Incident Laser Energy Research Articles

High-power fiber laser incident beam absorptivity variation of molten pool surfaces during their solid-liquid-gas state evolution

The thermal and ablation effects of lasers are widely used in laser additive manufacturing, laser welding, laser cleaning, and laser cladding technologies. Absorptivity is the core index of laser beam-thermal energy conversion efficiency. This paper analyzed the irradiated material's absorption behavior during the solid-liquid-gas evolution of laser-ablated metal surfaces by comparing the temperature fields and the corresponding weld cross-sections for different laser action periods, considering the fusion and vaporization latent heats. The solid-liquid-gas state evolution of high-power fiber laser-irradiated metal surfaces occurs during several milliseconds, with a short solid-phase existence period, since fusion or melting time is much shorter than the vaporization time. As the solid-liquid-gas state evolves, the average absorptivity and the share of thermal conduction energy loss decrease with the laser action time, while the fusion energy loss gradually increases. The solid-phase surface of the metal in laser processing absorbs significantly more incident laser energy than the liquid-phase one. The longer the solid-phase occurrence period of the metal surface during the laser action, the greater its average absorptivity of the incident laser. By increasing the laser spot area and scanning rate, the solid-phase period of the laser-irradiated metal surface can be increased, improving its incident laser absorptivity.

Anomalous hot electron generation from two-plasmon decay instability driven by broadband laser pulses with intensity modulations

We investigate the hot electrons generated from two-plasmon decay (TPD) instability driven by laser pulses with intensity modulated by a frequency Δω m using theoretical and numerical approaches. Our primary focus lies on scenarios where Δω m is on the same order of the TPD growth rate γ 0 ( Δωm∼γ0 ), corresponding to moderate laser frequency bandwidths for TPD mitigation. With Δω m conveniently modeled by a basic two-color scheme of the laser wave fields in fully-kinetic particle-in-cell simulations, we demonstrate that the energies of TPD modes and hot electrons exhibit intermittent evolution at the frequency Δω m , particularly when Δωm∼γ0 . With the dynamic TPD behavior, the overall ratio of hot electron energy to the incident laser energy, fhot , changes significantly with Δω m . While fhot drops notably with increasing Δω m at large Δω m limit as expected, it goes anomalously beyond the hot electron energy ratio for a single-frequency incident laser pulse with the same average intensity when Δω m falls below a specific threshold frequency Δω c . This anomaly arises from the pronounced sensitivity of fhot to variations in laser intensity. We find this threshold frequency Δω c primarily depends on γ 0 and the collisional damping rate of plasma waves, with relatively lower sensitivity to the density scale length. We develop a scaling model characterizing the relation of Δω c and laser plasma conditions, enabling the potential extention of our findings to more complex and realistic scenarios.

Design of first experiment to achieve fusion target gain > 1

A decades-long quest to achieve fusion energy target gain and ignition in a controlled laboratory experiment, dating back to 1962, has been realized at the National Ignition Facility (NIF) on December 5, 2022 [Abu-Shawareb et al., Phys. Rev. Lett. 132, 065102 (2024)] where an imploded pellet of deuterium and tritium (DT) fuel generated more fusion energy (3.15 MJ) than laser energy incident on the target (2.05 MJ). In these experiments, laser beams incident on the inside of a cylindrical can (Hohlraum) generate an intense ∼3 × 106 million degree x-ray radiation bath that is used to spherically implode ∼2 mm diameter pellets containing frozen deuterium and tritium. The maximum fusion energy produced in this configuration to date is 3.88 MJ using 2.05 MJ of incident laser energy and 5.2 MJ using 2.2 MJ of incident laser energy, producing a new record target gain of ∼2.4×. This paper describes the physics (target and laser) design of this platform and follow-on experiments that show increased performance. We show robust megajoule fusion energy output using this design as well as explore design modification using radiation hydrodynamic simulations benchmarked against experimental data, which can further improve the performance of this platform.

Deep learning with plasma plume image sequences for anomaly detection and prediction of growth kinetics during pulsed laser deposition

Materials synthesis platforms that are designed for autonomous experimentation are capable of collecting multimodal diagnostic data that can be utilized for feedback to optimize material properties. Pulsed laser deposition (PLD) is emerging as a viable autonomous synthesis tool, and so the need arises to develop machine learning (ML) techniques that are capable of extracting information from in situ diagnostics. Here, we demonstrate that intensified-CCD image sequences of the plasma plume generated during PLD can be used for anomaly detection and the prediction of thin film growth kinetics. We develop multi-output (2 + 1)D convolutional neural network regression models that extract deep features from plume dynamics that not only correlate with the measured chamber pressure and incident laser energy, but more importantly, predict parameters of an auto-catalytic film growth model derived from in situ laser reflectivity experiments. Our results demonstrate how ML with in situ plume diagnostics data in PLD can be utilized to maintain deposition conditions in an optimal regime. Further, the predictive capabilities of plume dynamics on the kinetics of film growth or other film properties prior to deposition provides a means for rapid pre-screening of growth conditions for the non-expert, which promises to accelerate materials optimization with PLD.

Quantitative investigation on the effects of eyeball geometry, blood perfusion and natural convection in retinal laser surgery

The photothermal effects of laser on tissue are widely used in ophthalmology to treat retinal diseases. The accurate modelling of retinal laser surgery will prove valuable guidance for clinic treatment. There are several important issues should be carefully evaluated during retinal laser surgery modelling, including the eye geometry (whole eye or fundus only, retinal pigment epithelium layer incorporated or not), blood perfusion and natural convection. In previous publications, the existence of the anterior part of eyeball and retinal pigment epithelium layer is neglected to simply the eye geometry due to low laser light absorption (anterior part of eyeball) or small volume occupation (retinal pigment epithelium layer), respectively. However, the justification of this treatment has not been conducted. In this study, we will quantitatively investigate these factors systemically in a large pulse duration range that covers all the retinal laser surgery used in clinic. To do this, we present a bioheat transfer model for the simulation of the thermal response of the eye to laser irradiation. In the simulation, the 3D whole-eye geometry is considered. The Pennes bioheat transfer equation is adopted for the whole-eye temperature modelling. Simultaneously, we treat the fundus as a multilayered medium that all primary components, including RPE layer, are considered for first time in whole eye modelling. Our model calculates the thermal responses of the eye during laser retinal surgery with the pulse duration from nanosecond to tens of second. The effects of eye geometry, blood perfusion and natural convection are clearly clarified based on the laser pulse duration and incident energy used. The results show that neglecting the anterior part of eyeball will introduce the computational error more than 27 %. The existence of the retinal pigment epithelium layer should always be considered in retinal laser surgery modelling in all pulse durations used in clinic, otherwise more than 20 % computational error will be generated. When the pulse duration is longer than 10s, the effect of blood perfusion and natural convection in vitreous body should both be considered.

Numerical simulation study on distinguishing nonlinear propagation regimes of femtosecond pulses in fused silica

We perform numerical simulations to investigate the nonlinear propagation dynamics of femtosecond Gaussian and vortex beams in fused silica. By analyzing the extent of spectral broadening, we are able to distinguish between the linear, self-focusing, and filamentation regimes. Additionally, the maximum intensity and fluence distribution within the cross-section of the vortex beams are analyzed for different incident laser energies. The results demonstrate a direct correlation between the spectral broadening and the peak intensity of the femtosecond laser pulse. As a result, this provides a theoretical foundation for distinguishing different propagation regimes, and determining critical powers for self-focusing and filamentation of both femtosecond Gaussian and structured beams.

High-Quality Cutting of Soda–Lime Glass with Bessel Beam Picosecond Laser: Optimization of Processing Point Spacing, Incident Power, and Burst Mode

Soda–lime glass has a wide range of applications in the fields of smart electronics, optical components, and precision originals. In order to investigate the effect of processing parameters on picosecond Bessel laser cutting of soda–lime glass and to achieve high-quality soda–lime glass cutting, a series of cutting experiments were conducted in this study. In this study, it was found that the machining point spacing, the incident laser energy, and the number of burst modes had a significant effect on the machining of the samples. The atomic force microscope (AFM) showed a better quality of roughness of the machined cross-section when the spacing of the machining points was 1 μm, a locally optimal solution was obtained when the number of burst modes was 2, and a locally optimal solution was also obtained when the incident laser power was 11.5 W. In this study, better machining quality was achieved for soda–lime glass of 1 mm thickness, with an average roughness of 158 nm and a local optimum of 141 nm.

Modulate the laser phase to improve the ns-LIBS spectrum signal based on orbital angular momentum.

Aiming to enhance the ns-LIBS signal, in this work, we introduced orbital angular momentum to modulate the laser phase of the Gaussian beam into the vortex beam. Under similar incident laser energy, the vortex beam promoted more uniform ablation and more ablation mass compared to the Gaussian beam, leading to elevated temperature and electron density in the laser-induced plasma. Consequently, the intensity of the ns-LIBS signal was improved. The enhancement effects based on the laser phase modulation were investigated on both metallic and non-metallic samples. The results showed that laser phase modulation resulted in a maximum 1.26-times increase in the peak intensities and a maximum 1.25-times increase in the signal-to-background ratio (SBR) of the Cu spectral lines of pure copper for a laser energy of 10 mJ. The peak intensities of Si atomic spectral lines were enhanced by 1.58-1.94 times using the vortex beam. Throughout the plasma evolution process, the plasma induced by the vortex beam exhibited prolonged duration and a longer continuous background, accompanied by a noticeable reduction in the relative standard deviation (RSD). The experimental results demonstrated that modulation the laser phase based on orbital angular momentum is a promising approach to enhancing the ns-LIBS signal.

Ultrafast Multistage Lattice Strain via Laser‐Excited Phonons in Lithium Niobate

AbstractUltrafast laser‐excitation of lithium niobate (LiNbO3) crystal has triggered numerous photonic applications through the structural transitions in LiNbO3. However, the explanations for ultrafast laser‐induced modification of LiNbO3 have remained phenomenological, lacking a convincing in‐depth understanding of the fundamental laser‐lattice interaction process. Based on ab initio simulations, it is demonstrated that photoexcited anharmonic phonons play a significant role in influencing the lattice structure of LiNbO3. Harnessing the real‐time time‐dependent density functional theory, it is revealed that the excitation of TO4 phonons via electric‐phonon coupling triggers displacement‐induced lattice oscillations during multiphoton ionization. These oscillations give rise to multistage structural strains, resulting in alterations of the refractive index. Significantly, these modifications exhibit sensitivity to the incident laser energy. Experimentally, using the waveguide technique and micro‐Raman spectroscopy, the correlation between local refractive index, lattice volume density, and phonon vibrational modes has been established, exhibiting good consistency with theoretical predictions. This work provides an effective means to understand the ultrafast excitation of phonons and relaxation processes of the lattice in dielectric crystals.

QERaman: An open-source program for calculating resonance Raman spectra based on Quantum ESPRESSO

We present an open-source program QERaman that computes first-order resonance Raman spectroscopy of materials using the output data from Quantum ESPRESSO. Complex values of Raman tensors are calculated based on the quantum description of the Raman scattering from calculations of electron-photon and electron-phonon matrix elements, which are obtained by using the modified Quantum ESPRESSO. Our program also calculates the resonant Raman spectra as a function of incident laser energy for linearly- or circularly-polarized light. Hands-on tutorials for graphene and MoS2 are given to show how to run QERaman. All codes, examples, and scripts are available on the GitHub repository. Program summaryProgram Title:QERamanCPC Library link to program files:https://doi.org/10.17632/ddcrrrmxw9.1Developer's repository link:https://github.com/nguyen-group/QERamanLicensing provisions: GNU General Public Licence 3.0Programming language: FortranExternal routines:Quantum ESPRESSO v7.2Nature of problem: Resonance Raman spectra with first-principles calculations.Solution method: The Raman intensity formula is given by the quantum theory, in which the electron-photon and the electron-phonon matrix elements are obtained from the modified Quantum ESPRESSO package.

Nonlinear optics properties of Ni-Sb2Se3 nanofilms in the near-infrared region.

S b 2 S e 3 is an emerging material in recent years, and past studies have shown that it has good optoelectronic properties when doped with metals. In this paper, pure S b 2 S e 3 films and N i-S b 2 S e 3 films with different doping contents (1, 2, 3W) were prepared by magnetron sputtering technology. The nonlinear optics properties of the sample films were investigated using femtosecond (fs) Z-scan technology under 800nm. The results showed that both pure S b 2 S e 3 and doped films exhibited reverse saturated absorption (RSA), and the occurrence of the reverse saturated absorption behavior of the doped films was mainly due to two-photon absorption (TPA), free carrier absorption (FCA), and the presence of defective energy levels. Compared with pure S b 2 S e 3 films, N i-S b 2 S e 3 films exhibit significantly enhanced nonlinear absorption properties and nonlinear refractive properties. By increasing Ni sputtering power and incident laser energy, the nonlinear optic properties of N i-S b 2 S e 3 films are enhanced. By testing the sample films using SEM, XRD, and UV-Vis techniques, we found that Ni metal doping greatly improved and optimized the crystallinity of the films and adjusted the optical band gap.

Laser-induced sonar: A promising approach for improved underwater acoustic sensing

This study presents the characteristics of acoustic pressure impulses generated by nanosecond laser-induced filamentation (ns-LIF) of water, focusing on the spatial, temporal, and spectral domains. In the time domain, the peak-to-peak (Pk-Pk) overpressures increase with higher incident optical energy while the arrival time remains constant. Notably, linearly polarized pulses exhibit higher Pk-Pk overpressures than circularly polarized pulses. Acoustic measurements of ns-LIF in water demonstrate a linear correlation between filament size and incident laser energy due to multiple plasma sources along the optical beam propagation. Alongside the temporal information, the spectrogram visualizes broad-spectrum underwater acoustic pressure impulses ranging from 10 to 800 kHz, perpendicular to the optical beam propagation. The low-frequency instantaneous underwater acoustic signals generated by ns-LIF is ∼90 kHz, offering advantages such as extended propagation distances and reduced attenuation in water. In addition to the experimental investigation, finite element analysis is employed to visualize the propagation and interaction of underwater acoustic signals across various interfaces. This integrated approach provides valuable insights into the behavior and characteristics of underwater signals. Eventually, our findings demonstrate the successful development of remote laser-induced sonar technology.

Massive laser pulling of graphene nanosheets in water.

Light manipulation of graphene-based materials attracts much attentions. As a new light manipulation concept, optical pulling develops rapidly in the past decade. However, optical pulling of graphene in liquid is rarely reported. In this work, laser pulling of graphene nanosheets (GN) in pure water by using common gauss beams is presented. This phenomenon holds for multiple incident laser wavelengths including 405 nm, 488 nm, 532 nm and 650 nm. A particle image velocimetry software PIVlab is adopted to analyze the velocity field information of GN. The laser pulling velocity of the GN is approximately ∼ 0.5 mm/s corresponding to ∼ 103 body length/s, which increases with an increase of the incident laser energy. This work presents a contactless mothed to massively pull microscale graphene materials in simple liquid, which supplies a potential manipulation technique for micro-nanofluidic devices and also provides a platform to investigate laser-graphene interaction in a simple liquid phase medium.

Research on the ablation characteristics of combined lasers for glass fiber reinforced plastic composites

Glass fiber reinforced plastic (GFRP) composites have been applied to the manufacture of missile shields and unmanned aerial vehicle (UAV) shells. It is of great significance to explore the ablation characteristics of different lasers for these composites. Currently, most existing studies on the ablation characteristics of lasers for Glass fiber reinforced plastic composites are conducted under a single laser output mode, such as continuous wave (CW) laser or pulsed laser. However, the ablation characteristics of combined lasers for Glass fiber reinforced plastic composites have not been clarified. Therefore, the ablation characteristics of single lasers (continuous wave, millisecond (ms) pulsed, or nanosecond (ns) pulsed laser) and combined laser (CW/ms or CW/ns combined pulsed lasers) were investigated by experimental and simulation methods in this study. Additionally, the ablation mechanisms of Glass fiber reinforced plastic under different laser irradiation conditions were compared and analyzed. The results demonstrated that the ablation rates of single lasers for Glass fiber reinforced plastic composites were all within an order of magnitude of 10 μg/J, which was not significantly correlated with the light source system. The ablation efficiency of the single laser was determined by the incident laser energy. The continuous wave laser was found to be the optimal light source for the ablation and destruction of Glass fiber reinforced plastic composites. Nevertheless, there were some obstacles in the ablation process of continuous wave lasers. Applying pulsed lasers during the irradiation of the continuous wave laser may generate a synergistic effect. Under the conditions in this study, the CW/ns pulsed combined laser increased the ablation efficiency by 53.8%.

Energy deposition in a telescopic laser filament for the control of fuel ignition.

The efficiency of energy coupled to plasma during femtosecond (fs) laser filamentation plays a decisive role in a variety of filament applications such as remote fabrication and spectroscopy. However, the energy deposition characterization in the fs laser filament formed by a telescope, which provides an efficient way to extend the filament distance, has not yet been revealed. In the present study, we show that when the distance between the two lenses in a telescope changes, i.e., the effective focal length changes, there exists an optimal plateau energy deposition region in which the energy deposited into the filament per unit length called the average lineic energy deposition (ALED) remains at high levels, exhibiting a remarkable difference from the monotonic change in a single-lens focusing system. As a proof of principle, we examined the influence of the energy deposition on the ignition of a lean methane/air mixture, and found that the use of the telescope can efficiently extend the ignition distance when compared with a single-lens focusing system under the same incident laser energy condition. Our results may help understand the energy deposition behaviors in a variety of telescopic filaments and provide more options to manipulating laser ignition at a desired distance.

Study of highly reflective ceramic glaze coating and laser irradiation effects

The rapid development and wide application of laser technology, together with the gradually increased output energy of lasers, pose a great threat to safety. Therefore, the demand for anti-laser is urgent. Highly reflective coatings can dissipate large incident laser energy through reflection. In this paper, zirconium silicate (ZrSiO4) highly reflective ceramic glaze coatings were prepared on both ceramic and metal substrates. Their macro and micro structure evolution treated at different heating temperatures were studied. The reflectivity of the coatings was on average 85% over the range of 400 and 2400 nm and reaches a maximum of 94% at 1064 nm. The coatings on ceramic substrates showed good stability when irradiated under high-energy continuous laser with power density of 2000 W/cm2 for 60 s. On the other hand, the coatings prepared on metal substrates formed a layer of dense, glaze-like material after high-energy continuous laser irradiation with power density of 500 W/cm2 for 10 s or 1000 W/cm2 for 5 s, demonstrating good anti-laser performance.

Study on the Duration of Laser-Induced Thin Film Plasma Flash

The accuracy of judging whether the film is damaged directly affects the accuracy of the measurement of the film laser damage threshold. When judging the film damage by the traditional plasma flash method, there is a problem of misjudgment caused by the failure to distinguish the film and air plasma flash. In order to eliminate misjudgment, the two flashes are accurately distinguished by the difference in the duration of the air and film plasma flash. This paper aims to obtain the theoretical and experimental values of the duration of the film plasma flash (tf) and analyze the factors affecting it. Firstly, taking single-layer hafnium oxide and aluminum oxide thin films as examples, when the wavelength of the incident laser is 1064 nm, the diameter of the laser focusing spot is 0.08 cm, the energy of the incident laser is 100 mJ, and the pulse width of incident laser is 10 ns, the tf of hafnium oxide, and aluminum oxide thin films are 542.7 and 299.6 ns, respectively. Secondly, the experimental study of tf was carried out. Through six experiments, the following results were obtained: (1) With the increase in incident laser energy, the tf of both films increases; (2) The tf of the hafnium oxide film is longer than that of the aluminum oxide film. (3) The experimental parameters are put into the calculation model, and the theoretical results are in good agreement with the experimental tf values. Finally, it is found that tf increases with the increase in incident laser energy and incident laser pulse width, and decreases with the increase in focusing spot diameter.

Copper Sulfide Small Nanoparticles as Efficient Contrast Agent for Photoacoustic Imaging

An experimental study on an innovative contrast agent is presented. This work demonstrates that copper sulfide in the form of small-sized nanoparticles can be exploited in photoacoustic imaging. An advantage of this material is strong light absorption in the near-infrared range, especially in the transparency windows of biological tissues. In order to yield a proper contrast, light absorption must be followed by heat release with high efficiency. Therefore, it is important to evaluate the photochemical conversion efficiency of the material. We applied a method that is strictly related to photoacoustic applications. The nanoparticles were produced according to a well-established synthesis. Subsequently, they were diluted in pure water to obtain an extinction <0.2/cm at 1064 nm. The photoacoustic signals, generated by 1064 nm laser excitation, were analyzed as a function of concentration and incident laser energy below 70 μJ /pulse. The signals were carefully compared with those of a reference aqueous solution, containing a light-absorbing ionic solute. Data analysis yielded a light-to-heat conversion efficiency 1.0 (±0.1). We discuss this result by comparison with related studies on other types of copper sulfide nanoparticles, where the conversion efficiency reportedly varied from 33% to 93%. The high value determined in the present study possibly indicates that resonant light scattering and luminescence are negligible for our material system.

Ultrafast nonlinear optical properties of Ag-CdTe thin films by co-sputtering in the near-infrared region

Metal-doped CdTe has become a heated topic in recent years due to its ultrafast nonlinear optical properties. Herein, Ag-CdTe thin films with different doping amount (1W, 3W and 5W) and pure CdTe thin films were fabricated on quartz substrates by magnetron sputtering. The effects of Ag doping on the morphology, optical band gap and nonlinear optical properties of the thin films were systematically investigated. The nonlinear optical properties were investigated by femtosecond(fs) Z-scan technique under 800 nm. The results indicated that the Ag-CdTe thin films exhibit good nonlinear optical properties in the near-infrared region due to free carrier absorption and two-photon absorption. The results showed that the dopant content and the variation of the incident laser energy can control the RSA intensity, the nonlinear absorption coefficient, and the nonlinear refractive index of Ag-CdTe thin films. These results can provide new support for their applications in the field of nonlinear optoelectronic devices.

Laser energy partitioning in nanosecond pulsed laser-induced air breakdown: effect of incident laser energy.

Air breakdown is generated by a 1064nm nanosecond pulsed laser beam, and laser energy deposited in the breakdown (E d), transmitted through the plasma region (E t) and carried away by the shock wave (E s) is estimated for the incident laser energy (E i) range of 60-273mJ. The E d is approximately 85% of E i at 60mJ, rapidly increasing to 92% at 102mJ. The shock wave front velocity and radius are measured as a function of E i and propagation distance. The shock wave velocity nicely follows the v∝E i0.3 trend predicted by the laser-supported detonation wave model. The Sedov-Taylor theory is used to estimate E s, which rapidly increases with E i, but E i to E s conversion linearly decreases from 83% to 48%. At lower values of E i, most of the laser energy is carried away by the shock wave, whereas the laser energy used in plasma heating or released in the form of electromagnetic and thermal radiation becomes important at higher laser energies. This implies that laser energy partitioning is highly dependent on the value of incident laser energy. These findings provide important insights into the fundamental physics of air breakdown and will be useful in a variety of applications such as laser-induced breakdown spectroscopy, laser ignition, and laser propulsion.