Masar Amplication of Incoherent Radiation by Interstellar OH

Molecular excitation model for maser amplification of emission lines of interstellar hydroxyl radica

On the theory of astronomical maser. II. Polarization of maser radiation

In this paper we investigate the polarization property of the radiation amplified by astronomical masers in the presence of a strong magnetic field. Our model explicitly takes into account the broadband nature of the radiation field and the interaction of the radiation with the maser transition J=1--0. The amplification of different realisations of the background continuum radition by the maser is directly simulated and the Stokes parameters of the radiation field are then obtained by averaging over the ensemble of emerging maser radiation. For isotropic pumping and partially saturated masers we find that the maser radiation is linearly polarized in two representative cases where the magnetic field {\bf B} makes an angle $\theta$=30$^0$ and $\theta$=90$^0$ to the maser axis. The linear polarization for maser radiation obtained in our simulations for both cases are in agreement with the results of the standard model. Furthermore, no instability during amplification is seen in our simulations. Therefore, we conclude that there is no problem with the previous numerical investigations of maser polarization in the unsaturated and partially saturated regime.Comment: 17 pages, 7 figures, to appear on MNRA

Theoretical Study of Fast Light with Short sech Pulses in Coherent Gain Media

We investigate theoretically the phenomenon of so-called fast light in an unconventional regime, using pulses sufficiently short that relaxation effects in a gain medium can be ignored completely. We show that previously recognized gain instabilities, including superfluorescence, can be tolerated in achieving a pulse peak advance of one full peak width.Comment: 7 pages, 8 figures; Replaced with revised version accepted by JOSA

On the theory of astronomical maser. I. Statistics of maser radiation

In this paper we re-analyse the amplification process of broadband continuum radiation by astronomical masers in one-dimensional case. The basic equations appropriate for the scalar maser and the random nature of the maser radiation field are derived from basic physical principles. Comparision with the standard radiation transfer equation allows us to examine the underlying assumptions involved in the current theory of astronomical masers. Simulations are carried out to follow the amplification of different realisations of the broadband background radiation by the maser. The observable quantities such as intensity, spectral line profile are obtained by averaging over an ensemble of the emerging radiation corresponding to the amplified background radiation field. Our simulations show that the fluctuations of the radiation field inside the astronomical maser deviates significantly from Gaussian statistics even when the maser is only partially saturated. Coupling between different frequency modes and the population pulsing are shown to have increasing importance in the transport of maser radiation as the maser approaches saturation. Our results suggest that the standard formulation of radiation transfer provides a satisfactory description of the intensity and the line narrowing effect in the unsaturated and partially saturated masers within the framework of one-dimensional model. Howerver, the application of the same formulation to the strong saturation regime should be considered with caution.Comment: 16 pages, 4 figures, to appear on MNRA

Superluminal Signals: Causal Loop Paradoxes Revisited

Recent results demonstrating superluminal group velocities and tachyonic dispersion relations reopen the question of superluminal signals and causal loop paradoxes. The sense in which superluminal signals are permitted is explained in terms of pulse reshaping, and the self-consistent behavior which prevents causal loop paradoxes is illustrated by an explicit example.Comment: 6 pages, 3 figure

Nonlinear interaction of light with Bose-Einstein condensate: new methods to generate subpoissonian light

We consider $\Lambda$-type model of the Bose-Einstein condensate of sodium atoms interacting with the light. Coefficients of the Kerr-nonlinearity in the condensate can achieve large and negative values providing the possibility for effective control of group velocity and dispersion of the probe pulse. We find a regime when the observation of the "slow" and "fast" light propagating without absorption becomes achievable due to strong nonlinearity. An effective two-level quantum model of the system is derived and studied based on the su(2) polynomial deformation approach. We propose an efficient way for generation of subpoissonian fields in the Bose-Einstein condensate at time-scales much shorter than the characteristic decay time in the system. We show that the quantum properties of the probe pulse can be controlled in BEC by the classical coupling field.Comment: 13 pages, 6 figures, 1 tabl

Transparent Anomalous Dispersion and Superluminal Light Pulse Propagation at a Negative Group Velocity

Anomalous dispersion cannot occur in a transparent passive medium where electromagnetic radiation is being absorbed at all frequencies, as pointed out by Landau and Lifshitz. Here we show, both theoretically and experimentally, that transparent linear anomalous dispersion can occur when a gain doublet is present. Therefore, a superluminal light pulse propagation can be observed even at a negative group velocity through a transparent medium with almost no pulse distortion. Consequently, a {\it negative transit time} is experimentally observed resulting in the peak of the incident light pulse to exit the medium even before entering it. This counterintuitive effect is a direct result of the {\it rephasing} process owing to the wave nature of light and is not at odds with either causality or Einstein's theory of special relativity.Comment: 12 journal pages, 9 figure