100-ps Laser Pulses Research Articles

Direct-drive measurements of laser-imprint-induced shock velocity nonuniformities.

Perturbations in the velocity profile of a laser-ablation-driven shock wave seeded by speckle in the spatial beam intensity (i.e., laser imprint) have been measured. Direct measurements of these velocity perturbations were recorded using a two-dimensional high-resolution velocimeter probing plastic material shocked by a 100-ps picket laser pulse from the OMEGA laser system. The measured results for experiments with one, two, and five overlapping beams incident on the target clearly demonstrate a reduction in long-wavelength (>25-μm) perturbations with an increasing number of overlapping laser beams, consistent with theoretical expectations. These experimental measurements are crucial to validate radiation-hydrodynamics simulations of laser imprint for laser direct drive inertial confinement fusion research since they highlight the significant (factor of 3) underestimation of the level of seeded perturbation when the microphysics processes for initial plasma formation, such as multiphoton ionization are neglected.

Tunable CO2 laser system with subnanosecond-pulse-train output

A CO2 laser system has been demonstrated that generates a train of subnanosecond pulses tunable over the P and R branches of the CO2 laser spectrum at 9–11µm. It utilizes optical free induction decay to generate a single ~100-ps laser pulse from a tunable transverse-excited-atmospheric CO2 laser. This laser pulse is injection-seeded into a high-pressure CO2 oscillator whose output consists of a train of amplified ~100-ps pulses with maximum pulse energy of 30mJ, corresponding to a peak power of 300MW. The ~100-ps, tunable, infrared laser pulses are needed for a new technique to remotely detect atmospheric gaseous molecules, which relies on the train of CO2 laser pulses selectively exciting the target molecules whose presence is then revealed using a separate terahertz probe beam.

Optimization of 4.7-keV X-ray titanium sources driven by 100-ps laser pulses

An experiment with thin titanium foils irradiated by two pulses delayed in time is conducted on the Shenguang-II laser facility. A prepulse induces an underdense plasma, 2-ns later a main pulse (λL = 0.35 μm, EL ≈ 120 J, τL ≈ 100 ps) is injected into the underdense plasma and produces strong line emission from the titanium K shell (i.e., Heα at 4.7 keV). Data show that the intensity of 4.7-keV X-ray emission with the prepulse is approximately twice more than without the prepulse, and can be used as a backlighting source satisfying the diagnostic requirements for dense plasma probing. High-quality plasma images are obtained with the backlighting 4.7-keV X-rays in a Rayleigh—Taylor hydrodynamic instability experiment.

Microdrilling of metals with an inexpensive and compact ultra-short-pulse fiber amplified microchip laser

We have investigated the ultra-fast microdrilling of metals using a compact and cheap fiber amplified passively Q-switched microchip laser. This laser system delivers 100-ps pulses with repetition rates higher than 100 kHz and pulse energies up to 80 μJ. The ablation process has been studied on metals with quite different thermal properties (copper, carbon steel and stainless steel). The dependence of the ablation depth per pulse on the pulse energy follows the same logarithmic scaling laws governing laser ablation with sub-picosecond pulses. Structures ablated with 100-ps laser pulses are accompanied only by a thin layer of melted material. Despite this, results with a high level of precision are obtained when using the laser trepanning technique. This simple and affordable laser system could be a valid alternative to nanosecond laser sources for micromachining applications.

Formation of laser plasma channels in a stationary gas

Plasma channels with nonuniformity of about ±3.5% have been produced by a 0.3J, 100ps laser pulses in a nonflowing gas, contained in a cylindrical chamber. The laser beam passed through the chamber along its axis via pinholes in the chamber walls. Plasma channels with an electron density in the range of 1018–1019cm−3 were formed in pure He, N2, Ar, and Xe. A uniform channel forms in an optimal pressure range at a certain time delay, depending on the gas molecular weight. The interaction of the laser beam with the gas leaking out of the chamber through the pinholes was not significant. However, the formation of the ablative plasma on the walls of pinholes by the wings of radial profile of the laser beam plays an important role in the plasma channel formation and its uniformity. A low-current glow discharge initiated in the chamber improves the uniformity of the plasma channel slightly, while a high-current arc discharge leads to overdense plasma near the front pinhole and further refraction of the laser beam. These results indicate the potential for using nonflowing gas targets to create uniform plasma channels.

Pump–probe studies of EUV and X-ray emission dynamics of laser-irradiated noble gas droplets

The interaction of high intensity 100-ps laser pulses with micron-sized noble gas (argon and krypton) droplets is experimentally investigated via a series of pump–probe experiments monitoring the delay-dependent X-ray and extreme ultraviolet (EUV) emission, and by imaging frequency-doubled probe light scattered from the interaction region. An understanding of the time scales for this interaction is important for optimization of EUV sources for next-generation lithography that utilizes laser-produced plasmas (LPP). Depending on the spectral region of interest, the type of emission, and the droplet characteristics, the effective emission lifetime was found to extend from a few hundred picoseconds to as long as several nanoseconds, in agreement with the expected plasma expansion, EUV excitation, and recombination emission time scales.

Specific Impulse and Other Characteristics of Elementary Propellants for Ablative Laser Propulsion

This work was performed as a continuation of ablative laser propulsion (ALP) research recently revived by the authors. An ALP-based space vehicle would be driven by high-energy, short-duration (10 -10 s and less) laser pulses, focused on a solid propellant. Under such irradiation conditions, plasma and laser pulses are temporally separated, and the direct ablation of solid propellant dominates other possible momentum transfer mechanisms. The studies were performed for various elemental targets, ablated at irradiances up to ∼3 × 10 13 W/cm 2 delivered by 100-ps laser pulses at a wavelength of 532 nm. The data, collected predominantly from time-of-flight technique, include ion velocities, number densities, ion density angular distributions, and mass-removal rates. From these data, specific impulses and net momenta exerted on targets by ablation were calculated. For the irradiation conditions used, the highest I sp and lowest mass-removal rates were achieved for low-Z targets, whereas high-Z elements provided higher momenta. The upper limits for I sp derived from ion velocity reached 2 x 10 4 s with carbon (graphite). Ion energies varied from element to element in the range of ∼0.4-2.0 keV, with ion fractions of ∼0.8-3.8% and mass-removal rates in the range of 0.1-3.0 μg per laser pulse. Ion velocities exhibit Maxwellian distributions, whereas ion number densities in the plasma plume were fitted to a cosine function. All studied characteristics indicate that high-I sp ALP must be based on solid low-Z propellants such as a carbon or aluminum.

A pump–probe investigation of laser-droplet plasma dynamics

The temporal character of the plasma formed by the irradiation of micron-sized argon and krypton droplets with intense 100-ps laser pulses is investigated using a pump–probe scheme. The evolution of the droplet plasma is assessed by monitoring delay-dependent x-ray and extreme ultraviolet (EUV) emission, and by imaging frequency-doubled probe light scattered from the interaction region. Depending on the spectral region of interest, the type of emission, and the droplet characteristics, the effective plasma lifetime can extend from a few hundred picoseconds to as long as several nanoseconds. Knowledge of the droplet plasma lifetime is important in optimizing the efficiency of this laser plasma as an EUV or x-ray source.

Time-resolved electron diffraction study of the Ge( [formula omitted])-(2×1)–(1×1) phase transition

The dynamics of the Ge(1 0 0)-(2×1)–(1×1) phase transition has been investigated by 100-ps time-resolved reflection high-energy electron diffraction (RHEED). A laser pulse heats the surface while a synchronized electron pulse is used to obtain a diffraction pattern of the surface. For slow heating, on a heated stage, the Ge(1 0 0)-(2×1) reconstruction is observed to lose its long-range order between 900 and 1000 K. For heating with 100-ps laser pulses, time-resolved electron diffraction shows that the Ge(1 0 0)-(2×1) reconstructed surface starts to disorder at 1027±44 K, 127±44 K above the onset temperature of 900 K for the disordering of Ge(1 0 0)-(2×1) observed for slow heating. The overheating of the Ge(1 0 0)-(2×1) structure by ultrafast laser pulses is indicative of the first-order nature of the transition, and provides support for the conclusion that the mechanism of the Ge(1 0 0)-(2×1)–(1×1) phase transition involves (2×1) domain wall proliferation instead of dimer break-up. RHEED intensity and profile analysis under slow heating shows increased surface vacancy density and roughness with temperature. The onset of the increase in surface roughness, as measured by RHEED profile width, correlates with the Ge(1 0 0)-(2×1)–(1×1) phase transition.

Time-resolved structural study of low-index surfaces of germanium near its bulk melting temperature

The structure of the low-index surfaces of germanium near its bulk melting temperature is investigated using 100-ps time-resolved reflection high-energy electron diffraction. The surface is heated by 100-ps laser pulses while a synchronized electron beam probes the structure. Ge(111) was observed to remain in its incomplete melting structure up to at least ${T}_{m}+134\ifmmode\pm\else\textpm\fi{}40\mathrm{K}$ when heated by a 100-ps laser pulse. Both the Ge(100) and Ge(110) surfaces are observed to melt near the bulk melting temperature when heated with 100-ps laser pulses. Because of the low-diffraction intensity-to-background ratio at high temperatures and because of the temperature uncertainty in the time-resolved experiments, we are unable to accurately identify the melting point of Ge(100) and Ge(110) when heated with a 100-ps laser pulse. The results, however, favor the lack of surface superheating of Ge(100) and, to some extent, Ge(110). The superheating of the incomplete melting state of Ge(111) could be due to the metallization of the top germanium bilayer and its interaction with the solid underneath causing an energy barrier sufficient to allow for transient surface superheating.

Direct-Drive Implosion Experiments on the SG-II Laser Facility

SG-II, a 8-beam Nd:glass laser with an output energy capability of 6kJ at 1.053μm, was built and direct-drive implosions were successfully performed early in 2000. Both exploding pusher and ablative targets were imploded using glass capsules with diameters of 200 and 500μm, and a wall thickness of about 1μm. The deuterium and tritium (DT) gas pressure filled in these capsules were 2.0 and 0.5MPa, respectively. Sophisticated diagnostics were deployed to measure laser absorption, hot electron temperature and fraction, thermal electron temperature, neutron yields, ion temperature, temporally resolved x-ray images, fuel areal density, alpha particle image, and so on. Significant results, such as neutron yields up to 4 × 109 for exploding pushers with 100-ps laser pulse irradiation and 6 × 108 for ablative targets with 1-ns pulses and clear x-ray images to see the compression process, were obtained. Numerical simulations were conducted to optimize target and laser parameter design and to duplicate the results afterwards with the specific shot parameters used in the experiment.

Surface morphology of laser-superheated Pb(111) and Pb(100)

The surface step density on the vicinal Pb(111) and the surface vacancy density on Pb(100) after laser superheating and melting are investigated using reflection high-energy electron diffraction. With {approximately}100-ps laser pulses, Pb(111) surface superheating does not significantly change the density of the steps and step-edge roughness. However, after laser surface melting, the average terrace width and the string length at the step edge become as large as those at room temperature. The average terrace width at 573 K changes from 38{plus_minus}15 to 64{plus_minus}19 {Angstrom} after laser surface melting, while the average string length at the step edge changes from 90{plus_minus}14 to 250{plus_minus}38 {Angstrom}. For Pb(100), the surface vacancy density remains unchanged when the surface is superheated without melting. However, when the laser fluence is high enough to cause surface melting, the surface vacancy density increases. This increase in vacancy density is attributed to fast diffusion of atoms in the liquid film formed on Pb(100) during laser melting. {copyright} {ital 1998} {ital The American Physical Society}

Laser-induced radiation leakage from microdroplets

Intensity distributions on the droplet surface and within the droplet are calculated and found helpful in explaining the observed laser-induced radiation leakage from various locations on or within the droplet. In an attempt to distinguish various possible mechanisms for laser-induced perturbation of micrometer-sized droplets, two types of experiment are conducted, and the measured intensity distribution is compared with that predicted by theoretical computation. The observed extra-light leakage of dye-laser radiation trapped in the droplet can be grouped into quasi-instantaneous and cumulative effects. One set of experiments uses a variable number of 100-ps laser pulses because of the laser’s low fluence at 0.1 GW/cm2. Quasi-instantaneous perturbations associated with an intensity-dependent index of refraction change are found to be important. Cumulative perturbations become evident as the number of 100-ps pulses increases. The second set of experiments uses one green laser beam and one near-IR laser beam, both with 7-ns duration, which because of the high fluence emphasize the electrostrictive- and temperature-related perturbations. Extra leakage from surface perturbations caused by the laser-induced temperature rise cannot be isolated but is found not to be dominant.

Nonlinear pulse propagation: a simple model and experiment

It is shown that the propagation of a light pulse in an optical fiber is accurately described using the proposed simplified equation containing only one parametric constant denned by the relationship between the phase self-modulation and the group velocity dispersion. The results of a numerical solution of this equation in its range of validity bounded by the Raman lasing threshold showed good quantitative agreement with the experimental data obtained for the propagation of a 100-ps YAG laser pulse in an optical fiber.

Softening of the H2+ molecular bond in intense laser fields.

We report a new kind of multiphoton dissociation in intense laser fields. ${\mathrm{H}}_{2}^{+}$ molecular ions were formed in above-threshold ionization of ${\mathrm{H}}_{2}$ gas by intense 532-nm, 100-ps laser pulses. Observations suggest that multiphoton couplings soften the molecular bond, resulting in dissociation at laser intensities over 50 TW/${\mathrm{cm}}^{2}$. The ion-dissociation spectra have multiple peaks caused by multiphoton transitions during dissociation of ${\mathrm{H}}_{2}^{+}$.

A laser-induced ultrasonic probe of the mechanical properties of aligned lipid multibilayers

The recently developed laser-induced phonon spectroscopy (LIPS) technique is applied to the determination of dynamic mechanical properties of aligned dilauroylphosphatidylcholine (DLPC) multibilayer arrays containing 2 and 20% water by weight. Sample excitation by two crossed 100-ps laser pulses generates a longitudinal ultrasonic wave whose wavelength depends on the crossing angle. In these experiments, the acoustic wave propagates parallel to the bilayer planes. The ultrasonic velocity and attenuation are monitored through the diffraction of a variably delayed probe pulse by the acoustic grating. The velocity measures the lateral area compressibility of the bilayers, while the attenuation is related to the viscosity. Velocities obtained in the gel and liquid crystal phases are compared with those found previously using Brillouin scattering. The acoustic attenuation is shown to be an order of magnitude more sensitive to the gel-liquid crystal phase transition than the velocity. The lipid area compressibility and viscosity of DLPC-20% water multilayers with and without 100 mM CaCl2 are found to be identical within our experimental error.

Time-Resolved X-Ray Spectroscopy of Laser-Produced Plasmas

X-ray emission spectra, produced by irradiation of neon-filled aluminum-coated glass microballoon and plane aluminum targets, with 100-ps Nd-glass laser pulses, have been time resolved with an x-ray streak camera. The time evolution of the $\mathrm{Ne}\ifmmode\times\else\texttimes\fi{}L\ensuremath{\alpha}$ and Si xiii $1^{1}S_{0}\ensuremath{-}3^{1}P_{1}$ linewidths has been recorded. The implosion delay time of microballoon targets and the emission histories of the Al xiii $1^{1}S_{0}\ensuremath{-}2^{1}P_{1}$ line and its satellites from plane targets have been measured.