TW Cm-2 Research Articles

Nonadiabatic Coupling Effects in the 800 nm Strong-Field Ionization-Induced Coulomb Explosion of Methyl Iodide Revealed by Multimass Velocity Map Imaging and Ab Initio Simulation Studies.

The Coulomb explosion (CE) of jet-cooled CH3I molecules using ultrashort (40 fs), nonresonant 805 nm strong-field ionization at three peak intensities (260, 650, and 1300 TW cm-2) has been investigated by multimass velocity map imaging, revealing an array of discernible fragment ions, that is, Iq+ (q ≤ 6), CHn+ (n = 0-3), CHn2+ (n = 0, 2), C3+, H+, H2+, and H3+. Complementary ab initio trajectory calculations of the CE of CH3IZ+ cations with Z ≤ 14 identify a range of behaviors. The CE of parent cations with Z = 2 and 3 can be well-described using a diatomic-like representation (as found previously) but the CE dynamics of all higher CH3IZ+ cations require a multidimensional description. The ab initio predicted Iq+ (q ≥ 3) fragment ion velocities are all at the high end of the velocity distributions measured for the corresponding Iq+ products. These mismatches are proposed as providing some of the clearest insights yet into the roles of nonadiabatic effects (and intramolecular charge transfer) in the CE of highly charged molecular cations.

Understanding laser desorption with circularly polarized light.

We present aspects of emerging optical activity in thin racemic 1,1'-Bi-2-naphthol films upon irradiation with circularly polarized light and subsequent resonant two-photon absorption in the sample. Thorough analysis of the sample morphology is conducted by means of (polarization-resolved) optical microscopy and scanning electron microscopy (SEM). The influence of crystallization on the nonlinear probing technique (second harmonic generation circular dichroism [SHG-CD]) is investigated. Optical activity and crystallization are brought together by a systematic investigation in different crystallization regimes. We find crystallization to be responsible for two counter-acting effects, which arise for different states of crystallization. Measuring crystallized samples offers the best signal-to-noise ratio, but it limits generation of optical activity due to self-assembly effects. For suppression of crystallization on the other hand, there is a clear indication that enantiomeric selective desorption is responsible for the generation of optical activity in the sample. We reach the current resolution limit of probing with SHG-CD, as we suppress the crystallization in the racemic sample during desorption. In addition, intensity-dependent measurements on the induced optical activity reveal an onset threshold (≈0.7 TW cm-2), above which higher order nonlinear processes impair the generation of optical activity by desorption with CPL.

Enhanced Ultrafast Nonlinear Optical Response in Ferrite Core/Shell Nanostructures with Excellent Optical Limiting Performance.

Nonlinear optical nanostructured materials are gaining increased interest as optical limiters for various applications, although many of them suffer from reduced efficiencies at high-light fluences due to photoinduced deterioration. The nonlinear optical properties of ferrite core/shell nanoparticles showing their robustness for ultrafast optical limiting applications are reported. At 100 fs ultrashort laser pulses the effective two-photon absorption (2PA) coefficient shows a nonmonotonic dependence on the shell thickness, with a maximum value obtained for thin shells. In view of the local electric field confinement, this indicates that core/shell is an advantageous morphology to improve the nonlinear optical parameters, exhibiting excellent optical limiting performance with effective 2PA coefficients in the range of 10-12 cm W-1 (100 fs excitation), and optical limiting threshold fluences in the range of 1.7 J cm-2 . These values are comparable to or better than most of the recently reported optical limiting materials. The quality of the open aperture Z-scan data recorded from repeat measurements at intensities as high as 35 TW cm-2 , indicates their considerably high optical damage thresholds in a toluene dispersion, ensuring their robustness in practical applications. Thus, the high photostability combined with the remarkable nonlinear optical properties makes these nanoparticles excellent candidates for ultrafast optical limiting applications.

Highly efficient second-harmonic generation of intense femtosecond pulses with a significant effect of cubic nonlinearity

A highly efficient (73%) second-harmonic generation of femtosecond pulses in a 1-mm-thick KDP crystal at a fundamentalharmonic peak intensity of 2 TW cm-2 has been demonstrated experimentally. In a 0.5-mm-thick KDP crystal, a 50% efficiency has been reached at a peak intensity of 3.5 TW cm-2. We examine the key factors that limit the conversion efficiency and present numerical simulation results on further temporal compression of second-harmonic pulses.

Influence of small-scale self-focusing on second harmonic generation in an intense laser field

Linearised equations for the amplitudes of harmonic perturbations of strong waves in quadratic and cubic nonlinear media are obtained within the model of plane monochromatic waves. The dependences of the gains of noise components of the first and second harmonic waves upon frequency doubling on the B integral are found. The maximally admissible noise level in the fundamental radiation beam is calculated by the example of a 0.5-mm-long KDP crystal at the peak intensity of 4.5 TW cm-2.

Depolarization of white light generated by ultrashort laser pulses in optical media

We report measurements of the extinction ratio (ER) of white light generated upon irradiation of BK7 glass by ultrashort (36 fs) laser pulses with incident power approximately 10(3) times larger than the critical power for self-focusing. At low incident powers, the continuum is symmetric about the incident laser wavelength; at high powers it becomes broader and distinctly asymmetric towards the blue side. We observe that ER degrades by 100-fold after the onset of multiphoton-induced free-electron generation (at incident intensity approximately 2 TW cm-2), which also corresponds to the onset of asymmetry in white-light spectra.

Three-photon near-threshold photoionization dynamics of isooctane

The electron survival probability following three-photon (9.3 eV total) near-threshold photoionization of neat isooctane is measured with sub-50 fs time resolution. The measured dynamics are nonexponential in time and are well described by a diffusion-controlled electron-cation recombination model. Excitation-power-dependent studies indicate that the unperturbed three-photon threshold ionization is only observed for pump irradiance below 0.5 TW cm2. At excitation fields above this level, the signal is no longer cubic in the excitation irradiance, and the observed electron survival probability dramatically changes, decaying as a single exponential in time.

Threshold of femtosecond laser-induced damage in transparent materials

The rate at which conduction-band electrons (CBE) absorb laser energy is calculated by both the quantum mechanical and the classical methods. Here fused silica irradiated with a 780-nm femtosecond-pulse laser is used as an example. It is found that the rate obtained by the quantum mechanical method is about one-third of that by the classical method, and it is much less than the direct-current limit. In the flux-doubling model, the avalanche rate in fused silica is 4 I ps-1 obtained by the quantum mechanical method, while it is about 13.7 I ps-1 by the classical method, where the laser intensity I is in units of TW cm-2. The differential equation of the evolution of CBE density is solved numerically, and it is found that the combination of CBE–hole recombination, CBE diffusion and initial CBE density (<1013 cm-3) is not important. The dependence of avalanche breakdown threshold on laser-pulse duration is presented. The threshold calculated by the quantum mechanical method agrees well with experimental results, while the threshold obtained by the classical method differs greatly from the experiments.

Time-resolved Stark effect in rapidly varying fields

The cycle-averaged ac Stark effect associated with the [A]2Σ+v′=2?[X]2Π1/2v′=0 two-photon absorption of NO at intensities between 7.7 and 15.2 TW cm-2 has been characterized in real time through a synergic combination of bichromatic laser experiments and quantum-dynamics calculations. Measurements of the fluorescence emitted by the Rydberg [A]2Σ+v′=2 level as a function of time between Stark and probe components of a bichromatic field exhibit a characteristic evolution in temporal peak structure with Stark-field intensity, which is interpreted in terms of a time-dependent Floquet analysis of the laser–matter interaction. The experimental observations are consistent with a dynamic Stark shift of Δes(ω1,ω2)≤0.23 eV of the optical transition at these intensities.

Resonance-enhanced high-harmonic generation

In this paper we present some results of a high-harmonic generation experiment in argon, performed with a Ti:sapphire laser (787 nm, 140 fs) and a spatially shaped focus (flat top). The range of intensities is 3-100 TW cm-2. The intensity dependence of the harmonic signal shows an interesting behaviour of the 13th harmonic, namely an initial suppression followed by a strong enhancement at 85 TW cm-2, which exceeds the magnitude of the 11th harmonic. This behaviour is a consequence of a resonant atomic process. Numerical calculations were also performed. They partly reproduced the experiment.

Numerical simulation of thin-foil acceleration with light-ion beams

The acceleration of metal foils under irradiation by powerful proton beams with power density 0.05-0.5 TW cm-2 and particle energy 0.5-5 MeV has been studied. The results of the numerical simulation have been compared with the experimental data on back surface velocity registration of the aluminum target (at the KALIF accelerator) and with simple analytical model calculations. The hydrodynamic efficiency of 10 mu m tungsten foil acceleration as a function of proton beam parameters has been investigated.

Transport/hydrodynamic simulation of intense ion beam diagnostics (abstract)

Sandia is working towards igniting an ICF target using MJ-level ion beams focused to intensities of up to 100 TW cm2. The diagnostics used to confirm the achieved focal intensities and study the beam-target interactions are sensitive to variations in both ion energy and angle of incidence. Therefore, to properly simulate time-integrated and time-dependent diagnostics, a coupled transport/diagnostic model is required. Further, intense ion beams cause hydrodynamic response of the target material. We have been developing both a transport/diagnostic code, PICDIAG, and a coupled 1-D rad/hydro/CRE capability to study these effects. We will describe the models used as well as the results of recent simulations of intense beam-target experiments on PBFA II. This will include simulations of diagnostics utilizing Rutherford scattering, inner-shell x-ray generation, and nuclear activation, as well as shadowboxes. Comparison with experimental data will be emphasized. This work was supported by the U. S. Department of Energy under contract No. DE-AC04-76-DP00789.

Above-threshold ionisation

Above-threshold ionisation (ATI) is the name given to the absorption of photons by an already ionised atomic electron. This is a new unexpected phenomenon in multi-photon physics. ATI has been found to occur readily at light intensities above 1 TW cm-2 in light pulses short enough to eliminate the influence of collisions. ATI arises from the simultaneous phase-coherent interaction of a single atomic electron with its parent ion and with laser light.

A laser-type light ion-beam-driven, reactor-size inertial fusion target

Numerical simulation results of compression and burn of a new inertial confinement fusion (ICF) target, which consists of a hollow DT shell followed by a low-Z Be pusher, are presented. This target contains 3.2 mg of DT fuel and is suitable for use in an ICF reactor system driven by 20–30 MeV lithium ions. A shaped input power pulse with an input energy of 4.7 MJ and a peak power of 250 TW (90 TW/cm2) has been used to implode this target. The overall energy gain is of the order of 120. The problem of symmetry and hydrodynamic stability of the target implosion is also discussed briefly.