
Alexander RubenchikLawrence Livermore National Laboratory | LLNL · National Ignition Facility
Alexander Rubenchik
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Publications (130)
This article is for students in grades 7-10 who are interested in physics (по-русски)
This study demonstrates the significant effect of the recoil pressure and Marangoni convection in laser powder bed fusion (L-PBF) of 316L stainless steel. A three-dimensional high fidelity powder-scale model reveals how the strong dynamical melt flow generates pore defects, material spattering (sparking), and denudation zones. The melt track is div...
There is a need in laser powder-bed fusion of metals to produce high quality
parts without pores by better understanding the complex interplay of process
parameters. This study considers the main physical phenomena involved in laser
powder interactions using a high fidelity three-dimensional mesoscopic
simulation model of 316L stainless steel powde...
The production of metal parts via laser powder bed fusion additive manufacturing is growing exponentially. However, the transition of this technology from production of prototypes to production of critical parts is hindered by a lack of confidence in the quality of the part. Confidence can be established via a fundamental understanding of the physi...
A compact system is developed to measure laser absorptivity for a variety of powder materials (metals, ceramics, etc.) with different powder size distributions and thicknesses. The measured results for several metal powders are presented. The results are consistent with those from ray tracing calculations.
The effect of rapid laser heating on the response of 7075-T6 aluminum has been characterized using 3-D digital image correlation and a series of thermocouples. The experimental results indicate that as the samples are held under a constant load, the heating from the laser profile causes non-uniform temperature and strain fields, and the strain-rate...
We present a method for direct calorimetric measurement of powder absorptivity using a thin laser illuminated disc. Powder porosity is measured independently and a scheme eliminating the effect of convective and radiative losses is implemented.
High speed microscopic imaging and thermography of stainless steel microsphere assemblies exposed to high power laser light is analyzed to understand the melt-flow dynamics associated with selective laser melting processes.
Introduction Experimental Procedure Results Discussion Conclusions Acknowledgements
In this research a high-power diode laser array was used to preheat HY-80 steel to determine the efficacy of using a diode laser array for preheating prior to friction stir welding in order to reduce frictional forces thereby reducing tool wear and increasing welding speeds. Using instrumented plates the temperature profile using diode heating alon...
Laser powder-bed fusion additive manufacturing of metals employs high-power focused laser beams. Typically, the depth of the molten pool is controlled by conduction of heat in the underlying solid material. But, under certain conditions, the mechanism of melting can change from conduction to so-called “keyhole-mode” laser melting. In this mode, the...
We have investigated optical absorption by a powder of metal spheres, via ray-trace calculations. The absorptivity significantly exceeds that for normal incidence, because of multiple scattering. The effect of beam size is also discussed.
A ground-based laser system for space-debris cleaning will use powerful laser pulses that can self-focus while propagating through the atmosphere. We demonstrate that for the relevant laser parameters, this self-focusing can noticeably decrease the laser intensity on the target. We show that the detrimental effect can be, to a great extent, compens...
Laser induced breakdown on the output surface of calcium fluoride produces distinctive visualization of the gaseous material ejection via time-resolved shadowgraphy microscopy. This characteristic behavior allowed a detailed investigation of the directional expansion of the plume at ambient atmospheric conditions and its spatial separation and infl...
Surface modification of fused silica windows caused by the laser ablation of surface-bound microparticles is investigated. Using optical and electron microscopies between laser pulses, we detail the ablation, fragmentation and dispersal of 2-150 µm diameter particles of various materials. Following complete ablation and ejection of all debris mater...
Surface modification of fused silica windows caused by the ablation of surface-bound microparticles under short pulse laser irradiation is investigated and related to beam propagation effects. Particle material dispersal and subsequent surface pitting after multiple pulses of 351 nm, ∼ 9 J / cm 2 laser light were found to depend strongly on materia...
Interpretation of spatial and time resolved images of rear surface ns laser damage in dielectrics requires understanding of the dynamic interaction of the incoming laser beam with the confined expanding plasma in the material. The detailed kinetics of the plasma, involving both expansion and retraction, depends on details of reflection and absorpti...
Crater formation that accompanies laser-induced damage is the result of material ejection following the rapid, localized
heating to temperatures on the order of 1 eV. The objective of this work is to compare the material ejection behavior in
fused silica and KDP crystals as captured using time-resolved shadowgraphy. These two materials are of funda...
With the long-term goal in mind of investigating possible designs for a “universal, solid-sample comminution technique” for elemental analysis of debris and rubble, we have studied pulsed-laser ablation of solid samples that were submerged in water. Using 351-nm, 15-ns laser pulses with energy between 1 J and 0.35 J, intensities between 500 MW/cm2...
Among the approaches to the proposed mitigation and remediation of the space
debris problem is the de-orbiting of objects in low Earth orbit through
irradiation by ground-based high-intensity pulsed lasers. Laser ablation of a
thin surface layer causes target recoil, resulting in the depletion of orbital
angular momentum and accelerated atmospheric...
We develop models of laser interactions with composite materials consisting of fibers embedded within a matrix. A ray-trace model is shown to determine the absorptivity, absorption depth, and optical power enhancement within the material, as well as the angular distribution of the reflected light. We also develop a macroscopic model, which provides...
The material response following nanosecond, UV laser induced breakdown inside of the exit surface of fused silica is investigated using multimodal time resolved microscopy. The study spans up to about 75 ns delay from the onset of material modification during the laser pulse through the observation of material ejection. A number of distinct process...
The self-focusing characteristic of 355 nm, 3.3 ns pulses propagating through phosphate glass samples is found to significantly change during repeated exposure. The results indicate this change is related to the formation of color centers in the material as well as the generation of a transient defect population during exposure to the laser pulses....
Orbital debris in low Earth orbit (LEO) are now sufficiently dense that the use of LEO space is threatened by runaway collisional cascading. A problem predicted more than thirty years ago, the threat from debris larger than about 1cm demands serious attention. A promising proposed solution uses a high power pulsed laser system on the Earth to make...
Primary candidate classes of materials for future nuclear power plants, whether they be fission, fusion or hybrids, include oxide dispersion strengthened (ODS) ferritic steels which rely on a dispersion of nano-oxide particles in the matrix for both mechanical strength and swelling resistance, or tantalum alloys which have an inherent neutron-induc...
Orbital debris in low Earth orbit (LEO) are now sufficiently dense that the
use of LEO space is threatened by runaway collisional cascading. A problem
predicted more than thirty years ago, the threat from debris larger than about
1 cm demands serious attention. A promising proposed solution uses a high power
pulsed laser system on the Earth to make...
The modulation instability (MI) in optical fiber amplifiers and lasers
with anomalous dispersion leads to CW beam breakup and the growth of
multiple pulses. This can be both a detrimental effect, limiting the
performance of amplifiers, and also an underlying physical mechanism in
the operation of MI-based devices. Here we revisit the analytical the...
The modulation instability (MI) in optical fiber amplifiers and lasers with anomalous dispersion leads to cw radiation breakup. This can be both a detrimental effect limiting the performance of amplifiers and an underlying physical mechanism in the operation of MI-based devices. Here we revisit the analytical theory of MI in fiber optical amplifier...
Proposals for ground-based laser remediation of space debris rely on the
creation of appropriately directed ablation-driven impulses to either divert
the fragment or drive it into an orbit with a perigee allowing atmospheric
capture. For a spherical fragment, the ablation impulse is a function of the
orbital parameters and the laser engagement angl...
Tissue that has undergone significant yet unknown amount of ischemic injury is frequently encountered in organ transplantation and trauma clinics. With no reliable real-time method of assessing the degree of injury incurred in tissue, surgeons generally rely on visual observation which is subjective. In this work, we investigate the use of optical...
The modulation instability (MI) is one of the main factors responsible for the degradation of beam quality in high-power laser systems. The so-called B-integral restriction is commonly used as the criteria for MI control in passive optics devices. For amplifiers the adiabatic model, assuming locally the Bespalov-Talanov expression for MI growth, is...
The use of reduced nicotinamide adenine dinucleotide (NADH) fluorescence to gain metabolic information on kidneys in response to an alteration in oxygen availability has previously been experimentally demonstrated, but signal quantification has not, to date, been addressed. In this work the relative contribution to rat kidney autofluorescence of th...
Laser-induced growth of optical damage often determines the useful
lifetime of an optic in a high power laser system. We have extended our
previous work on growth of laser damage in fused silica with
simultaneous 351 nm and 1053 nm laser irradiation by measuring the
threshold for growth with various ratios of 351 nm and 1053 nm fluence.
Previously...
Growth of laser initiated damage plays a major role in determining optics lifetime in high power laser systems. Previous measurements have established that the lateral diameter grows exponentially. Knowledge of the growth of the site in the propagation direction is also important, especially so when considering techniques designed to mitigate damag...
We apply well known nonlinear diffraction theory governing focusing of a powerful light beam of arbitrary shape in medium with Kerr nonlinearity to the analysis of femtosecond (fs) laser processing of dielectric in sub-critical (input power less than the critical power of self-focusing) regime. Simple analytical expressions are derived for the inpu...
Growth of laser initiated damage is a potential lifetime limiter of laser optics. While laser initiated damage occurs most often on the exit surface of optical components, some damage sites can occur on the input surface. We have investigated the growth of laser initiated damage in fused silica when the damage occurs on the input surface of the opt...
Material processing using high-intensity femtosecond (fs) laser pulses is a fast developing technology holding potential for direct writing of multi-dimensional optical structures in transparent media. In this work we re-examine nonlinear diffraction theory in context of fs laser processing of silica in sub-critical (input power less than the criti...
We present theoretical and numerical analysis of the femtosecond processing of silica by laser beam with power below self-focusing threshold.
The Feit-Rubenchik size-selection damage model has been extended in a number of ways. More realistic thermal deposition profiles have been added. Non-spherical shapes (rods and plates) have been considered, with allowance for their orientation dependence. Random variations have been taken into account. An explicit form for the change of absorptivit...
In laser systems using frequency conversion, multiple wavelengths will be present on optical components. We have investigated the growth of laser initiated damage in fused silica in the presence of multiple wavelengths. In particular, we measured growth at 351 nm in the presence of 1053 nm near the threshold of growth for 351 nm alone. The data sho...
Bulk damage sites in frequency conversion crystals scatter and/or absorb laser light leading to interference and downstream intensification .We find that laser induced bulk damage sites in DKDP exhibit a "shell" of structurally and/or chemically modified material surrounding a central core as indicated by SEM and optical micrographs and micro Raman...
The effective lifetime of a laser optic is limited by both laser-induced damage and the subsequent growth of laser initiated damage sites. We have measured the growth rate of laser-induced damage on polished fused silica surfaces in 10 torr of air at 1053 nm at 10 ns. The data shows exponential growth in the lateral size of the damage site with sho...
An experimental technique has been developed to measure the damage
density ρ(Φ) variation with fluence from scatter maps of bulk
damage sites in plates of KD2PO4 (DKDP) crystals
combined with calibrated images of the damaging beam's spatial profile.
Unconditioned bulk damage in tripler-cut DKDP crystals has been studied
using 351 nm (3ω) light at p...
Cracks can affect laser damage susceptibility in three ways. These are field intensification due to interference, enhanced absorption due to trapped material in the cracks, and increased mechanical weakness. Enhanced absorption is the most important effect.
In a solid-state heat capacity laser (SSHCL), waste heat is stored in the lasing slabs, minimizing temperature gradients and optical distortions. After the maximum number of pulses is reached, the slabs are cooled or rapidly exchanged with cool slabs. During the past several years, our laboratory at LLNL has built a number of SSHCLs, demonstrating...
Collagen possesses a strong second order nonlinear susceptibility; when
it is irradiated with intense laser light, some of the reflected and
transmitted light will have twice the frequency of the incident beam, a
phenomenon known as second harmonic generation (SHG). Polarization
modulation of an ultra-short pulse laser beam can be used to
simultane...
The second-harmonic signal in collagen, even in highly organized samples such as rat tail tendon fascicles, varies significantly with position. Previous studies suggest that this variability may be due to the parallel and antiparallel orientation of neighboring collagen fibrils. We applied high-resolution second-harmonic generation microscopy to co...
Laser-induced pinpoint bulk damage of deuterated potassium dihydrogen phosphate at 351 nm is shown to depend on the propagation direction relative to the crystallographic axes and on growth temperature in addition to the previously reported dependence on continuous filtration. Pulse-length scaling is also consistent with earlier reports. The leadin...
Pressure waves generated by the surface ablation of water with ultrashort laser pulses in the 140 fs–10 ps duration range were studied with a Mach–Zehnder interferometry. Formation and propagation of spherical and, in ablation with longer pulses, combination of spherical and cylindrical pressure waves were observed. Measurements of the amplitude an...
The effects of structural perturbation on second harmonic generation in collagen were investigated. Type I collagen fascicles obtained from rat tails were structurally modified by increasing nonenzymatic cross-linking, by thermal denaturation, by collagenase digestion, or by dehydration. Changes in polarization dependence were observed in the dehyd...
The raster scan technique is used for large optics damage tests and laser conditioning. We show that the "effective area" concept enables the possibility to compare various scanning schemes and to use raster scan experiments for NIF optics damage prediction. It is shown that the hexagonal lattice of laser beam imprints yields optimal use of each sh...
Nonlinear processes associated with plasma heating using radiofrequencies near the lower hybrid frequency are considered. In this frequency range, parametric instabilities are easily excited and the resulting plasma turbulence leads to increased absorption of RF power. Study of the spectrum of this turbulence enables the absorbed energy distributio...
Collagen possesses a strong second-order nonlinear susceptibility, a nonlinear optical property characterized by second harmonic generation in the presence of intense laser beams. We present a new technique involving polarization modulation of an ultra-short pulse laser beam that can simultaneously determine collagen fiber orientation and a paramet...
We use polarization-modulated second harmonic generation to image fiber orientation in collagen tissues, with an axial resolution of about 10 micrometers and a transverse resolution of up to 1 micrometers . A linearly polarized ultra-short pulse (200 fs) Ti:Sapphire laser beam is modulated using an electro-optic modulator and quarter-wave plate com...
Using scanning confocal microscopy, we measure the backscattered second harmonic signal generated by a 100 fs laser in rat-tail tendon collagen. Damage to the sample is avoided by using a continuous scanning technique, rather than measuring the signal at discrete points. The second harmonic signal varies by about a factor of 2 across a single cross...
High power laser pulses can produce damage in high quality fused silica
optics that can lead to its eventual obscuration and failure. Current
models suggest the initiation of a plasma detonation due to absorbing
initiators and defects, leading to the formation of shock waves. Recent
experiments have found a densified layer at the bottom of damage s...
Laser damage of large fused silica optics initiates at imperfections. Possible initiation mechanisms are considered. We demonstrate that a model based on nanoparticle explosions is consistent with the observed initiation craters. Possible mechanisms for growth upon subsequent laser irradiation, including material modification and laser intensificat...
We have investigated the flash of light that accompanies laser damage
using time-resolved spectroscopy. Damage events were initiated in the
bulk of both fused silica and DKDP crystals with 355-nm 3-ns pulsed
radiation. Spectra from the accompanying flash were recorded in the
200-500 nm wavelength range with 5-ns temporal resolution. Ten ns
followin...
The purpose of this study was to evaluate the ablation of ossicular tissue using a 1,053 nm Ti:Sapphire chirped pulse amplifier laser system configured to deliver ultrashort pulses of 350 femtoseconds (fs) (3.5x10(-13) seconds) in cadaver temporal bone.
Ablation of the formalin-fixed incus and stapes was performed using an ultrashort pulse laser (U...
The measurement of the second order nonlinear susceptibility of collagen
in various biological tissues has potential applications in the
detection of structural changes which are related to different
pathological conditions. We investigate second harmonic generation in a
rat-tail tendon, a highly organized collagen structure consisting of
parallel...
Results are reported from recently performed bulk-damage, pulse-scaling experiments on DKDP tripler samples taken from NIF-size, rapid-growth boule BD7. The tests were performed on LLNL's Optical Sciences Laser. A matrix of samples was exposed to single shots at 351 nm (3(omega) ) with average fluences from 4 to 8 J/cm2 for pulse durations of 1, 3...
The effective lifetime of optics in the UV is limited both by laser induced damage and the subsequent growth of laser initiated damage sites. We have measured the growth rate of laser induced damage in fused silica in both air and vacuum. The data shows exponential growth in the lateral size of the damage site with shot number above threshold fluen...
An extended Rayleigh model for laser generated bubbles in water and soft tissue is presented. This model includes surface tension, viscosity, a realistic equation of state, material strength and failure, stress wave emission, and linear growth of interface instabilities. The model is validated by comparison to detailed compressible hydrodynamic sim...
Femtosecond laser ablation has been shown to produce well-defined cuts and holes in metals with minimal heat effect to the remaining material. Ultrashort laser pulse processing shows promise as an important technique for materials processing. We will discuss the physical effects associated with processing based experimental and modeling results. In...
We report the effects of the repetition rate and the beam size on the threshold for ultrashort laser pulse induced damage in dentin. The observed results are explained as cumulative thermal effects. Our model is consistent with the experimental results and explains the dependence of the threshold on repetition rate, beam size, and exposure time. ©...
A Mach-Zehnder interferometer was used for analysis of pressure waves
generated by ultrashort laser pulse ablation of water. It was found that
the shock wave generated by plasma formation rapidly decays to an
acoustic wave. Both experimental and theoretical studies demonstrated
that the energy transfer to the mechanical shock was less than 1%.
The use of femtosecond lasers allows materials processing of practically
any material with extremely high precision and minimal collateral
damage. Advantages over conventional laser machining (using pulses
longer than a few tens of picoseconds) are realized by depositing the
laser energy into the electrons of the material on a time scale short
comp...
A confocal microscopy imaging system was devised to selectively detect second harmonic signals generated by biological tissues. Several types of biological tissues were examined using this imaging system, including human teeth, bovine blood vessels, and chicken skin. All these tissues generated strong second harmonic signals. There is considerable...
Expansion of ablation plumes created by intense ultrashort lasers is determined by various complicated physical processes which have very different spatial and time scales. Since complete simulation by one model is practically impossible, we suggest using two models describing initial and final stages that can be matched at an intermediate time. Th...
Recent LLNL experiments reported elsewhere at this conference explored the pulse length dependence of 351 nm bulk damage incidence in DKDP. The results found are consistent, in part, with a model in which a distribution of small bulk initiators is assumed to exist in the crystal, and the damage threshold is determined by reaching a critical tempera...
We describe simulations of experiments invovling laser illumination of a metallic knife edge in the Optical Sciences Laboratory (OSL) at LLNL, and pinhole closure in the Beamlet experiment at LLNL. The plasma evolution is modeled via LASNEX. In OSL, the calculated phases of a probe beam are found to exhibit the same behavior as in experiment but to...
Laser modulated scattering (LMS) is introduced as a non- destruction evaluation tool for defect inspection and characterization of optical surface sand thin film coatings. It allows simultaneous measurement of the DC and AC scattering signals of a probe laser beam from an optical surface. by comparison between the DC and AC scattering signals, one...
Plasma luminescence spectroscopy was used for precise ablation of bone tissue without damaging nearby soft tissue using an ultrashort pulse laser. Strong contrast of the luminescence spectra between bone marrow and spinal cord provided the real time feedback control so bone tissue is selectively ablated while preserving the spinal cord.
Temperature and shock wave propagation in water (as a model of tissue)
irradiated by sub-picosecond and nanosecond pulses were modeled. The
high temperature and pressure generated during sub-picosecond
irradiation did not penetrate deeply into the water due to quickly
ejected plasma while significant pressure and temperature increases were
observed...
Assuming the observed scaling of laser damage threshold fluence with the (almost) square root of pulse duration is due to thermal conduction, we develop a formalism for directly comparing pulses of different shapes and durations. We find, for example, that a top hat pulse leads to 15% higher temperature (presumably 15% lower damage threshold) than...
Starting from the absorption of laser energy at a subsurface
nanoparticle in fused silica, we simulate the consequent buildup of
stresses and resulting mechanical material damage. The simulation
indicates the formation of micropits with size comparable to a
wavelength, similar to experimental observation. Possible mechanism for
enhanced local light...