Initial Pulse Duration Research Articles

Extending the supercontinuum spectrum down to 200 nm with few-cycle pulses

By focusing 805 nm pulses of low energy (0.2–1 mJ) into atmospheric-pressure argon, a supercontinuum is generated with a short-wavelength cutoff of 640, 250 and 210 nm for initial pulse durations of 45, 10 and 6 fs, respectively. It is shown numerically that the large shift of the UV cutoff and many features of the spectrum are caused by terms beyond the slowly-varying-envelope approximation (SVEA). Their effect on pulse compression and filament length is also discussed.

Development of a high-efficiency pulsed slow positron beam for measurements with orthopositronium in vacuum

We have developed a high-efficiency pulsed slow positron beam for experiments with orthopositronium in vacuum. The new pulsing scheme is based on a double-gap coaxial buncher powered by an RF pulse of appropriate shape. The modulation of the positron velocity in the two gaps is used to adjust their time-of-flight to a target. This pulsing scheme allows to minimize non-linear aberrations in the bunching process and to efficiently compress positron pulses with an initial pulse duration ranging from ∼ 300 to 50 ns into bunches of 2.3 to 0.4 ns width, respectively, with a repetition period of 1 μ s . The compression ratio achieved is ≃ 100 , which is a factor 5 better than has been previously obtained with slow positron beams based on a single buncher. Requirements on the degree, to which the moderated positrons should be mono-energetic and on the precision of the waveform generation are presented. Possible applications of the new pulsed positron beam for measurements of thin films are discussed.

Spall behavior of aluminum with varying microstructures

A series of plate-impact spall experiments was conducted to study the spall strength of seven microstructural conditions of aluminum, including three grain sizes of 6061 Al alloy, both ultrapure and commercially pure (1060) polycrystalline aluminum, and single-crystal Al with two different orientations, over the stress range of 4–22 GPa. The pullback velocity, which is a characteristic signature of spall strength, is observed to depend on initial microstructure, impact stress, pulse duration, and loading rate. The pullback velocity generally increases over the stress range of 4–14 GPa and achieves a maximum as the impact stress approaches 22 GPa. The pullback velocity of [100] single-crystal Al is higher than that for both polycrystalline samples and [111] single-crystal samples, indicating that grain orientation strongly affects material response. Experimental results also show that the spall behavior is strongly dependent on sample thickness, while the effect of shock pulse duration was observed to be less significant. Comparison among three 6061 materials indicates that the observed differences depend on initial yield strength. The results also show that initial microstructures and impurities have a diminishing effect on the pullback velocity at stresses near 22 GPa. However, initial properties are observed to have a pronounced effect on the detailed structure of the pullback velocity profile at all stress levels. In particular, an interesting feature, i.e., a sharp slope during pullback followed by a distinct transition to a slower slope, is consistently observed. The occurrence of this change in slope is observed to depend on impact stress, loading rate, and grain size.

Phase-matched four-wave mixing of isolated waveguide modes of high-intensity femtosecond pulses in a hollow photonic-crystal fiber

Hollow-core photonic-crystal fibers with a special dispersion profile are shown to allow phase-matched nonlinear optical interactions of isolated air-guided modes of high-intensity femtosecond laser pulses confined in the hollow fiber core. We present theoretical and experimental studies of the four-wave mixing of fundamental and second-harmonic pulses of a Cr:forsterite laser with an initial pulse duration of about 50 fs and an intensity on the order of 1014 W/cm2 in waveguide modes of a hollow photonic-crystal fiber with a core diameter of about 13μm.

Spatiotemporal dynamics of high-intensity femtosecond laser pulses propagating in argon

The spatiotemporal dynamics of high-intensity femtosecond laser pulses propagating in argon have been investigated experimentally and the results compared with numerical calculations. After the beam expands and refocuses, the pulse splits in two. While the subcomponent expands immediately, the main component propagates as a filament. The pulse duration of the main component is less than half the initial pulse duration and the spectrum is broadened by a factor of two.

Electron pulse broadening due to space charge effects in a photoelectron gun for electron diffraction and streak camera systems

The electron pulse broadening and energy spread, caused by space charge effects, in a photoelectron gun are studied analytically using a fluid model. The model is applicable in both the photocathode-to-mesh region and the postanode electron drift region. It is found that space charge effects in the photocathode-to-mesh region are generally unimportant even for subpicosecond pulses. However, because of the long drift distance, electron pulse broadening due to space charge effects in the drift region is usually significant and could be much larger than the initial electron pulse duration for a subpicosecond electron pulse. Space charge effects can also lead to a considerable electron energy spread in the drift region. Temporal broadening is calculated for an initial electron pulse as short as 50 fs with different electron densities, final electron energies, and drift distances. The results can be used to design electron guns producing subpicosecond pulses for streak cameras as well as for time resolved electron diffraction.

Mechanisms of laser ablation from molecular dynamics simulations: dependence on the initial temperature and pulse duration

The effect of the initial sample temperature and laser pulse duration on the mechanisms of molecular ejection from an irradiated molecular solid is investigated by large-scale molecular dynamics simulations. The results of simulations performed for two initial temperatures are found to be consistent with the notion of two distinct regimes of molecular ejection separated by a threshold fluence. At low laser fluences, thermal desorption from the surface is observed and the desorption yield is described by an Arrhenius-type dependence on the laser fluence. At fluences above the threshold, a collective multilayer ejection or ablation occurs and the ablation depth follows a critical density of the deposited energy. The same activation energy for desorption and critical energy density for ablation provide a good description of the fluence dependence of the total yield in simulations with different initial temperatures. Comparison of the simulation results for two pulse durations is performed to elucidate the differences in the ejection mechanisms in the regimes of thermal and stress confinement. We find that in the regime of stress confinement, high thermoelastic pressure can cause mechanical fracture/cavitation leading to energetically efficient ablation and ejection of large relatively cold chunks of material.

Mechanisms of laser ablation from molecular dynamics simulations: dependence on the initial temperature and pulse duration

Mechanisms of laser ablation from molecular dynamics simulations: dependence on the initial temperature and pulse duration

Electrospray Ionization High-Resolution Ion Mobility Spectrometry−Mass Spectrometry

A hybrid atmospheric pressure ion mobility spectrometer is described which exhibits resolving power approaching the diffusion limit for singly and multiply charged ions (over 200 for the most favorable case). Using an electrospray ionization source and a downstream quadrupole mass spectrometer with electron multiplier as detector, this ESI-IMS-MS instrument demonstrates the potential of IMS for rapid analytical separations with a resolving power similar to liquid chromatography. The first measurements of gas-phase mobility spectra of mass-identified multiply charged ions migrating at atmospheric pressure are reported. These spectra confirm that collision cross sections are strongly affected by charge state. Baseline separations of multiply charged states of cytochrome c and ubiquitin demonstrate the improved resolving power of this instrument compared with previous atmospheric pressure ion mobility spectrometers. The effects of electric potential, initial pulse duration, ion-molecule reactions, ion desolvation, Coulombic repulsion, electric field homogeneity, ion collection, and charge on the resolving power of this ion mobility spectrometer are discussed.

Adiabatic stabilization of a circular state: Theory compared to experiment

The probability that an atom of hydrogen, initially in the 5g(m=4) state, survives a 90-fs pulse of 620 nm wavelength is calculated both by direct integration of the time-dependent Schroedinger equation and by a Floquet calculation. The two methods give virtually identical results. The survival probability calculated for a one-electron model of neon, for the same initial state, pulse duration and wavelength, is in fair quantitative agreement with the experimental data of van Druten et al [Phys. Rev. A 55, 622 (1997)].

Time reversal in multiply scattering media

Abstract The application of time-reversal mirrors (TRM) to media with very high-order multiple scattering is presented. Random sets of up to 2500 steel rods are considered. When a pulsed wave traverses such a medium, it undergoes many scatterings before reaching the TRM. The resulting pressure field spreads in time, up to 300 times the initial pulse duration; it is recorded, time-reversed and retransmitted through the same disordered medium. Surprisingly, the time-reversed waves are found to converge to their source and recover their original waveform and duration, unlike one could have expected given the high order of multiple scattering involved and the usual sensitivity to initial conditions of time-reversal processes. In addition to this, the observed resolution of the time-reversed waves was greatly increased, and found to be smaller than the theoretical limit for the array's aperture. Theoretical limits of time-reversed experiments are discussed.

A new approach to molecular classical optimal control: Application to the reaction HCN→HC+N

We present a new method for classical control theory of Hamiltonian systems. This approach is based on a special treatment of the adjoint or Lagrange multiplier equations of motion. The latter function is only asked to preserve the mean of the ensemble of molecular trajectories. In the present case only four such equations are involved greatly simplifying the field design process and making it faster and more stable. Good results are obtained for the selective photodissociation of HCN. The objective is to control the intramolecular reaction HCN→HC+N (i.e., break the stronger bond). Hamilton’s equations of motion are employed to model the HCN molecule, initially in its ground state. The control equations are integrated to obtain a high degree of selectivity in the unimolecular dissociation. The robustness of the results to changes in the initial conditions and pulse durations are investigated. An increase of the pulse duration beyond a certain point makes it more difficult to dissociate the N atom due to strong intramolecular coupling. The resultant pulse fields may serve as a basic indicator for future experimental selective dissociation of HCN→HC+N using high power lasers.

Experimental demonstration of compression of dispersed optical pulses by reflection from self-chirped optical fiber Bragg gratings

Dispersion compensation is demonstrated experimentally by pulse compression with the use of chirped optical fiber Bragg gratings. The gratings chirp is self-induced by the Gaussian intensity profile of the 240-nm wavelength beam used for holographic sidewriting of the grating. Chirped pulses generated by a 1.55-microm gain-switched distributed-feedback laser with an initial pulse duration of 21 ps and a spectral width of 0.7 nm are compressed to 13 ps, in good agreement with theory.

Scaling law for temporal dispersion of a short electron pulse across a diode in space-charge regime

The space-charge phenomenon in the diode of a photoinjector induces simultaneously temporal and spatial dispersions of the electrons burst in pulsed regime. They are higher when the initial pulse duration is shortened. Here, we present the results of temporal dispersion obtained by computer simulation and we give a simple scaling law accounting for it. This law can be used to compare durations of electron pulses at the cathode and anode in any well-defined operation set, in space-charge regime.

Stress Pulse Attenuation in Cloth-Laminate Quartz Phenolic

Using flat plate impact techniques and quartz stress gages, a study of the propagation of short-duration stress pulses in a cloth-laminate quartz phenolic composite was made. Propagation characteristics, wave shape and peak stress attenuation, were measured for 14.5 and 7 kbar initial impact stresses and 0.1, 0.2, and 0.3 μsec initial pulse durations for propagation distances up to 52 times the impactor thickness. The experimental results showed marked dispersive spreading of the wave profiles and very strong attenuation of stress, but no evidence of elastic-plastic effects. The amount of attenuation was much greater than could be attributed to "normal" atten uation in homogeneous materials with a modulus equivalent to the com posite modulus. Experimental results were compared to calculations based on a rate dependent analogy using a generalized Maxwell model. The instantaneous and equilibrium response of the model was defined solely in terms of bulk properties of the components of the composite; while a single relaxation time of 0.023 μsec was determined from experiment. Model predictions agreed extremely well with the measured profiles, giving ± 10% agreement to the measured half-amplitude wave velocities. Attenuation was predicted to better than 20% in stress, which is quite good. Quantitative agreement was found between calculations and the measured results of other investigations, as well.