigated using Z-scan technique, employing 10ns laser pulses, at 532nm. The systems were found to exhibit strong nonlinear optical response, dominated by nonlinear refraction. The effect of organic and inorganic composition ratio on the nonlinear susceptibility is studied experimentally. In all cases, the nonlinear absorption and refraction have been determined. The corresponding third-order susceptibilities and second-order hyperpolarizability are determined to be as large as 10−6(esu) and 10−28(esu) under ns laser excitation respectively. Showing large third-order optical nonlinearity in CH3NH3PbBr3 thin films, suggesting their potential for photonics applications.

Laser-welded fused silica substrates using a luminescent fresnoite-based sealant

The laser welding of two fused silica substrates using a fresnoitic glass thin film as a sealant by irradiation with a ns laser is studied. Two different laser parameter sets were compared in terms of bond quality, which include two different laser beam trajectories: linear and wobble (circular) trajectory. The composition of the glass sealant changes with the course of the laser welding, incorporating silica from the substrates. After joining, the bonded samples were exposed to UV light and a very intense emission in the blue spectral range is observed by naked eye, which is due to the crystallization of the fresnoite glass upon the laser irradiation. EDX analysis confirms the crystallization of fresnoite, together with a great enrichment in silica. The formation of a eutectic between both is very plausible. Bond quality and bond strength were evaluated by scanning acoustic microscopy (SAM) and tensile test, which results in a tensile stress of 7MPa.