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 3×10¹⁴W/cm².     

Laser-induced Alignment and Coulomb Explosion of CO2

实验研究了CO₂分子在飞秒强激光脉冲作用下的动力学过程,包括分子取向,隧穿电离和库仑爆炸,激光强度从1×10¹³W/cm²变化到6×10¹⁴W/cm². 当激光强度小于分子的电离阈值时,CO₂分子的非绝热转动激发形成一个相干转动波包,波包演化导致分子沿激光电场方向取向. 激光脉冲结束后,分子取向可以周期性地再现,利用另一束激光可以对取向结构进一步进行修饰. 当激光强度大于分子     

Electron-positron pair production by electromagnetic pulses

Electron-positron pair production in vacuum by a single focused laser pulse and by two counter-propagating colliding focused pulses is analyzed. A focused laser pulse is described using a realistic three-dimensional model based on an exact solution of Maxwell’s equations. In particular, this model reproduces an important property of focused beams, namely, the existence of two types of waves with a transverse electric or magnetic vector (e-or h-polarized wave, respectively). The dependence of the number of produced pairs on the radiation intensity and focusing parameter is studied. It has been shown that the number of pairs produced in the field of a single e-polarized pulse is many orders of magnitude larger than that for an h-polarized pulse. The pulse-intensity dependence of the number of pairs produced by a single pulse is so sharp that the total energy of pairs produced by the e-polarized pulse with intensity near the intensity I _S = 4.65 × 10²⁹ W/cm² characteristic of QED is comparable with the energy of the pulse itself. This circumstance imposes a natural physical bound on the maximum attainable intensity of a laser pulse. For the case of two colliding circularly polarized pulses, it is shown that pair production becomes experimentally observable when the intensity of each beam is I ～ 10²⁶ W/cm², which is one to two orders of magnitude lower than that for a single pulse.     

Linear photogalvanic current excited in sillenite crystals by femtosecond laser pulses

We observe electric pulses generated in sillenite crystals (Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀) by 100-fs laser pulses at the wavelength of 400 nm (below the band gaps of both crystals). The peak value of the current pulses scales linearly with the intensity of laser pulses up to ∼45 GW/cm². The direction of the induced current depends on the polarization state of the laser pulse. This polarization dependence and features of the current detection via charge accumulation at the sample electrodes allow us to conclude that the electric pulses are generated due to the linear photogalvanic effect.     

Vacuum ultraviolet luminescence from a micro-pulling-down method grown Nd:(La0.9,Ba0.1)F2.9

Vacuum ultraviolet (VUV) luminescence from a Nd³⁺:(La_1−x,Ba_x)F_3−x (x=0.1) and a Nd³⁺:LaF₃ single crystal grown by the micro-pulling-down method modified for fluoride crystal growth is discussed. Emission resulting from excitation with 157 nm pulses of a F₂ laser and by 290 nm femtosecond pulses of a Ti:sapphire laser show that the luminescence spectral and temporal characteristics are similar for both excitation cases and that they have good prospects as a VUV laser material.     

Experimental preparation of entangled Bell’s vacuum-single exciton and vacuum-biexciton states for pair centers of neodymium ions in a crystal

The process of low temperature laser excitation of neodymium ion M pair centers in CaF₂ crystals at the ⁴I_9/2-⁴G_5/2 optical transition is analyzed. It is shown that maximally entangled Bell’s vacuum-single exciton and vacuum-biexciton states are experimentally prepared when irradiating these crystals by nanosecond laser pulses.     

Dense laser-driven electron sheets as relativistic mirrors for coherent production of brilliant X-ray and γ-ray beams

Several techniques exist to obtain brilliant X-ray beams by coherent reflection from relativistic electrons (E _e=γ mc ²) with Doppler frequency upshift of 4γ ². We describe a new approach starting with an ultra-thin solid target. Larger ‘driver’-laser intensities with high contrast are required to produce dense electron sheets. Their acceleration in vacuum results in a transverse momentum component besides the dominant longitudinal momentum component. The counter-propagating ‘production’ laser for optimum Doppler boost in X-ray production by reflection has to be injected opposite to the electron direction and not opposite to the driver laser. Different measures to increase the reflectivity of the electron sheet via laser trapping or free-electron-laser-like micro-bunching are discussed, extending the photon energy into the MeV range. Here, first-order estimates are given.     

Coherent optical pulse evolution in a CO2 amplifier

The coherent reshaping of short duration (2–5 nsec) CO₂ laser pulses in a low-pressure (∽ 5 torr), longitudinal discharge CO₂ amplifier is experimentally studied in the linear regime for a variable number of gain lengths (αL?7). Single pulses grow considerably in duration as well as amplitude in agreement with theoretical considerations. Analysis of the observed pulse evolution is used to obtain the transverse relaxation parameter T₂. Zero-degree pulses {∫^+∞_-∞E ( z, t) dt = 0} are observed to terminate much of the long tail which occurs in single-pulse amplification. Off-resonant amplification of short-duration pulses is shown to lead to dramatic changes in the zero-degree pulse evolution. Numerical calculations relating to pulse amplification in the nonlinear regime for high-pressure CO₂ amplifiers are also presented.     

Reactions induced in (CF3I) n clusters by femtosecond UV laser pulses

The excitation and ionization of CF₃I molecules and their clusters by femtosecond UV laser pulses is studied. It is concluded that the types of excitation of free CF₃I molecules and their clusters by femtosecond UV laser pulses are different. The composition and kinetic energy of ion products observed upon the ionization of (CF₃I) _n clusters by femtosecond pulses are found to differ considerably from those obtained upon ionization by nanosecond pulses. It is shown that the molecular I ₂ ⁺ ion is produced in reactions induced in (CF₃I) _n clusters by UV radiation. Using the pump-probe method, we found the two channels of producing I ₂ ⁺ ions with characteristic times ??₁ ?? 1 ps and ??₂ ?? 7 ps. A model of the reactions under study proposed in the paper is consistent with our experimental results.     

Generation of characteristic x rays by a terawatt femtosecond chromium-forsterite laser

Characteristic K _α x rays arising when a metallic target is irradiated by femtosecond infrared pulses that are generated by a terawatt chromium-forsterite laser system (1240 nm, 90 mJ, 80 fs) are studied. The absolute yield (up to 3 × 10⁸ photons/sr pulse) and the coefficient of the transformation of laser radiation to K _α radiation (maximum value ≈0.03%) are measured for an iron target. The dependence of the radiation intensity on the angle of incidence of p polarized laser radiation is analyzed. The mechanisms of the production of fast electrons responsible for generating characteristic x rays are discussed.     

Influence of Na-related defects on ArF laser absorption in CaF2

ArF laser pulse transmission through commercial high purity CaF₂ is determined by measuring the energy of each pulse before and behind the sample up to an incident fluence H of 10 mJ/cm². The steady state transmission of ArF laser pulses decreases with increasing fluence. The related absorption coefficients α ^st(H) are proportional to H and rationalized by effective 1- and 2-photon absorption coefficients 2.4×10^?4 cm^?1≤α ^eff≤16.8×10^?4 cm^?1 and 1.7×10^?9 cm?W^?1≤β ^eff≤9.3×10^?9 cm?W^?1, respectively. The α ^eff and β ^eff values increase with the Na content of the CaF₂ samples as identified by the fluorescence of Na-related M _Na centers at 740 nm. This relation is simulated by a rate equation model describing the ArF laser induced M _Na generation in the dark periods between the laser pulses and their annealing during laser irradiation. M _Na generation starts with intrinsic 2-photon absorption in CaF₂, yielding self-trapped excitons (STE). These pairs of F and H centers move upon thermal activation and the F centers combine with F _Na to form M _Na centers. M _Na annealing occurs by its photo dissociation into a pair of F and F _Na centers.     

Subpicosecond photon echoes by using nanosecond laser pulses

Subpicosecond time resolutions have been obtained in photon echoes when a sample was excited by two nanosecond dye laser pulses with a smooth and broad spectrum. The dye laser was pumped by second harmonics of a Q-switched Nd:YAG laser, and the pulse width was 10 ns. The sample was 3% Nd³⁺-doped silicate glass, and the center frequency of the dye laser was tuned at 5910 Å on resonance with the ⁴I_{$\text{9}\text{2}$} ? ²G_{$\text{7}\text{2}$}, ⁴G_{$\text{5}\text{2}$} transition of Nd³⁺. The homogeneous transverse relaxation time T₂ was measured to be 91 ps at 10 K in agreement with the previous measurements by picosecond pulses.     

Field-induced ionization and Coulomb explosion of nitrogen

Femtosecond-laser field-induced ionization and Coulomb explosion of diatomic nitrogen were systematically investigated using time-of-flight mass and photoelectron spectrometry. Both linearly and circularly polarized femtosecond laser pulses were used at intensities varying from 5×10¹³ to 2×10¹⁵ W/cm². Strong N₂ ⁺, N₂ ²⁺, N⁺, N²⁺ and N³⁺ ion signals were observed for horizontally polarized pulses. Moreover, signals from the atomic ions exhibited a double-peak structure. Suppression of ionization was observed for circularly polarized pulses, while for vertically polarized pulses, only N₂ ⁺ and N₂ ²⁺ ions were observed. The angular distributions of the ions were measured under zero-field conditions in the ionization zone. The atomic ions N⁺, N²⁺ and N³⁺ exhibited highly anisotropic distributions, with maxima along the laser polarization vector and zeroes normal to the laser polarization vector. In contrast to the atomic ions, N₂ ⁺ exhibited a strong isotropic angular distribution. These observations indicate that dynamic alignment is responsible for the observed anisotropic angular distribution of the atomic ions. The kinetic energy spectrum of the photoelectrons is featureless and broad, extending above the ponderomotive potential of the laser pulse. The angular distribution is markedly anisotropic, with a maximum along the laser polarization vector. These observations further support the notion that the field-ionization mechanism is dominant under our experimental conditions. Received: 29 January 2002 / Revised version: 15 March 2002 / Published online: 12 July 2002     

Isotope-selective ionization utilizing molecular alignment and non-resonant multiphoton ionization

We demonstrate laser nitrogen isotope separation, which is based on field-free alignment and angular-dependent ionization of ¹⁴N₂ and ¹⁵N₂ isotopologues. A linearly polarized short laser pulse (???~?795?nm, ?????~?60?fs) creates rotational wave packets in the isotopologues, which periodically revive with different revival times as a result of different moments of inertia. Another linearly polarized short laser pulse (???~?795?nm, ?????~?60?fs) ionizes one of the isotopologues selectively as a result of their different angular distributions. In the present experiments, the ion yield ratio R [=I(¹⁵N₂ ⁺)/I(¹⁴N₂ ⁺)] can be changed in the range from 0.85 to 1.22, depending on the time delay between the two laser pulses.     

Second-and third-harmonic generation by carbon nanotubes irradiated with femtosecond laser pulses

Femtosecond pulses of a Cr:forsterite laser are used to study second-and third-harmonic generation in a layer of single-wall carbon nanotubes produced by low-velocity spraying. The harmonic amplitude in our experiments scales as (I _p)ⁿ as a function of the pump intensity I _p, with n=2 and 3 for the second and third harmonics, respectively. This scaling law holds up to pump intensities on the order of 10¹²W/cm². The ratio of the maximum signal to the averaged background in the spectra of the second and third harmonics is estimated as 50 and 30, respectively. The second and third harmonics produced by a linearly polarized pump field are also linearly polarized, with their polarization vectors oriented along the polarization direction of the pump field. The capabilities of nonlinear-optical methods for structural and morphological analysis of carbon nanotubes are discussed, as well as ways to create solid-state carbon-nanotube generators of optical harmonic.     

Laser pulses designed in time by adaptive hydrodynamic modeling for optimizing ultra-fast laser–metal interactions

Determining optimal temporal pulse shapes is an essential aspect for controlling the nature and the energetic characteristics of the ablation products following laser irradiation of materials on ultra-fast scales. In this respect, adaptive feedback loops based on temporal pulse manipulation have been inserted into a hydrodynamic code. The procedure enables us to reach the theoretical maximal temperature at a certain energy input. Several regimes have been considered with fluences ranging from the ablation threshold (F _th=0.34 J/cm²) up to 10 J/cm², proposing an optimal coupling for laser–solid and laser–plasma interactions in these fluence regimes. We determine shapes of optimal pulses on ultra-short and short scales (up to 42 ps) and forecast optimized interaction scenarios with fundamental control factors difficult to access experimentally. Simulations performed on aluminum reveal that ultra-short pulses are the natural better solution for localizing energy in space and time for F～F _th. For higher fluences, pulses spread over tens of picoseconds and ended by a final peak enable a better impulsive coupling with the nascent plasma, optimizing its maximal temperature.     

Nonlinear absorption at 10.6 μm in Hg1−xCdxTe

Near band-edge illumination of Hg_1?xCd_xTe (x ≈ 0.2) by short bursts of CO₂ laser radiation has been shown to lead to highly nonlinear absorption and saturation. For sufficiently high infrared intensities the transmission through an absorber is enhanced more than three orders of magnitude. The effect should be useful in temporal shaping of high intensity CO₂ laser pulses.     

Generation of CO2 pulses by free induction decay in KCI:KReO4

Subnanosecond pulses have been produced by optical free induction decay in KCI:KReO₄. Here the ν₃ vibrational mode of the perrhenate ion (ReO^-₄) acts as a resonant absorber for the 10.6μm, P(26) CO₂ laser line. As the phase relaxation time T₂ of the substitutional molecule ion is very small, efficient picosecond pulses can be produced by this medium if faster shuttering can be devised.     

Neutron generation in dense femtosecond laser plasma of a structured solid target

We report neutron production by the ²H(d, n)³He reaction induced upon the illumination of a solid nanostructured target by femtosecond laser pulses of intensity 20 PW/cm² (1 PW = 10¹⁵ W). The target was structured through the preliminary illumination by a laser pulse of the same intensity.     

Third-order nonlinear optical response in transparent solids using ultrashort laser pulses

The third-order optical nonlinearity, χ ⁽³⁾, is measured in transparent glasses (BK7 and fused silica) and crystals (BaF₂ and quartz) using 36-fs, 800-nm laser pulses and the optical Kerr gate (OKE) technique; values are found to lie in the range 1.3–1.7×10^-14 esu, in accordance with theoretical estimates. We probe the purely electronic response to the incident ultrashort laser pulse in fused silica and BK7 glass. In BaF₂ and quartz, apart from the electronic response we also observe contribution from the nuclear response to the incident ultrashort pulses. We observe oscillatory modulations that persist for ～400 fs. The response of the media (glasses and crystals) to ultrashort pulses is also measured using two-beam self-diffraction; the diffraction efficiency in the first-order grating is measured to be in the range of 0.06–0.13 %. Third harmonic generation due to self-phase matching in the transient grating geometry is measured as a function of temporal delay between the two incident ultrashort pulses, yielding the autocorrelation signal.