Threshold Laser Fluence Research Articles

Investigation of the laser fluence and wavelength dependence in surface-assisted laser desorption/ionization mass spectrometry using gold nanoparticles.

Surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) builds on the use of nanostructured surfaces (e.g., coatings of colloidal nanoparticles) to promote analyte desorption and ionization. The SALDI process is believed to occur mainly through thermal processes, resulting from heating of the nanosubstrate upon absorption of the photon energy, and by assisting ionization steps. Mostly due to the accessibility of the respective hardware, the majority of SALDI-MS studies use standard laser wavelengths for MALDI (i.e., 337 or 355 nm), even though peak absorption of the SALDI nanosubstrate might completely differ from these values. Here, we investigated the wavelength dependence in SALDI-MS to determine if wavelength adjustment would be beneficial, and to provide new experimental data for a better understanding of the SALDI mechanism. To this end, gold nanoparticles (AuNPs) sprayed onto microscope glass slides were employed as SALDI nanosubstrates and L-arginine as a model analyte. In addition, we used 2,5-dihydroxyacetophenone (2,5-DHAP) for classical MALDI-MS using the same experimental setup. Arginine ion signals were recorded as a function of laser wavelength and laser fluence. Mass spectra were acquired in the wavelength range between 310 and 630 nm, including the absorption maximum of the sprayed AuNPs around 550 nm and that of 2,5-DHAP around 380 nm. Laser fluence thresholds for the generation of arginine ions were found to be dependent on the laser wavelength and to inversely correlate with the absorbance profiles of the deposited AuNPs and 2,5-DHAP, respectively. Very differently to MALDI, in SALDI ionization efficiency was found to strictly linearly decrease with increasing laser wavelength. Our results, therefore, corroborate the general assumption that material ejection in SALDI-MS is mainly driven by thermal processes in the low laser fluence range and add new evidence that the ionization process is directly influenced by photon energy when AuNPs are employed as nanosubstrates.

Femtosecond pulse train-facilitated periodic nanostructuring on TiN films via laser-oxidation

Laser-induced periodic surface structure is a universal phenomenon observed when solid materials are irradiated with pulsed lasers. Typically, these structures are generated using Gaussian pulses. Here, we investigate the formation of periodic nanostructures on titanium nitride thin films using two different laser sources: Gaussian pulses and femtosecond pulse trains generated through a Fabry-Perot cavity. Our findings indicate that the formation of self-organized nanostructures on the thin films can occur through either laser-induced oxidation or laser ablation processes, depending on laser fluence, scanning speed, and pulse duration. Interestingly, when utilizing femtosecond pulse trains, we observe that the range of laser fluence thresholds for oxidative nanostructure formation is significantly wider compared to Gaussian pulses. To comprehend the underlying mechanisms driving these observations, we employ a two-temperature model to calculate the ultrafast dynamics of hot electrons when exposed to laser irradiation. Our results reveal that the initial sub-pulse generates a substantial population of hot electrons, exciting surface plasmons and resulting in the creation of a periodic energy distribution. Before these hot electrons can dissipate, subsequent sub-pulses reach the thin film, re-exciting a large number of hot electrons. Consequently, the periodic energy distribution persists for an extended duration, creating more favorable conditions for the formation of periodic oxidative nanostructures when using pulse trains. These findings contribute to a comprehensive understanding of the mechanisms involved in the formation of periodic nanostructures under femtosecond laser irradiation.

Study on Radiation Damage of Silicon Solar Cell Electrical Parameters by Nanosecond Pulse Laser

This experimental study investigates the damage effects of nanosecond pulse laser irradiation on silicon solar cells. It encompasses the analysis of transient pulse signal waveform characteristics at the cells’ output and changes in electrical parameters, such as I–V curves before and after laser irradiation under varying laser fluence and background light intensities, and explores the underlying action mechanisms of laser irradiation. The study reveals that as the laser fluence increases up to 4.0 J/cm2, the peak value of the transient pulse signal increases by 47.5%, while the pulse width augments by 88.2% compared to the initial transient pulse signal. Furthermore, certain parameters, such as open-circuit voltage, short-circuit current, and peak power obtained, from the measured I–V curve indicate a threshold laser fluence for functional degradation of the solar cell at approximately 1.18 ± 0.42 J/cm2. Results obtained from laser irradiation under different background light intensities underscore the significant influence of background light on laser irradiation of silicon cells, with the most severe damage occurring in the absence of light. Moreover, findings from laser irradiation at multiple locations on the silicon cell demonstrate a linear decrease in the output voltage of the silicon cell with an increase in the number of irradiation points.

Minimally destructive laser-induced breakdown spectroscopy of brass assisted by a low-power atmospheric pressure plasma jet

Minimizing sample damage is crucial in laser-induced breakdown spectroscopy (LIBS) for applications involving valuable samples and elemental mapping. In this study, we introduced a low-power atmospheric pressure plasma jet (APPJ) to reduce sample damage by obtaining LIBS signals at significantly lower laser fluences. The proposed technique, APPJ-assisted LIBS (APPJ-LIBS), utilized an argon APPJ to provide seed electrons and enhance the excitation. The APPJ was generated by a 10 kHz alternating current power supply and made contact with the surface of a brass sample at a 30° angle. An infrared nanosecond Nd:YAG laser was focused onto the contacting zone, allowing the resulting laser-induced plasma to evolve within the surrounding APPJ and produce optical emission. The optimized APPJ-LIBS system reduced the laser fluence threshold for spectral detection of the brass sample by 97 %, from 1.43 J/cm2 to 0.05 J/cm2, which represented the lowest laser fluence threshold reported in LIBS studies on copper-based materials. Micrographs of the sample surface showed no visible damage after the APPJ-LIBS measurement at a near-threshold laser fluence and an APPJ input power as low as 6.0 W. Furthermore, gated images showed the plasma evolution in APPJ-LIBS and confirmed the excitation capability of the APPJ for the laser-ablated materials.

A study beyond laser fluence threshold on WO3 nanoparticle, employing pulsed laser ablation in liquid

A study beyond laser fluence threshold on WO3 nanoparticle, employing pulsed laser ablation in liquid

3D Printing of LuAG:Ce transparent ceramics for laser-driven lighting

The fabrication of transparent ceramics with freely designed geometries via 3D printing represents a paradigm shift in molding processes. The application of luminescent transparent ceramics (LTCs) in high-power laser-driven solid-state lighting has been significantly limited by traditional molding methods. Herein, dense cerium activated lutetium aluminum garnet (LuAG:Ce) transparent ceramics with good light transmittance (∼40%) and complex structures were successfully fabricated using a digital light processing 3D printing method assisted by vacuum sintering. We developed an ink with a high solid content (∼50 vol%) and good shear thinning property, and the optimal dye content (0.002 wt%) and exposure dose (80 mJ cm−2) to achieve 50 μm high-resolution printing were determined. A superhemispherical ceramic device with a convex curvature radius (R) of 11 mm, a concave curvature radius (r) of 10 mm, and a thickness of 1 mm was printed and sintered, which exhibited excellent laser-driven lighting properties and a high laser fluence threshold (19.22 W mm−2) owing to the special structure of surface guide grooves formed by 3D printing. Optical components prepared using 3D-printed LuAG:Ce LTCs with excellent physical and chemical properties have promising applications in next-generation high-power laser-driven solid-state lighting devices.

Numerical study of ultrashort laser-induced microjet formation on the metal film based on the Navier–Stokes equation

A two-temperature model (TTM) for the electron-phonon thermal equilibrium is used to determine the heat distribution and laser fluence threshold for melting a thin metal film coated on a glass substrate and irradiated by an ultrashort laser pulse. This study proposes a novel model based on the Navier–Stokes equation to explain the formation of jet-shaped structures in the film's molten region. By solving this equation and obtaining the temporal evolution of the velocity distribution and displacement in the molten region, the Marangoni convection effect can be numerically demonstrated, and the circular motion of the fluid can describe the formation of a jet-shaped structure in the central region of the radiation. The results are compared to those obtained by numerically solving the thermo-elastoplastic equations, and also, to the previously reported experimental results to ensure the accuracy of the microjet height calculated by the Navier–Stokes equation. Good agreement is observed, particularly when the temperature of the irradiated area is significantly over the film's melting temperature. In addition, several calculations are performed for various pulse fluences. In both models, increasing the pulse fluences leads to an increase in the height of microjets.

Transient dynamics of the phase transition in VO2 revealed by mega-electron-volt ultrafast electron diffraction

Vanadium dioxide (VO2) exhibits an insulator-to-metal transition accompanied by a structural transition near room temperature. This transition can be triggered by an ultrafast laser pulse. Exotic transient states, such as a metallic state without structural transition, were also proposed. These unique characteristics let VO2 have great potential in thermal switchable devices and photonic applications. Although great efforts have been made, the atomic pathway during the photoinduced phase transition is still not clear. Here, we synthesize freestanding quasi-single-crystal VO2 films and examine their photoinduced structural phase transition with mega-electron-volt ultrafast electron diffraction. Leveraging the high signal-to-noise ratio and high temporal resolution, we observe that the disappearance of vanadium dimers and zigzag chains does not coincide with the transformation of crystal symmetry. After photoexcitation, the initial structure is strongly modified within 200 femtoseconds, resulting in a transient monoclinic structure without vanadium dimers and zigzag chains. Then, it continues to evolve to the final tetragonal structure in approximately 5 picoseconds. In addition, only one laser fluence threshold instead of two thresholds suggested in polycrystalline samples is observed in our quasi-single-crystal samples. Our findings provide essential information for a comprehensive understanding of the photoinduced ultrafast phase transition in VO2.

Enhanced surface effects and optical property modulation of Ge2Sb2Te5 by pulsed laser irradiation

In this work, Ge2Sb2Te5 (GST) thin films are irradiated by a 1064 nm pulsed laser heat treatment system with different beam profiles. The surface effects induced by different laser conditions are studied systematically by atomic force microscope, spectroscopic ellipsometry, and Raman spectroscopy. It is found that a top-hat beam profile with uniform intensity distribution demonstrates the advantages of a non-destructive and homogeneous surface, which is critical for large-scale processing uniformity. The threshold laser fluence for the amorphization process is predicted by simulation and further proved by the laser irradiation experiment to be 27.9 mJ/cm2 at 1 ns pulse width. We further show that modulation of complex refractive indices of GST thin films can be achieved with different duty ratios (spatial ratio of amorphization part) from 0% to 100%. Our approach paves the way for the precise control of the optical properties of PCMs in emerging optical applications such as photonic switches, optical memories, and all-optical neural networks.

Ultrafast Laser Patterning of Metals Commonly Used in Medical Industry: Surface Roughness Control with Energy Gradient Pulse Sequences

Ultrafast laser ablation is widely used as a versatile method for accurate micro-machining of polymers, glasses and metals for a variety of industrial and biomedical applications. We report on the use of a novel process parameter, the modulation of the laser pulse energy during the multi-scan texturing of surfaces. We show that this new and straightforward control method allows us to attain higher and lower roughness (Ra) values than the conventional constant pulse energy irradiation sequence. This new multi-scanning laser ablation strategy was conducted on metals that are commonly used in the biomedical industry, such as stainless steel, titanium, brass and silver samples, using a linear (increasing or decreasing) gradient of pulse energy, i.e., varying the pulse energy across successive laser scans. The effects of ablation were studied in terms of roughness, developed interfacial area ratio, skewness and ablation efficiency of the processed surfaces. Significantly, the investigation has shown a global trend for all samples that the roughness is minimum when a decreasing energy pulse sequence is employed, i.e., the irradiation sequence ends up with the applied laser fluences close to threshold laser fluences and is maximum with increasing energy distribution. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis on single craters with the three different energy deposition conditions revealed a chaotic and random material redistribution in the cases of uniform and increasing energy distributions and the presence of regular laser-induced periodic surface structures (LIPSS) at the bottom of the ablation region in the case of decreasing energy distribution. It is also shown that the ablation efficiency of the ablated surfaces does not significantly change between the three cases. Therefore, this novel energy control strategy permits the control of the roughness of the processed surfaces without losing the ablation efficiency.

Influence of photothermal and plasma-mediated nano-processes on fluence thresholds for ultrafast laser-induced cavitation around gold nanoparticles.

Laser fluence thresholds of ultrafast excitation of vapor bubbles around gold nanoparticles are determined experimentally. An optical scattering technique of limited minimum bubble size resolution is employed and analyzed for that purpose. Measurements were performed for spherical gold nanoparticles of varying sizes (40-200 nm) and for laser pulses of varying pulse width (55 fs to 4.3 ps) to estimate the limits where the evaluated thresholds are attributed to either plasma-mediated or photothermal cavitation. Furthermore, thresholds were obtained by double 55 fs pulsed excitation (varying delay 0.0-4.3 ps), providing insights into the dynamics of the excited plasma. A relationship is established between particle properties, (size, near-field amplification factor, and absorption efficiency) and the crossover pulse width of the transition from plasma-mediated to photothermal cavitation. Further, by comparing theory and experiments, we examine the approximative optical breakdown density of ∼10-21 cm-3 at a distance of 1-2 nm from the particle surface as a criterion of plasma-mediated cavitation around gold nanoparticles in analogy to the spinodal criterion for photothermal cavitation. For a given pulse width, the breakdown density appears to be nearly size-independent, establishing the aforesaid criterion applicable. However, a small pulse width dependence of the breakdown density is still observed. Based on these criteria, a comparison is further provided between theoretical thresholds of cavitation and the ones of detectable bubbles. An increasing discrepancy is observed between them with decreasing size for the case of photothermal cavitation. For plasma-mediated cavitation, the latter discrepancy is seemingly smaller, presumably due to the highly nonlinear nature of the process.

An experimental study on laser ablation of Ultra-thin SiNx layer of PERC solar cell

In this work, a nanosecond green laser (532 nm) is used to generate narrow openings by removing an ultra-thin (85 nm) SiN x layer that is coated on a silicon substrate for application in the fabrication of Passivated Emitter and Rear Contact (PERC) solar cells. An experimental analysis is presented to identify the optimal range of laser parameters for an efficient ablation with minimal damage to the silicon substrate. The ablated samples were characterized using a 3D profilometer to obtain the surface profiles and scanning electron microscope imaging to observe the surface quality. Further, energy-dispersive X-ray line analysis and atom probe tomography were performed to evaluate the nitrogen content on the surface and along the depth, respectively. The experimental results suggest that the SiN x layer starts to ablate only above a threshold laser fluence of 1.4 J/cm2, while the surface bulged out for laser fluence slightly below the ablation threshold. The central part of the ablated region was clean with a negligible nitrogen concentration at the surface, about ∼0.03% at a fluence of 2.4 J/cm2. Nitrogen concentration reduces continuously and almost becomes zero at 80 nm depth, suggesting complete ablation of the SiN x layer for establishing electrical contacts. The ablation width was close to the laser spot diameter only at lower values of the laser fluence. The lowest value of ablation depth was about 180 nm, suggesting that only about 95 nm layer of the silicon is ablated. The study demonstrates that nanosecond laser ablation is a potential technique for ablation of the SiN x layer of PERC solar cells but requires choosing the optimal parameters.

Particle Generation from Aluminum Alloy Surfaces Irradiated by 355nm Pulsed Laser and Multi-shots Laser Pretreatment Method at Low Fluences

The studies focus on particle generation from aluminium alloy surfaces irradiated by a Q switched Nd:YAG laser at 355 nm wavelength at low fluencies. The particles number increase with laser fluence but reduces with pulse number. The average initial threshold laser fluence for particles generation is 26.64 mJ/cm2. Larger particle with a diameter more than 5μm are found for fluencies above 76.54 mJ/cm2. After 40 pulsed laser shots at 72 mJ/cm2, it can effectively reduce the generation of metal particles. Our results demonstrated that 7075 aluminum alloy has the best laser resistance and the method of multi-shots laser pretreatment can reduced the generated particles by 76.92%. The SEM results shows that laser pretreatment process lead the passivation effct which smoothed the microstructure on the initial surface. The XPS results shows that zinc aluminate was generated during laser pretreatment process which has good thermodynamic properties. This study has important value for reducing metal particle contamination due to laser irradiation on metal surface in high power laser facilities and improving the service life of optical components.

Femtosecond large-area fabrication of multi-phase titanium oxide LIPSS on thin films

The effect of the laser beam scanning velocity and overlap on the large-area fabrication of Laser-Induced Periodic Surface Structures (LIPSS), on 400 nm thickness titanium films, is investigated. The experiments were performed in ambient air with well below ablation threshold laser fluence by using a femtosecond laser at a wavelength of 1030 nm with a pulse duration of 270 fs, operating at a repetition rate of 20 kHz.For this purpose, single-line experiments were performed to study the LIPSS formation process as a function of fs-laser-beam scanning velocity. Based on these results, the fabrication of large-area LIPSS pattern was investigated by unidirectional scanning of the fs-laser beam across the sample surface using different lateral spacing. The results show the formation of LSFL and TLIPSS, with different orientation with respect to the polarization of the incident radiation. The analysis of the chemical composition, by microRaman spectroscopy, indicates the formation of a mixture of TiO2 (anatase phase) and Ti2O3 (rutile phase) for high scanning velocities and TiO2 (rutile phase) for low scanning velocities. The chemical composition, spatial period, and depth of the LIPSS show a clear dependence with the laser-beam scanning velocity and overlap.

Histological analysis of tattoo removal by water cavitation bubbles and jet formation using Nd: YAG and nanosecond laser pulses

Background: For efficient laser tattoo removal, photodisruption of tissue can ensure a very powerful means to shutter ink granules. At very high laser intensity level, photodisruption dominates and selective photothermolysis will have secondary effect in clearing the tattoo ink. Lower laser fluence is all what to trigger non-linear photons absorption and the generation of exploding cavitation bubbles that can tremendously hammer the ink granules.Subjects, materials, and methods: Three domestic white rabbits; each received simultaneous injections of a color pigment tattoo under general anesthesia, followed by a single session of (1064) Q.S Nd: YAG nanosecond laser pulses for tattoo removal.Results and Discussion: Spectroscopic properties of black, dark brown and red tattoo inks were studied. Near threshold laser fluence was selected to select the optimum conditions for obtaining scar-free treatment. Histological images of the biopsies, taken after thirty days of laser treatment of black, dark brown and red tattoos showed a marked reduction in pigment granules size with no appearance of hyperplasia or inflammatory cells. Coexistence of macrophages was suggested to be responsible for actively phagocytizing the laser-dispersed tattoo fragments.Conclusion: skin biopsies have demonstrated ink granules local redistribution. Photodisruption at 1064nm laser effectively targeted black and dark brown tattoo pigments by the generation of cavitation bubbles. The weaker laser light absorption of red pigments at 1064nm only showed tattoo clearance when using 532nm wavelength.

Multi-phase titanium oxide LIPSS formation under fs laser irradiation on titanium thin films in ambient air

The formation of laser-induced periodic surface structures (LIPSS) on Ti thin films, their phase and stoichiometric evolution, as a function of well below ablation threshold laser fluence, and the number of pulses, is investigated. The experiments were carried out in ambient air by using a femtosecond laser at a wavelength of 1030 nm with a pulse duration of 270 fs, operating at a repetition rate of 18.6 kHz. On the one hand, the formation of LSFL composed by an either single phase (C-Ti2O3 or r-TiO2-x) or a two-phase mixture of titanium oxide (C-Ti2O3 +r-TiO2-x) is reported. The titanium oxide phases were obtained by micro-Raman spectroscopy. The orientation of the LSFL, either parallel or perpendicular with respect to the polarization of the incident beam, correlate well with the electronic nature of the titanium oxide phases. On the other hand, the results also show the formation of HSFL, this under low cumulative fluence, with periods of 398 and 460 nm with a two-phase mixture of titanium oxide in the form ofa-Ti2O3+ C-Ti2O3and C-Ti2O3+ r-TiO2-x, respectively. To the best of our knowledge this is the first time that single phase C-Ti2O3 or r-TiO2-x LIPSS are reported.

Simple and robust method for determination of laser fluence thresholds for material modifications: an extension of Liu's approach to imperfect beams.

The so-called D-squared or Liu's method is an extensively applied approach to determine the irradiation fluence thresholds for laser-induced damage or modification of materials. However, one of the assumptions behind the method is the use of an ideal spatial Gaussian beam that can lead in practice to significant errors depending on beam imperfections. In this work, we rigorously calculate the bias corrections required when applying the same method to Airy-disk like profiles. Those profiles are readily produced from any beam by insertion of an aperture in the optical path. Thus, the correction method gives a robust solution for exact threshold determination without any added technical complications as for instance advanced control or metrology of the beam. Illustrated by two case-studies, the approach holds potential to solve the strong discrepancies existing between the laser-induced damage thresholds reported in the literature. It provides also an appropriate tool for new studies with the most extreme laser radiations.

A comparative study of KrF laser ablation of aluminum and alumina in oxygen ambiance

This paper is dedicated to a comparative study of aluminum and alumina plasmas, induced under the same conditions of laser fluence and oxygen gas pressure in the range of 0.02 to 5 mbar. In this comparative analysis, measurements of ablated mass versus laser fluence and optical emission spectroscopy diagnostic of the plasmas are studied. The results from ablated mass measurements reveal the existence of two regimes of ablation according to a laser fluence threshold for phase explosion mechanism. This laser fluence threshold was found different for Al and Al2O3. A difference in the amount of ablated mass/pulse is also observed and is assigned to the optical (reflectivity and absorption) and thermodynamical properties (evaporation temperature) of the targets. Below a certain laser fluence threshold, thermal and non-thermal processes are involved. Above this laser fluence threshold, the mechanism of ablation is assigned to phase explosion. For plasma diagnostic by optical emission spectroscopy, two species were followed; the neutral Al (396.15 nm) for its persistence in time and distance and the AlO (the band head emission at 484.21 nm) molecule that is probably the precursor in alumina thin films formation. The spectra revealed a disparity in the intensity emission of neutral Al and AlO molecules, which were, related to the target's properties. The origin of AlO molecules recorded in aluminum and alumina plasma under oxygen ambiance is discussed. Their translational temperature for neutral Al, molecular vibrational temperature, and free linear velocities, are also compared.

Simple and robust method for determination of laser fluence thresholds for material modifications: an extension of Liu’s approach to imperfect beams

The so-called D-squared or Liu’s method is an extensively applied approach to determine the irradiation fluence thresholds for laser-induced damage or modification of materials. However, one of the assumptions behind the method is the use of an ideal Gaussian profile that can lead in practice to significant errors depending on beam imperfections. In this work, we rigorously calculate the bias corrections required when applying the same method to Airy-disk like profiles. Those profiles are readily produced from any beam by insertion of an aperture in the optical path. Thus, the correction method gives a robust solution for exact threshold determination without any added technical complications as for instance advanced control or metrology of the beam. Illustrated by two case-studies, the approach holds potential to solve the strong discrepancies existing between the laser-induced damage thresholds reported in the literature. It provides also an appropriate tool for new studies with the most extreme laser radiations.

Efficacy evaluation and treatment parameter optimization for laser surgery of Ota's nevus based on an advanced non-equilibrium bio-tissue heat transfer model.

This study was performed to better understand the laser-tissue interaction mechanism and optimize the laser wavelength and pulse duration for the laser treatment of Ota's nevus, thereby providing the precise theoretical guidance for clinician to improve the therapeutic effect. A non-equilibrium bio-tissue heat transfer model coupled with thermo-mechanical effect for the explosive vaporization of melanin granule induced by laser heating was developed to investigate the temperature and thermal damage distribution using alexandrite (755 nm) and Nd:YAG (1064 nm) lasers with the pulse width of 10-120 ns. Cryogen spray cooling (CSC) was introduced to prevent the epidermal thermal damage due to competitive laser absorption between epidermal and dermal melanin. Thermal injury of the epidermis with temperature in the melanin zone reaching 132.3 °C was induced by alexandrite laser. The optimal pulse durations were 50 and 30 ns, and the corresponding incident laser fluence thresholds were 6.45 and 19.5 J·cm-2 for alexandrite and Nd:YAG laser, respectively, with the pre-cooling of R32 spray. Using R32 spray cooling, the 1064-nm laser fluence threshold of melanin can be increased by 56.0%, resulting in an indirect increase (80.4%) of injury thickness. Nd:YAG laser (1064 nm) was highly suitable for the removal of melanin in the deep tissue owing to its deep penetration. Too short pulse duration should be avoided to prevent the epidermal thermal damage. The improvement of therapeutic effect by CSC demonstrates its high clinical application potential.