Short Pulse Limit Research Articles

Strong-field short-pulse nondipole dynamics

We present a quantitative investigation of strong-field short-pulse nondipole dynamics in laser-matter interactions. We find excellent agreement between ab initio numerical and analytic results obtained using the Magnus expansion. We show that in the short-pulse limit, ultrafast transfer and control of population can be achieved using nondipole effects. The relative importance of nondipole to dipole effects depends on the displacement imparted to a free classical electron.

Time domain formulation of the equivalent fluid model for rigid porous media

A set of equations has been derived which corresponds to the time domain formulation of the equivalent fluid model. It models the propagation of an acoustic pulse in rigid frame porous material accounting for both viscous and thermal effects. It has been shown analytically and confirmed numerically that the equations can be reduced to those published previously in the limit of long and short duration pulses. Numerical solutions have been found for different pulse durations and the results have been compared with other time domain models.

Effects of polydispersity on PGSE NMR coherence features

Real systems always contain some degree of polydispersity and yet the effects of this real and very important problem have not been studied in great detail in NMR diffusion experiments. The effects of polydispersity become even less clear when we are outside the short gradient pulse (SGP) limit (which we generally are). Here we investigate the effects of polydispersity, in the form of a Gaussian distribution of characteristic distances, on the coherence features of PGSE NMR experiments of a model system. Characteristic pore sizes were determined from the coherence features and compared to characteristic distances determined from Fourier transforms of the second derivative.

Multidimensional short-pulse space-charge-limited flow

The two-dimensional models of the space-charge-limited (SCL) current density at the short pulse limit for which the electron pulse length is comparable or smaller than the electron transit time across the gap (i.e., XCL⩽1) have been developed. In particular, the scaling laws for short-pulse SCL electron emission in a planar diode with a circular emitting strip and a cylindrical diode with a finite length have been obtained and verified with particle-in-cell simulation. It is found that the enhancement (in terms of the long-pulse SCL current density) is proportional to XCL−1 for small XCL for both planar and cylindrical cases. The enhancement of the cylindrical short-pulse SCL current density is also found to be larger for the convergent flow (cathode outside) than divergent flow (cathode inside). Multidimensional effects are important only for small emitting strips with size comparable to the effective penetration distance (into the gap) of the short-pulse electron beam. Smooth transition between the short-pulse regime and the long pulse (steady-state) regime is demonstrated.

Comparison of wakefield and relativistic effects on the self-phase modulation and frequency shift of intense laser pulse propagation

The relativistic effect and wakefield effect on the propagation of an intense laser pulse in plasma are compared in the resonant and non-resonant cases in detai l. The effect of wakefield, which depends on the initial pulse shape and width, leads to the asymmetry of pulse self-phase modulation. In the long pulse limit, the wakefield effect is negligible compared to the relativistic effect; in the short pulse limit, it balances the relativistic effect. Under the resonant cond ition, most of the photons within the pulse decelerates.

Suppressing the Spreading of Continuum Wave Packets via Chirped Laser Pulses

We investigate the focusing, that is, the reversing of spreading of continuum wave packets created with chirped laser pulses in the weak-field limit. Focusing can be accomplished when a negative position−momentum correlation is created in the wave packet. Specializing to constant and linear (repulsive) potentials, we show analytically that in a constant potential wave packet spreading can be compensated with positively chirped laser pulses, whereas in the linear potential negatively chirped laser pulses are required, in the short-pulse limit. The analytical results for the linear potential are supported by numerical simulations. The results are discussed in the context of a classical model for wave packet focusing.

NMR diffusometry and the short gradient pulse limit approximation

In NMR diffusometry, one often uses the short gradient pulse (SGP) limit approximation in the interpretation of data from systems with restricted diffusion. The SGP limit approximation means that the gradient pulse length, δ, is so short that the spins do not diffuse during the pulse duration, but this condition is rarely met. If the length scale of the pores corresponds to the molecular mean square displacement during the gradient pulse, the measured echo intensities become a function of the gradient pulse length. Here, we have studied highly concentrated emulsions to show how the length of the gradient pulse influences NMR diffusion experiments. We have focused on molecules confined to one pore and molecules that can migrate through the porous system. For the former the echo decays give smaller pores than the actual case and for the latter we show large changes in echo decay depending on the gradient pulse length, everything else being equal.

Nonadiabatic coupling effects on the short time signal in four-wave mixing experiments

The transient four-wave mixing signal from a three-level system coupled to a bath of harmonic oscillators is calculated in the short-pulse limit and analyzed at short times. The two excited states of the system are coupled by a constant nonadiabatic interaction V which is treated exactly. This allows an examination of the influence of V on the dynamics in the excited levels at short times. We show that the positions of the minima of the potential energy surfaces of the excited states with respect to the one of the fundamental state play an important role in the relaxation processes. We also show that for high nonadiabatic coupling it is possible to minimize the effect of the bath and hence to minimize, at short times, the coherence losses of the system. The identification of the molecular parameters required to take advantage of this effect should be of interest in the selection of materials for applications in nonlinear optics.

On the dynamics of rotationally broad, spatially aligned wave packets

The problems of rotational excitation and molecular alignment in moderately intense laser fields are addressed theoretically and numerically. Several scaling propensities are derived, which connect the achievable alignment with the underlying field and molecular parameters. An analytical model is developed, which exposes the origin of alignment in the short-pulse limit. A numerical study is presented, which quantifies the notions of adiabatic and sudden alignment and illustrates the behavior of the post-pulse alignment subsequent to slow turn-on and rapid turn-off of the laser pulse. Several new potential applications of laser alignment are proposed.

Time-dependent alignment and orientation of molecules in combined electrostatic and pulsed nonresonant laser fields.

We examine the time evolution of states created by the nonadiabatic interaction of a polar molecule with combined electrostatic and pulsed nonresonant laser fields and show that the orientation due to the electrostatic field alone can be greatly enhanced by restricting the angular amplitude of the molecule by the pulsed laser field. An analytic model indicates that in the short-pulse limit the interaction is governed by an impulsive transfer of action from the radiative field to the molecule.

Recurring Molecular Alignment Induced by Pulsed Nonresonant Laser Fields

We examine the rotational wavepackets created by the nonadiabatic interaction of a linear molecule with a pulsed nonresonant laser field. We map out the recurrences of the wavepackets and of the concomitant alignment as a function of the duration and intensity of the laser pulse. We derive an analytic solution to the time-dependent Schrödinger equation in the short-pulse limit and find it to agree quantitatively with our numerical computations. This indicates that the recurrences are favored under an impulsive transfer of action from the radiative field to the molecule. The recurring wavepackets afford field-free alignment of the molecular axis.

Influence of population decay on the short time signal in four-wave mixing experiments

Oscillatory dephasing is a characteristic of non-Markovian relaxation phenomena when the coherence losses are essentially due to pure dephasing processes. In this paper, we wish to consider the case where the phase relaxation is essentially due to population decay. To this end, we calculate the transient four-wave mixing signal in the short pulse limit from a three-level sytem nondiagonally coupled to a bath of harmonic oscillators. We will show that no oscillatory dephasing is observed in the non-Markovian regime, only a nonexponential decay. Therefore, oscillatory dephasing appears to be a signature of pure dephasing processes.

Switching behavior of a Stoner particle beyond the relaxation time limit

Results of the switching properties of Stoner-like magnetic particles, subject to short magnetic field pulses, obtained by numerical investigations, are reported. The switching properties are discussed as a function of the external field pulse strength and direction, pulse length, and the pulse shape. For field pulses long compared to the precession time, the switching behavior is governed by the magnetic damping term. In the limit of short field pulses, switching properties are dominated by the details of the magnetic precession. In the latter case, the magnetic damping term is of minor importance and ultrafast switching can be achieved by choosing the right field pulse parameters. It is also possible to choose pulse parameters in order to provide switching over a wide range of applied field directions.

Amplification of Dense Spectra of XeCl Excimer

Amplification process in an excimer laser of dense modes is studied. Rate-equations with wavelength dependence are used to reproduce its spectra. Two extreme cases are tried and in spite of the duration of the output pulse relative to the relaxation time, the short pulse limit is proved to describe well the oscillation process of individual mode.

Formulation of the criterion of thermoelastic laser damage of transparent dielectrics and the dependence of the damage threshold on the pulse duration

An analysis of the thermoelastic problem is used to formulate the criterion of mechanical damage of a transparent dielectric by laser radiation. The criterion is suitable for calculation of the threshold characteristics of the damage associated with absorbing inclusions and that involving intrinsic mechanisms. It is shown that in the limit of short laser pulses the threshold energy density tends to a constant value for any radiation absorption mechanism. A study is reported of the dependence of the threshold of damage initiated by absorbing inclusions on the pulse duration in a wide range of pulse durations. The theoretical results are compared with the published experimental data.

Oscillatory dephasing in the presence of inhomogeneous broadening.

Oscillatory dephasing is a characteristic signature of non-Markovian relaxation phenomena. From its analysis one can obtain valuable information about the dynamics of the surroundings of a given system. In this paper we calculate the transient four-wave mixing signal in the short pulse limit from two-level systems coupled to a bath of harmonic oscillators for arbitrary coupling strengths and in the presence of inhomogeneous broadening. The strong and weak coupling cases having already been analyzed, we focus on the study of the intermediate case where oscillatory dephasing is observed. We specifically show that even in the presence of inhomogeneous broadening, information about the surroundings can still be extracted in the intermediate coupling regime. The formation and the evolution of the photon echo in such experiments are also analyzed. \textcopyright{} 1996 The American Physical Society.

Non-Markovian effects on quantum beats.

The transient four-wave mixing signal from a three-level system coupled to a bath of harmonic oscillators is calculated in the short-pulse limit. This allows an examination of the non-Markovian effects on the relaxation processes in the excited levels between which beating occurs. In particular, it is possible to distinguish between oscillations which are due to oscillatory dephasing, a consequence of the memory effects, and oscillations due to the quantum beats. This also allows one to analyze how these two kinds of oscillations are mixed together. We show that we not only have an amplitude modulation of the beats as in the Markovian case but also deformations of the beats for short delays between the exciting light pulses. These deformations are a consequence of the nonequilibrium motion of the surroundings of the system under optical excitation. \textcopyright{} 1996 The American Physical Society.

Observation of surface enhanced multiphoton photoemission from metal surfaces in the short pulse limit

Photoelectrons with excess kinetic energy corresponding to several absorbed photons above the work function have been measured from atomically clean Cu(110) and Cu(100) surfaces under ultrahigh vacuum conditions. The power dependence of the photoemission yield does not follow a simple power law dependence corresponding to the number of photons absorbed. This behavior is reminiscent of other above threshold ionization (ATI) or tunnel ionization (TI) processes observed for atoms in the gas phase. The photoelectrons are generated with laser pulsewidths less than 100 fs in duration and peak powers as low as 100 MW/cm2. These intensities are on the order of 105 times lower than that required to observe similar phenomena in the gas phase. The relatively low intensities and correlation with surface roughness suggests a contribution from a surface enhancement mechanism. Thermal heating and space charge effects have been ruled out, and the possibility of electric field enhancement at the surface due to the coupling of photons into surface plasmons is discussed. The nonlinear yield and enhancement of the photoemission produced by short pulse excitation needs to be considered when discussing photoinduced hot electron reaction channels at metal surfaces.

Effects of correlation between inhomogeneously broadened transitions on quantum beats in transient four-wave mixing.

The transient four-wave-mixing signal from a three-level system is calculated in the short-pulse limit including a general distribution function for the energies of the levels between which beating occurs. This allows an examination of the effects of correlation between the energetic splitting of the levels. As expected, the results demonstrate that the degree of correlation can significantly alter the four-wave-mixing signal. However, the results also show that in the case where the level energies are completely correlated, in a rigorous sense, but that the correlation is not a simple offset, a strong modification of the beat amplitude occurs. This case describes the type of correlation expected in semiconductor quantum wells. Examination of several representative cases demonstrates that these effects can easily be incorrectly ascribed to dephasing.

On stabilization by high-frequency, short-pulsed fields

We examine the effects of a high-frequency electric field on a on one-dimensional hydrogen atom in the case that the interaction time is shorter than the classical orbital period and the field frequency, Omega , is sufficiently high that one or two photon ionization is possible. In this short-pulse limit we can obtain simple analytic approximations to the ionization probability. For one-photon ionization we find that after an initial rise, at relatively weak fields, the quantal ionization probability reaches a local maximum at that field for which the energy transfer Omega h(cross) becomes classically accessible. These results are consistent with the numerical results obtained by substantially more elaborate calculations. They show that for short-pulsed, high-frequency fields stabilization can be interpreted as a rainbow phenomenon, the low intensity side of the maximum being on the dark side of the rainbow, with oscillations on the bright side. We also briefly consider some two-frequency fields and show that the ionization probability can be sensitive to the phase difference between field components.