Threshold Fluence Research Articles

Cumulative material damage from train of ultrafast infrared laser pulses

We developed a systematic experimental method to demonstrate that damage threshold fluence (DTF) for fused silica changes with the number of femtosecond laser (800 nm, $65\pm 5~\text{fs}$, 10 Hz and 600 Hz) pulses. Based on the experimental data, we were able to develop a model which indicates that the change in DTF varies with the number of shots logarithmically up to a critical value. Above this value, DTF approaches an asymptotic value. Both DTF for a single shot and the asymptotic value as well as the critical value where this happens, are extrinsic parameters dependent on the configuration (repetition rate, pressure and geometry near or at the surface). These measurements indicate that the power of this dependence is an intrinsic parameter independent of the configuration.

Influence of processing parameters on characteristics of laser-induced periodic surface structures on steel and titanium

Laser-induced periodic surface structures (LIPSS) can appear due to interaction between polarized laser pulses and surface. They can be used to improve or change the surface functionality. Here, titanium and steel surfaces were irradiated by linearly polarized, 30-ps-laser pulses (wavelength of λ=1064 nm) and 45-ns-laser pulses (wavelength of λ=1060 nm), respectively. We investigated how pulse fluence, number of pulses, and pre-existing defects influence LIPSS formation and period. The results show that fluences lower than single-pulse fluence threshold for ablation lead to LIPSS with periods within 150-500 nm, while increased fluence results in larger spatial period between 800-1100 nm. A slight change in processing parameters can also result in different topographical and/or chemical characteristics of the top surface layer, making LIPSS undetectable by SEM.

Femtosecond laser induced nano-meter size surface structures on ZnSe film

We realize femtosecond laser-induced high spatial frequency nano-meter size periodic surface structures with a periodicity about 140nm-170 nm on ZnSe film in one step. Compared with bulk ZnSe, the periodicity and fluence threshold of the surface structure on ZnSe film are smaller and lower. We propose the shortened melting duration, which caused by the higher thermal conductivity of the sapphire substrate, is the origin of the difference between the bulk ZnSe and ZnSe film. Surface capillary wave is easier frozen with shorter melting duration leaves smaller periodicity. Meanwhile, we also found stripes on the surface of sapphire with much lower threshold than previously reported values. The results can benefit advanced optoelectronic device fabrication and fundamental research.

Measuring the ablation threshold fluence in femtosecond laser micromachining with vortex and Bessel pulses.

Femtosecond laser micromachining holds significant promise for advanced manufacturing, however uptake has been limited by the low processing speed. Altering the beam shape from its typical Gaussian profile has been attempted to improve efficiency, however virtually all reliable methods for quantifying the efficiency assume a Gaussian beam shape. Here, we describe an approach for quantifying the ablation threshold fluence - the key parameter for comparing efficiency - suitable for weakly focused non-Gaussian beams. We successfully demonstrate this method for Bessel and vortex beams, finding that the ablation threshold depends not just on the material, but the beam shape as well.

Mass ablated controlled laser induced V2O5 plasma parameters for controllable VO2 films growth

The effect of the laser fluence on V2O5 plasma dynamics, composition, and ionization state was studied. By combining three plasma diagnostic techniques, fast imaging, optical emission spectroscopy, and Langmuir probe, two ablation regimes have been identified. These ablation regimes depend on the evolution of the amount of the ablated mass that was measured by the method of mass loss. The transition between the two regimes at 1.3 J cm−2. For fluences lower than this threshold value, the expansion velocity of the plasma elements, the plasma dimensions, and the ionic current increase rapidly, unlike the fluences higher than the threshold fluence. Reverse behavior was observed for the ablated mass. This effect of the ablated mass would have a significant impact on the control of the properties of vanadium dioxide layers deposited by pulsed laser deposition in a reactive atmosphere of oxygen.

Effect of the electronic kinetics on graphitization of diamond irradiated with swift heavy ions and fs-laser pulses

Diamond preliminary damaged with neutrons was irradiated with swift heavy ions (SHI, 1030 MeV 209Bi) decelerated in the electronic stopping regime as well as with fs-laser pulses. The initial excess electronic energy densities appearing in the nanometric vicinity of the SHI trajectories and within the absorbing layers in laser spots were comparable (∼1024 eV cm−3).Graphitization of diamond in the central parts of the lased spots was observed above the threshold fluence of 15–30 J/cm−2. It was also found that the lower threshold fluence is required for initiating graphitization as well as destruction of the pre-damaged crystal by laser pulses in comparison to that for undamaged diamond. This indicates a noticeable effect of an existing defect ensemble on the kinetics of diamond transformations in laser spots.However, X-ray diffraction, atomic-force microscopy, and electron microscopy detected no graphitic domains within the SHI-irradiated pre-damaged crystal.The research demonstrated that the density of the initial excess electronic energy cannot be treated as the sole parameter governing subsequent structure transformations in diamond. Large differences between the spatial as well as temporal scales finally results in different pathways of the relaxation kinetics of this excess energy in laser spots and SHI tracks in diamond.

Enhancement of pulsed laser ablation assisted with continuous wave laser irradiation

Continuous wave (CW) laser irradiation is employed to enhance the pulsed laser ablation of silicon and stainless steel (316L) samples. Different surface temperatures generated by the CW laser irradiation are set as the initial working circumstance for the pulsed laser ablation. The diameter and depth of laser-ablated craters are measured to study threshold fluence, pulse incubation coefficient and ablation rate under different surface temperatures. Numerical simulation employing Heat Transfer in Solid and Deformed Geometry Interface modules in COMSOL is performed to estimate ablation rate theoretically based on Hertz-Knudsen equation. The realized crater-related data are analyzed to further obtain their dependences on surface temperature. The parametric and morphological studies indicate that the weakened plasma shielding effect and thermal diffusion in the ablated region induced by the CW laser irradiation lead to the enhanced pulsed laser ablation significantly.

Ultrashort pulsed laser ablation of stainless steels

Abstract This research investigated the ablation process of commercial stainless steels with ultrashort pulsed laser. Square hollows were ablated on stainless steel sheets using a picosecond laser with pulse duration of 0.25, 1 and 10 ps, respectively, and fluence ranging from 0.125 J/cm2 to 5 J/cm2. For each setting the surface quality and ablation efficiency were determined by optical microscopy and scanning electron microscopy (SEM). Processing windows producing high quality surfaces at high ablation efficiency were found at a fluence around 0.75 J/cm2 for the two shorter pulse lengths tested. Individual cones and periodic cone-like protrusions were found in the low fluence regime ( 0.875 J/cm2), respectively, both of which make the ablated surface rough. Emphasizing on the individual cones, it is found that the cones are caused by inclusions in the base materials, attributed to a higher fluence threshold required for ceramic-like inclusions. This novel theory explaining the creation mechanism is verified by multiple analysis methods like SEM and energy dispersive X-ray spectroscopy and stop-motion SEM imaging.

Long-Term Influence of Laser-Processing Parameters on (Super)hydrophobicity Development and Stability of Stainless-Steel Surfaces.

Controlling the surface wettability represents an important challenge in the field of surface functionalization. Here, the wettability of a stainless-steel surface is modified by 30-ns pulses of a Nd:YAG marking laser (λ = 1064 nm) with peak fluences within the range 3.3–25.1 J cm−2. The short- (40 days), intermediate- (100 days) and long-term (1 year) superhydrophilic-to-(super)hydrophobic transition of the laser-textured surfaces exposed to the atmospheric air is examined by evaluating its wettability in the context of the following parameters: (i) pulse fluence; (ii) scan line separation; (iii) focal position and (iv) wetting period due to contact angle measurements. The results show that using solely a short-term evaluation can lead to wrong conclusions and that the faster development of the hydrophobicity immediately after laser texturing usually leads to lower final contact angle and vice versa, the slower this transition is, the more superhydrophobic the surface is expected to become (possibly even with self-cleaning ability). Depending on laser fluence, the laser-textured surfaces can develop stable or unstable hydrophobicity. Stable hydrophobicity is achieved, if the threshold fluence of 12 J cm−2 is exceeded. We show that by nanosecond-laser texturing a lotus-leaf-like surface with a contact angle above 150° and roll-off angle below 5° can be achieved.

Effect of chemical composition on the structure, optical properties and femtosecond laser ablation performance of Ge-Sb-Se glasses

The effects of chemical composition on the physical and optical properties and femtosecond laser ablation performance of Ge–Sb–Se glasses with compositions of Ge18SbxSe82-x [x = 5, 10, 15, 17.5, 20, 21.5, 23, 25 (series A)] and GexSb25Se75-x [x = 10, 12.5, 15, 18, 20 (series B)] were systematically studied. With the increase in Sb and Ge in the series A and B glasses, respectively, the glass transition temperature increased at first and then decreased with increasing mean coordination number. The density and refractive index of the sample were mainly affected by Sb content. The third-order nonlinearity was measured at 3.5 μm by Z-scan technique. Raman spectra analysis showed that the proportion of Se–Se bonds declined and that of Sb–Sb and Ge–Ge bonds increased with the increase in Sb or Ge content. The enhancement of nonlinearity was associated with the presence of Sb–Sb homopolar bonds. Series A and B samples were irradiated by femtosecond laser (150 fs, 1 kHz) at 3 μm with different laser powers. The damaged area increased and ablation threshold fluence (Fth) was reduced gradually with the increase in Sb or Ge concentration. Thus, the addition of Sb or Ge can reduce the resistance of Ge–Sb–Se glasses from laser damage.

Epidermal Fluence Threshold Determination by Real-Time Melanin Measurements.

Epidermal preservation is essential during laser treatment for vascular, hair, and benign pigment dyschromias. Epidermal tolerance is determined by epidermal melanin content, fluence, pulse width, wavelength, skin cooling, and spot size. The authors' objective was to determine the maximum epidermal tolerance for the long-pulse alexandrite 755 nm and the long-pulse neodymium-doped yttrium aluminum garnet (Nd:YAG) 1064-nm lasers for varying epidermal melanin content. Skin melanin measurements were performed at the test sites with a melanin reader, and 0.5 to 1 second of refrigerated air precooled the skin. Then, alexandrite and Nd:YAG laser test spots of 5 to 18 mm were delivered in a series of ascending fluences using 5-, 20-, and 50-ms pulse widths. Skin response at 24 to 48 and 96 hours was scored from 0 to 15 varying from "no reaction" to "severe scabbing." Alexandrite laser, mean threshold fluences increased by a factor of 1.2 increasing from 5 to 20 ms, and by a factor of 1.4 increasing from 5 to 50 ms, among subjects with a melanin index (MI) from 9 to 25 (Fitzpatrick skin phototype I-III). The Nd:YAG fluence to reach epidermal tolerance was 6X the fluence with the alexandrite laser for the same MI in subjects with MI 26 to 35. Epidermal melanin measurements are quantitative and objective, therefore, improving treatment setting determination by decreasing the risk of overtreatment or undertreatment.

Formation of composite material based on proteins, chitosan and nanotubes by nanosecond laser pulses

Aqueous dispersions with albumin, collagen, chitosan and single-walled carbon nanotubes (SWCNTs) in different combinations were studied. It was found, that an increase in the fluence above threshold leads to a significant increasing in the total absorption of radiation, moreover the main part of the energy was taken up by the SWCNTs. Thus, the thermal effect on proteins and chitosan decreased. This was confirmed by the nonlinear absorption coefficients, which were determined for the studied aqueous dispersions. Low threshold values of fluence for albumin 0.006 J/cm2 and collagen 0.009 J/cm2 are due to the onset of the denaturation process. This parameter increased up to ~0.05 J/cm2 after addition of SWCNTs that was caused by the formation of microbubbles due to the water evaporation from the heating of nanotubes. The reverse situation occurs in samples with chitosan (0.2 J/cm2) in which there was no denaturation. Adding SWCNTs leads to a decrease in the threshold fluence threshold to 0.15 J/cm2, which can be explained by the rapid heat transfer from nanotubes compared with chitosan. It was established, that optimal energy density range from 1 to 50 J/cm2 for the formation of a composite material using laser pulses with duration 16 ns.

Optical and thermal simulations for the design of optodes for minimally invasive optogenetics stimulation or photomodulation of deep and large cortical areas in non-human primate brain

The use of optogenetics or photobiomodulation in non-human primate (NHP) requires the ability to noninvasively stimulate large and deep cortical brain tissues volumes. In this context, the optical and geometrical parameters of optodes are critical. Methods and general guidelines to optimize these parameters have to be defined. Objective. We propose the design of an optode for safe and efficient optical stimulation of a large volume of NHP cortex, down to 3–5 mm depths without inserting fibers into the cortex. Approach. Monte Carlo simulations of optical and thermal transport have been carried out using the Geant4 application for tomographic emission (GATE) platform. Parameters such as the fiber diameter, numerical aperture, number of fibers and their geometrical arrangement have been studied. Optimal hardware parameters are proposed to obtain homogeneous fluence above the fluence threshold for opsin activation without detrimental thermal effects. Main results. The simulations show that a large fiber diameter and a large numerical aperture are preferable since they allow limiting power concentration and hence the resulting thermal increases at the brain surface. To obtain a volume of 200–500 mm3 of brain tissues receiving a fluence above the opsin activation threshold for optogenetics or below a phototocixity threshold for photobiomodulation, a 4 fibers configuration is proposed. The optimal distance between the fibers was found to be 4 mm. A practical implementation of the optode has been performed and the corresponding fluence and thermal maps have been simulated. Significance. The present study defines a method to optimize the design of optode and the choice of stimulation parameters for optogenetics and more generally light delivery to deep and large volumes of tissues in NHP brain with a controlled irradiance dosimetry. The general guidelines are the use of silica fibers with a large numerical aperture and a large diameter. The combination of several fibers is required if large volumes need to be stimulated while avoiding thermal effects.

Prediction of femtosecond laser ablation profile on human teeth.

To predict the laser ablation profile on dental hard tissue which will enable the user to optimize laser parameters so as to carry out the laser treatment with minimal tissue damage. The present study constructs a mathematical model to predict the ablation profile based on Gaussian beam distribution of laser intensity and correlates the model with experimentally obtained ablation parameters (effective Gaussian beam radius, ablation threshold fluence, and effective energy penetration depth). To obtain the ablation parameters, laser ablation experiments were carried out on dental hard tissues using Ti:Sapphire femtosecond laser (800nm, 100fs, 10kHz). The method is further extended to predict the ablation rate and efficiency as well. The profile predicted from the mathematical model is compared with that of experimental results. It is found that the predicted ablation profile agrees well with the experimental profile for both enamel and dentin except a slight deviation at higher fluence for dentin. The calculated ablation rate is comparable to that of experimental results whereas for ablation efficiency appreciable deviation is observed in the case of dentin. The model succinctly predicts the ablation profile, ablation rate, and ablation efficiency which will enable to perform dental surgery at optimized laser processing conditions with high precision thus reducing the tissue damage appreciably. Once the details of lesion are known through proper diagnostic tools, the method enables the user to readily obtain optimum laser parameters. It can be used as a handy reference for dentists to perform damage-free surgery, ensuring quicker healing.

Damage Threshold of Ni Thin Film by Terahertz Pulses

Damage in a thin nickel film irradiated by subpicosecond pulses of terahertz (THz) radiation in the range of 1–3 THz at electric-field strengths up to 20 MV/cm at the center of a focal spot is observed. The damage threshold fluence is evaluated for single-pulse experiments. The damage pattern induced by multiple THz pulses has the appearance of a complex periodic structure in the form of elongated channels of metal film discontinuity that are perpendicular to the in-plane electric field direction of THz radiation.

Understanding the performance of corona aged epoxy nano micro composites

Corona ageing of the epoxy nanocomposites surface exhibits a high influence on contact angle of the material. A reduction in corona inception voltage due to water droplet, upon corona ageing, is less with epoxy composites, which has Wollastonite as filler followed with nano-micro silica filler added epoxy composites. Charge accumulation studies indicate that charge retention time drastically reduces with corona aged epoxy composite specimen. Epoxy composites with Wollastonite as filler have shown higher mean charge lifetime. Adoption of laser induced breakdown spectroscopy (LIBS) technique for characterisation of samples is unique. Measure of threshold fluence and plasma temperatures through LIBS studies enables to classify the ageing condition of the composite material. Plasma temperature and threshold fluence are clear indicators to classify different materials. Plasma temperature is also an indicator of the hardness of the material. Epoxy composite with Wollastonite as filler is not affected by laser abrasion, which is in accordance with its superior performance with corona ageing, proving as discharge resistant material.

Identification and formation mechanism of the transient ion fragments produced in laser-induced dissociation of 1, 1-diamino-2, 2-dinitroethylene

AbstractDetailed knowledge of dissociation behavior and dissociation products is necessary to understand the stability, sensitivity, and the reactive mechanism of explosives under laser initiation. A time-of-flight mass spectrometer was utilized to detect the transient products of 1,1-diamino-2,2-dinitroethylene (FOX-7) produced under 532 nm pulse laser ablation, the possible attribution of intermediate ion fragments were confirmed. The laser fluence threshold for detectable fragments is about 3.6 J/cm2. The peak intensities of main ions (CN, CNO/C2H4N, NO2, C2N2O, HCN, C2NH2, etc.) increase with the increasing of laser fluence, and reach the maximum at 11.5 J/cm2. Moreover, time-depend changes of ion intensity indicate that the type and degree of reactions are different in different periods. According to the molecular structure of FOX-7 and the intermediate ions, the laser-induced dissociation mechanisms were proposed to illustrate the cause of the fragments which might throw some light on the laser initiation of FOX-7.

Numerical study of material decomposition in ultrafast laser interaction with metals

A study of ultrafast laser-induced ablation of metals is presented based on an improved two-temperature model. Material decomposition and the resultant energy loss from the sample are considered through the dynamic description of the ablation process. The boiling temperature is adopted as the ablation initiation temperature to capture the contributions from multiple ablation mechanisms. Ablation occurrence period, ablation depth, threshold fluence, residual thermal energy and melting layer thickness have been studied for aluminum, copper, and gold. The simulation results agree well with the experimental measurements for different materials and laser parameters. The electron-phonon coupling strength and thermal conductivity are found to be critical factors determining ablation behaviors. With strong electron-phonon coupling and low thermal conductivity (like aluminum), ultrafast ablation can be triggered within hundreds of femtoseconds, leading to a more efficient laser energy deposition, high ablation depth, and thin melting layer.

Dual photoacoustic monitoring in laser ablation synthesis of silver nanoparticles to find in situ their fluence threshold formation

Abstract In this study, a dual pulsed photoacoustic technique was applied to monitor, in real time, the laser ablation synthesis in solution (LASiS) of silver nanoparticles. It was found that, at low optical fluence values, the ultrasound signal associated to the laser ablation process exhibited constant amplitude, independently of the ablation time. Further analysis confirmed that, under these conditions, the ablation process provided only suspended micro-sized Ag fragments. However, fluence values above 400 mJ cm−2 promoted the formation of nanoparticles, accompanied by amplitude changes in the photoacoustic signal. Determination of this threshold condition establishes a fingerprint in the photoacoustic signal that can be used to identify, in situ and in real time, the beginning of nanoparticle synthesis. Supplementary, the nanoparticle formation was also photoacoustically monitored during the ablation process introducing an additional laser pulse train of low fluence value propagating perpendicularly to the pulse train used for ablation. These laser pulses did not interfere with the nanoparticle LASiS process, but renders an ultrasound signal enhancement due to the surface plasmon resonance of metal nanoparticles. The characterization of the LASiS process through the photoacoustic technique can provide information about nanoparticle concentration and other synthesis features of interest.

Investigation of Helium Behavior in RAFM Steel by Positron Annihilation Doppler Broadening and Thermal Desorption Spectroscopy

The behavior of helium in reduced-activation ferritic/martensitic steels was investigated systematically with positron annihilation Doppler broadening measurement and thermal desorption spectroscopy. Specimens were irradiated with helium ions with different energies to various fluences at different temperatures. A threshold fluence was observed above which the rate of formation and growth of helium bubbles dramatically increased. Irradiation at higher temperature could suppress the formation and growth of HenVm clusters with low binding energies and enhance that of helium bubbles and HenVm clusters with high binding energies. Different changes of S parameters were observed in various depth after the irradiation temperature was increased from 523 K to 723 K. Irradiation of 18 keV-He+ enhanced the growth of HenVm clusters and helium bubbles compared with 100 keV-He+ irradiation. A possible mechanism is discussed.