Laser Field Strength Research Articles

Nonlinear response of a molecular gas caused by orientation effects in the field of an intense femtosecond laser pulse

The orientation anisotropy of a molecular gas induced by a high-power femtosecond pulse is studied by the method of direct numerical integration of the nonstationary Schrödinger equation. The contribution of the anisotropy to the nonlinear polarisation of the gas is calculated. It is shown that the orientation nonlinearity caused by the efficient alignment of molecules along the laser field is accompanied by a considerable repopulation of the rotational levels of a molecule and cannot be described by the perturbation theory. It is found that the inertia of this nonlinearity depends on the laser-field strength.

Calculating electron current in a tight-binding model of a field-driven molecular wire: Application to xylyl-dithiol

A recently developed Floquet theory-based formalism for computing electron transport through a molecular bridge coupled to two metal electrodes in the presence of a monochromatic ac radiation field is applied to an experimentally relevant system, namely a xylyl–dithiol molecule in contact at either end with gold electrodes. In this treatment, a nondissipative tight-binding model is assumed to describe the conduction of electric current. Net current through the wire is calculated for two configurations of the electrode–wire–electrode system. In one, symmetric, configuration, the electrodes are close (∼2 Å) and equidistant from the bridge molecule. In the other, asymmetric configuration, one electrode is farther away (∼5 Å), representing the tip of a scanning tunneling microscope located at this distance from the bridge molecule (the other end being chemisorbed to a gold substrate). For both configurations, electron current is calculated for a range of experimental inputs, including dc bias and the intensity and frequency of the laser. Via absorption/emission of photons, resonant conditions may be achieved under which electron transport is significantly enhanced compared to the unilluminated analog. Calculations show that this can be accomplished with experimentally accessible laser field strengths.

Boundary-free scaling calculation of the time-dependent Schrödinger equation for laser-atom interactions

The time-dependent Schrodinger equation for atoms in an intense laser field is solved in scaled coordinates. Starting with the time propagation of a free particle, we show that scaling removes the spreading and rapid phase variation of a wave packet. By solving the one-dimensional model atom in an intense laser field, we show that stable numerical results are best calculated in the acceleration gauge using scaled coordinates. The wave function thus calculated has the least oscillations and does not spread at large time so that there is no need to introduce absorbers at the boundaries, We show the method Is suitable for calculations at any laser field strength. We also show that the scaling method permits propagation to large times thus revealing above threshold ionization structure directly in the wave function.

Approximate model of intense field stabilization for hydrogen atom

An approximate model is proposed to study the stabilization problem of hydrogen atoms under monochromatic intense laser field. The stabilization regime for system parameters such as laser field strength, laser field frequency and atomic magnetic quantum number are obtained by stability analysis of fixed points of the model. The results are consistent with those obtained by other methods.

The effect of the van der Waals interaction on the excited state for atom guiding in metal-coated, hollow-core optical fibers

The study of atom guiding in a metal-coated, hollow-core optical fiber has been extended to the case of moderate to strong coherent excitation. Significant populations of the upper level are produced and the effect of the increased van der Waals interaction of the excited state with the inner wall is represented as a multiplicative scaling factor of the ground state interaction. This scaling factor is justified by a simple, exact calculation for atomic hydrogen where the van der Waals interaction for H(4p) is a factor of 400 larger than that for the H(1s). By combining the optical dipole potential and the population-weighted van der Waals level shifts, an overall guiding barrier is obtained. The barrier functions are maximized in terms of both the detuning and the radial position to give the maximum barrier heights. For given laser field strengths at the inner wall, the resulting barriers depend strongly on the van der Waals scaling factors.

Resonances and dichroism in the scattering of an electron in an intense laser field by the Coulomb potential

We use a new type of Hamiltonian representing the electron-proton interaction in an intense laser field for an ab initio calculation of the differential scattering cross section. We give the diagrams of the results of calculations of the shapes and widths of the resonances in the cross section of electron scattering by the “field-dressed” Coulomb potential. The resonances emerge because of re-emission of photons by the electron. We also give the angular distribution of the scattered electrons as a function of circular dichroism for different values of the laser field strength and frequency.

Effect of external parameters on interface-LO-phonon amplification inquantum wire

Under a laser field, the interface-LO-phonon amplification effect in a quantum wire is presented in this paper. The rate of change of the phonon population is calculated, and the amplification condition is given. It is shown that in the low temperature region, there are lower and upper thresholds of the laser field strength for achieving phonon amplification in a quantum wire.

Ultrafast multiphoton ionization dynamics and control of NaK molecules

The multiphoton ionization dynamics of NaK molecules is investigated experimentally using one-color pump–probe femtosecond spectroscopy at 795 nm and intermediate laser field strengths (about 10 GW/cm2). Both NaK+ and Na+ ions are detected as a function of pulse separation time, pulse intensities, and strong pulse–weak pulse order. To aid in the analysis, the potential energy curves of the two lowest electronic states of NaK+ and the electronic transition dipole moment between them are calculated by the GAUSSIAN94 UCIS method. Different ionization pathways are identified by Franck-Condon analysis, and vibrational dynamics in the A 1Σ+ and 3 1Π states, as well as in the ground state, is observed. Further, the existence of a highly excited (above the adiabatic ionization limit) neutral state of NaK is proposed. By changing the strong pulse–weak pulse order of the pulses, the ionization pathways for production of both ions can be varied and thus controlled.

On the effects of absolute laser phase on the interaction of a pulsed laser with polar versus nonpolar molecules

The absolute laser phase dependence of the time-dependent populations of the molecular states, including the steady-state (long time) populations of the states, associated with the interaction of a molecule with a pulsed laser is investigated using illustrative two-level examples. One-photon transitions, including the effects of permanent dipoles, are discussed as a function of the pulse duration, intensity, and (absolute) laser phase, for selected laser frequencies. The effects of laser phase can be large, depending on the values of the pulse duration for a given frequency and intensity. The effects of permanent dipoles, relative to no permanent dipoles, are significant for large laser field strengths ε0. When the laser-molecule coupling parameter b=μ12ε0/E21⩾0.2, where μ12 and E21 are the transition dipole and energy difference between the ground and excited states, respectively, the dynamics of the pulse-molecule interaction are (strongly) phase dependent, independent of pulse duration, whereas the corresponding steady-state populations of the molecular states may or may not be phase-dependent depending on the pulse duration. Analytical rotating wave approximations for pulsed laser-molecule interactions are useful for interpreting the dynamics and the steady-state results as a function of field strength and pulse duration, including the effects of permanent dipole moments. The results reported in this paper are based on molecular parameters associated with an S0→S1 electronic transition in a dipolar molecule. However, they are presented in reduced form and therefore can be scaled to other regions of the electromagnetic spectrum. Short, intense pulses at or beyond the limits of current laser technology will often be required for the types of absolute laser phase effects of this paper to be appreciable for electronic excitations. The discussion, in the UV-VIS, also suffers from the use of a two-level model and from the requirement of field intensities that can be beyond the Keldysh limit. For other spectral regions, these absolute laser phase effects will be much more readily applicable.

Laser-induced quasicrystalline order in charge-stabilized colloidal systems

We have studied the ordering of a two-dimensional charge-stabilized colloidal system in the presence of a stationary one-dimensionally modulated laser field formed by the superposition of two modulations with wave vectors $q_o\tau$ and $q_o /\tau$, where $q_o$ is the wave vector corresponding to the first peak of the direct correlation function of the unperturbed liquid, and $\tau$ is the golden mean. In the framework of the Landau-Alexander-McTague theory we find that a decagonal quasicrystalline phase is stabler than the liquid or the triangular lattice in certain regions of the phase diagram. Our study also shows a reentrant melting phenomenon for larger laser field strengths. We find that the transition from the modulated liquid to the quasicrystalline phase is continuous in contrast to a first order transition from the modulated liquid to the triangular crystalline phase.

Ionization of Rydberg atoms in THz-laser fields at the transition from low to high scaled frequencies

We have studied the ionization of Rydberg-excited xenon atoms in THz-laser fields and by quantum dynamical calculations. The experimental threshold laser field strength for 10% ionization probability follows an n*-1.68 (\(\)1.04 THz) dependence (n* effective principal quantum number) with additional weak resonance structures and shows that ionization does not occur by a Landau-Zener mechanism. At scaled frequencies of \(\) to 5.6 the simulated threshold fields for ionization in oscillatory fields show a dependence on the principal quantum number n of n-4.1 to n-1.35.

The physics experiment for a laser-driven electron accelerator

A physics experiment for laser-driven, electron acceleration in a structure-loaded vacuum is being carried out at Stanford University. The experiment is to demonstrate the linear dependence of the electron energy gain on the laser field strength. The accelerator structure, made of dielectric, is semi-open, with dimensions a few thousand times the laser wavelength. The electrons traverse the axis of two crossed laser beams to obtain acceleration within a coherence distance. We predict that the demonstration experiment will produce a single-stage, electron energy gain of 300 keV over a 2.5 mm distance. Ultimately, acceleration gradients of 1 GeV m −1 should be possible.

Effect of four-photon interactions on coherent population trapping in Λ-systems

The resonance fluorescence spectrum of a Λ-system excited by two resonant light fields is calculated using a Markov analysis. Analytical formulas are derived in the strong-field limit within and beyond the rotating wave approximation. It is shown that the resonance fluorescence of the system does not vanish during coherent population trapping. Its spectrum consists of two multiplets which are similar to a triplet in the resonance fluorescence spectrum of a two-level atom and lie at the electronic transition frequencies, together with two triplets located at the frequencies of four-photon processes involving the optical excitation fields. The latter are fundamental in character and impose limits on the lower bound of the dephasing rate for the Raman resonance owing to the effect of radiative decay of the dipole transitions on the dynamics of the ground state. The effect of four-photon dephasing on the absorption spectrum of a Λ-system is analyzed and found to lead to a substantial reduction in the depth of a dip in the absorption spectrum which vanishes as the laser field strength is increased.

Protonium formation in the excited state in laser assisted antiproton-hydrogen atom collisions

A detailed analysis using the Born approximation is given of protonium formation in the first excited state ( n = 2) in laser assisted antiproton hydrogen atom collisions adopting the semi-classical model. The projectile-laser-field interaction and the dressing effects on the hydrogen atom and the protonium atom have been considered to first order in the A · p gauge. The differential cross sections for protonium ( n = 2) formation have been presented for no-photon and one-photon exchange ( l = 0, ± 1) at 50, 100 and 200 keV energies with laser frequency ω = 0.043 a.u. and laser field strength E 0 = 0.002 a.u. along with the field free results. Two interesting characteristics are noted. The differential cross sections increase slowly with the scattering angle unlike the case of electron capture by heavy particles. Secondly, contributions from the entire angular region (0°–180°) are important for the calculation of cross sections, which is a familiar characteristic in light particle scattering.

High power laser semiconductor interactions: A Monte Carlo study for silicon

In this article, we use Monte Carlo methods to study the interaction of high power laser pulses with electrons in the conduction band of semiconductors. The laser field is represented by a sinusoidal electric field which tends to cause an oscillatory motion in the electrons. The scattering of electrons from the lattice force the electrons to lose phase coherence with the field. The approach is applied to silicon. We use the approach to examine the carrier energy distribution and material breakdown due to the transfer of energy from the laser to the electrons followed by impact ionization. The impact ionization coefficient, α, and its dependence on the laser frequency and field strength is examined and compared to the values in a dc field. In general, the ac value is smaller than the dc value, but at low frequencies and high field strengths, the ac impact ionization coefficient approaches the dc value at the same rms field value. The importance of collisions in the energy transfer process is elucidated.

Positronium formation in the excited state in the presence of a laser field

A straightforward approach for the evaluation of the rearrangement scattering amplitude in the first Born approximation has been given for the study of positronium (Ps) formation in the first excited state (n = 2) in e+ – H(1S) collisions in the presence of a homogeneous, single mode, linearly polarized laser field embodying the full interaction potential. The projectile-laser-field interaction and the dressing effects on the target hydrogen atom and the Ps atom in the final channel have been considered to first order. The A · p gauge has been adopted in constructing the dressed wave functions. The FBA scattering amplitudes have been obtained in closed analytic forms for field direction parallel to the incident positron momentum. The differential cross sections for Ps (n = 2) formation have been presented for no photon and one photon exchange (l = 0, ±1) at 50, 100 and 200 eV energies with laser field amplitude ℰ0 = 1.0 × 108 V cm−1 and the angular frequency ℏω = 1.17 eV. The relations between the differential cross sections and the laser field strength, angular frequency and incident energy have also been demonstrated.

Two-color above-threshold ionization of atoms.

A nonperturbative method is used to study the two-color multiphoton ionization of atoms by solving the Schr{umlt o}dinger equation in momentum space, which facilitates the extraction of the rapidly varying part of the wave function. Our method generalizes the problem for cases where an arbitrary number of ionization photons are absorbed, and also takes into account the threshold shifts due to both the high- and low-frequency fields. The photoelectron energy spectra of the hydrogen atoms are calculated and the dependence on the laser field strengths and frequencies is discussed. {copyright} {ital 1996 The American Physical Society.}

Multiphoton ionization of cobalt atoms produced by multiphoton dissociation of Co(CO) 3NO and Co(CO) 2(NO)PR 3 (R=CH 3, n-C 3H 7 and n-C 4H 9)

Resonance-enhanced multiphoton ionization is used to probe the cobalt atomic state distributions produced in the multiphoton dissociation of the title compounds in the 445–453 nm range under various laser field strengths. At relatively weak laser field strengths, exclusive production of Co(a 2F) is achieved from Co(CO) 2(NO)P( n-C 4H 9) 3. Ground state a 4F and the b 4F multiplets are observed as the size of the PR 3 ligand decreases or if the PR 3 ligand is replaced by CO. Increasing the laser field strength greatly enhances production of the 4F multiplets to the point where it is independent of the molecular precursor. The dissociation mechanism is discussed in respect to the above observations.

Resonant tunneling in double-barrier heterostructures tunable by long-wavelength radiation.

It is suggested that resonant-tunneling currents of electrons through ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As-GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructures can be tuned through a pulsed low-energy long-wavelength laser. This new tuning mechanism is based on the variation of the quasibound states of the quantum well with increasing intensity of the external laser field. The resonant-tunneling tuning conditions are discussed as well as the behavior of resonant-tunneling currents for different laser field strengths.

Tuning of the two-dimensional electron density in modulation-doped quantum wells by longwavelength radiation

We suggest that the two-dimensional electron density in a symmetric modulation-doped quantum well can be tuned through a pulsed long-wavelength laser field. This new mechanism arises from the blue-shift of the bound-state energy of the quantum well due to the laser-dressing of the heterostructure potential. The tuning conditions associated with the proposed mechanism are discussed within the framework of a self-consistent calculation as well as the behaviour of the two-dimensional electron density for different laser field strengths.