Laser-induced Dielectric Breakdown Research Articles

Optical and X-ray Emission Spectroscopy of High-Power Laser-Induced Dielectric Breakdown in Molecular Gases and Their Mixtures

Large-scale plasma was created in molecular gases (CO, CO2, N2, H2O) and their mixtures by high-power laser-induced dielectric breakdown (LIDB). Compositions of the mixtures used are those suggested for the early earth's atmosphere of neutral and/or mildly reducing character. Time-integrated optical spectra emitted from the laser spark have been measured and analyzed. The spectra of the plasma generated in the CO-containing mixtures are dominated by emission of both C2 and CN radicals. A vibrational temperature of approximately 10(4) K was determined according to an intensity distribution in a vibronic structure of the CN (B2Sigma(+)u-X2Sigma(+)g) violet band. For comparison, the NH3-CH4-H2-H2O mixture has been irradiated as a model of the strongly reducing version of the early earth's atmosphere. In this mixture, excited CN seems to be significantly less abundant than C2. The LIDB experiments were in the molecular gases carried out not only in the static cell but also using a large, double stream pulse jet (gas puff target) placed in the vacuum interaction chamber. The obtained soft X-ray emission spectra indicate the presence of highly charged atomic ions in the hot core of high-power laser sparks.

Mechanisms of femtosecond laser-induced breakdown and damage in MgO

Single-shot laser damage threshold of MgO for 40–986fs, 800nm laser pulses is reported. The pump–probe measurements with femtosecond pulses were carried out to investigate the time-resolved electronic excitation processes. A theoretical model including conduction band electrons (CBE) production and laser energy deposition was applied to discuss the roles of multiphoton ionization (MPI) and avalanche ionization in femtosecond laser-induced dielectric breakdown. The results indicate that avalanche ionization plays the dominant role in the femtosecond laser-induced breakdown in MgO near the damage threshold.

Laser-triggeredlightning discharge

Advances in ultrafast optics in recent years have revived a keeninterest in laser-induced dielectric breakdown study. While itis widely accepted that femtosecond laser pulses with peakpowers reaching gigawatts can propagate over tens of metresunder laboratory conditions, the dynamics underlying thishighly nonlinear phenomenon is yet not fully understood.Although initial research on laser-triggered lightning wasstarted with infrared lasers, it was found that they are notsuitable to initiate lightning. Recent published literature andexperimental work favour the use of ultraviolet (UV) laser pulses asthe appropriate means for laser-induced lightning discharge. Ananalytical solution based on Maxwell's equations has beendeveloped for UV filamentation in air, arising from adynamic oscillating balance between self-focusing, diffractionand plasma defocusing. This model suggests that UV(220-420 nm) 200 ps laser pulses with a peak power ofaround 50 MW (or 12.5 mJ input energy) and a beam size of100 µm are the optimal tool to trigger outdoor lightning.The laser beam size remains relatively small (less than 0.3 mm)after a propagation distance of 200 m up into the normallycloudy and damp atmospheric conditions.

Modeling of laser-induced breakdown in dielectrics with subpicosecond pulses

Theoretical study of ultrafast laser induced damage by short pulses (τ<1 ps) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker–Planck type equation for free electron production is discussed in detail. The calculation shows that the collisional avalanche ionization competes with the multiphoton ionization even for pulse length shorter than 25 fs. Sensitivity tests of all the rates in the equation are performed and the most critical ones are identified. From these tests we obtain valuable information in developing new materials that have the desired damage fluence for specific applications. To describe the relaxation of electron plasma, a three body recombination rate is included. Thus, the temporal behavior of the electron density due to a single pulse is treated, as well as the case of exposure to two laser pulses with a time delay between them. The model is only partially successful in reproducing the recent experimental data. Effect of the presence of a linear decay term and optical defects on the damage threshold is considered in the context of the rate equation input.

Dehydrochlorination of 1,2-Dichloropropane by CO2 Laser-Induced Breakdown; Comparison with Thermal Elimination

Dehydrochlorination of 1,2-dichloropropane (DCP) was conducted by thermal elimination and CO2 laser-induced dielectric breakdown, which was induced by focusing the 10.6 μm lines of a CO2 TEA laser. Thermal dehydrochlorination proceeded by a four-centered mechanism at > 425 °C and by a surface-catalyzed radical process at 2000 °C.

TEA CO 2 laser induced breakdown versus selective photochemistry in C 2F 5H and C 2F 5H/C 2F 5T mixtures

In the context of laser separation of tritium isotope using C 2F 5T in mixtures with C 2F 5H, we have studied the laser induced dielectric breakdown (LIDB) as a function of the system pressure using 10.6 μm radiation of a TEA CO 2 laser. By taking also into account our isotope selective results, we have found that LIDB does not imply difficulties by affecting the laser selective process. Analysis of the experimental results shows that LIDB works by cascade ionization and the recombination constitutes the major loss process. The LIDB threshold fluence Φ th for the tritiated mixture was found to be lower than the corresponding value in pure C 2F 5H indicating preionization in the medium due to β emission of tritium.

Nanosecond-to-femtosecond laser-induced breakdown in dielectrics.

We report extensive laser-induced damage threshold measurements on dielectric materials at wavelengths of 1053 and 526 nm for pulse durations $\ensuremath{\tau}$ ranging from 140 fs to 1 ns. Qualitative differences in the morphology of damage and a departure from the diffusion-dominated ${\ensuremath{\tau}}^{\frac{1}{2}}$ scaling of the damage fluence indicate that damage occurs from ablation for $\ensuremath{\tau}<~10$ ps and from conventional melting, boiling, and fracture for $\ensuremath{\tau}>50$ ps. We find a decreasing threshold fluence associated with a gradual transition from the long-pulse, thermally dominated regime to an ablative regime dominated by collisional and multiphoton ionization, and plasma formation. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in quantitative agreement with the experimental results.

Particle Formation by CO2 Laser-Induced Breakdown. (II). Formation of B4C Fine Powder from Boron Bromide-Methane-Hydrogen System.

Fine B4C powder was synthesized by the TEA-CO2 laser-induced dielectric gas breakdown of a BBr3 + CH4 + H2 gas mixture. The product was characterized by SEM, ICP atomic emission spectroscopy, and electron and X-ray diffractions. The particles show a narrow size distribution with a mean particle size of 0.18 μm. The yield of particles was increased linearly with increasing laser pulse energy. About 90% of incident pulse energy was absorbed by plasma in the reactant gas.

Structure of amorphous boron fine particles produced by laser-induced dielectric breakdown

Structure of amorphous boron fine particles produced by laser-induced dielectric breakdown

An Analysis of the Effect of System Variables on the Quality of Thin Films and Powders Produced by Laser-Breakdown Chemical Vapor Deposition

ABSTRACTA gas phase process for large area depositions on an ambient temperature substrate using laser-induced dielectric breakdown of gas phase precursors has recently been developed.1 Deposits of nickel alloys show excellent grain refinement (<10 nm) and metastable phase incorporation due to rapid quenching from the gas phase. Particle size distribution and compositional variance within the deposited films have been studied using electron microscopy and electron diffraction. Kinetic expressions to explain homogeneous gas phase nucleation and growth of the deposited materials have been developed in an effort to better understand this process. The effect of system variables on film and powder grain sizes has been studied. This analysis gives insight into the fluid flow/heat transfer patterns involved in the system and their effect on the final deposited material. The effect of system pressure, gas phase composition, and laser pulse energy, on particle size, surface area coverage, and deposition thickness are discussed.

Cr, Mo and W alloy powders produced by laser-induced breakdown of metal carbonyl vapors

Very fine ( < 100 Å) CrMoW alloy powder particles were produced from a mixture of metal (Cr, Mo, W) carbonyl vapors via laser-induced dielectric breakdown. Extensive STEM analyses revealed two types of particles, differentiated by morphology and chemistry. The much more abundant particles were basically amorphous, and rich in Cr and Mo. The second particle type was rich in Mo and depleted of Cr and consisted of very fine crystallites dispersed within an amorphous matrix.

Spectroscopy of laser-induced dielectric breakdown in gas mixtures

Laser-induced dielectric breakdown has been used to initiate fragmentation in molecular gases and gas mixtures. The emission spectra of the atoms, ions, and molecular fragments has been assigned and used to identify specific components of given mixtures. While this study only reports on mixtures of hydrocarbons and halogenated hydrocarbons, the technique is general and does not rely on laser absorption in any component of the mixtures.

Laser induced dielectric breakdown studies of the reaction UF 6 + H 2

Laser induced dielectric breakdown (LIDB) has been documented in UF 6 at pressures ranging from 8–100 torr. A high power, line selectable TEA CO 2 laser has been used as the source to induce the dielectric breakdown (DB). Reactions of the fragmented UF 6 with H 2 have been studied at various pressure ratios. In all cases a rapid and large pressure drop and heavy deposits of suspended particulates were observed and attributed to the LIDB driven reaction 2UF 6 + H 2 → 2UF 6 + H 2 → 2UF 5 + 2HF.

Laser-induced breakdown and nonlinear refractive index measurements in phosphate glasses, lanthanum beryllate, and Al2O3

We present results of measurements of the laser-induced dielectric breakdown threshold in two phosphate composition laser glasses, in lanthanum beryllate:Nd, and in Al2O3. The nonlinear refractive indices for these materials are also presented. The measurements were made with single 30-psec 1.064-μm pulses. The results are compared with those for other laser materials in the 1.06-μm spectral region.

Laser-induced dielectric breakdown and mechanical damage in silicate glasses

Dielectric breakdown and mechanical damage in silicate glasses under high-intensity laser radiation are investigated in detail. From thermoelastic considerations a criterion for the validity of possible damage mechanisms is established. It is shown that stimulated Brillouin scattering cannot give rise to an effective absorption of 50 cm−1 in the focal volume as required by the thermoelastic considerations. It is proposed and established that the mechanical damage is caused by the acceleration of primary electrons produced by multiphoton ionization, leading to a fully developed instability in a few nanoseconds. At this electron density, the absorption in the focal volume is 105 cm−1, and is responsible for the complete absorption of the laser pulse at intensities above the threshold for breakdown. The diffusion and recombination of electrons are found to be negligible, the only rate-limiting process being the loss of electron energy to the lattice. The variation in the threshold intensity for breakdown in different glasses is due to the variations in the elastic scattering cross section.