Pump-noise-induced Hanle effect in forward four-wave mixing

This is an addendum to our previous publication [Phys. Rev. A 36, 143 (1987)] dealing with cross-correlation-induced resonances in four-wave mixing. The temporal fluctuations of the source lead to well-resolved Hanle resonances in forward four-wave mixing which is in contrast to the dephasing-induced resonance in the same geometry

Modeling of Protostellar Clouds and their Observational Properties

A physical model and two-dimensional numerical method for computing the evolution and spectra of protostellar clouds are described. The physical model is based on a system of magneto-gasdynamical equations, including ohmic and ambipolar diffusion, and a scheme for calculating the thermal and ionization structure of a cloud. The dust and gas temperatures are determined during the calculations of the thermal structure of the cloud. The results of computing the dynamical and thermal structure of the cloud are used to model the radiative transfer in continuum and in molecular lines. We presented the results for clouds in hydrostatic and thermal equilibrium. The evolution of a rotating magnetic protostellar cloud starting from a quasi-static state is also considered. Spectral maps for optically thick lines of linear molecules are analyzed. We have shown that the influence of the magnetic field and rotation can lead to a redistribution of angular momentum in the cloud and the formation of a characteristic rotational velocity structure. As a result, the distribution of the velocity centroid of the molecular lines can acquire an hourglass shape. We plan to use the developed program package together with a model for the chemical evolution to interpret and model observed starless and protostellar cores.Comment: Accepted to Astronomy Report

Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling

This review paper outlines background information and covers recent advances made via the analysis of spectra and images of prominence plasma and the increased sophistication of non-LTE (ie when there is a departure from Local Thermodynamic Equilibrium) radiative transfer models. We first describe the spectral inversion techniques that have been used to infer the plasma parameters important for the general properties of the prominence plasma in both its cool core and the hotter prominence-corona transition region. We also review studies devoted to the observation of bulk motions of the prominence plasma and to the determination of prominence mass. However, a simple inversion of spectroscopic data usually fails when the lines become optically thick at certain wavelengths. Therefore, complex non-LTE models become necessary. We thus present the basics of non-LTE radiative transfer theory and the associated multi-level radiative transfer problems. The main results of one- and two-dimensional models of the prominences and their fine-structures are presented. We then discuss the energy balance in various prominence models. Finally, we outline the outstanding observational and theoretical questions, and the directions for future progress in our understanding of solar prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a better resolution in the published version. New version reflects minor changes brought after proof editin

Dipole radiation in the presence of a rough surface. Conversion of a surface-polariton field into radiation

The characteristics of the radiation produced by a dipole, located near the rough surface of a material medium, are examined. The field distribution is calculated at any point outside the medium for arbitrary orientation of the dipole moment, so that one may obtain the electromagnetic Green's function in the presence of surface roughness. The medium can have either local or a nonlocal dielectric function and the results are valid to first order in roughness. The surface roughness converts the surface polariton field, created even in the absence of roughness, into radiation and thus leads to well-defined resonances in the farfield radiation pattern. Numerical results for the case of metallic as well as dielectric gratings are given. The effect of the nonlocality of the dielectric function on the resonances in the radiation is shown to be significant in certain cases. For metallic gratings the dominant effect of the nonlocality is to shift the position of the angular resonances

Four-wave mixing in stochastic fields: fluctuation-induced resonances

The effect of pump fluctuations on various coherent processes that arise in three-level systems interacting with two external fields is examined. Such coherent processes include the forward Hanle effect and various four-wave mixing effects such as the generation of phase-conjugate signals. A general formulation that enables one to calculate the influence of laser linewidth on the coherent signals produced in various directions is presented. Ensemble averages, over laser temporal fluctuations, of various physical quantities, such as atomic polarization, are calculated. The spectrum of polarization fluctuations is shown to consist of several new features which lead to coherent radiation in different directions depending on the resonant frequencies in the polarization fluctuations. The influence of pump linewidth on pressure-induced extra resonance (PIER) is treated in detail. The possibility of producing a laser-fluctuation-induced coherent signal at one of the atomic frequencies is examined. This new signal, which is produced in a direction different from that of the PIER signal, but has the same type of resonant character as PIER, is found to have significantly different pressure dependence than the PIER signal. The results of our numerical computations are qualitatively explained in terms of the convolutions of products of third-order susceptibilities and pump field-correlation functions

Fluctuation-induced Hanle resonances in phase conjugation

We demonstrate the existence of Hanle resonances in four-wave mixing with pump and probe fields derived from the same fluctuating source. A theory of these fluctuation-induced Hanle resonances is developed following the work of Agarwal and Kunasz [Phys. Rev. A 27, 996 (1983)] on four-wave mixing in stochastic fields. Such Hanle resonances are compared with those induced by dephasing collisions

Pump-probe cross-correlation-induced resonances in four-wave mixing

A rigorous theory of the stochastic field-induced resonances in four-wave mixing with cross-correlated pump and probe fields is developed. The behavior of the extra resonance for a range of fluctuation and collisional parameters is discussed. Our results differ significantly from those obtained from a decorrelated theory

Comparative study of four-wave mixing in chaotic and phase-diffusing fields

We consider the dependence of the four-wave-mixing signal from an ensemble of homogeneously broadened, two-level atoms on the statistics of the pump field. Two models for the pump field are considered: the chaotic field and the phase-diffusing field. A Monte Carlo simulation procedure is used to integrate the equations describing the four-wave-mixing process numerically. This technique allows consideration of arbitrary bandwidths for the pump and incorporates pump-induced saturation effects. Results are presented for the strength of the four-wave-mixing signal as a function of the pump intensity for both resonant and off-resonant cases. It is shown that the four-wave-mixing signal is sensitive to the statistics of the pump, particularly for strong fields