Ns Laser Research Articles

Influence of nanosecond laser processed surface textures on the tribological characteristics of diamond films sliding against zirconia bioceramic

CVD Diamond films have shown much potential in bioimplantable applications. Aiming at enhancing the tribological properties of CVD diamond and bioceramic tribopair, two types of surface texture, namely groove surface and square surface, are fabricated on free-standing diamond films by nanosecond (ns) laser processing technology. Reciprocating friction and wear experiments against zirconia bioceramic (ZBC) balls in simulated body fluid (SBF) lubrication are performed to evaluate the influence of as-processed surface textures on the tribological behavior of diamond films. It is demonstrated that the counterpart balls sliding against surface-textured diamond films present obviously lower wear rate than that sliding against the untreated diamond films. This can be ascribed to the reduction of adhesive wear and abrasive wear by the surface texture. On the other hand, the heat-affected zone (HAZ), which is caused by the ns laser process, can greatly degrade the friction behavior and wear resistance of as-fabricated surface textures. In this work, after the HAZ is worn out, the groove surface exhibits lower friction coefficient than the untreated surface and square surface. And yet owing to more severe in-depth HAZ, the square surface shows higher friction coefficient and much more abrasion loss than the groove surface.

Biomimetic Surface Structuring Using Laser Based Interferometric Methods

This review investigates the capabilities of laser-based interferometric methods for producing structures with multiple-scaled surface features imitating natural examples. Firstly, laser interference lithography is used to produce hierarchical patterns with length-scales in the micrometer and sub-micrometer range. Different strategies are discussed to produce a wide variety of periodic arrays, depending on the number of resist lasers used as well as the way in which the exposure steps are organized. After that, periodic patterns are fabricated on polymers using ns laser pulses from an UV-laser system. Additionally in this case, multiple-scale patterns are produced by using different strategies. A similar approach is described to treat metallic surfaces of steel X6Cr17 and a titanium alloy Ti6Al4V. The geometry of the produced microstructures was characterized using scanning electron microscopy and confocal microscopy. Measurement of water contact angle is performed for both polymer and metallic surfaces.

Intensity enhancement of LIBS of glass by using Au thin films and nanoparticles

We investigate the features of Au nanoparticles (NPs) formation by laser irradiation of thin films and its application in Nanoparticle Enhancement Laser Induced Breakdown Spectroscopy (NELIBS). Ns laser pulsed irradiation of Au thin films previously deposited on glass, allowed simultaneously the modification of the surface morphology of the metal film and the enhancement of the LIBS signal of the glass. Our results demonstrate that the presence of Au either as thin film or NPs favors the formation of a plasma composed by Au and the elements of underlying substrate. Thus, it is possible observing the optical emission of the glass at much lower irradiance than in conventional LIBS. The effect of the thickness of the starting film, laser fluence and number of pulses on the nanostructuration of the surface and the enhancement of the emission signal were analyzed. Under the studied conditions, the first pulse produces the formation of Au NPs and a LIBS spectrum with dominant emission of Au lines over the lines from the substrate. The subsequent pulses modify the NPs morphology and decrease the amount of Au on the substrate, thus the emission of the glass elements dominates the spectrum. After a certain number of pulses, the NPs are removed leaving an apparently clean surface after the analysis.

Sub-bandgap photo-response of non-doped black-silicon fabricated by nanosecond laser irradiation.

Non-doped black silicon (b-Si) is fabricated on the surface layer of a near-intrinsic Si substrate by nanosecond (ns) laser direct writing in an argon (Ar) atmosphere. The non-doped samples exhibit a near-unity sub-bandgap (1100∼2500 nm) absorptance of more than 50%. Amazingly, the resistivity of the ns laser irradiated b-Si layer is about five orders of magnitude lower than that of the unprocessed Si substrate. The carrier density of the b-Si layer is about 1×1018 cm-3, according to the Hall effect measurement. Temperature-dependent Hall effect measurements show that the non-doped b-Si layer exhibits an energy level of 0.026eV below the conduction band minimum (CBM). At last, Si infrared photodiodes are made based on the difference of carrier concentration between the ns laser-processed b-Si layer and the high-resistivity Si substrate. The responsivity of the b-Si photodiode for 1310nm is up to 256 mA/W at a 10-V reverse bias, which is much higher than that of the reported pure Si bulk-structure photodiodes.

Ultraviolet Pulsed Laser-Induced Fluorescence Nonlinearity in Optically Thick Organic Samples.

A simple two-component model is worked out to investigate pulsed laser-induced fluorescence in complex organic samples, like biological tissues, optically thick at the excitation wavelength. Expression for emitted fluorescence signal is obtained. Saturation process is shown to be determined not only by fluorophores excited by the laser, but non-fluorescent chromophores with overlapping absorption band as well. For homogeneous samples the forms of saturation curves are determined by fluorophore's features. Experimental saturation curves of bulk paper and mice tissues ultraviolet pulsed laser-induced fluorescence are discussed considering this model. For the ns and shorter laser pulse durations with wavelength in 200-300nm region, pulse energy density should be less than 200μJ/cm2 for correct quantitative comparison of fluorescence spectra of biological tissues with primarily tryptophan fluorescence.

Modeling plasma heating by ns laser pulse

The transition to breakdown of a weakly ionized gas, considering inverse bremsstrahlung, has been investigated using a state-to-state self-consistent model for gas discharges, mimicking a ns laser pulse. The paper is focused on the role of the initial ionization on the plasma formation. The results give the hint that some anomalous behaviors, such as signal enhancement by metal nanoparticles, can be attributed to this feature. This approach has been applied to hydrogen gas regarded as a simplified model for LIBS plasmas, as a full kinetic scheme is available, including the collisional-radiative model for atoms and molecules. The model allows the influence of different parameters to be investigated, such as the initial electron molar fraction, on the ionization growth.

Investigation of Plasma Formation with ns Laser by Using Focused Sub-ns Laser Probe

A laser ablation plasma formation process was investigated by shooting another laser beam. The initial plasma was created by a mildly focused nanosecond laser beam and the probe laser, which has about ten times shorter pulse length with tightly focussed condition, was irradiated on it. By analysing high temperature plasma created by the probe laser, we could detect an interaction between the initial low temperature plasma and the probe laser. The interaction caused that the velocity of high temperature plasma generated by the sub-ns laser became smaller and the amount of the highly charged ions decreased. We found that the interaction does not occur during the irradiation of the first half of the ns laser. This fact indicates that the plasma is not produced during the first half of the ns laser.

Characterization of X-ray emission from laser generated plasma

X-ray emission from laser generated plasma was studied at low (1010 W/cm2) and high (1018 W/cm2) intensity using ns and fs laser, respectively. Plasma characteristics were controlled trough the laser parameters, the irradiation conditions and the target properties. The X-ray spectra were acquired using fast detection technique based on SiC diodes with different active regions. The X-ray yield increases with the atomic number of the target, both at low and high intensity, and a similar empirical law has been obtained. The X-ray emission mechanisms from plasma are correlated to the plasma temperature and density and to the Coulomb charge particle acceleration, due to the charge separation effects produced in the non-equilibrium plasma. Functional dependences, theoretical approaches and interpretation of possible mechanism will be presented and discussed.

High-Sensitivity Differential Interface for the Detection of Energy Variations of Nanosecond Laser Pulses for Spectroscopic Applications

This paper reports on a photodiode bridge interface circuit that is able to perform differential measurements of the energy variations of 10-ns laser pulses obtained by differentially detecting the photocurrents generated by a reference and a signal photodiode. A complete characterization of the interface circuit is reported by varying its gain and the energy-per-pulse difference between the signal and the reference laser pulses at a 20-Hz operating frequency (i.e., the laser repetition rate). Under these conditions, a maximum detection sensitivity of about 7 mV/fJ has been achieved. The functionality of the interface circuit has been tested by detecting the Rhodamine-6G photoluminescence emission after pumping the sample with 10-ns laser pulses at a 20-Hz repetition rate. In particular, the Rhodamine-6G was diluted in ethanol, and the interface circuit has been used to measure the photoluminescence emission as a function of the variations of its molar concentration. The experimental findings demonstrate the possibility to achieve interface circuit sensitivities up to 1225 mV/ $\mu \text{M}$ corresponding to a minimum detection resolution of 0.8 pM with a measured rms output noise level of about $1~\mu \text{V}$ . It is also reported that these results enhance more than three orders of magnitude, the sensitivity achieved by using a standard/commercial lock-in amplifier in a differential measurement configuration.

Absorption modification by laser irradiation in DKDP crystals

Laser-induced modification at 355 nm of deuterated potassium dihydrogen phosphate (DKDP) crystals following exposure to nanosecond (ns) and sub-ns laser irradiation is investigated in order to probe the absorption mechanism in damage initiation. Laser damage resistance is greatly improved by sub-ns laser conditioning, whereas only a little improvement occurred after ns laser conditioning at the same laser fluence. Moreover, scattering and transmittance variations after the two types of laser conditioning indicate similar reduction of linear absorption. However, by contrast, large differences on nonlinear absorption modification are discovered using Z-scan measurement. This characteristic absorption modification by laser irradiation provides evidence that a nonlinear absorption mechanism plays a key role in damage initiation at 355 nm.

Improved in situ Sr isotopic analysis by a 257 nm femtosecond laser in combination with the addition of nitrogen for geological minerals

In situ Sr isotope analysis of geological materials by laser ablation multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) is a powerful tracer technique for tracking magmatic source components and geological processes. However, the accuracy and precision of the 87Sr/86Sr ratio are limited in the analysis of natural minerals because of the low-Sr concentration, the isobaric interference or small grains with complex textural contexts, especially for transparent minerals such as feldspars. In this study, analytical results demonstrated that ablation rates in fs laser ablation were consistent for various samples (0.08–0.11μm per pulse), but those in ns laser ablation were obviously material properties-dependent, such as the rates of 0.026μm per pulse and 0.144μm per pulse for feldspar and pyrite, respectively. In addition, at similar energy fluences, the sensitivities of Sr in feldspars analyzed by the fs laser were 3.4 times higher than those analyzed by the ns laser due to the higher ablation efficiency of the fs pulse. These advantages of the fs laser not only offer the benefit of eliminating or weakening the matrix effect during the laser ablation processes but also help to improve the analytical precision for transparent minerals. We also demonstrated that the isobaric interferences of calcium dimers and argides (CaAr++CaCa+) and Kr+ were dramatically reduced by factors of 6.5–11.7 and 5–12.5 in the presence of 6–12mlmin−1N2, respectively. Furthermore, with the addition of N2 (12mlmin−1), the sensitivity of Rb was inhibited, resulting in a decrease of 1.47 times in Rb/Sr signal ratios. Due to the effect of suppressing interferences by adding N2, both the stability and accuracy of the 87Sr/86Sr and 84Sr/86Sr ratios show improvement, especially for the Rb-rich feldspars.Combining the advantages of the fs laser system with the addition of nitrogen, an improved in situ Sr isotope analytical method is then developed. The satisfactory accuracy and precision of the 87Sr/86Sr ratio from natural plagioclases, a K-feldspar with high Rb/Sr ratios (0.46) and a low-Sr clinopyroxene were obtained, demonstrating the reliability of the proposed method. Four feldspars, which have different contents of the major elements, Sr and Rb, showed homogeneous Sr isotope compositions and were recommended as potential suitable reference materials for in situ Sr isotope analysis. As an application, two plagioclases in mafic microgranular enclaves (MMEs) with small grain sizes (200–300μm) and wide ranges of Rb/Sr ratios were analyzed and showed obvious variations of the 87Sr/86Sr ratios from core to rim, which indicated that the proposed method in this study can provide high spatial resolution geochemical information for a single mineral.

Energy distributions of electrons emitted by a biased laser-produced plasma at 1013 W cm−2

In this paper, we present the electron bunches extracted from a dense plasma produced by the interaction of a ns laser at an intensity of 1013 W cm−2 with a solid target, when this plasma expands in an electric field. The plasma expansion dynamics measured in the previous works [M. Comet et al., J. Appl. Phys. 119, 013301 (2016)] is used to determine the electron charge density profile at different instants after the laser shot. When applying the electric field, a few 1013 electrons are extracted with continuous energy distributions. Particle-In-Cell simulations are run to understand the extraction process and reproduce the measured energy distributions, with the electron charge density profiles used as inputs. These simulations show that the electron extraction proceeds from the plasma front edge throughout the plasma expansion.

Tunable Nanosecond and Femtosecond Nonlinear Optical Properties of C–N–S-Doped TiO2 Nanoparticles

Third-order nonlinear optical (NLO) properties of undoped and C–N–S doped TiO2 nanoparticles (NPs) were investigated using the Z-scan technique in the nanosecond (ns) and femtosecond (fs) regimes. Undoped TiO2 NPs exhibited reverse saturable absorption (RSA) behavior with both ns and fs laser pulse excitation, whereas C–N–S-doped TiO2 NPs demonstrated switch over from RSA to saturable absorption (RSA → SA) behavior with fs excitation. Our detailed NLO studies have clearly demonstrated the tunable nonlinear absorption behavior of C–N–S-doped TiO2 NPs. Such versatile tunability is important for various practical applications. The interesting nonlinear absorption (NLA) properties of these NPs are attributed to the interplay between ground and excited state absorption of the free carriers. The results obtained with ns laser excitation demonstrated that the nonlinear absorption coefficient (β) and nonlinear refractive index (n2) for these materials varied with input irradiance, thus indicating the possibility ...

LSPR based optical fiber sensors treated with nanosecond laser irradiation for refractive index sensing

This article reports on the application of localized surface plasmon resonance (LSPR) based optical fiber probes in the form of U-shape geometry, as refraction index (RI) sensors in sucrose and alcohol solutions. For their fabrication, the immobilization of gold nanoparticles (AuNP) on the probe’s active area was achieved by a dip-coating procedure and the formed AuNP decoration was tailored by ns laser irradiation at 532nm. The procedure was monitored and controlled in real time by recording the induced changes into the optical fiber extinction spectra. For the optimization of the fabrication technique, an irradiation protocol is established while the dependence of RI sensitivity on the AuNP size and their abundance on the fiber have been explored. Sensors with RI sensitivities, comparable to the best reported in the literature (up to ΔΕ/ΔRI=45.2) were developed from mono-dispersed colloidal solutions of AuNP. Finally, it should be stressed that following the same procedure, sensors with similar RI sensitivity (ΔΕ/ΔRI=33.9) were fabricated by using a mixture of different size (20–100nm) AuNP bypassing the necessity of mono-dispersed solutions.

Femtosecond and nanosecond laser decontaminations of biocidal-loaded wooden artworks

Until the end of the 1980s, many wooden artworks underwent surface treatment by liquid preservatives, e.g., Hylotox-59. DDT (dichlorodiphenyltrichloroethane) crystal structures are formed on the wooden surfaces by the “blooming” of chlorine compounds by time. In addition to an aesthetic disturbance, it is assumed that DDT represents a health risk. Therefore, the removal of DDT crystals from the surfaces is requested. Contaminated wood with natural biocide ageing, gilded and wood carved elements, and wooden samples with paint layers were provided by the Schlossmuseum Sondershausen, Germany. Laser cleaning on selected surface areas on the objects was done by means of femtosecond and nanosecond laser pulses. For the same object, cleaning results using 30-fs laser pulses at 800-nm wavelength are compared to findings utilizing 10-ns laser pulses at 1064-nm wavelength. Before and after laser treatment, chlorine measurements at the same surface position were done by X-ray fluorescence analysis (XRF) as an indicator for the presence of DDT. In this way, pointwise chlorine depletion rates can be obtained for the different pulse duration regimes and wavelengths. In addition, the object surfaces were examined using optical microscopy and multi-spectral imaging analysis.

Revealing and Identifying Laser-Induced Damages in CIGSe Solar Cells by Photoluminescence Spectroscopy

Laser-based patterning for serial interconnection of chalcopyrite (i.e., Cu(In ${}_{\rm{x}},\text{Ga}_{\rm{1-x}}$ )Se2 or CIGSe) solar cells was obtained by 1) laser ablation using picosecond (ps) pulses, 2) local phase transformation using nanosecond (ns) laser pulses, and 3) conventional needle-based patterning. All three patterning approaches cause a modification of the material properties in the vicinity of the actual P2 scribing lines, which affects and limits the electrical functionality of the interconnection, and thus has to be considered for positioning the P3 scribe. Thus, the extension and the properties of the affected zone aside the P2 scribe was investigated through spectral and spatial photoluminescence (PL). From the depletion of the PL intensity when approaching the scribing line and a peak shift analysis it is concluded that the laser-affected zone is distinctively larger than visual inspections suggest. Even putatively ultrashort, nonthermal ps pulses cause material modifications which might be facilitating recombination losses and thus limiting solar cell efficiencies. For ps laser patterning the affected area is even larger than for the ns laser patterning, due to a modification of the band structure and to thermal decomposition. Evolving subpeaks at the low energy tails are found to originate from Cu-related flat defect levels, i.e., Cu vacancies ( $V_{{\rm{Cu}}}$ ) and antisites (Cu In), created upon laser impact. These findings provide insights into laser-based material modification and provide beneficial information for minimizing the dead area resulting from laser-based monolithic interconnection.

Benzophenone as a photoprobe of polymer films

The review article is devoted to kinetics of fast reactions following photoexcitation of benzophenone in polymer films. We observed three processes by ns laser flash photolysis in elastomers: (i) decay of a triple state of benzophenone with hydrogen abstraction from polymer matrix, (ii) formation and decay of geminate radical pairs, (iii) cross-termination of the formed radicals in the polymer bulk. Application of external magnetic field (MF) of B=0.2T essentially affects recombination of geminate (G-) and a bimolecular recombination of free radicals, which escaped polymer cage (F-pairs). Theoretical calculation of MF effects on G- and F-pairs is in agreement with corresponding experimental data. Elongation of elastomer leads to an unexpected observation: recombination in the bulk becomes slower. An explanation of this phenomenon based on elastomer free volume Vf approach was suggested.

On the Development of a Chip Breaker in a Metal-Matrix Polycrystalline Diamond Insert: Finite Element Based Design With ns-Laser Ablation and Machining Verification

This paper reports the development of an original design of chip breaker in a metal-matrix polycrystalline diamond (MMPCD) insert brazed into a milling tool. The research entailed finite element (FE) design, laser simulation, laser fabrication, and machining tests. FE analysis was performed to evaluate the effectiveness of different designs of chip breaker, under specified conditions when milling aluminum alloy (Al A356). Then, the ablation performance of an MMPCD workpiece was characterized by ablating single trenches under different conditions. The profiles of the generated trenches were analyzed and fed into a simulation tool to examine the resultant thickness of ablated layers for different process conditions, and to predict the obtainable shape when ablating multilayers. Next, the geometry of the designated chip breaker was sliced into a number of layers to be ablated sequentially. Different ablation scenarios were experimentally investigated to identify the optimum processing conditions. The results showed that an ns laser utilized in a controllable manner successfully produced the necessary three-dimensional feature of an intricate chip breaker with high surface quality (Ra in the submicron range), tight dimensional accuracy (maximum dimensional error was less than 4%), and in an acceptable processing time (≈51 s). Finally, two different inserts brazed in milling tools, with and without the chip breaker, were tested in real milling trials. Superior performance of the insert with chip breaker was demonstrated by the curled chips formed and the significant reduction of obtained surface roughness compared to the surface produced by the insert without chip breaker.

Surface damage induced by a combined millisecond and nanosecond laser.

The surface damage morphologies of single-crystal silicon induced by a combined pulse laser (CPL) and a single millisecond laser are investigated, respectively. The CPL includes a millisecond (ms) laser superposed by a nanosecond (ns) laser. Inspected by an optical microscope, it was found that the surface damage was more serious when the sample was irradiated by the CPL than by a single ms laser with the same incident laser energy. Besides surface cleavage, obvious ablation and fold areas were discovered by CPL irradiation. A two-dimensional spatial axisymmetric model was established to assess the difference between single ms laser and CPL irradiation and explain the generation mechanism of the different surface damage. This was attributed to the preheating effect by the ms laser and the surface damage caused by the ns laser.

Laser-induced periodic surface structures on bismuth thin films with ns laser pulses below ablation threshold

We demonstrate the formation of laser-induced periodic surface structures (LIPSS) in bismuth (Bi) thin films by irradiation with nanosecond laser pulses. We report on the formation and the destruction of the LIPSS as a result of the delivered number of pulses; both the formation and destruction threshold were very well determined. Results show that the obtained LIPSS are perpendicular to the laser polarization, and their ripple periodicity is on the order of the irradiation wavelength. Although all the irradiation experiments were done in ambient air, Raman micro-spectroscopy indicates that the LIPSS are constituted by metallic bismuth, i. e. the LIPSS formation is oxidation free.