Laser-sustained Plasma Research Articles

Bright compact ultrabroadband source by orthogonal laser-sustained plasma

Laser-sustained plasma (LSP) source featuring high brightness and broadband spectral coverage is found to be powerful in various fields of scientific and industrial applications. However, the fundamental limit of low conversion efficiency constrains the system compactness and widespread applications of such broadband light sources. In this paper, we propose an innovative orthogonal LSP to break through the conversion efficiency limitation. Driven by the elevated conversion efficiency from absorbed laser power to ultraviolet (UV) emission, a compact broadband source (250–1650 nm) with UV spectral radiance exceeding 210 mW/(mm2⋅sr⋅nm)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${mW}/({{mm}}^{2}\\,\\cdot\\, {sr}\\,\\cdot\\, {nm})$$\\end{document} is achieved with >100 W pump laser. With the plot of a two-dimensional refractive index model, we report an important conceptual advance that the orthogonal design eliminates the influence of the negative lensing effect on laser power density. Experimental results unambiguously demonstrate that we achieve a bright compact UV-VIS-NIR source with negligible thermal loss and the highest conversion efficiency to our knowledge. Significant enhancement of 4 dB contrast-to-noise ratio (CNR) in spectral single-pixel imaging has been demonstrated using the proposed ultrabroadband source. By establishing the quantitative link between pumping optics design and plasma absorption, this work presents a compact broadband source that combines superior conversion efficiency and unprecedented brightness, which is essential to high-speed inspection and spectroscopy applications.

Multiphysics modeling and simulations of laser-sustained plasmas

Laser-sustained plasma (LSP), which can be utilized for a novel radiation light source, has advantages such as high irradiance, broad spectral range, and stable emission, demonstrating significant applications in wafer inspection in the field of the semiconductor industry. This paper revisits the historical development of LSP research and introduces fundamental physical processes in LSP. The mathematical description equations for LSP and methods of calculating plasma parameters are provided, thereby a time-dependent two-dimensional fluid model is established by taking into consideration a laser-thermal-hydrodynamic coupling effect. The propagation of the laser in plasma is investigated based on the established model, and the fundamental processes in LSP, including the initial evolution process, laser energy deposition, steady-state characteristics, and instability, are explored. The effectiveness of the simulation model is confirmed through comparing with the experimental results of high-pressure Xe LSP. The findings indicate that the mode, power, <i>F</i>-number of incident lasers, as well as parameters including components, pressure, and flow velocity of gas, can all affect the steady-state properties of LSPs. Under the identical power and <i>F-</i>number conditions, Gaussian mode laser and annular mode laser both produce LSPs with different shapes and positions. Notably, under the conditions of high-power annular laser incidence, large laser <i>F-</i>number, and high flow velocity, the simulation results reveal temporal and spatial instability in LSP. These simulation results contribute significantly to a more in-depth understanding of the underlying physical mechanisms of the LSP. Furthermore, they provide a theoretical basis for designing the light source system and optimizing the multiple parameters. The influence of laser parameters on LSP properties elucidated in this study not only advances the fundamental understanding of LSP but also offers crucial insights for designing and optimizing the light source systems in various applications, particularly in the field of optical detection for semiconductor wafer inspection.

Optical emission spectroscopy of fiber laser sustained Xe plasma

Laser-sustained plasma (LSP), a unique method of obtaining high brightness broadband light sources, has been used for rapid optical inspection of wafer defects and has thus attracted great interest. Then Xe plasma sustained under high pressure by a continuous fiber laser was investigated using optical emission spectroscopy (OES). Emission spectra of spatial integration diagnosis have shown that with the increase of laser power and the decrease of Gaussian beam focusing number F, the electron temperature fitted by Boltzmann plot method increased from 0.45 eV to 0.85 eV, the corresponding electron density was roughly an order of magnitude of 1016-1017 cm-3, and the calculated absorption coefficient of plasma was distributed in 0.9–1.5 cm-1. To further analyze the spatial distribution of plasma parameters, the spatial resolved OES of LSP was acquired, and the two-dimensional electron temperature and electron density were determined. The non-uniform distribution of the plasma parameters was analyzed due to the existence of thermal gravity convection and forced convection. The serious self-reversal of emission lines of 881.94 nm and 823.16 nm was observed and analyzed.

Characteristics of Krypton and Krypton-Based Mixed Gases Laser-Sustained Plasma Using a 4-kW Laser Diode

Characteristics of Krypton and Krypton-Based Mixed Gases Laser-Sustained Plasma Using a 4-kW Laser Diode

Laser-sustained plasma of high radiance in the ultraviolet spectral range based on the reservoir effect of the annular beam.

Laser-sustained plasma (LSP) featured by high radiance in the ultraviolet (UV) spectral range is found to be powerful in high resolution inspection and spectroscopy applications. In this paper, we demonstrate significant enhancement of UV radiance in LSP excited by a high power focused annular laser beam of small F-number. A two-dimensional refractive index distribution in the plasma region is established to obtain the propagation path of the annular beam. Subsequently, the spatial distribution of the annular beam power density is calculated to establish the quantitative relationship between laser power and plasma dimension. Theoretical analysis of a "reservoir" effect constructed by the annular beam is carried out. Abrupt plasma dimension change is observed when the laser power exceeds the "reservoir" boundary. The new model is then validated by experiments. LSP with the highest reported UV radiance to our knowledge has been achieved. This work paves what we believe is a novel way of generating high radiance UV sources by spatial manipulation of excitation laser beam.

Investigation of the laser-sustained plasma of a Xenon lamp driven by an annular beam.

Higher brightness UV light sources based on laser-sustained plasma (LSP) finds critical applications in high-resolution optical inspection and measurement. An annular beam focused with a low F-number is proposed for the first time to investigate its effect in realizing high-brightness LSP compared with Gaussian beams. We experimentally verified that the annular laser beam sustained a nearly laterally symmetric plasma with a longitudinal length less than 600 µm plasma length, where the length-width ratio was four times smaller than that driven by a TEM00 Gaussian beam. Theoretical analysis further explored that the annular beam's transmission path could limit the plasma's longitudinal growth, like a "reservoir". Furthermore, the "reservoir" effect is favorable for effectively increasing the plasma temperature and the effective power ratio of UV from 8% to 15%. This work paves a novel way of generating high-brightness UV sources by spatial manipulation of infrared pumping beam.

Oscillations of convective flow around a continuous optical discharge in high-pressure xenon

Laser-sustained plasma or continuous optical discharge (COD), predicted and first obtained in pioneering works initiated and inspired by Professor Raizer et al (1970 JETP Lett. 11 120–3; 1970 JETP Lett. 11 302–4; 1972 Sov. Phys.—JETP 34 763–9), is still a unique method of sustaining localized dense plasma in the laboratory and some special devices such as high-brightness broadband radiation sources [6–10]. Although the parameters determining the COD state can be stabilized, in practice, COD tends to regular oscillations. As stated in the paper, the oscillations may be attributed to the regular pulsing of thermal gravitational convective plumes around the COD. Application problems arising from instability of this type is pulsing of the emission from promising radiation sources utilizing COD plasma. This paper presents a detailed analysis of the experimental data on COD convective plume radii and oscillation frequencies depending on the plasma-forming gas pressure. The analysis leads to a similarity relation for the oscillation frequency. The similarity relation turns out to be common for optical discharges and laminar flames under conditions of prevailing buoyancy forces. This indicates the hydrodynamic nature of the instability considered, regardless of whether the energy input method is in the form of absorbed laser radiation or a chemical reaction. To support the idea of a hydrodynamic origin of the pulsations, a numerical simulation of a convective plume from a concentrated heat source was carried out. The thermal dissipation power of the heat source was equal to that of the COD. The results of numerical simulations demonstrate good agreement with those of the experiments. The dynamic temperature, density and gas velocity distributions obtained show that the direct cause of the oscillations is the dynamics of the toroidal vortices developed in the convection plume. The pulsing frequency equal to the frequency of the vortex formation may be increased under incoming additional forced gas flow that may also suppress the convective oscillations. The results obtained may be useful for studying optical discharges, and improving the parameters of high-brightness broadband laser-plasma radiation sources based on them.

Research of continuous fiber laser sustained Xe plasma

High-speed optical inspection of wafers requires a high-brightness and broadband light source. The brightness of traditional arc discharge lamps is difficult to increase, which is limited by the cathode current density. Laser sustained plasma (LSP) is a promising method to overcome the limitations of existing light source technologies. Then the characteristics of fiber laser sustained plasma in a xenon bulb were investigated, the absorption coefficient was measured, and the spectrum was analyzed. The results show that with the increase of laser power, the volume of plasma gradually increases and moves towards the direction of laser incident. With the increase of Gaussian beam focusing number F, the threshold laser power required to maintain the stable existence of plasma decreases gradually. There are upper and lower limits for laser power at which the LSP could be sustained. When the laser incident at the optimal incident angle, the laser propagation in the plasma is the optimal path for laser energy deposition, and the plasma has higher absorption coefficient, temperature and brightness. When F ≥ 10, the Xe plasma will show spatial instability due to refraction effect. Compared with the traditional short-arc xenon lamp, the LSP spectrum has a higher proportion in short wavelength.

Hyperspectral infrared imaging of surface phonon-polaritons in SrTiO3

Polaritons have a demonstrated impact on nanophotonic applications in the midinfrared through visible spectral range. Surface phonon-polaritons (SPhPs) offer a way to bring the potential of polaritons to the longer infrared wavelengths. Strontium titanate (STO) is a perovskite polar dielectric with diverse technologically advantageous properties and it can support SPhPs in a uniquely broad spectral range of the far infrared. Despite these advantages, STO has mostly been overlooked as a nanophotonic material. In this work we investigate SPhP propagation in STO in the far-infrared through midinfrared spectral range using broadband, near-field nanospectroscopy. We developed a tabletop, laser sustained plasma light source that enabled us to obtain amplitude and phase resolved hyperspectral line scan maps of SPhPs across the surface of the STO sample. Analytical modeling of experimental data reveals the dispersion characteristics of SPhPs in STO. This work establishes STO as a platform for perovskite-based broadband far-infrared and terahertz nanophotonics.

Laser gas alloying of Ti-6Al-4V in air for tribological applications

Hard and wear-resistant TiOxNy/α-Ti composite coatings were fabricated on Ti-6Al-4V alloy via laser gas alloying in air for tribological applications. The effects of laser power, scan speed, and hatch spacing on the surface morphology, geometry and microstructure of the composite coatings were investigated systematically. The formation of laser-sustained plasma above the melt surface could lead to the dissociation of N2 and O2 molecules into N and O atoms. The atoms diffused and mixed into the melt, and subsequently reacted with molten Ti to form TiOxNy dendritic reinforcements, while the remaining melt solidified to form the α-Ti solid solution matrix. A compositional gradient was observed in the transitional zone between the TiOxNy dendrite and α-Ti matrix. The transitional zone was also found to have a crystallographic relationship with the TiOxNy dendrite. The microhardness and wear resistance of the composite coating were 2.8 and 10.8 times those of Ti-6Al-4V base metal, respectively. The excellent wear resistance of the composite coatings was attributed to the formation of hard TiOxNy reinforcements and α-Ti solid solution, as well as the strong TiOxNy/α-Ti interface.

Argon–Xenon Laser-Sustained Plasma Using 1-kW Diode Laser

Argon–xenon laser-sustained plasma (LSP) was generated using a 1-kW class diode laser. At a total pressure of 1 MPa, the threshold laser power of the xenon–argon LSP was higher than that of the pure xenon LSP, whereas the former was lower than the latter at a total pressure of 0.1 MPa because of homonuclear associative ionization effect. The minimum xenon pressure ratio at which LSP could be generated was 7.67% at a total filled pressure of 3 MPa. The maximum value of fractional absorption was 37.7% at a total filled pressure of 0.5 MPa.

Feasibility study of a laser launch system for picosatellites and nanosatellites in low-earth orbits

Laser propulsion is a space transportation system that obtains thrust from a laser beam that is transmitted from the ground as an energy source. In this study, we evaluated the launch performances of two types of laser propulsion systems in a circular orbit at an altitude of 500 km. One propulsion used the heating system of the laser-sustained plasma (LSP) type, and the other propulsion had the heating system of the porous-carbon heat exchange (hereinafter PHX) type. Assuming a laser power of 500 kW, which is the maximum laser power that is commercially available, the payload masses were estimated to be 0.679 and 0.640 kg for the LSP and PHX systems, respectively. This result shows that the laser power level that is currently available enables the launch of a picosatellite. The laser power values required to launch a nanosatellite less than 10 kg were estimated to be 6.8 and 6.4 MW for the LSP and PHX systems, respectively. Finally, the launch cost was estimated as a function of the number of launches. Our results show that, when the picosatellite was launched 1000 times, the launch cost became comparable with that of a conventional nanosatellite launch. Subsequently, the launch cost decreased to one-twentieth the cost of a conventional chemical rocket.

Generating conditions of argon laser-sustained plasma by disk, fiber and diode lasers

Laser-sustained plasma (LSP) is a promising method for high-pressure plasma generation that overcomes the problems of arc discharge. Then, the generating conditions of argon LSP were investigated using a kilowatt class disk, fiber and diode lasers. For the disk and fiber lasers, the threshold laser power for LSP generation was found to decrease with pressure. The minimum laser power of the disk and fiber lasers was 1.1 kW at a pressure of 1.5 MPa and 0.45 kW at a pressure of 2.0 MPa. The threshold power of the disk laser was higher than that of the fiber laser. The fiber laser might have a lower threshold because atomic line absorption occurs at 1067 nm and 1068 nm since both absorption coefficients achieved through the Bremsstrahlung process are almost the same. Since LSP could not be generated by a diode laser at a pressure of less than 6.5 MPa, the generating conditions were estimated based on the product of the absorption coefficient of laser intensity. A pressure of more than 8 MPa was found to be necessary for a laser with 1 kW of power, and laser power of more than 109 kW is necessary for a pressure of 0.1 MPa.

Laser-Sustained Plasma (LSP) Nitriding of Titanium: A Review

Titanium and its alloys possess several attractive properties that include a high strength-to-weight ratio, biocompatibility, and good corrosion resistance. However, due to their poor wear resistance, titanium components need to undergo surface hardening treatments before being used in applications involving high contact stresses. Laser nitriding is a thermochemical method of enhancing the surface hardness and wear resistance of titanium. This technique entails scanning the titanium substrate under a laser beam near its focal plane in the presence of nitrogen gas flow. At processing conditions characterized by low scan speeds, high laser powers, and small off-focal distances, a nitrogen plasma can be struck near the surface of the titanium substrate. When the substrate is removed, this plasma can be sustained indefinitely and away from any potentially interacting surfaces, by the laser power and a cascade ionization process. This paper presents a critical review of the literature pertaining to the laser nitriding of titanium in the presence of a laser-sustained plasma, with the ultimate objective of forming wide-area, deep, crack-free, wear-resistant nitrided cases on commercially pure titanium substrates.

A new method for isolating plasma interactions from those of the laser beam during plasma nitriding

A new method for separating the interactions of a laser plasma from those of a laser beam with a titanium substrate was developed. A laser-sustained plasma (LSP) was generated by striking the plasma on a titanium plate in a flowing nitrogen atmosphere at ambient pressure and subsequently removing the strike plate. The resultant nitrogen plasma was sustained indefinitely around the laser focal plane until the nitrogen gas flow and/or the laser power were switched off. The laser-sustained nitrogen plasma was then used as a “reactor” to study the effects of a nitrogen plasma, independent of the laser beam, when the plasma was brought into close proximity, (2 to 3mm), parallel to a titanium substrate. There was no interaction of the parallel laser beam with the substrate. Heat, nitrogen and titanium were exchanged between the nitrogen LSP and substrate resulting in melting, nucleation and growth of faceted and dendritic TiN crystals, rectangular hollow TiN morphologies and particulate TiN deposits. Dense TiN layers up to 300μm thick formed within 5s. The results demonstrated that compositionally graded layers of nitrogen-enriched titanium, and titanium nitride could be formed very rapidly on model, commercially-pure titanium substrates, with promise for extension to commercial alloys such as Ti-6Al-4V.

Enhancement of CP-titanum wear resistance using a two-step CO2 laser-sustained plasma nitriding process

In this paper, a method of forming hard, wide-area, crack-free, and wear-resistant nitrided cases on commercially-pure titanium using a 3.5kW CO2 laser-sustained plasma is described. This surface hardening method was comprised of two steps: (1) a laser-sustained nitrogen plasma was first used to nitride the titanium substrate; and (2) a laser-sustained argon plasma was then employed to remelt the nitrided layer deposited in the first step. Previous research using single laser trail experiments had shown that the (second) remelting step can eliminate cracks formed during the (first) nitriding step and homogenize the nitrided layer. In this work, the two-step nitriding-remelting process was extended to wider surface areas by depositing multiple overlapping trails at four different nitriding speeds and a constant remelting speed. The hardened layer was characterized using x-ray diffraction (XRD), optical metallography, and hardness testing. Reciprocating ball-on-flat wear tests were conducted to assess the wear resistance of the nitrided case, with the wear scar being characterized using scanning electron microscopy (SEM) and optical profilometry. Crack-free, hard cases of depths up to 600μm and average hardness values up to 641±86HV0.3 were observed. The LSP nitriding-remelting treatment was found to improve the wear resistance of the base metal (CP-Ti) by up to 80%.

A two-step laser-sustained plasma nitriding process for deep-case hardening of commercially pure titanium

A two-step, laser-sustained plasma (LSP) process was developed to form deep, hard, nitrided cases on commercially pure titanium. A laser-sustained plasma is plasma generated in a gaseous atmosphere that can be sustained indefinitely by the laser beam away from any potentially interacting surface. The first step utilized a nitrogen LSP associated with a defocused CO2 laser beam in pure nitrogen gas flow to melt and nitride the titanium surface. The second step used an argon LSP to remelt and refine the nitrided layer. Twenty experimental cases with varying nitriding and remelting speeds were studied. Optical and scanning electron microscopy, energy dispersive spectroscopy, optical profilometry, and x-ray diffraction were used to characterize the remelted nitrided layer. An analytical moving heat source solution was used to model the heat transfer in the melt pool during the nitriding and remelting processes. The mass flux of nitrogen into the melt pool and the efficiency of nitrogen intake were estimated. The remelting treatment was found to eliminate the surface cracks formed during nitriding, decrease the surface roughness of the nitrided trail, homogenize the hardened surface layer, and refine the microstructure. The microstructures and hardness of the nitrided layers could be tailored by changing the combination of LSP nitriding and remelting scan speeds. Case depths of up to 0.8mm and average case hardness values in the range of 475–729HV0.3 were achieved in this study.

Effect of processing parameters on microstructure during laser-sustained plasma (LSP) nitriding of commercially-pure titanium

A nitrogen plasma can be sustained in air using a high-power CO2 laser. If nitriding of a Ti substrate is performed in the presence of such laser-sustained plasma (LSP) in open atmosphere, the following effects have been previously reported: increase in nitrogen intake, wider surface coverage, reduction in surface oxidation, and no attenuation of energy transfer to the substrate. This investigation expands on previous work by systematically studying the effects of varying process parameters on microstructure during LSP nitriding of CP-titanium. Microstructural features are reported as a function of off-focal distance, scan speed of the laser, and nitrogen:argon gas flow ratio. The top surfaces and transverse cross sections of these single trail nitriding runs were observed using optical and scanning electron microscopy. X-ray diffraction (XRD) and optical profilometry techniques were used to characterize the nitrided surface layer. The presence of LSP increased the solubility of nitrogen in liquid titanium leading to gas recombination and expulsion on the surface. This tendency decreased with increasing speed. Nitrogen dilution by argon was found to reduce crack formation. Further, nitrogen dilution reduced Marangoni convection in the melt pool. This changed the nitrogen transport mechanism in the melt pool from convection-dominated to diffusion-dominated, thus reducing nitrogen incorporation into the melt pool. Argon addition was also found to reduce surface roughness and surface oxidation. XRD and scanning electron microscopy revealed the presence of phases such as TiN, TiN0.3 and acicular martensitic titanium (α′-Ti). Future work will focus on depositing multiple overlapping nitride trails to form wide-area titanium nitride surface layers.

レーザー維持プラズマ風洞の作動特性の数値解析的調査

レーザー維持プラズマ風洞の作動特性の数値解析的調査

Alumina reduction by laser sustained plasma for aluminum-based renewable energy cycling

A novel alumina (Al2O3) reduction technique for a renewable energy cycling system based on aluminum is proposed. Al2O3 powder was fed into laser-sustained plasma and thermally dissociated. The produced Al was expanded to supersonic speeds through a nozzle. From the Al and argon line distributions in the flow direction, it was found that Al remained in the dissociated state. A water-cooled copper tube was inserted in the flow to collect Al. X-ray analysis indicated that elemental Al was observed on the surface of the tube. The maximum value of the estimated reduction efficiency was 5%.