Incident Laser Flux Research Articles

1064nm nanosecond laser induced ring structure on SiO2 films with different thickness in air and vacuum environments

The ring structure induced by 1064 nm nanosecond laser on SiO2 films with different film thickness was investigated in air and vacuum environments. The damage depth and ring spacing versus the incident laser flux were analyzed. It is found that vacuum induced high defect density as a result of low defect damage threshold, compared with the air environment. And for the same laser fluence, the ring spacing was reduced while the damage depth was increased once the surrounding air was removed. Our research results also indicate that the defects logging in sub-surface of coating layer, play a much more crucial role in the formation process of laser-induced ring structures.

CuInSe2 semiconductor formation by laser annealing

One industrially relevant fabrication method for CuInSe2 absorber layers begins with electrodeposition, which is highly resource efficient and easily up-scalable, followed by a high temperature annealing step. Commonly a furnace is used for annealing, although it is possible to reduce the duration of this step by 2–3 orders of magnitude using laser heating. Past work demonstrated that rastering a 1064nm, continuous wave, neodymium-doped yttrium aluminium garnet laser beam over an electrodeposited precursor film with 1s dwell time, promoted CuInSe2 formation, grain growth, and gave a photovoltaic device with 1.6% power conversion efficiency. However, this device showed inhomogeneous current collection correlating approximately to the Gaussian flux profile of the laser beam. This work demonstrates how deviations in the incident laser flux on the precursor lead to local temperature variation in the film. The temperature affects the rates of atomic diffusion, grain growth and chemical reactions, with the hottest region having the largest crystallites and a stoichiometric and constant composition through the film depth. Spatially resolved photoluminescence yield shows a positive correlation to the temperature profile caused by the Gaussian flux. It is expected that with a homogeneous laser beam flux, spatial variations in absorber layer properties would be eliminated, leading to uniform device properties.

Laser–plasma interaction studies in the context of shock ignition: the regime dominated by parametric instabilities

The shock ignition concept for inertial confinement fusion includes launching a strong shock with a high-intensity laser spike into an imploding shell. The laser intensity in the plasma corona is above the threshold for parametric instabilities, thus providing conditions for strong non-linear effects. Here we present a series of one-dimensional kinetic simulations of laser–plasma interactions in such a regime. After a transient period of strong non-stationary scattering, the laser–plasma interaction enters an asymptotic regime where a significant part of the incident laser flux is absorbed in the plasma and is transformed into hot electrons. The repartition of the absorbed energy and spectral characteristics of the scattered radiation are presented for laser intensities in the range 2.4–24 PW cm−2. For a laser intensity of 8 PW cm−2, the total absorption is 69% ; about 50% of absorption takes place at quarter critical density and the remaining 19% at 1/16th of the critical density. 52% of the total laser pulse energy are absorbed due to stimulated Raman scattering, which produces electrons with a temperature of about 30 keV, and 17% is absorbed due to cavitation, which produces a more isotropic distribution of hot electrons with a temperature of about 10 keV.

A finite element model to predict the ablation depth in pulsed laser ablation

This work presents development of a two-dimensional finite element model to predict temperature distribution and ablation depth in a laser ablation process. The model considers a number of aspects of the process, which hitherto have been considered independently in the literature. The aspects considered include: temperature dependent material properties of the target material, effect of plasma shielding on the incident laser flux, and temperature dependent absorptivity and absorption coefficient of the target. It was evident that these considerations have resulted in a significant improvement in the ability of the model to predict the ablation depth. Finally, the predicted ablation depth was found to match extremely well with experimental results at lower laser fluences, though at higher fluences there is a marginal overestimation.

High energy electron transport in dense plasma in fast ignition scenario

We investigate the high energy electron transport in dense plasma for different laser intensity by using 3D PIC (Three-Dimensional Particle-In-Cell) simulation. We have observed dependence of electron energy flux on incident laser intensity. The electron energy fluxes normalized by the incident laser flux, I/I0, become ≈ 0.2 for the normalized laser electric field of a = 3, ≈ 0.1 for a = 1 and no visible flux for a = 0.2 in dense plasma region, where a = eE/meω0c. For the case of a = 3, we investigated space and time resolved velocity distribution functions and induced longitudinal electric field. We show that the high energy electrons have beam-like velocity distribution with relativistic thermal spread near the energy flux front. We also discuss the electromagnetic instabilities associated with the high energy electron beam taking the relativistic thermal spread of the beam into account.

Transient Raman observations of heme vibrational dynamics in five-coordinate iron porphyrins

Transient resonance Raman spectroscopy has been used to study vibrational dynamics in five-coordinate, high-spin FeII octaethyl porphyrin with a 2-methyl imidazole axial ligand. Vibrational populations of the porphyrin ground electronic state were probed by examining Stokes and anti-Stokes Raman scattering as a function of incident laser flux using ∼10 nanosecond pulses in resonance with the Soret electronic transition. Within a single pulse, each molecule goes through several excitation-decay cycles, building up a non-equilibrium, excited vibrational energy distribution that is exquisitely sensitive to the vibrational mode lifetimes and to the incident laser flux. A kinetic model illustrates these ideas and provides strong support for the interpretation of the results. The flux dependence of the Raman intensities, positions and linewidths suggests that ν3 and ν4 act as “bottleneck” vibrational states, while νCH and ν7 couple more effectively to the environment.

An eigenvalue method for computing the burning rates of HMX propellants

A mathematical model for a three-tiered system consisting of solid, liquid, and gas is derived for studying the combustion of HMX propellants. The resulting nonlinear two-point boundary value problem is solved by Newton’s method with adaptive gridding techniques. In this study the burning rate is computed as an eigenvalue, which removes the uncertainty associated with employing evaporation and condensation rate laws in its evaluation. Results are presented for laser-assisted and self-deflagration of HMX monopropellants and are compared with experimental results. The burning rates are computed over a wide range of ambient pressures and compare well with experimental results from 1 to 90 atmospheres. The burning rate is found to be proportional to the pressure raised to the 0.82 power. Sensitivity of the burning rate to initial propellant temperature is calculated and found to be extremely low, in agreement with past theoretical predictions and experimental data. Results for laser-assisted combustion show a distinct primary and secondary flame separated by a dark zone, the length of which is dependent upon the incident laser flux intensity.

Transient Raman Observations of Heme Electronic and Vibrational Photodynamics in Deoxyhemoglobin

Transient resonance Raman spectroscopy has been used to probe the vibrational dynamics of the heme active site of deoxyhemoglobin during photoexcitation. Near UV pulses of approximately 35 ps in duration were used to both excite the sample and generate resonance Raman spectra of the heme during its rapid electronic and vibrational relaxation. The behavior of the Stokes and anti-Stokes transitions as a function of incident laser flux directly reflects net heme vibrational populations and permits the isolation and characterization of ground and excited electronic state phenomena. Scattering from excited electronic states significantly influences the spectra only at the highest excitation fluxes used in this study. A simple model that accounts for the flux-dependent manifestations of the electronic and vibrational contributions to the heme transient resonance Raman spectra is discussed. In addition, the data presented here clearly show mode selectivity in the vibrational energy distribution associated with t...

An Eigenvalue Method for Computing the Burning Rates of RDX Propellants

A mathematical model for a three-tiered system consisting of solid, liquid and gas is derived for studying the combustion of RDX propellants. The resulting nonlinear two-point boundary value problem is solved by Newton's method with adaptive gridding techniques. In this study the burning rate is computed as an eigenvalue, which can remove the uncertainty associated with employing evaporation and condensation rate laws in its evaluation. Results are presented for laser-assisted and self-deflagration of RDX monopropellants and are compared with experimental results. The burning rates are computed over a wide range of ambient pressures and compare well with experimental results from one to ninety atmospheres. The burning rate is found to be proportional to the pressure raised to the 0.76 power. Sensitivity of the burning rate to initial propellant temperture is calculated and found to be extremely low, in agreement with past theoretical predictions and experimental data. Results for laser-assisted combustion show a distinct primary and secondary flame separated by a dark zone, the length of which is dependent upon the incident laser flux intensity.

Picosecond soft-x-ray source from subpicosecond laser-produced plasmas

Short-pulse high-intensity laser–plasma interactions are investigated experimentally with temporally and spectrally resolved soft-x-ray diagnostics. We demonstrate that, by adjustment of the incident laser flux, the pulse width of the laser-produced x rays emitted from solid targets may be varied to as short as the picosecond time scale. Bright, picosecond, broadband emission characteristic of a short-scale-length high-density plasma is produced only when a high laser contrast (1010) is used. The results are found to be in qualitative agreement with both the predictions of a simple model of radiation from a collisionally dominated atomic system and the results obtained from a numerical simulation.

Characterisation of GaLaS chalcogenide glass thin-film optical waveguides, fabricated by pulsed laser deposition

The fabrication of stoichiometric thin-film optical waveguides of GaLaS via a pulsed laser deposition technique is reported. Stoichiometric films are grown by ablating GaLaS bulk glass with a KrF excimer laser (λ = 248 nm) at an incident laser flux ⩾ 3.5 J/cm 2. The composition of the films is determined by energy-dispersive X-ray analysis and the refractive index is measured by a dark-mode prism coupling technique. Photoinduced structural rearrangement of the as-deposited films leads to a blueshift in the visible absorption edge and a permanent refractive index change, Δ n, of −1%. On the basis of these results, grating structures have been written with both blue light, and e-beam addressing, and their suitability for integrated optical structures assessed.

Picosecond x-rays from subpicosecond-laser-produced hot-dense matter

Short-pulse, high-intensity laser-plasma interactions are investigated experimentally with temporally and spectrally resolved soft x-ray diagnostics. The emitted x-ray spectra from solid targets with various atomic numbers are characterized for a laser pulse width τ 1 ~ 400 fs. With low contrast (10 5), the x-ray spectrum in the λ = 40–100 A ̊ spectral region is dominated by line emission, and the x-ray pulse duration is found to be characteristic of a long-scalelength, low-density plasma. Bright, picosecond, broadband emission, characteristic of a short-scale-length, high-density plasma, is produced only when a high laser contrast (10 10) is used. It is demonstrated experimentally that the pulse width of laser-produced x-ray radiation may be varied down to the picosecond time scale by adjusting the incident ultrashort-pulse laser flux. The results are found to be in qualitative agreement with the predictions of both a code-independent model of radiation from a collisionally dominated two-level ion and a hydrodynamics code coupled to a detailed-configuration atomic physics model. X-ray film measurements of conversion efficiency, along with pinhole camera measurements of the emission region, reveal very high x-ray brightness.

Generation of self-pulsed multiple charged ions by an XeCl excimer laser

Multiple charge heavy ion pulses were extracted from a plasma produced by an XeCl excimer laser beam at a relatively low flux (∼ 30 MW/cm 2) focused on Si, Ge, Mg and Zn targets. An output peak current of Si 3− ions of 375 mA was recorded at an acceleration voltage of 200 V only. The insertion of a variable capacitance between the target holder and the acceleration electrode allows a self-bunching of the ion beam. When a W target was used, a higher incident laser flux (∼ 90 MW/cm 2) was necessary in order to produce ions from it.

Effect of Oxidation and Plume Formation on Low Power Nd-Yag Laser Metal Interaction

The effect of oxide formation and plume formation on laser energy absorption by metallic targets has been studied. The change in directional-hemispherical spectral reflectance of metallic targets during single-shot Nd-Yag laser pulse irradiation was measured using an integrating sphere under controlled environments of both oxygen and argon gas. The spectral transmittance of the plume formed over the targets was also measured using a He-Ne probe laser. The metal targets studied included A16061, Cu, 304 stainless steel, and low-carbon steel. Results obtained show that once a target is hot enough to form a vapor plume, in either argon or oxygen, absorption by the plume significantly limits the amount of laser energy available for absorption at the target surface. Prior to plume formation, the amount of laser energy absorbed by the target is determined by the target surface optical properties. Under the conditions of the present study (incident laser flux of the order of 106 W/cm2 over 0.5–1.0 ms), in the case of absorbing metals (αλ>0.3), such as stainless and low carbon steel, the intrinsic absorptivity of the metal is high enough that the effect of ambient gas (oxygen or argon) on absorptivity is insignificant. In the case of nonabsorbing metals (αλ<0.3), such as Cu and A16061, the intrinsic metal absorptivity is low enough that the ambient gas does have a significant effect on the target absorptivity. The formation of an oxide layer that occurs in an oxidizing environment generally has the effect of increasing the target absorptivity. The relative magnitude of the absorptivity enhancement due to oxide formation depends on the respective absorptivities of the oxide and metal. For metals that form oxides that are intrinsically absorbing in the solid state, such as Cu2O or CuO, the enhancement in absorptivity due to oxide formation can be as much as an order of magnitude. For metals that form oxides that are intrinsically nonabsorbing in the solid state, such as MgO or Al2O3, the enhancement in absorptivity due to oxide formation is more modest but still significant (40 percent). The enhancement in absorptivity in the latter case (MgO or Al2O3) is postulated to be associated either with a thin, absorptive, transition region composed of a mixture of metal and substoichiometric solid oxide just below target surface or with the melting of the oxide at the surface.

Multiphoton ionization spectroscopy of organometallics: The Cr(CO)6, Cr(CO)3C6H6, Cr(C6H6)2 series

The multiphoton ionization (MPI) spectra of the title compounds have been studied in the 3533–6000 Å region in an effort to elucidate the fragmentation and ionization pathways. At wavelengths longer than 4000 Å, the compounds dissociate more rapidly than they are ionized, thereby forming Cr(I) which then can be ionized in a (2+1) resonant process. The resonance wavelengths are identical in the three compounds, and only Cr+ is observed in the MPI-produced mass spectra. At wavelengths shorter than 4000 Å, both Cr+ and CrO+ appear in the mass spectra of Cr(CO)6 and Cr(CO)3C6H6. Surprisingly, however, no ionization of the unbound ligands was observed in any of the complexes studied. Electron time-of-flight measurements in Cr(CO)6 show that at wavelengths shorter than 4000 Å ionization of the parent–molecule resonant state and fragmentation to neutrals with eventual ionization of Cr(I) are important ionization paths. The observation of severe ac Stark broadening of the Cr(I) MPI resonances implies that a rate-equation approach to ion production will be inadequate, except at very low flux. Additionally, the exclusive production of Cr+ indicates extremely rapid dissociation of the intermediate molecular fragments. The breakdown of the rate-equation approach leads to effective power exponents which are smaller than those required by the known Cr+ mass-spectrometric appearance potentials; this indeed is found experimentally. The branching ratios for production of various excited states of Cr(I) are dependent upon the precursor molecule and the incident laser flux.

Laser Ignition of Plasma Off Aluminum Surfaces

The prompt initiation of a plasma above metal surfaces irradiated by a CO2 laser pulse in an intensity range of 106 -109 W/cm2 is modeled. The initiation mechanism is assumed to be the vaporization of flakes or surface defects that are thermally insulated from the bulk surface, followed by laser-induced breakdown in the vapor. The fluid dynamics of the expansion in an air background is modeled in one- and three-dimensional regimes. Breakdown of the vapor due to inverse bremsstrahlung absorption of the laser radiation is calculated specifically for aluminum by use of a Bolt/maim code. Results are presented in the form of a map of breakdown time vs incident laser flux and compared with available experimental data.

High-intensity laser photolysis of OCS at 157 nm: S(1S) production, photoionization, and loss

We have observed the production of significant concentrations of electrons in high-intensity pulsed-laser photolysis of OCS to S(1S) at 157 nm. These results imply a photoionization cross section for S(1S) of 2.6×10−19 cm2. We have measured the quenching of S(1S) for a variety of conditions, including incident laser fluences of 100 mJ/cm2 and initial S(1S) densities of 3×1016 cm−3. Quenching by electrons can be minimized by adding diluents such as SF6 or CF4. We interpret our observed S(1S) decay rates in terms of quenching by S(3P) with a rate constant of 3×10−11 cm3 sec−1 and by CS [and/or O(3P)] with a rate constant near 2×10−10 cm3 sec−1. The measured S(1S) lifetime decreases with increasing initial S(1S) density, reaching a value of about 1 μsec at a S(1S) density of 3×1016 cm−3. The quantum yield for S(1S) production is found to be high (near 0.8) and independent of incident laser flux or added diluent pressure. These storage times and production efficiencies are consistent with the possible use of the S(1S) →S(1D) transition as an efficient high-energy-storage laser.

Laser heating and melting of thin films on low-conductivity substrates

A laser beam, incident on a highly absorbent thin film supported by a poorly conductive substrate, causes that film to heat and melt. The time required to reach the melting point and that required to complete the melting process are calculated as a function of the incident laser flux. The calculations neglect heat losses arising from lateral diffusion, convection, and thermal radiation, but they account for a possible reflectivity change at the melting point. They yield a criterion for the minimal absorbed flux necessary to maintain stable monotonic melting.

Observation of highly ionised species in plasma produced by picosecond laser pulses

Results are presented which show that in the case of the light elements highly ionised species can be detected in laser produced plasmas using a normal incidence spectrograph, the plasmas being formed by picosecond pulses from a high power Nd: glass laser. In particular hydrogenic ions of carbon, oxygen and fluorine have been observed when the incident laser flux density onto a solid target was about 3×10 14 W/ cm 2.

WA4--Intracavity breakdown in CO and CO2lasers

The threshold for intracavity breakdown in large CO and CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> lasers has been calculated from solutions for the electron-energy distribution function as a function of incident laser flux. For fluxes of high intensity and short duration, a threshold fluence of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ft_{p} = 1</tex> J/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for He/N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> mixtures and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ft_{p} \approx 4</tex> J/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for He/CO is predicted. For breakdown at lower fluxes with pulses <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t_{p} \gg 10^{-9}</tex> s, the threshold becomes flux dependent with values of f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cw</inf> between <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4 \times 10^{8} - 10^{9}</tex> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for typical He/N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> mixtures and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_{cw} \sim 5 \times 10^{9}</tex> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for He/CO mixtures. Breakdown by pulses traversing laser-amplifier stages is also investigated using both the rate equation and coherent propagation formulation of the problem. It is found that in most cases of interest, breakdown occurs in the short-pulse regime and the coherent solution for the temporal structure is required.