Laser Fluence Research Articles

An In Vitro Study of 355‐nm Laser Ablation of Atherosclerotic Lesions Model

ABSTRACTA study of 355 nm laser with high pulse energy across various types of atherosclerotic lesion models is presented. The 355 nm laser pulses (10 ns) are delivered via a single fiber (600 μm diameter), and the ablation of calcified tissue, lipid tissue, and thrombus‐like tissue are studied under varied laser fluence (40–70 mJ/mm2) and repetition rate (5–30 Hz). The contact and noncontact ablation processes of chicken tibia samples (calcified tissue) are compared at 60 mJ/mm2 and 30 Hz, and the size of ablation particles is in the range of 0.1–1 μm. At the same repetition rate, the advancement rate of tricalcium phosphate samples reaches 150 μm/s at 70 mJ/mm2. Calcified and lipid models demonstrate predictable increases in ablation with higher laser fluence and repetition rate. The fresh porcine blood clot samples exhibit high‐quality ablation with good channel effect at 50 mJ/mm2 and 30 Hz.

Impact of High Power Laser Fluence on the Chemical Signatures of Silica Aerogel Films

Impact of High Power Laser Fluence on the Chemical Signatures of Silica Aerogel Films

Ultrafast thermal modulation dynamics during ripples formation induced by femtosecond laser multi-pulse vortex beam

Abstract This work theoretically investigated the ultrafast thermal modulation dynamics during early formation of ripples on Au film induced by femtosecond laser vortex beam irradiation. An extended two-temperature dynamics model that comprehensively considers the optical interference modulation for the formation of seed ripples, transient reflectivity, and non-nonequilibrium thermal transfer was self-consistently built to predict high-contrast ripples formation. The 2D evolutions of the electron and phonon temperature modulations during ripples formation in high non-nonequilibrium state of Au film were obtained via femtosecond laser vortex beam irradiation. It was revealed that the ripples contrast in can be significantly amplified by shortening the laser wavelength, increasing the pulse number, or enlarging the laser fluence of vortex beam. Moreover, the electron-phonon coupling time during ripples formation was fully explored in detail. The studies provide valuable insights into optimizing laser parameters for controlled high-contrast ripple formation on Au films.

Dynamic tunability of NIR absorption in a plasmonic grating through nonlinear kerr-effect of graphene-oxide and photothermal phase transition of GSST

In this paper, we present an in-depth examination of the absorption dynamics of a silver plasmonic grating incorporating Kerr-type nonlinear graphene-oxide (GO) nanoslits and a Ge2Sb2Se4Te1 (GSST) phase-change material, both in linear and nonlinear regimes. Linear absorption approaches unity at the resonant wavelength when GSST is amorphous. Upon exposure to a nanosecond Gaussian pulse laser irradiation with appropriate fluences, the amorphous GSST partially crystallizes through a thermoplasmonic-induced process, resulting in a significant reduction of the linear on-resonance absorption in the form of a step-like curve. Additionally, the weak off-resonance linear absorption of the system can be enhanced owing to the non-uniform phase-transition within the GSST. Transitioning to a nonlinear regime, considering the third-order nonlinearity of GO nanoslits, reveals distinct absorption behaviors. Temporal analysis reveals that at the on-resonance wavelength, the unit absorption decreases with increasing laser intensity, reaching a minimum at the pulse peak due to the Kerr nonlinearity in the GO. Intriguingly, the absorption exhibits a U-shaped curve at lower fluences, while at higher fluences, it stabilizes at a lower value post-pulse peak where the amorphous GSST undergoes a thermally driven partially crystallization phase transformation. In the off-resonance mode, the weak absorption enhances dramatically by increasing the fluence, tracing an imperfect parabolic trajectory that reaches nearly unit at the trailing edge of the pulse, attributed to the Kerr nonlinearity within the GO nanoslits. By further increasing the laser fluence, the photothermal response of the GSST emerges as the dominant factor, diminishing the unity absorption observed at the trailing edge of the pulse. These findings underscore the dynamic responsiveness of the proposed grating to pulsed laser illumination, driven by the nonlinear Kerr effect and GSST reconfigurability, offering promising opportunities for developing active optical switching devices.

Effect of machining environments on the crack behavior of ZrO2-Al2O3 composite during short-pulsed laser processing

The ZrO2-Al2O3 exhibits distinct behavior compared to monolithic ceramics when exposed to stress. The compelling quality of this trait makes it well-suited for any demanding supporting application that necessitates resilience. However, under a thermal process, it might cause functional concerns such as cracking patterns, which pose a threat to the endurance of orthopedic implants. This issue has lately attracted medical scrutiny. Being a thermal process, fiber laser treatment of ZrO2-Al2O3 is more complex than monolithic ceramic because of its unique thermal characteristics and varied rates of absorption, which rely on the matrix and the reinforcement material. This research aims to scrutinize the divergent characteristics of ZrO2-Al2O3 in terms of crack behavior while treating it with the same laser fluence under auxiliary environments. It has been found that ZrO2-Al2O3 prone to form cracks when processed under high-temperature environments due to the development of stress during phase transformation because of prolonged exposure, as evidenced by the surface characterization results. Meanwhile, when it was processed at low-temperature environments like water and ice, the detrimental effect of laser fluence factor appeared to be meager by reducing the likelihood of phase transformation and crack quantity. With this, the research demonstrates a promising approach that effectively maintains the overall structural integrity of ZrO2-Al2O3 by impeding the progression of the cracks along with a smooth, flawless surface during laser processing.

Low-power red laser and blue LED modulate telomere maintenance and length in human breast cancer cells.

Cancer cells have the ability to undergo an unlimited number of cell divisions, which gives them immortality. Thus, the cancer cell can extend the length of its telomeres, allowing these cells to divide unlimitedly and avoid entering the state of senescence or cellular apoptosis. One of the main effects of photobiomodulation (PBM) is the increase in the production of adenosine triphosphate (ATP) and free radicals, mainly reactive oxygen species (ROS). Existent data indicates that high levels of ROS can cause shortening and dysfunctional telomeres. Therefore, a better understanding of the effects induced by PBM on cancer cell telomere maintenance is needed. This work aimed to evaluate the effects of low-power red laser (658nm) and blue LED (470nm) on the TRF1 and TRF2 mRNA levels and telomere length in human breast cancer cells. MCF-7 and MDA-MB-231 cells were irradiated with a low-power red laser (69J cm-2, 0.77W/cm-2) and blue LED (482J cm-2, 5.35W/cm-2), alone or in combination, and the relative mRNA levels of the genes and telomere length were assessed by quantitative reverse transcription polymerase chain reaction. The results suggested that exposure to certain red laser and blue LED fluences decreased the TRF1 and TRF2 mRNA levels in both human breast cancer cells. Telomere length was increased in MCF-7 cells after exposure to red laser and blue LED. However, telomere length in MDA-MB-231 was shortened after exposure to red laser and blue LED at fluences evaluated. Our research suggests that photobiomodulation induced by red laser and low-power blue LED could alter telomere maintenance and length.

Pulse Duration Dependent Formation of Laser‐Induced Periodic Surface Structures in Atomic Layer Deposited MoS2

AbstractIn this paper, the formation of laser‐induced periodic surface structures (LIPSS) on atomic‐layer deposited MoS2 layers are studied experimentally. The process parameters (laser fluence and the pulse overlap) corresponding to formation of low‐ and high‐spatial frequency LIPSS as well as ablation and modification of the layers are identified for different pulse durations in the range from 0.2 to 10 ps. The role of the temperature accumulation is evaluated by changing the repetition rate from 0.2 to 2 MHz. The negative accumulation effect, i.e., the ablation of the layers becomes more difficult at higher laser pulse overlaps, is also observed. A simple model explaining the transition between different types of the LIPSS and the decrease of the ablation efficiency with the pulse overlap is suggested.

Optoplasmonic tuneable response by femtosecond laser irradiation of glass with deep-implanted gold nanoparticles

The manipulation of the optical properties of plasmonic nanocomposites is of high interest for the development of advanced optical devices with tailored unique properties. Achieving these objectives requires a combination of synthesis techniques and post-fabrication strategies. Here, we combine the use of two well-established physical strategies: MeV ion implantation and femtosecond laser processing. Firstly, we synthesize Au-doped soda lime glass nanocomposite through ion beam implantation (Au2+ at 1.8 MeV) followed by thermal annealing. This synthesis procedure results in a peculiar optical response based on the combination of Au-nanoparticle plasmonic resonance and a Fabry-Perot interference, caused by the deep implantation (centered at 480 nm). Secondly, this dual response is demonstrated to be highly tuneable by non-resonant femtosecond laser irradiation (800-nm wavelength and 130-fs pulse duration). Depending on the laser fluence, three transformation regimes are distinguished: supressing the interferometric response by spallative ablation, inducing vivid blue colors by surface swelling, and producing red-shifted color changes by multi-shot irradiation at low fluences. The proposed method is very versatile, since it is applicable to any dielectric matrix or implanted element. This work paves the way to a new route for the development of scalable and tuneable nanocomposites with several potential applications in optics.

Ultrafast antiferromagnetic switching of Mn2Au with laser-induced optical torques

Ultrafast manipulation of the Néel vector in metallic antiferromagnets most commonly occurs by generation of spin-orbit (SOT) or spin-transfer (STT) torques. Here, we predict another possibility for antiferromagnetic domain switching by using novel laser optical torques (LOTs). We present results of atomistic spin dynamics simulations from the application of LOTs for all-optical switching of the Néel vector in the antiferromagnet Mn2Au. The driving mechanism takes advantage of the sizeable exchange enhancement, characteristic of antiferromagnetic dynamics, allowing for picosecond 90 and 180-degree precessional toggle switching of the Néel vector with laser fluences on the order of mJ/cm2. A special symmetry of these novel torques greatly minimises the over-shooting effect common to precessional spin switching by SOT and STT methods. We demonstrate the opportunity for LOTs to produce deterministic, non-toggle switching of single antiferromagnetic domains. Lastly, we show that even with sizeable ultrafast heating by laser in metallic systems, there exist a large interval of laser parameters where the LOT-assisted toggle and preferential switchings in magnetic grains have probabilities close to one. The proposed protocol could be used on its own for all-optical control of antiferromagnets for computing or memory storage, or in combination with other switching methods to lower energy barriers and/or to prevent over-shooting of the Néel vector.

Synergic Effect of N and Se Facilitates Photoelectric Performance in Co-Hyperdoped Silicon.

Femtosecond-laser-fabricated black silicon has been widely used in the fields of solar cells, photodetectors, semiconductor devices, optical coatings, and quantum computing. However, the responsive spectral range limits its application in the near- to mid-infrared wavelengths. To further increase the optical responsivity in longer wavelengths, in this work, silicon (Si) was co-hyperdoped with nitrogen (N) and selenium (Se) through the deposition of Se films on Si followed by femtosecond (fs)-laser irradiation in an atmosphere of NF3. The optical and crystalline properties of the Si:N/Se were found to be influenced by the precursor Se film and laser fluence. The resulting photodetector, a product of this innovative approach, exhibited an impressive responsivity of 24.8 A/W at 840 nm and 19.8 A/W at 1060 nm, surpassing photodetectors made from Si:N, Si:S, and Si:S/Se (the latter two fabricated in SF6). These findings underscore the co-hyperdoping method's potential in significantly improving optoelectronic device performance.

Numerical study of multi-phase flow in ultrashort pulse laser processing

Ultrashort pulse laser processing is highly valued due to its enormous potential for high micromachining quality while it comprises complicated physical mechanisms especially when the pulse width falls in the regime about 10 ps. This study presents a complete mathematical modeling on multi-phase phenomena in ultrashort pulse laser processing, such as melting, vaporization, and solidification. The volume of fluid (VOF) technique is used to capture the interface during phase transition. Based on this hybrid model, the transport of energy and mass are investigated to study the influence of laser parameters on the formation of laser ablation hole. This work develops the incompressible multiphase laser ablation solver based on PIMPLE algorithm. The calculation results are validated by different references under different laser processing parameters which emphasize the importance of mechanism on thermal ablation and hydrodynamics in picosecond laser processing. More calculation results about the trends in hole depth over pulse laser width, laser fluence, and the number of pulses show the intricate influence of laser parameters.

Laser-initiated electron and heat transport in gold-skutterudite CoSb3 bilayers resolved by pulsed x-ray scattering

Abstract Electron and lattice heat transport have been investigated in bilayer thin films of gold and CoSb3 after photo-excitation of the nanometric top gold layer through picosecond x-ray scattering in a pump-probe setup. The kinetics of heat transfer are detected by thermal lattice expansion and compared to simulations based on the two-temperature model of coupling of electron and phonon degrees of freedom. The unexpected observation of a larger portion of the deposited heat being detected in the underlying CoSb3 layer before the topmost gold layer is heated supports the picture of transport of the photo-excited electrons from gold to the underlying layer to be converted into lattice heat. The change of partition of heat between the gold and CoSb3 layer with laser fluence and wavelength (either exciting intraband transitions or additionally interband transitions) is rooted in the amplitude of electron temperature. Higher electron temperatures result in a longer equilibration time with the lattice and thus a larger proportion of ballistic electron transport across the interface.

Liquid crystal-based temperature-controlled recirculating flat jet system.

In this paper, the design and implementation of a temperature-controlled recirculating flat jet system for liquid crystals (LCs)-based experiments are presented. In these experiments, the target liquid is usually exposed to medium to high laser fluences, possibly resonant with specific excitation, thus resulting in a change of local temperature and sudden degradation. To overcome this problem, each laser pulse must interact with a new volume of liquid, preferably with flat surfaces, while avoiding the use of substrates. A well-established solution consists of impinging two identical laminar jets that force the liquid into a radial expansion perpendicular to the plane formed by the jets, resulting in a consecutive chain of flat sheets bound by thick rims. In this context, LCs pose several challenges considering their viscosity, non-Newtonian behavior, and mesophase nature. Here, a precise control of temperature, thus mesophase, and pressure is demonstrated enabling the use of LCs in an impinging jet system. In particular, the system presented here delivers stable fluid chains of different sizes and thicknesses. The viscosity and non-Newtonian behavior of the LCs have a significant impact on the thickness of the chains as a function of the nozzle inner diameter, impinging angle, and radial distance from the impinging point. The flow rate, on the other hand, primarily affects the width and length of the liquid sheet.

Fabrication of Multiscale and Periodically Structured Zirconia Surfaces Using Direct Laser Interference Patterning

AbstractThe functionalization of zirconia surfaces by accurate and fast printing of periodical patterns embedding sub‐micrometric features is of great interest to many engineering fields and is yet to be explored. This study aims to assess the influence of the Direct Laser Interference Patterning processing parameters on the morphology and microstructure of zirconia surfaces using a 532 nm 10 ps‐pulsed laser source. Well‐defined linear structures with a period of 3 µm are successfully produced. Depending on the laser parameters, the structures are developed at or below the surface level, with higher depths (≈1 µm) being seen for increasing values of laser fluence and pulse overlap. Line‐like hierarchical structures with smaller interference spatial structures (3 µm period) and higher secondary structures with different periods (18, 15, and 12 µm) and heights (7, 5, and 3 µm, respectively) are also obtained on zirconia surface. Ablated regions presented few traces of molten material, nano‐droplets, and sub‐micrometric (<1 µm) pores, while no (sub) micrometric cracks are detected. A slight amount of tetragonal to monoclinic phase transformation (≈5%) is detected by X‐ray diffractometry. A processability map for ps‐laser processing of zirconia is proposed based on the experimental data.

The ablation behavior of diamond/Cu composites by infrared nanosecond pulsed laser

This paper investigates the material removal behavior of diamond/Cu composites by multi-passes infrared nanosecond pulsed laser ablation, focusing on the formed surface cracks, periodic ripples, profile evolution and deposition. Combining nanosecond laser ablation experiments with simulations based on thermophysical properties is then undertaken to effectively reduce the maximum thermal stress and inhibit crack initiation and propagation. Three types of cracks were identified in nanosecond laser ablation, which were influenced by the diverse thermophysical properties of the material and the temperature gradient. The periodic ripples of 300–1800 nm on the sidewalls of the machined groove on diamond grains were induced when the laser fluence approached the ablation threshold, which did not even appear on the copper matrix as the thermal effects dominate. The groove profile showed a limited change as the ablation passes further increase to be over 40 due to the plasma shielding effect inside the groove, the laser defocusing phenomenon and the increased difficulty of sputtering the molten material. In addition, the different ablation mechanisms of diamond and copper resulted in the formation of overlapping deposition layers with changed surface characteristics at varying laser energy densities. The uneven island-like recast layer affected the uniformity of the ablated grooves, which was dependent on the material microstructure and accumulated laser energy.

Femtosecond Laser Ablation and Delamination of Functional Magnetic Multilayers at the Nanoscale.

Laser nanostructuring of thin films with ultrashort laser pulses is widely used for nanofabrication across various fields. A crucial parameter for optimizing and understanding the processes underlying laser processing is the absorbed laser fluence, which is essential for all damage phenomena such as melting, ablation, spallation, and delamination. While threshold fluences have been extensively studied for single compound thin films, advancements in ultrafast acoustics, magneto-acoustics, and acousto-magneto-plasmonics necessitate understanding the laser nanofabrication processes for functional multilayer films. In this work, we investigated the thickness dependence of ablation and delamination thresholds in Ni/Au bilayers by varying the thickness of the Ni layer. The results were compared with experimental data on Ni thin films. Additionally, we performed femtosecond time-resolved pump-probe measurements of transient reflectivity in Ni to determine the heat penetration depth and evaluate the melting threshold. Delamination thresholds for Ni films were found to exceed the surface melting threshold suggesting the thermal mechanism in a liquid phase. Damage thresholds for Ni/Au bilayers were found to be significantly lower than those for Ni and fingerprint the non-thermal mechanism without Ni melting, which we attribute to the much weaker mechanical adhesion at the Au/glass interface. This finding suggests the potential of femtosecond laser delamination for nondestructive, energy-efficient nanostructuring, enabling the creation of high-quality acoustic resonators and other functional nanostructures for applications in nanosciences.

Picosecond photoacoustic generation of ultrasounds with composites of graphene-decorated gold nanoparticles

Photoacoustic or light-to-pressure transducer materials exhibit tremendous potential across various disciplines and applications, embracing theragnostic ultrasounds and permeabilization of biological barriers for drug delivery or for gene transfection. Here, we report the photoacoustic response in the picosecond excitation regime of graphene and gold nanoparticles decorated graphene dispersed in a polydimethylsiloxane polymer matrix. A novel decoration method was developed to nucleate Au nanoparticles of 25 nm on the graphene surface without reducing agent. We observed that the picosecond excitation enforced by the stress confinement generates high-frequency waves, with bandwidths of −6 dB and −20 dB of around 70 MHz and 130 MHz respectively, and peak pressure > 5 MPa. Further, the presence of gold surface decoration leads to a better dispersibility of the flake into the polymer matrix and to an increasing bandwidth at −6 dB up to 85 MHz. The decrease in source thickness leads to an increase in ultrasound frequency, which retains its value even when the laser fluence is increased up to 150 mJ cm –2. This enables the generation of high pressures while keeping the same bandwidth. We experimentally show that the photoacoustic waveform can be tailored by changing the temporal duration of the laser light, the geometry of the acoustic source, and the nature of the graphene absorber. Flexible decoration and fabrication methods of thin films, along with the generation of high-frequency and high-pressure ultrasound waves, offer new perspectives for the use of physical methods to permeabilize biological barriers and develop high-resolution photoacoustic imaging systems.

Understanding spin-dependent vibrational frequencies in Fe(II) metal organic coordination complexes

We investigated the compositional and temperature (T) dependences of vibrational frequencies in Hofmann-type Fe(L)2[M(CN)4] spin-crossover (SCO) coordination polymers in which {M = Ni, Pd or Pt with L = pyridine (py)}, or {L = 3-Cl-py or 3-methylpy with M = Ni}, using Raman spectroscopy. The SCO-driven peak shifts (in wavenumber) ranged up to ∼170 cm−1, manifesting significant spin-dependent structural evolutions. Furthermore, there appear clear HS signatures even at T ≪ TSCO for L = 3-Cl-py or 3-methylpy implying the steric effects of the organic ligands on the HS trapping. Meanwhile, for L = py, such HS trapping at the low temperature was not significant although some spectra taken under high laser fluence exhibit light-induced excited spin state trapping (LIESST) effect. The mechanism of the LIESST is discussed in detail in terms of the M d – C 2sp hybridization effects.

From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge-Transfer Phase Transition.

We study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 ⋅ 0.2H2O material, induced at room temperature by a single laser shot. This system exhibits a charge-transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn3+-N-C-Fe2+ tetragonal phase to the high temperature Mn2+-N-C-Fe3+ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out-of-equilibrium dynamics leading to this phase is multi-scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N-C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn3+-N-C-Fe2+ phase creates small charge-transfer polarons [Mn2+-N-C-Fe3+]* within ≃250 fs. The local trapping of photoinduced intermetallic charge-transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn2+-N-C-Fe2+ species. Above a threshold fluence, when a critical fraction of small CT-polarons is reached, the macroscopic phase transition to the persistent Mn2+-N-C-Fe3+ cubic phase occurs within ≃ 100 ps. This non-linear photo-response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.

From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge‐Transfer Phase Transition

AbstractWe study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 ⋅ 0.2H2O material, induced at room temperature by a single laser shot. This system exhibits a charge‐transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn3+−N−C−Fe2+ tetragonal phase to the high temperature Mn2+−N−C−Fe3+ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out‐of‐equilibrium dynamics leading to this phase is multi‐scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N−C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn3+−N−C−Fe2+ phase creates small charge‐transfer polarons [Mn2+−N−C−Fe3+]* within ≃250 fs. The local trapping of photoinduced intermetallic charge‐transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn2+−N−C−Fe2+ species. Above a threshold fluence, when a critical fraction of small CT‐polarons is reached, the macroscopic phase transition to the persistent Mn2+−N−C−Fe3+ cubic phase occurs within ≃ 100 ps. This non‐linear photo‐response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.