Impact of the lasr wavelength and fluence on the ablation rate of aluminium

The dependence of the ablation rate of aluminium on the fluence of nanosecond laser pulses with wavelengths of 532 nm and respectively 1064 nm is investigated in atmospheric air. The fluence of the pulses is varied by changing the diameter of the irradiated area at the target surface, and the wavelength is varied by using the fundamental and the second harmonic of a Q-switched Nd-YAG laser system. The results indicate an approximately logarithmic increase of the ablation rate with the fluence for ablation rates smaller than ∼6 μm/pulse at 532 nm, and 0.3 μm/pulse at 1064 nm wavelength. The significantly smaller ablation rate at 1064 nm is due to the small optical absorptivity, the strong oxidation of the aluminium target, and to the strong attenuation of the pulses into the plasma plume at this wavelength. A jump of the ablation rate is observed at the fluence threshold value, which is ∼50 J/cm² for the second harmonic, and ∼15 J/cm² for the fundamental pulses. Further increasing the fluence leads to a steep increase of the ablation rate at both wavelengths, the increase of the ablation rate being approximately exponential in the case of visible pulses. The jump of the ablation rate at the threshold fluence value is due to the transition from a normal vaporization regime to a phase explosion regime, and to the change of the dimensionality of the hydrodynamics of the plasma-plume.      

High-vacuum time-resolved laser-induced incandescence of flame-generated soot

We have measured time-resolved laser-induced incandescence of flame-generated soot under high-vacuum conditions (4.1×10⁻⁶ mbar) at an excitation wavelength of 532 nm with laser fluences spanning 0.06–0.5 J/cm². We generated soot in an ethylene/air diffusion flame, introduced it into the vacuum system with an aerodynamic lens, heated it using a pulsed laser with a spatially homogeneous and temporally smooth laser profile, and recorded LII temporal profiles at 685 nm. At low laser fluences LII signal decay rates are slow, and LII signals persist beyond the residence time of the soot particles in the detection region. At these fluences, the temporal maximum of the LII signal increases nearly linearly with increasing laser fluence until reaching a plateau at ∼0.18 J/cm². At higher fluences, the LII signal maximum is independent of laser fluence within experimental uncertainty. At these fluences, the LII signal decays rapidly during the laser pulse. The fluence dependence of the vacuum LII signal is qualitatively similar to that observed under similar laser conditions in an atmospheric flame but requires higher fluences (by ∼0.03 J/cm²) for initiation. These data demonstrate the feasibility of recording vacuum LII temporal profiles of flame-generated soot under well-characterized conditions for model validation.     

Room-temperature continuous wave laser oscillations in Nd:YAG ceramic waveguides produced by carbon ion implantation

We report on the generation of continuous wave lasers at a wavelength of ∼1064 nm in a Nd:YAG ceramic waveguide at room temperature. The waveguide was fabricated by using 6 MeV carbon ion implantation at a fluence of 3×10¹⁴ ions/cm². Laser operation has been realized with a slope efficiency as high as ∼11%. The pump threshold of an 808-nm laser beam for the waveguide laser oscillation is 19.5 mW.     

Optical properties of pulsed laser heated soot

To investigate the transient change of soot optical properties resulting from pulsed laser heating of soot in a cooled exhaust plume we have simultaneously performed cw light extinction at 405 and 830 nm and elastic light scattering at 1064 nm. A reversible increase to the 830-nm light extinction of up to 7%, observed during the time period where the soot was hot, suggests a temperature-dependent light absorption refractive index function, E(m _λ ). At low fluence, small permanent increases of E(m _λ ) of <2% were also observed. 405-nm extinction measurements revealed that the soot likely contained material which continued to absorb 405-nm radiation when desorbed, thus complicating measurement interpretation. 1064-nm light scattering measurements showed a gradual decrease of scattering propensity with increasing laser fluence up to the point of material loss, which is consistent with the expected decrease of the structure factor of the soot aggregates as they expand. It is concluded that variations of the optical properties are occurring at the time of laser-induced incandescence (LII) emission, which should be accounted for in time-resolved LII measurement interpretation.     

Laser induced incandescence determination of the ratio of the soot absorption functions at 532 nm and 1064 nm in the nucleation zone of a low pressure premixed sooting flame

In this work, the two-excitation wavelength laser induced incandescence (LII) method has been applied in a low-pressure premixed methane/oxygen/nitrogen flame (equivalence ratio 2.32) to determine the variation of the ratio of the soot absorption functions at 532 nm and 1064 nm E(m,532 nm)/E(m,1064 nm) along the flame. This method relies on the comparison of LII signals measured upon two different excitation wavelengths (here 532 nm and 1064 nm) and with laser fluences selected in such a way that the soot particles are equally laser-heated. The comparison of the laser fluences at 532 nm and 1064 nm leads to an easy determination of E(m,532 nm)/E(m,1064 nm). The reliability of the method is demonstrated for the first time in a low pressure flame in which the soot nucleation zone can be spatially resolved and which contains soot particles acting differently with the laser fluence according to their residence time in the flame. The method is then applied to determine the profile of E(m,532 nm)/E(m,1064 nm) along the flame. A very important decrease of this ratio is observed in the region of nascent soot, while the ratio remains constant at high distance above the burner. Implication on temperature determination from spectrally resolved measurement of flame emission is studied.     

Optical absorption in early stage low temperature laser produced plasma

We describe a new technique to measure the UV/visible absorption spectrum of the ablated material during the laser pulse. The technique utilizes the continuum emission from one laser produced plasma as a light source to measure the absorption properties of a second laser produced plasma which is formed on a semi-transparent target with an array of 40 μm holes. A 6 ns, 1064 nm laser was used to ablate a Ag target and the plasma absorption was measured in the range 450–625 nm for a laser fluence of 1 J cm⁻². The total absorption cross-section is (0.5–1.5)×10⁻¹⁷ cm² in the range 450–540 nm. By comparing the measured absorption with a calculation using the plasma spectroscopy code FLYCHK it can be concluded that, in the wavelength region examined here, the absorption is mainly due to bound-bound transitions.     

Dynamics of two-photon picosecond absorption in ZnWO<Subscript>4</Subscript> and PbWO<Subscript>4</Subscript> crystals

A method has been proposed to analyze the dynamics of interband two-photon absorption in a nonlinear medium excited by a sequence of picosecond laser pulses of variable intensity and continuous probe radiation. Induced absorption leading both to hysteresis in the dependence of the absorption on the intensity of laser pump radiation and to the opacity of crystals at the pump wavelength has been revealed in initially transparent ZnWO₄ and PbWO₄ crystals irradiated by a train of 523.5-nm pulses with a duration of 20 ps at pump intensities of 5 to 140 GW/cm². The kinetics of an increase in absorption and its subsequent relaxation at a 523.5-nm picosecond excitation of the crystals have been measured with continuous 633-nm probe radiation. An exponential component of the increase in absorption with the time constant τ = 2−3.5 and 8–9.5 μs depending on the direction of the linear polarization of pump radiation has been revealed at 300 K in ZnWO₄ and PbWO₄ crystals, respectively. The absorption relaxation kinetics in the crystals are complicated and approach an exponential at a late stage with the constant τ = 40−130 and 12–80 ms for the ZnWO₄ and PbWO₄ crystals, respectively.     

Formation of highly organised,periodic microstructures on steel surfaces upon pulsed laser irradiation

We present results on the growth of highly organised, reproducible, periodic microstructure arrays on a stainless steel substrate using multi-pulsed Nd:YAG (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz, beam quality factor of M ²∼1.5) laser irradiation in standard atmospheric environment (room temperature and normal pressure) with laser spot diameter of the target being ∼50 μm. The target surface was irradiated at laser fluence of ∼2.2 J/cm² and intensity of ∼0.31×10⁹ W/cm², resulting in the controllable generation of arrays of microstructures with average periods ranging from ∼30 to ∼70 μm, depending on the hatching overlap between the consecutive scans. The received tips of the structures were either below or at the level of the original substrate surface, depending on the experimental conditions. The peculiarity of our work is on the utilised approach for scanning the laser beam over the surface. A possible mechanism for the formation of the structures is proposed.     

Patterning of PLZT and PSZT thin films by excimer laser

The patterning of lanthanum-doped lead zirconate titanate (PLZT) and strontium-doped lead zirconate titanate (PSZT) thin films has been examined using a 5-ns pulsed excimer laser. Both types of film were deposited by rf magnetron sputtering with in situ heating and a controlled cooling rate in order to obtain the perovskite-structured films. The depth of laser ablation in both PSZT and PLZT films showed a logarithmic dependence on fluence. The ablation rate of PLZT films was slightly higher than that of PSZT films over the range of fluence (10–150 J/cm²) and increased linearly with number of pulses. The threshold fluence required to initiate ablation was ∼ 1.25 J/cm² for PLZT and ∼ 1.87 J/cm² for PSZT films. Individual squares were patterned with areas ranging from 10×10 μm² up to 30×30 μm² using single and multiple pulses. The morphology of the etched surfaces comprised globules which had diameters of 200–250 nm in PLZT and 1400 nm in PSZT films. The diameter of the globules has been shown to increase with fluence until reaching an approximately constant size at ≤ 20 J/cm² in both types of film. The composition of the films following ablation has been compared using X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. PACS 79.20.Ds; 82.80.Pv; 82.80.Ej     

Experimental study of signal trapping of OH laser induced fluorescence and chemiluminescence in flames

The absorption of OH^∗ chemiluminescence and laser-induced fluorescence (LIF) in the exhaust gas of confined premixed laminar CH₄/air flames at atmospheric pressure was investigated. One flame was used as source and a second as absorber. OH LIF was excited in the ν″=0→ν′=1 band of the A–X electronic system around ≈283 nm and spectrally resolved detected in the (0,0) and (1,1) vibrational bands around 305–320 nm. For OH^∗ chemiluminescence, spectrally resolved detection was performed in the wavelength range 280–340 nm. For an absorption path of 54 mm and at T≈2000 K, signal trapping on the order of 10–40% was observed. Signal trapping was most pronounced in the (0,0) band, as expected from the thermal population distribution of OH in the electronic ground state. The spectral distribution of the signals and the wavelength dependence of the signal trapping are addressed in this paper. Implications from the results with respect to detection strategies and chemiluminescence-based equivalence ratio measurements are discussed.     

Enhanced soft X-ray emission from carbon nanofibers irradiated with ultra-short laser pulses

A comparative experimental study of the X-ray emission in the water-window spectral region has been performed using carbon nanofibers (CNFs) of different sizes and graphite plate targets, irradiated with ultra-short (Ti:sapphire) laser pulses. More than an order of magnitude enhancement in the X-ray yield is observed from CNFs of 60-nm diameter with respect to graphite targets. The X-ray emission from CNFs of 160-nm diameter was also high. The integrated X-ray yield of these carbon-based targets scales with the laser intensity (I _L) as I_L ^{~ 1.3-1.4}I_{\mathrm{L}}^{\sim 1.3-1.4} in the intensity range of 4×10¹⁶–4×10¹⁷ W/cm². The effect of the laser pulse duration on the X-ray emission from the CNFs was also studied by varying the pulse duration from 45 fs up to 3 ps at a constant fluence of 2×10⁴ J/cm². The optimum laser pulse duration for maximum X-ray emission increases with the diameter of the CNFs used. The results are explained from physical considerations of heating and hydrodynamic expansion of the CNF plasma in which resonance field enhancement takes place while passing through two times the critical density. The results add to the efforts towards achieving an efficient low-cost water-window X-ray source for microscopy.     

The effect of particle aggregation on the absorption and emission properties of mono- and polydisperse soot aggregates

This study concerns the effect of soot-particle aggregation on the soot temperature derived from the signal ratio in two-color laser-induced incandescence measurements. The emissivity of aggregated fractal soot particles was calculated using both the commonly used Rayleigh–Debye–Gans fractal-aggregate theory and the generalized Mie-solution method in conjunction with numerically generated fractal aggregates of specified fractal parameters typical of flame-generated soot. The effect of aggregation on soot temperature was first evaluated for monodisperse aggregates of different sizes and for a lognormally distributed aggregate ensemble at given signal ratios between the two wavelengths. Numerical calculations were also conducted to account for the effect of aggregation on both laser heating and thermal emission at the two wavelengths for determining the effective soot temperature of polydisperse soot aggregates. The results show that the effect of aggregation on laser energy absorption is important at low fluences. The effect of aggregation on soot emissivity is relatively unimportant in LII applications to typical laminar diffusion flames at atmospheric pressure, but it can become more important in flames at high pressures due to larger primary particles and wider aggregate distributions associated with enhanced soot loading.     

On the dependence of the laser-induced incandescence (LII) signal on soot volume fraction for variations in particle size

“The laser-induced incandescence (LII) signal is proportional to soot volume fraction” is an often used statement in scientific papers, and it has – within experimental uncertainties – been validated in comparisons with other diagnostic techniques in several investigations. In 1984 it was shown theoretically in a paper by Melton that there is a deviation from this statement in that the presence of larger particles leads to some overestimation of soot volume fractions. In the present paper we present a detailed theoretical investigation of how the soot particle size influences the relationship between LII signal and soot volume fraction for different experimental conditions. Several parameters have been varied; detection wavelength, time and delay of detection gate, ambient gas temperature and pressure, laser fluence, level of aggregation and spatial profile. Based on these results we are able, firstly, to understand how experimental conditions should be chosen in order to minimize the errors introduced when assuming a linear dependence between the signal and volume fraction and secondly, to obtain knowledge on how to use this information to obtain more accurate soot volume fraction data if the particle size is known. PACS 42.62.-b; 44.40.+a; 61.46.Df; 78.70.-g; 65.80.+n     

Preparation and characterization of La0.8Sr0.2MnO3 thin films by an excimer laser MOD process for a bolometer

La_0.8Sr_0.2MnO₃ (LSMO) films were prepared on LaAlO₃ substrates by excimer laser metal organic deposition (ELMOD) at 500 °C. The temperature dependence of resistance of the LSMO films was investigated by changing the laser fluence, irradiation time, and film thickness. It was found that the resistance of the LSMO films 80 nm in thickness that were irradiated by an ArF laser at a fluence of 100 mJ/cm² for 60 min showed a metallic temperature dependence, and the maximum temperature coefficient of resistance of the films (defined as 1/R×dR/dT) was 3.4% at 265 K. PACS 81.15.-z; 81.15.Fg; 81.15.Np; 73.61.-r; 71.30.+h     

Comparison of LII derived soot temperature measurements with LII model predictions for soot in a laminar diffusion flame

Laser-induced incandescence (LII) was used to derive temperatures of pulsed laser heated soot particles from their thermal emission intensities detected at two wavelengths in a laminar ethylene/air co-annular diffusion flame. The results are compared to those of a numerical nanoscale heat and mass transfer model. Both aggregate and primary particle soot size distributions were measured using transmission electron microscopy (TEM). The model predictions were numerically averaged over these experimentally derived size distributions. The excitation laser wavelength was 532 nm, and the LII signal was detected at 445 nm and 780 nm. A wide range of laser fluence from very low to moderate (0.13 to 1.56 mJ/mm²) was used in the experiments. A large part of the temporal decay curve, beginning 12–15 nsec after the peak of the laser excitation pulse, is successfully described by the model, resulting in the determination of accommodation coefficients, which varies somewhat with soot temperature and is in the range of 0.36 to 0.46. However, in the soot evaporative regime, the model greatly overpredicts the cooling rate shortly after the laser pulse. At lower fluences, where evaporation is negligible, the initial experimental cooling rates, immediately following the laser pulse, are anomalously high. Potential physical processes that could account for these effects are discussed. From the present data the soot absorption function, E(m), of 0.4 at 532 nm is obtained. A procedure for correcting the measured signals for the flame radiation is presented. It is further shown that accounting for the local gas temperature increase due to heat transfer from soot particles to the gas significantly improves the agreement in the temperature dependence of soot cooling rates between model and experiments over a large range of laser fluences.     

Characterization of Ag and Au nanoparticles created by nanosecond pulsed laser ablation in double distilled water

Pulsed laser ablation of Ag and Au targets, immersed in double-distilled water is used to synthesize metallic nanoparticles (NPs). The targets are irradiated for 20 min by laser pulses at different wavelengths—the fundamental and the second harmonic (SHG) (λ = 1064 and 532 nm, respectively) of a Nd:YAG laser system. The ablation process is performed at a repetition rate of 10 Hz and with pulse duration of 15 ns. Two boundary values of the laser fluence for each wavelength under the experimental conditions chosen were used—it varied from several J/cm² to tens of J/cm². Only as-prepared samples were measured not later than two hours after fabrication. The NPs shape and size distribution were evaluated from transmission electron microscopy (TEM) images. The suspensions obtained were investigated by optical transmission spectroscopy in the near UV and in the visible region in order to get information about these parameters. Spherical shape of the NPs at the low laser fluence and appearance of aggregation and building of nanowires at the SHG and high laser fluence was seen. Dependence of the mean particle size at the SHG on the laser fluence was established. Comments on the results obtained have been also presented.     

Generation of CdS clusters using laser ablation: the role of wavelength and fluence

The formation of cationic clusters in the laser ablation of CdS targets has been investigated as a function of wavelength and fluence by mass spectrometric analysis of the plume. Ablation was carried out at the laser wavelengths of 1064, 532, 355, and 266 nm in order to scan the interaction regimes below and above the energy band gap of the material. In all cases, the mass spectra showed stoichiometric Cd _n S _n ⁺ and nonstoichiometric Cd _n S _n−1⁺, Cd _n S _n+1⁺, and Cd _n S _n+2⁺ clusters up to 4900 amu. Cluster size distributions were well represented by a log-normal function, although larger relative abundance for clusters with n=13, 16, 19, 34 was observed (magic numbers). The laser threshold fluence for cluster observation was strongly dependent on wavelength, ranging from around 16 mJ/cm² at 266 nm to more than 300 mJ/cm² at 532 and 1064 nm. According to the behavior of the detected species as a function of fluence, two distinct families were identified: the “light” family containing S₂⁺ and Cd⁺ and the “heavy” clusterized family grouping Cd₂⁺ and Cd _n S _m ⁺. In terms of fluence, it has been determined that the best ratio for clusterization is achieved close to the threshold of appearance of clusters at all wavelengths. At 1064, 532, and 355 nm, the production of “heavy” cations as a function of fluence showed a maximum, indicating the participation of competitive effects, whereas saturation is observed at 266 nm. In terms of relative production, the contribution of the “heavy” family to the total cation signal was significantly lower for 266 nm than for the longer wavelengths. Irradiation at 355 nm in the fluence region of 200 mJ/cm² has been identified as the optimum for the generation of large clusters in CdS.     

Laser ablation of liquid surface in air induced by laser irradiation through liquid medium

The pulse laser ablation of a liquid surface in air when induced by laser irradiation through a liquid medium has been experimentally investigated. A supersonic liquid jet is observed at the liquid–air interface. The liquid surface layer is driven by a plasma plume that is produced by laser ablation at the layer, resulting in a liquid jet. This phenomenon occurs only when an Nd:YAG laser pulse (wavelength: 1064 nm) is focused from the liquid onto air at a low fluence of 20 J/cm². In this case, as Fresnel’s law shows, the incident and reflected electric fields near the liquid surface layer are superposed constructively. In contrast, when the incident laser is focused from air onto the liquid, a liquid jet is produced only at an extremely high fluence, several times larger than that in the former case. The similarities and differences in the liquid jets and atomization processes are studied for several liquid samples, including water, ethanol, and vacuum oil. The laser ablation of the liquid surface is found to depend on the incident laser energy and laser fluence. A pulse laser light source and high-resolution film are required to observe the detailed structure of a liquid jet.     

1064-nm laser fragmentation of thin Au and Ag flakes in acetone for highly productive pathway to stable metal nanoparticles

The laser-induced fragmentation of thin Au and Ag flakes in acetone by 1064-nm nanosecond laser (with the fluence typically ∼2 J/cm²) potentially offers a highly productive pathway to stable metal nanoparticles in liquid. Acetone serves as a superior liquid medium that keeps fine metal nanoparticles free from precipitation even in such concentrated nanoparticle solutions exceeding ∼0.1 M. Thin metal flakes have good capability to absorb the 1064-nm laser energy as efficiently as in the visible region. A part of the thus laser-heated molten flakes explosively split into submicroparticles, and some other significant part directly into fine nanoparticles. Both kinds of product particles have minor absorption cross-sections for subsequent laser pulses at 1064 nm, and thus no longer fragment further. One of the two kinds of Ag flakes studied in this work yielded fine Ag nanoparticles at a remarkable high production rate of 1.1 mg/min for the input laser power of only ∼0.65 W.