Pulsations and chaos in laser-produced plasmas

A general criterion is proposed predicting the onset of chaotic behavior for parametric processes in a laser-produced plasma. The conditions when the transition to the stochastic regime occur are determined for various parametric instabilities excited when a high intensity laser interacts with a plasma. The complicated temporal structure observed in 3/2₀, 2₀, 1₀, and fast electron emission in experiments using high-intensity (I10^15–17 W/cm²), short ( _L 40–200 psec) Nd laser pulses is attributed to the reflection seeded SBS instability being driven into this regime.Lebedev Physics Institute, Leninsky Prospect 53, Moscow 117924 Russia.     

Nonlinear refraction and nonlinear absorption of As2S3 aqueous solution

We report for the first time to our knowledge on the preparation of colloidal solution of chalcogenide semiconductor As₂S₃ by laser ablation and the measurements of its nonlinear-optical characteristics using Z-scan method at the wavelength of Nd:YAG laser radiation ( = 1064 nm, = 25 ns). The nonlinear refractive index was measured to be –7.5 × 10^–18 m² W^–1. Nonlinear absorption coefficient of chalcogenide solution was measured to be 1 cm GW^–1.     

Nonlinear effects and the transformation of wave energy in an inhomogeneous plasma

This paper discusses the problem: under what conditions does the quasilinear transformation of waves in plasma with an inhomogeneous beam remain a dominant effect if the nonlinear interaction of waves is respected. It is shown that in the region of densities _p ² _B ² the nonlinear interaction between the high-frequency and the newly excited waves can already affect the initial stage of the process and can result in the establishment of the Rayleigh-Jeans distribution for the two groups of waves. If, on the other hand, the low-frequency waves grow first with a quasilinear increment, the nonlinear transformation of energy inside a group of low-frequency waves is again apt to affect substantially their final amplitude and thus also the total amount of energy that can be transformed into those waves. The conditions under which a particular nonlinear effect predominates are derived for various characteristic values of the density of a cold plasma.     

Optical properties of low-temperature magnesium laser plasma

Results are reported of an experimental study of ehe optical properties of the plasma produced when magnesium vapor is irradiated by resonant laser emission ( = 285.2 nm). The spectral and temporal characteristics of the vapor luminescence were measured. An additional laser source was used to study the kinetics of the plasma formation and decay. The conditions for obtaining a plasma with a large ion content were determined. At 1 kW resonance radiation and at a magnesium-atom concentration 4·10¹⁵ cm^-3 the ion concentration was 2·10¹⁴ cm^-3. Lasing on the 4³S₁–3³P₂ transition of the magnesium atom ( = 518 nm) was obtained for the first time and was the result of rapid transfer of the excitation from the singlet system of levels to the unperturbed triplet system via secondary processes accompanying the particle collisions.Translated from Lazernye Sistemy, pp. 35–45, 1982.     

Gas laser excitation by an electron beam formed at open discharge

The method of formation of powerful electron beams (EBs) with nanosecond duration has been described (electron energyu10 to 20 keV and beam currentl _EB>1 kA) in the medium pressure gas which is simultaneously the laser active medium. The value ofl _EB=7 kA has been achieved atU=22 kV, =20 ns andp _Ne=300 Pa in a coaxial cell 330 mm long and 99 mm in diameter. The 3 kA current from the cathode with 60 cm² square in the shape of a segment of a sphere 140 mm in diameter has been achieved atU=23 kV, =15 ns andp _Ne=300 Pa. With the help of a magnetic field this beam has been transported to a distance of up to 100 cm.Applications of the beams for pumping various lasers were broadened. Lasing in He + Cd⁺ mixtures at the optimal He pressure of 10 to 12 kPa and in the triple mixture He-Cd-Kr has been obtained. Using the method of doubled excitation pulses it has been shown that EB pumping provides preservation of the lasing pulse parameters in the Eu atom laser in comparison with glowing discharge, the sequential frequency being an order of magnitude greater. Lasing at a significantly greater ion concentration than in gas discharge has been obtained in the ion laser on the self-terminating transition of Ca⁺,=866 nm. Quasicontinuous lasing has been realized in the mixture He + Sr + Kr in the plasma laser on Sr⁺,=430.6 nm.     

Characterization of preformed plasmas with an interferometer for ultra-short high-intensity laser-plasma interactions

The evolution of an Al preformed plasma produced by a prepulse was observed before and after the arrival of the main pulse by an interferometer using a femtosecond probe pulse. A central density depression due to the ponderomotive force of the main laser pulse in the preformed plasma with a 100 m scale length was clearly visible after the main pulse irradiation at an intensity of 5×10¹⁶ W/cm². The temporal profiles of the prepulse, characterized by a cross-correlation in conjunction with a precise density profile measurement by an interferometer, contribute to the better understanding of femtosecond laser-matter interactions. PACS 52.38.-r; 52.50.Jm; 52.70.-m     

Time-resolved UV absorption of polyimide

The 355-nm transient absorption of polyimide thin films has been measured following excitation withsubablative, 24-ps long, 355-nm laser pulses. The 355-nm absorption increases by 25% following 355-nm, 20 mJ/cm² excitation and recovers with a fast time constant 34 ps, and a slow time constant which is much longer than 6 ns. The data are fitted by a three-level rate equation model incorporating the temperature dependence of the ground state absorption coefficient. The fast component is attributed to the decay ofS ₁ and the slow component results from increased ground state absorption caused by a laser-induced temperature rise. The nonlinear intensity dependence is attributed to excited state (S ₁) absorption. These results indicate the importance of considering the dynamic absorption in modelling ablation.     

Laser Annealing of SiO2 Glass with Copper Nanoparticles

Composite layers formed in SiO₂ by implantation of 50keV Cu⁺ ions with a dose of 8·10¹⁶ cm^–2 at an ion current density of 10 A/cm² have been investigated. It is shown that ion implantation carried out under the chosen conditions allows one to synthesize copper nanoparticles in the surface region of a dielectric. The exposure of composites to highpower pulses of an excimer krypton laser at a wavelength in the SiO₂ transmission region has been investigated. In the absence of effective optical absorption by a glass substrate, the dynamics of the change in the structure of the layer with metallic nanoparticles is determined by the number of laser pulses. It has been established that at the initial stage of pulsed irradiation fragmentation of the largest nanoparticles occurs followed by the inverse process of their agglomeration as a result of slight heating of the glass matrix; further exposure to the laser irradiation leads to an effective accumulation of energy in the particles and, as a consequence, to their melting and dissociation into small clusters and individual atoms.     

Studies on excimer laser doping of GaAs using sulphur adsorbate as dopant source

Excimer laser doping of GaAs using sulphur adsorbate as a dopant source is demonstrated. Box-like n-type layers of depths of about 100 nm with carrier concentration as high as (23)×10¹⁹ cm^–3 are formed. Passivation of GaAs using a (NH₄)₂S_x solution for 40 min followed by sublimation of the excess sulphur atoms in high vacuum result in an effective dopant for controllable n-type doping. The samples are irradiated using a KrF excimer laser in a N₂ gaseous environment. Secondary ion mass spectrometry (SIMS) measurements show that sulphur is successfully incorporated in the GaAs. The sheet resistance is controlled by adjusting the laser energy fluence and number of laser pulses. Rutherford backscattering spectrometry with channeling (RBS/C) alignment measurement indicates that lattice damage is undetectable for N₂ gas pressures of 760 Torr.     

Resonance lonization mass spectroscopy with a pulsed thermal atomic beam

Resonance ionization mass spectroscopy (RIMS) and pulsed-laser induced desorption (PLID) have been combined for ultrasensitive detection and spectroscopy of very small samples of refractive elements. The method has been tested and applied to laser spectroscopy of 5×10⁹ atoms (1.5 pg) of¹⁹⁵Au (T _1/2= 183d) implanted at the ISOLDE online mass separator with 60 keV into graphite. A pulsed thermal atomic beam was formed by laser desorption with a 10 ns NdYag laser pulse. Subsequently the atoms were photoionized in a three-colour, three-step resonant excitation to an autoionizing state. The selectivity was enhanced by a time-of-flight measurement of the photo ions. In resonance, one ion was detected per 10⁵ atoms implanted resulting in a gain in detection efficiency by three orders of magnitude in comparison to the use of a continuous atomic beam. In the course of the experiments several unknown autoionizing states were found, and the lifetime of the 6d ² D ^3/2 state of gold was determined to be=10.7(6) ns.     

Laser cooling of trapped Yb+

¹⁷²Yb⁺ ions in an rf trap have been laser cooled for the first time by driving the ² S _1/2–² P _1/2 transition at 369.5 nm. It was necessary to irradiate the ions with 2.438 m infra-red radiation to depopulate the metastable ² D _3/2 state. An upper limit on ion energies was determined by observing the size of the trapped cloud and corresponds to a temperature below 2 K. Cooled ion lineshapes were compared with simulations and coherence nulls were observed in the infra-red frequency scans.     

An argon ion ring laser as a gyroscope

As a prototype gyroscope for a precision measurement of the Earth's rotation _E, we set up an argon ion ring laser with an area of 1.4 m². Different cavity geometries were tested in order to achieve a cancellation of the effect of the plasma flow. In a set-up with two laser tubes of the same type facing towards opposite directions, a stability of the beat frequency of 2–3 _E was measured. In a configuration with a double transition of the laser beam through the tube, the lock-in threshold was too high for the Earth's rotation to be measured, however, no effect of the plasma flow was observed.     

Simultaneous detection of atomic and molecular hydrogen using a tunable ArF excimer laser

We report preliminary experiments which demonstrate the simultaneous detection of both atomic and molecular hydrogen using a single tunable ArF laser. The tunable ArF laser was modified to lase simultaneously at two wavelengths by the addition of a second grating to the oscillator stage as in earlier work by Ketterle et al. for KrF operation. To our knowledge, this is the first demonstration of dual-wavelength capability with a tunable ArF excimer. The H atom diagnostic utilized tunable ArF excimer laser output at 193.29 nm which was Raman shifted in D₂ (first Stokes) to generate 205.14 nm radiation which then excited the H atom to its (3s, 3d) states via a two-photon transition. Fluorescence was detected at 656 nm on the (3s, 3d) 2p transition. The second wavelength from the tunable excimer was used to excite several rovibronic LIF transitions of hydrogen molecule via two-photon absorption on the E ¹ _g ⁺ X ¹ _g ⁺ (2,0) band. Fluorescence occurs at 750 nm and 830 nm on the (E ¹ _g ⁺ B ¹ _g ⁺ ) transitions.     

Saturated absorption and nonlinear refraction of silicate glasses doped with silver nanoparticles at 532 nm

The nonlinear optical characteristics of silicate glasses doped with silver nanoparticles are investigated by Z-scan technique using second harmonic radiation of picosecond Nd:YAG laser (=532 nm, =55 ps). The real and imaginary parts of third-order nonlinear susceptibility of silver-contained glasses were measured. It was found that the sign of Im ⁽³⁾was negative due to saturated absorption and the sign of Re ⁽³⁾ was changed from negative (self-defocusing) to positive (self-focusing) with growth of laser radiation intensity. The mechanisms responsible for saturated absorption and nonlinear refraction are discussed.     

X-ray emission from short-pulse laser plasmas

Highly intense picosecond and subpicosecond laser pulses interacting with solids can create hot and dense plasmas which emit x-ray pulses in a broad spectral range from 100 eV up to MeV. The duration of these x-ray pulses depends on the transient behaviour of the relaxation and recombination mechanisms, as well as on the lifetime of energetic electrons produced via nonlinear processes in the plasma. This paper reports experiments using a 1.5-ps laser pulse with high constrast ratio (up to 10¹⁰) and intensities up to 10¹⁸ W cm^-2 irradiating solid targets. Both the line spectrum characteristics of a magnesium plasma, recorded using crystal spectrometers with high spectral resolution, and kinetic calculations have allowed the deduction of plasma parameters in the process of plasma evolution. In addition, hard x-ray pulses from a tantalum plasma were measured and their scaling was explained as bremsstrahlung emission from energetic electrons. Absolute dose values of x-ray pulses are given.     

Determination of optical constants and nonlinear optical coefficients of Violet 1-doped polyvinyl alcohol thin film

The optical properties of Violet 1-doped polyvinyl alcohol (PVA) have been investigated using Wemble and Didomenico (WD) method. The optical constants such as refractive index n, the dispersion energy E _d, the oscillation energy E ₀, the lattice dielectric constant \(\varepsilon _{\infty } \), light frequency dielectric constant ε ₀ and the ratio of carrier concentration to the effective mass N/m* have been determined using reflection spectra in the wavelength range 300–900 nm. The single- beam Z-scan technique was used to determine the nonlinear optical properties of Violet 1:polyvinylalcohol (PVA) thin film. The experiments were performed using continuous wave (cw) laser with a wavelength of 635 nm. The calculated nonlinear refractive index of the film, n ₂ = ?2.79×10^?7 cm²/W and nonlinear absorption coefficient, β = 6.31×10^?3 cm /W. Optical limiting characteristics of the dye-doped polymer film was studied. The result reveals that Violet 1 can be a promising material for optical limiting applications.     

Spectral studies of Nd3+ and Er3+ ions in POCl3:SnCl4 laser liquid

The optical absorption spectra of triply ionized neodymium and erbium ions in POCl₃SnCl₄ laser liquid have been studied for the first time in the UV-VIS and NIR regions. Spectroscopic and Judd-Ofelt intensity parameters are evaluated from the observed band positions and their intensities. Radiative lifetimes and the luminescent branching ratios for the excited fluorescent levels of Nd³⁺ and Er³⁺ ions are theoretically estimated and the possible laser transitions are indicated. From the observed splittings of certain bands in the second-derivative spectrum of the Nd³⁺ ion, the crystal field (A ₂₀,A ₄₀) parameters are evaluated assumingC _3h symmetry for the ion.     

Ion–polymer and ion–ion interaction in PEO‐based polymer electrolytes having complexing salt LiClO4 and/or ionic liquid, [BMIM][PF6]

Ion–polymer and ion–ion association in polymer electrolyte films of PEO complexed with salt LiClO₄, ionic liquid (1‐butyl‐3‐methylimidazolium hexafluorophosphate, BMIMPF₆) and (LiClO₄ + BMIMPF₆) have been studied by laser Raman spectroscopy. The cations (Li⁺ and/or BMIM⁺) of the dopant salt/IL are shown to complex with the ether oxygen of the polymer backbone (i.e. C O C bond of PEO). The polymer–cation complexation results in the appearance of an additional peak at ∼1131 cm⁻¹ apart from the C O C stretching vibrations of PEO at ∼1062 and 1141 cm⁻¹. This peak due to polymer–cation complexation is relatively strong for LiClO₄ than BMIMPF₆, indicating stronger interaction for the former. In the PEO:LiClO₄ and PEO:BMIMPF₆ spectra, Raman peaks at 937 and 747 cm⁻¹, respectively related to Li⁺· ClO and BMIM⁺· PF ‘contact ion pairs’, have also been observed as a result of ion–ion association. In the polymer electrolyte PEO:LiClO₄ + BMIMPF₆ which contained two different anions, viz. ClO and PF, an interesting observation of the formation of ‘cross contact ion pairs’ viz. Li⁺· PF and BMIM⁺· ClO is also reported. Copyright © 2011 John Wiley & Sons, Ltd.     

Gas-phase vibrational spectroscopy and ab initio calculations of Rb(H2O)n and Rb(H2O)nAr cluster ions

The competition between ion–water electrostatic interactions and water–water hydrogen bonding in cluster ions depends on several factors, including charge density of the ion and temperature of the system. Infrared photodissociation spectra of Rb⁺(H₂O)_n=2–5 and Rb⁺(H₂O)_n=1–5Ar are presented here and compared to previous experiments involving potassium and cesium. The temperature, or internal energy, of hydrated rubidium cluster ions is controlled by varying the evaporative path available for cluster formation. Warmer clusters (with effective temperatures of 250–500 K) are formed by the evaporation of water, while colder clusters (40–120 K) can be formed by argon evaporation. Colder cluster ions tend to favor conformers with more hydrogen bonds compared to those cluster ions at warmer temperatures. Previous work from this laboratory has shown significant and dramatic differences between the spectra of hydrated potassium and cesium ions. With a charge density intermediate between that of K⁺ and Cs⁺, Rb⁺ plays an important role in bridging the gap in our previous studies.     

Preionization of TEA CO2 laser by sliding discharge

TEA CO₂ laser preionization by plasma sheet formed by discharge sliding over a dielectric surface is described. The preionization electron number density in order of 10⁹cm^–3 was measured in the CO₂ N₂ He=113 gas mixture. The plasma sheet was also tested as a main discharge electrode in TEA CO₂ laser.The authors would like to acknowledge with thanks the current interest and the help of P. Gavrilov and V. Krajíek in experiments.