Strong Resonant Field Research Articles

Effects of crossings and quasi-crossings of quasi-energy levels in spectra of molecules placed in a strong resonant field

Response of a molecule placed in resonant electromagnetic field is considered basing on the quasi-energy method and assuming that the intensity of the field is enough (10 3 - 10 5 V/cm) for the Stark splitting to exceed the energy intervals in the fine structures of the levels which are in resonance (such as hyperfine structure, l-doubling). An attention is paid to effects caused by quasi-crossings of the quasi-energy levels.

Stability and phase-dependent dynamics of an atom driven by a strong resonant field and a tunable moderate-strength perturber field.

We have studied the transient response of undamped two-level atoms to excitation by a classical bichromatic field consisting of a strong resonant component and a moderate-strength, tunable, ``perturber'' component. It is found that the strong field tends to stabilize the atom against the influence of the perturber field. Exceptions to this general result occur when the perturber is detuned from the strong field by the Rabi frequency of the latter or its subharmonics. We believe this stabilization phenomenon is a semiclassical analog of the effect that leads to the dynamical modification of spontaneous emission [M. Lewenstein and T. W. Mossberg, Phys. Rev. Lett. 59, 775 (1987)], and the presence of detunings at which the stabilization fails provides a simple motivation for the Rabi subharmonic resonances found in certain cw bichromatic spectra. Interestingly, when the perturber field is relatively weak, detuned from the atomic resonance by the strong-field Rabi frequency, and turned on with a specific initial phase relative to the strong field, one finds that the bichromatic field can be employed to dressed-state polarize initially ground-state atoms. In a related vein, we find that perturbations at the subharmonic detunings lead to interesting phase-dependent atomic dynamics. Results were obtained using both numerical and approximate analytical techniques entirely within the context of the rotating-wave approximation. The validity of the latter is ensured by demanding that all detunings and Rabi frequencies are vanishingly small compared to the atomic transition frequency.

Collective resonance fluorescence in a squeezed vacuum.

The resonance fluorescence of a small sample of N two-level atoms driven by a strong resonant coherent field and damped by a squeezed vacuum is examined. The steady-state spectrum is shown to be dramatically dependent on the relative phase of the pumping field and on the squeezed-vacuum field in both the strong- and weak-squeezing limits.

ESR pumping experiments in spin-polarized atomic hydrogen

Pumping experiments were carried out in atomic hydrogen using an ESR technique. A model of the decay of the four hyperfine states in the presence of a moderately strong resonant microwave field is described and compared with experimental results. The results indicate that because of spin-exchange relaxation, it is difficult to influence the nuclear polarization of the lower hyperfine states by means of ESR pumping. The model is applied to the design of an ESR-operated beam source of spin-polarized atomic hydrogen. We estimate that in our experiments the maximum flux of particles ejected from the source was 5\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$.

Splitting in the absorption spectrum produced by strong and two-photon resonant fields

We demonstrate in the present article that it is possible to observe a two-photon resonance splitting by the use of probe fields coupled to one or both of the degenerate levels.

Quasiequilibrium state of semiconductors in a strong resonant electromagnetic field

Quasiequilibrium state of semiconductors in a strong resonant electromagnetic field

Non-perturbative theory of four-wave light scattering in strong fields

Nonlinear four-wave light scattering (FWLS) on a three-level system is analyzed, adopting the density operator approach. A strong field Ω 1 is mixed by the material system with two fields Ω 2 and Ω 3 to yield an output field peaked at the frequency Ω 4 = Ω 1 + Ω 3 - Ω 2. The theory is developed within the dipole and Markov approximations. The strong resonant field is treated “exactly”, while the other fields are regarded as perturbations. The generated intensity is obtained as the expectation value of the photon number operator, estimated at the sixth order in the weak-field expansion. The generation of a coherent field at the frequency Ω 4 is due to processes where either a single molecule or a couple of molecules are involved. The total intensity is interpreted as the superposition of four contributions: a four-photon scattering process (FPS), a three-photon process (TPS), and two more processes related to resonance Raman scattering (RRS) and resonance fluorescence (RF). The method allows the deduction of nonlinearities in the generated intensity which are not included in fully perturbative treatment, and which, in the strong-field limit, originate the Rabi intensity dip of the lines.

Squeezed states in N-atom time-dependent resonance fluorescence

The dependence of squeezing on the number of atoms is discussed in the time-dependent collective resonance fluorescence of N two-level atoms, coherently driven by a strong resonant laser field. It is shown that, for strong driving field, squeezing appears only in the transient regime of resonance fluorescence. The location of squeezing in time is independent of the number of atoms and the initial atomic populations. It is found that despite the reduction in the amount of squeezing per single atom, squeezing increases in the whole resonance fluorescence field as the number of atoms increases, and attains its maximal value for moderate N. Moreover, the maximal value of squeezing is dependent on the intensity of the driving field. As the latter increases, the maximum of squeezing increases and shifts towards larger numbers of atoms. For a very large number of atoms, however, squeezing is altogether removed Dependance de la compression des fluctuations du champ en fonction du nombre d'atomes dans le cas de la fluorescence de resonance collective dependant du temps, lorsque N-atomes a deux niveaux sont soumis de maniere coherente a un champ laser resonnant et intense

Non perturbative theory of electronic resonant coherent Raman scattering (CARS, CSRS)

Resonant coherent Raman scattering is investigated in a non perturbative way using the rotating wave approximation and modeling the molecule as an n-level system. Only the strong resonant field at Ω1 is treated exactly being the other fields considered as perturbations. The theory use the density matrix formalism and the Markov approximation for the decay operator.

Molecular dynamics in intense fields. III. Nonadiabatic effects

It is shown that coupled equations for multiphoton processes can be written with either momentum transition moments or dipole transition moments. In perturbation (low field intensity) theories, nonadiabatic corrections are necessary to make the adiabatic momentum electronic transition probabilities nonzero and equivalent to electronic dipole transition probabilities in infrared transitions. For strong fields, nonadiabatic interactions can become important due to multiphoton resonances, and both approaches necessitate full coupled equations. In the case of electronic excitations, adiabatic, nuclear dipole transition probabilities become nonzero and equivalent to nuclear momentum transitions probabilities only after nonadiabatic corrections are included. These corrections should become important at strong fields for electronic states coupled nonadiabatically. It is shown further using a representation which introduces field modified electronic functions and electronic potentials that unusual field effects will occur at photon frequencies in resonance with adiabatic electronic potential energy separations at pseudo (avoided) crossings. Thus, in such cases, electronic energy gaps are increased by the field and nonadiabatic couplings at these avoided crossings are reduced by the field. Strong resonant fields should therefore quench nonadiabatic couplings at avoided crossings, rendering nonadiabatic reactions more adiabatic.

Off-axis sample spinning in the slow-spinning regime

The past few years have witnessed an increasing interest in the study of solids by means of high-resolution NMR techniques (I, 2). Of the various approaches that have been developed, the combination of polarization transfer and dipolar decoupling (3) together with magic-angle sample spinning (MASS) (4, 5) has become a popular tool in the study of a wide variety of amorphous and polycrystalline samples (6). In this case, heteronuclear dipolar broadening is attenuated by irradiation of the proton spectrum by a strong resonant rf field, and residual broadening due to the chemicalshift anisotropy of the dilute spins is removed by spinning the sample around the magic angle. When the frequency, UR, is fast compared to the shift anisotropy (in hertz), then a high-resolution “liquid-like” spectrum, devoid of all information other than the isotropic chemical shifts, is observed. Because these experiments lead to a suppression of the chemical-shift anisotropy, they have stimulated a search for methods to reintroduce this information in a manner consistent with the goal of high resolution. The two methods most frequently discussed for this purpose involve application of a train of pulses synchronized with the sample spinning (7-11) or analysis of the sideband intensities in the slowspinning regime (12, 13). The former approach results in a distorted (but calculatable) powder lineshape or sideband pattern which must be resolved in some cases by twodimensional techniques. In the latter case, the spinning speed is reduced so that the condition VR AC this approach leads to a centerband consisting of a powder pattern of much reduced breadth and which changes sign as the spinning angle moves from one side of 54.7” to the other. However, in the limit vR < AU we have observed some unexpected results. In particular, the centerband and sidebands consist of

Spectrum of absorption of a weak signal by an atom in a strong field

An analysis is made of the spectrum of absorption of a weak probe electromagnetic by two-level atoms in a strong resonant laser field, undergoing collision with buffer gas atoms. The analysis is made using an approach that allows for the direct influence of a strong electromagnetic on the dynamics of an elastic collision between an active atom and a buffer gas atom. Rate equations are analyzed for a combined atom–strong electromagnetic field system (an atom dressed by the field) allowing for spontaneous and optical collisional transitions, and also for the interaction with the probe field. In the steady-state case, an expression is derived for the electric susceptibility of the medium at the small-signal frequency. This expression contains the rates of the optical collisional transitions that depend nontrivially on the parameters of the strong electromagnetic field. The phenomenological characteristics of optical collisional transitions generally used are only valid at low intensities and for small frequency detunings of the strong electromagnetic field, i.e., in the impact limit.

Cooperative and interference effects in the resonance fluorescence of two identical three-level atoms at high photon densities

Numerical calculations are presented for the excitation spectrum of two identical three-level atoms interacting with a strong resonant laser field. The atoms interact through their dipole-dipole interaction and radiate to each other as well. The spectrum of the symmetric and antisymmetric modes due to the radiative decay from the higher to the lower excited state of the interacting atoms is considered without and with taking into account the effects of the strong pumping process. In the absence of strong pumping, the dipole-dipole interaction in the spectrum of the symmetric modes gives rise to an asymmetric doublet whose ratio of intensities is 2:1, while the spectrum of the antisymmetric modes consists of two peaks one of which represents a stable mode indicating the trapping of a photon between the atoms and a radiative one which has a lifetime one-half that of the isolated atom. In the presence of strong pumping, asymmetries due to the dipole-dipole interaction arise enhancing certain peaks while diminishing the intensity of others and a new pair of sidebands is induced as well. The computed spectra are presented graphically for different values of the Rabi frequencies and the dipole-dipole interaction, respectively.

Higher harmonics in collective resonance fluorescence

The Stark representation of a strong external resonant field and the third order of perturbation theory of the fluorescence field are used to show that the probability of emission of the (ωa + 4Ω) harmonic (ωa and Ω are the atomic and Rabi frequencies) of collective resonance fluorescence of a pair of two-level atoms is equal to γ2/384, where γ is the width of the spontaneous emission line of a single atom.

Effect of Interatomic Interactions on Resonance Fluorescence of Two Atoms Coherently Driven by a Strong Resonant Laser Field

Effect of Interatomic Interactions on Resonance Fluorescence of Two Atoms Coherently Driven by a Strong Resonant Laser Field

An experimental study of the (J,K)=(3,3) inversion spectral line of NH3under the influence of a strong resonant pumping field

A microwave bridge is described which permits radiation from two sources to be propagated through a gas and to be detected separately even when the sources are operating at the same frequency. This bridge is used to study the shape of the (J,K)=(3,3) inversion line in N14H3 when the molecules are subjected to a strong resonant or near-resonant pumping field. The shape of the line is studied by measuring the absorption of a much weaker probe field. For sufficiently strong pump fields, power can be removed from the pump and transferred to the probe field, causing the latter to be amplified. The results are in quantitative agreement with theory. However, the development of a theory which satisfactorily includes the consequences of the degeneracy of the molecular energy levels has not yet been accomplished.

Rabi frequency splitting in stimulated Raman scattering

Stimulated Raman scattering (SRS) from a system that is both Raman active and infrared active has been investigated theoretically when a strong coherent resonant infrared light field, in addition to a pumping field, is applied. In the density matrix formulation, it is found that the gain function in steady state is redistributed as a result of the modulation on the populations and transition probabilities by the presence of the strong resonant field. The redistribution curve shows Rabi frequency splitting for the Stokes component and a new origin of anti-Stokes components of SRS that is anticipated in the present mechanism.

Cooperative state and resonance fluorescence

Using the exact solution of the Schrödinger equation for a system of N two-level atoms driven by a strong resonant field, the spectrum of cooperative resonance fluorescence consisting of the (ω 0 + mΩ)-harornics has been found.

Penning and associative ionization in crossed-beam Na/Na collisions assisted by strong resonant laser fields

We observe the production of Na2+ and Na+ arising from single collisions between crossed beams of sodium atoms when a laser field is tuned near the Na(3p 2P3/2) and Na(3p 2P1/2) transitions. Measurements of ion intensity vs laser intensity show that at moderately high power true laser-induced processes dominate over purely collisional effects. Relative intensity of mass-selected ions produced at either member of the Na resonance doublet shows conclusively that Na+ does not arise simply from photodissociation of Na2+ but must result from a direct, laser-induced collisional ionization.

Resonance fluorescence spectrum of two atoms, coherently driven by a strong resonant laser field

In Lehmberg's approach, we consider the resonance fluorescence spectrum of two radiatively interacting atoms. In the strong field limit we have obtained analytical solutions for the spectrum of the symmetric and antisymmetric modes without decoupling approximation. Our solutions are valid for all values of the distance r 12 separating the atoms. The spectrum of the symmetric modes contains additional sidebands in 2Ω (Ω is the Rabi frequency) with amplitude dependent on ( a/Ω) 2, where a is a parameter dependent on r 12. The antisymmetric part of the spectrum has no additional sidebands in 2Ω. For small distances r 12 ( a=1) our results for the symmetric modes are identical with those of Agarwal et al. apart from the so-called scaling factor. For large distances r 12 ( a=0) the spectra of the symmetric and antisymmetric modes are identical with the well-known one-atom spectrum.