Laser Pulse Fluence Research Articles

Characterization of cobalt oxide nanoparticles produced by laser ablation method: Effects of laser fluence

Abstract The role of laser fluence in producing cobalt oxide nanoparticles in distilled water by laser ablation method has been studied experimentally. Output of a pulsed Nd:YAG laser of 1064 nm wavelength and 7 ns pulse width with 10 Hz repetition rate was employed to irradiate a high purity cobalt bulk in distilled water. According to results, size of produced nanostructures as well as their oxidation was strongly under the influence of laser energy. Although a high purity Co bulk was used as the target of laser irradiation in the experiment but produced nanoparticles were mainly cobalt oxides. X ray diffraction pattern indicates that produced nanostructures are Co3O4. The excitonic absorption peak of Co3O4 nanoparticles was occurred in UV region. Results show that with increasing the fluence of laser pulse, size of nanoparticles was decreased while their aggregation was increased noticeably.

Improvement of palladium limit of detection by microwave-assisted laser induced breakdown spectroscopy

Detecting elements such as heavy metals is important in many industrial processes. The techniques currently used are time consuming and require excessive sample preparation. In this paper, we demonstrate microwave-assisted laser-induced breakdown spectroscopy (MW-LIBS) to detect palladium (Pd) in solid samples at ambient conditions. Microwave radiation was introduced by a near field applicator to couple the microwave radiation with the plasma. The results were a 92–fold enhancement in palladium signal with 8-fold improvement in the limit of detection at laser energy levels below 5 mJ (1250 J/cm2 laser pulse fluence). We also investigate the optimum experimental parameters of palladium detection for both laser-induced breakdown spectroscopy (LIBS) and MW-LIBS. The maximum signal to noise ratio improvement was achieved at microwave power of 750 W and laser pulse fluence of 157 J/cm2 for Pd I 340.46 nm. Finally, we examine the location of the near field applicator (NFA) with respect to the sample to show that the MW-LIBS signal strength was significantly affected by the vertical position compared to the horizontal. The detection limits of palladium with LIBS and MW-LIBS were 40 ppm and 5 ppm respectively.

Hybrid magnetron sputtering and pulsed laser ablation for the deposition of composite ZnO-Au films

A hybrid technique that combines magnetron sputtering (MS) of zinc oxide and pulsed laser deposition (PLD) of gold was used to synthesize ZnO-Au composite thin films. Three different laser pulse fluence values of 4.5, 13.6 and 20.9 J cm−2 were used. Films of ZnO and Au were deposited separately by MS and PLD respectively to compare the growth by the individual and hybrid techniques. For the hybrid technique, no significant changes on the Zn concentration are observed when varying the laser pulse fluence, whilst the concentrations of O decreased and Au increased. The gold was incorporated uniformly throughout the thicknesses of the films as a second phase of nanoparticles. The presence of the gold did not modify the crystal orientation of the ZnO, which was a hexagonal wurtzite-phase with the c-axis perpendicular to the substrate. The ZnO-Au films were found to be thicker than the sum of the thicknesses of the Au and ZnO produced individually by MS and PLD, indicating that the hybrid technique incremented the net deposition rate. The emission of the laser ablation plume species, specifically neutral atoms of gold, was studied by optical emission spectroscopy to compare the individual PLD and MS-PLD processes.

Ultrafast magnetization dynamics in an epitaxialNi54.3Mn31.9Sn13.8Heusler-alloy film close to the Curie temperature

The influence of the amplitude of an external magnetic field ($H$) and femtosecond laser pulse fluence ($F$) on ultrafast magnetization dynamics has been investigated in a ferromagnetic ${\mathrm{Ni}}_{54.3}{\mathrm{Mn}}_{31.9}{\mathrm{Sn}}_{13.8}$ Heusler-alloy film using the time-resolved magneto-optical Kerr effect. A large slowing down of the demagnetization process was observed and characteristic parameters of magnetization precession were determined for a wide range of $H$ and $F$ values. Long demagnetization times of the order of hundreds of picoseconds have been found and explained as a result of the Curie temperature (${T}_{\mathrm{C}}$) proximity in the alloy film studied. Effective magnetic anisotropy field (${H}_{\text{k}}^{\text{eff}}$) and Gilbert damping parameter dependencies were determined. A significant reduction of the precession frequency versus $F$ of the uniform Kittel mode was found. A strong decrease of ${H}_{\text{k}}^{\text{eff}}$ with $F$ was well simulated in the frame of an extended version of the microscopic three-temperature model (eM3TM), and explained by the ${T}_{\mathrm{C}}$ proximity effect. The estimated low values of the eM3TM model parameters, the demagnetization rate and electron-lattice coupling constant, appeared essential to explain the slowing down effect of demagnetization. Precession amplitude dependencies were explained by a phenomenological approach taking into account ${H}_{\text{k}}^{\text{eff}}$ and changes of the equilibrium magnetization angles induced by pump-pulse excitation.

Surface plasmon assisted laser ablation of stainless steel

Colloidal Au nanoparticles (NPs) were decorated on stainless steel for surface plasmon enhanced laser ablation. A comparative study of the laser ablation efficiency was carried out on stainless steel samples with and without the Au NPs decoration at a variable pulsed laser fluence and laser pulse number. Higher ablation efficiency was clearly demonstrated in the former as illustrated from the larger diameter, maximum depth and the cross-sectional area of the crater generated by the laser ablation under the same conditions. Additionally, both the maximum depth and efficiency enhancement were found to depend on the laser fluence and pulse number. The maximum enhanced ablation efficiency of 36% based on the cross-sectional area of the crater was obtained at 1 pulse number of laser fluence 1.53 J cm−2. The efficiency enhancement of laser ablation is attributed to the highly enhanced surface plasmon field at the interface between Au NPs and stainless steel.

Laser-induced phase transition processes of amorphous Ge2Sb2Te5 films

Amorphous thin films of Ge2Sb2Te5, sputter-deposited on polycarbonate (PC) substrates, are investigated for purpose of understanding the phase change process by transient optical reflectivity measurements. It was found that phase transition dynamics induced by nanosecond laser pulses were complex optical evolution processes. To understand the effect of heat flow conditions, the aluminium (Al) film with the thickness of 200 nm was introduced as a heat sink layer. The result indicates that the phase change process can be controlled and tunneled by heating conditions due to the addition of Al under-layer with larger thermal conductivity. Finally, a three-level reflectivity was achieved induced by well-selected multi-pulse with identical laser pulse fluence.

Periodic surface structures on dielectrics upon femtosecond laser pulses irradiation.

The formation of laser-induced periodic surface structures (LIPSS) on two different dielectrics of K9 glass and fused silica upon irradiation in ambient conditions and in vacuum with multiple femtosecond (fs) laser pulse sequences at different pulse durations (35fs, 260 fs, and 500 fs) was studied experimentally. Three types of LIPSS, so-called high-spatial-frequency LIPSS (HSFL), low-spatial-frequency LIPSS (LSFL), and supra-wavelength periodic surface structures (SWPSS) with different spatial periods and orientations were identified. The appearance was characterized with respect to the experimental parameters of laser fluence and number of laser pulses per spot. The crater morphologies - including nanoripples, periodic microgrooves, quasiperiodic microspikes, and central smooth zone - were observed by scanning electron microscope (SEM). The supra-wavelength structures exhibit periodicities, which are markedly, even multiple times, higher than the laser excitation wavelength. The SWPSS were formed with a broader range of laser fluences, upon the longer laser pulse durations (260 fs and 500 fs) and/or on the lower band-gap dielectrics (K9 glass), due to the deeper effective light penetration depths and thicker viscous surface layers formation. The HSFL were observed on the higher band-gap dielectrics (fused silica) and within a certain narrow laser parameter window. The formation mechanisms of LIPSS were also discussed.

Molybdenum nanoparticles generation by pulsed laser ablation and effects of oxidation due to aging

Molybdenum oxides (MoOx) nanoparticles (NPs) have great optical and electronic features that make them suitable for potential applications such as surface enhanced Raman spectroscopy (SERS) and energy systems. However, there are very few papers that report the synthesis of MoOx NPs by using the laser ablation of solids in liquids (LASL) technique and they lack the explanation for the oxidation process. This work reports on the generation of NPs composed of molybdenum trioxide hydrated (MoO3 · xH2O) (x = 1, 2) by using this method and its oxidation due to aging. A picosecond Nd:YAG laser was used and the per pulse laser fluence was varied from 5 to 20 J/cm2. Spherical NPs were obtained with average diameters from 48 to 141 nm, respectively. The absorption evolution of the obtained colloids was characterized by optical absorption spectroscopy, TEM was used to study the MoOx NPs morphology, size and structure and Raman spectroscopy to determine the material chemical composition.

Femtosecond laser assisted generation of micro-dimples on moly-chrome film for improving its tribology

Femtosecond laser ablation of the moly-chrome deposited piston ring and the influence of laser fluence and number of laser pulses on dimple morphology and geometry were analysed. The increase of laser fluence and pulse numbers beyond 8.4 J/cm2 affected the dimple surface by inducing severe cracks due to heat accumulation. Based on the experimental investigation, the ablation threshold for moly-chrome film was found to be 0.36 J/cm2. Textured surface was formed on the moly-chrome film with laser fluence near the ablation threshold and the tribological characteristics were studied. The piston ring surface was textured with dimple diameter of 100 μm and dimple aspect ratio of 0.2. Effect of different dimple area ratios, such as 2%, 16% and 38% were analysed by reciprocating type tribology testing. The textured surface with 16% dimple area ratio showed a reduction in friction compared to the plain and the remaining textured piston rings.

飞秒脉冲激光对硅基多层膜损伤特性

In order to understand the ultrafast laser-induced damage mechanisms of typical imaging sensor's film structures, the damage characteristics of Si-based multi-layer films irradiated by a femtosecond pulsed laser were investigated, and the laser pulse fluence ranges and threshold conditions corresponding to various damage phenomena were evaluated. Si-based multi-layer films that were similar in structure of CCD were prepared by electron beam deposition. The damage characteristics of these films irradiated by a femtosecond pulsed laser with wavelength of 800 nm and pulse width of 100 fs under different pulse fluences and numbers were investigated using a metallurgical microscope. Experimental results showed that the laser-affected zone size increased linearly with pulse fluence in the range of 1.01 to 24.7 J/cm2. Surface damage caused by oxidation/amorphization, non-thermal ablation, and laser-induced plasma ablation could be observed in the laser irradiated zone, which tightly depended on the pulse fluence. Multi-layer damage could be observed and the damage probability increased from 1% to 51% in the pulse fluence range from 2.42 to 24.7 J/cm2. Irradiated by sequent pulses at a fluence of 1.01 J/cm2, the laser affected zone remained almost unchanged and the ablated depth increased with the pulse number. From the single pulse damage experiment data, the femtosecond pulse laser-induced surface damage threshold was evaluated to be 0.543 J/cm2 and laser-induced multi-layer stress damage threshold was linearly fitted to be 2.16 J/cm2. Sequent pulse irradiation with low fluence(≤ 1.01 J/cm2) also could lead to deep damage on the multi-layer film.

Direct Laser Fabrication of Cylindrical Lenses in Wide Band Gap Materials

The Fluorine laser microstructuring technique enables the direct laser fabrication of cylindrical lenses and lens arrays thereof in wide band gap materials. For the mask projection technique, a special mask geometry was calculated, which allows the fabrication of cylindrical lenses with a nearly optimum curved surface. Based on our results of processing fused silica and borosilicate glass, we investigated the possibility to apply these microstructuring technique to wide band gap materials like calcium fluoride. The radius of curvature (ROC) can be adjusted by the process parameters laser pulse fluence and pulse-to-pulse overlap in a range of 130 to 450 µm micrometer. A minimal surface roughness of 100 nm RMS can be reached.

Optically induced magnetization reversal in [Co/Pt]N multilayers: Role of domain wall dynamics

All-optical switching (AOS) of magnetization in ferri- and ferromagnetic thin films has in recent years attracted a strong interest since it allows magnetization reversal in the absence of applied magnetic field. Here we investigate AOS in ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ multilayers. The coercivity (${H}_{C}$) of the multilayers was tuned either by varying the bilayer repetition number ($N$) or the sample temperature ($T$). During the AOS experiments, we first illuminated the multilayers by a sequence of femtosecond laser pulses with varying fluence, light polarization, and repetition rate. The optically affected area was then imaged with magneto-optical Kerr microscopy. Our results indicate that the optical pulses can trigger either AOS or initiate an all-optical domain formation (AODF). The laser fluence required for AOS scales linearly with ${H}_{C}$ and depends on a precise tuning of laser pulse fluence, repetition rate, and light polarization. Furthermore, the magnetic response of the samples at a varying ambient temperature (down to 50 K) and for different time intervals between subsequent laser pulses point to the crucial role of domain wall dynamics in optical control of magnetization in ferromagnetic multilayers.

Laser surface texturing of copper and variation of the wetting response with the laser pulse fluence

We report an experimental investigation on laser surface texturing of copper targets by Ti:Sa femtosecond laser pulses addressing their wetting response to water droplets. In particular, fs laser surface processing is used to developed hierarchical surface structures by writing parallel micro-trenches with a period of 50 μm at different laser pulse fluences. The laser irradiation simultaneously induces both the formation of laser induced periodic surface structures (LIPSS), in form of periodic ripples, and the random decoration with nanoparticles, resulting in the formation of a multiscale surface morphology. The morphological features of the samples are investigated and correlated with their wetting response through static contact angle measurements. Our findings evidence a progressive increase of the contact angle with the laser pulse fluence. The combination of the microscale trenches, written by laser line scanning, with the ripples patterns and the random nanoparticles decoration, formed on the surface, allow developing highly hydrophobic copper samples with contact angles reaching values around 160°, presenting potential interest for wettability applications.

Laser Fluence Dependence of the Electrical Properties of MoO2 Formed by High Repetition Femtosecond Laser Pulses

Molybdenum oxides have gained attention in the last few years due to their vast variety of polymorphs. These materials relate to technological applications in several devices to exploit their chromic and electrical features, among others. Molybdenum oxide (MoO2 and Mo4O11) tracks are obtained in molybdenum thin films, deposited on glass substrates, by a previously reported (by our research group) optical technique based on femtosecond pulses from a Ti:sapphire laser oscillator. The present work reports on both the electrical resistance and resistivity of MoO2 tracks as a function of the per pulse laser fluence (Fp) used for the oxide synthesis. It is found that the electrical resistance, as well as the resistivity, of the MoO2 tracks drops as the delivered laser fluence is increased. The resistivity was determined to drop from 1.7 × 10−3 Ω cm to 5 × 10−4 Ω cm. This result agrees well with resistivity measurements reported in the literature for MoO2 nanosheets and films, respectively. This is explained by the fact that at low laser fluence the MoO2 forms a very thin surface layer, while for high laser fluences the MoO2 will get thick.

Hole-like surface morphologies on the stainless steel surface through laser surface texturing underwater

Laser-matter interaction in the liquid environment can produce some special structures on the target surface under certain conditions. In this paper, group micro holes have been fabricated on 304 stainless steel surfaces through laser surface texturing underwater with nanosecond laser. The effects of pulse duration, scanning speed, laser fluence and scanning times on the surface morphologies have been investigated. The surface morphologies have been characterized through scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM). Energy dispersive spectrometer (EDS) has been used to analyze the chemical compositions within the holes. For further clarifying the forming mechanisms of micro holes, high-speed camera was used to monitor the texturing process. The main conclusions are as follows. The group micro holes were fabricated through the water jet interaction due to laser-induced bubble collapsing. The sizes of these micro holes can reach below 2 μm. Increasing laser pulse width, the sizes of holes will increase accordingly due to laser-induced bigger bubbles. Before the target surface is full of the micro holes, lower laser scanning speed, higher laser fluence and more scanning times produce more micro holes on the target surface. While after the hole number is saturated, further increasing laser fluence and scanning times, or decreasing laser scanning speed tend to deepen the holes or make their inner surfaces smoother due to the multiple impingement of water jet, rather than increase the number of the micro holes. With this method, group through holes with a diameter of a few microns have been successfully prepared on the 304 stainless steel foil. So besides surface texturing, this method also demonstrate another great potential application in the field of drilling ultra-small through holes.

A photoacoustic pulse-echo probe for monitoring surface stone mechanical properties: Validation tests in consolidation of Carrara marble

The present work focuses on the development and validation of a laser-based photoacoustic sensor for non-destructive monitoring of surface stone mechanical properties. The sensor is constituted of a carbon black target excited by fiber-coupled Q-Switched Nd:YAG(1064 nm) laser with variable pulse duration between 10 and 50 ns, a superimposed PVDF film transducer, and gel coupler. The high optical absorption of the target and its relatively large lateral size allowed generating planar longitudinal pressure transients of amplitude around 1 MPa using a laser pulse fluence of about 8 mJ/cm2. The probe provided an almost flat spectral response over a bandwidth of 7.5–30 MHz. Thanks to the high pulse-to-pulse laser emission stability, the pressure transients were highly reproducible in amplitude and waveform. These features and the FFT analysis of the back reflected signal were exploited to characterize the surface mechanical properties of decayed Carrara marble upon consolidation. Treatment tests were carried out using three commercial products: acrylic-siloxane polymer, a fluoroelastomer-acrylic polymer, and ethyl-silicate. The acoustic reflection and then surface sound speed spectra were measured and analyzed for the first time within a spectral window of 1–30 MHz. Surface speed spectra and dephasing derived from reflectance measurements resulted to be essential parameters for characterizing and monitoring stone consolidation. The results achieved provide evidence that the technique proposed offers significant advantages with respect to traditional approaches and can represent an effective tool in conservation-restoration of stone artifacts, wall paintings, and other.

The influence of hohlraum dynamics on implosion symmetry in indirect drive inertial confinement fusion experiments

High laser energy (>1.2 MJ) implosion experiments on the National Ignition Facility show that low mode implosion symmetry is highly dependent on an expanding high-Z wall “bubble” plasma feature. The bubble is caused by the early time deposition of laser beams incident on the interior near the entrance of the cylindrical hohlraum (outer cone beams). It absorbs beams designated for the waist of the hohlraum (inner cone beams) causing a redistribution of x-ray flux on the capsule. From measurements, we are able to quantify the absorption and expansion of this bubble. Measurements show that the resulting hot spot is more oblate when there is more inner beam absorption in the bubble. We find absorption in the bubble to be between 51 ± 3% and 62 ± 2%. This bubble absorption is found to evolve predictably as a function of the early time outer cone laser pulse fluence and the pulse length. From this, a phenomenological model of the effective drive symmetry and subsequent implosion shape is found indicating a very strong dependence of implosion shape on early time laser fluence and laser pulse duration.

Fabrication of self-assembled spherical Gold Particles by pulsed UV Laser Treatment

We report on the fabrication of spherical Au spheres by pulsed laser treatment using a KrF excimer laser (248 nm, 25 ns) under ambient conditions as a fast and high throughput fabrication technique. The presented experiments were realized using initial Au layers of 100 nm thickness deposited on optically transparent and low cost Borofloat glass or single-crystalline SrTiO3 substrates, respectively. High (111)-orientation and smoothness (RMS ≈ 1 nm) are the properties of the deposited Au layers before laser treatment. After laser treatment, spheres with size distribution ranging from hundreds of nanometers up to several micrometers were produced. Single-particle scattering spectra with distinct plasmonic resonance peaks are presented to reveal the critical role of optimal irradiation parameters in the process of laser induced particle self-assembly. The variation of irradiation parameters like fluence and number of laser pulses influences the melting, dewetting and solidification process of the Au layers and thus the formation of extremely well shaped spherical particles. The gold layers on Borofloat glass and SrTiO3 are found to show a slightly different behavior under laser treatment. We also discuss the effect of substrates.

High repetition-rate femtosecond stimulated Raman spectroscopy with fast acquisition.

Time-resolved femtosecond stimulated Raman spectroscopy (FSRS) is a powerful tool for investigating ultrafast structural and vibrational dynamics in light absorbing systems. However, the technique generally requires exposing a sample to high laser pulse fluences and long acquisition times to achieve adequate signal-to-noise ratios. Here, we describe a time-resolved FSRS instrument built around a Yb ultrafast amplifier operating at 200 kHz, and address some of the unique challenges that arise at high repetition-rates. The setup includes detection with a 9 kHz CMOS camera and an improved dual-chopping scheme to reject scattering artifacts that occur in the 3-pulse configuration. The instrument demonstrates good signal-to-noise performance while simultaneously achieving a 3-6 fold reduction in pulse energy and a 5-10 fold reduction in acquisition time relative to comparable 1 kHz instruments.

Study of LIPSS formation on bismuth thin films deposited by Pulsed Laser Deposition

In this work, we report the fabrication of bismuth thin films on glass substrates by Pulsed Laser Deposition. A focused beam of a Nd:YAG laser (9 ns, 10 Hz, 1064 and 532 nm) was utilized to carry out the ablation of a high purity Bi target. The per pulse laser fluence was varied between 0.95 and 1.35 J/cm2, depending on the laser wavelength used. The deposits were characterized by SEM and XRD to analyze the surface morphology of thin films and their crystalline structure, respectively. The SEM micrographs showed differences in roughness, which increased with the laser wavelength. The diffractograms revealed the formation of Bi thin films with a polycrystalline structure or with a preferential orientation, depending on the mean kinetic energy of the plasma ions. The as deposited bismuth thin films were irradiated with a non-focused laser beam at low energies below the ablation threshold. In the laser treated area, it could be observed the formation of periodic structures, known as LIPSS (laser induced periodic surface structures). Depending on the crystallographic structure of the irradiated film, the formed LIPSS showed different characteristics, such as width and shape.