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Emission from an oscillator ensemble

Sergei A. Akhmanov, S YU Nikitin

Published Apr 3, 1997• 0 citations
Publisher

Abstract

Abstract From the dynamics of an oscillator to the statistics of an ensemble. Non-laser sources of light: intensity, polarization, and radiation directivity pattern. Radiation statistics of independent oscillators. The radiation spectrum. Mechanisms of spectral line broadening. Natural broadening. Doppler broadening. Collision-induced broadening and the shift of a spectral line. Dicke’s effect: collision-induced narrowing of a Doppler spectral line in a dense gas.

DOI
10.1093/oso/9780198517955.003.0006

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Abstract From the dynamics of an oscillator to the statistics of an ensemble. Non-laser sources of light: intensity, polarization, and radiation directivity pattern. Radiation statistics of independent oscillators. The radiation spectrum. Mechanisms of spectral line broadening. Natural broadening. Doppler broadening. Collision-induced broadening and the shift of a spectral line. Dicke’s effect: collision-induced narrowing of a Doppler spectral line in a dense gas.
The paper is devoted to the study of the influence of self-similar correlations on the Doppler and pressure broadening within the non-equilibrium Boltzmann gas. The diffuse model for the thermal motion of the radiator and the self-similar mechanism of interference of scalar perturbations for phase shifts of an atomic oscillator are developed. It is shown that taking into account self-similar correlation in a description of the spectral line shape allows one to explain, on the one hand, the additional spectral line Dicke-narrowing in the Doppler regime, and, on the other hand, the asymmetry in wings of the spectral line in a high pressure region.
The absorbed/emitted radiation will be treated classically, as a monochromatic plane wave, whereas the atomic system that can absorb/emit such radiation will be described quantum-mechanically. However, no incident radiation beam is needed, and the atom “falls down” after a certain time interval to the ground state without any external reason. In the spontaneous emission processes the emitting atom can be thought of as a classical oscillating dipole, whose amplitude decreases due to the energy loss through the radiation. If an ensemble of identical atoms is considered and if, for simplicity, each atom emits radiation with the same spectral shape and full width at half maximum, it is impossible to ascribe a definite spectral component with intensity to a definite atom. In this case, such a broadening is identical for all atoms in the ensemble. On the microscopic scale, absorption of radiation is reduced to the transitions between pairs of energy levels of a discrete atomic spectrum.
We investigate the effect of collisional broadening, of population trapping and repumping, and of detuning of the atomic levels on the quantum-statistical properties of Raman laser radiation. We find that under suitably chosen operating conditions the effect of such additional incoherent relaxation mechanisms for the atomic polarization and populations can be made very small. One obtains highly nonclassical values for the Mandel Q parameter and the output-intensity fluctuation spectrum S(\ensuremath{\omega}) for experimentally reasonable parameters. Including detuning of the laser and the pump light from the atomic levels in certain cases can lead to further narrowing of the intracavity photon number distribution, as compared to the resonant case, down to a Mandel Q parameter close to the optimum possible value of Q=-1. A subsequent preliminary analysis prevails that inhomogeneous Doppler broadening of the atomic lines, both for collinear and for counterpropagating pump and laser light, is not detrimental to the nonclassical intracavity state.
The effect of collisions on the resonance fluorescence spectrum of two-level atoms excited by monochromatic resonance radiation is studied. Analysis is performed for systems where the Doppler broadening is small compared to the collision frequency (high buffer gas pressures) and for the general case where collisions arbitrarily change the phase of the radiation-induced dipole moment (from completely interrupted to completely unaffected phase of the dipole moment). Both at a relatively low and at a high excitation radiation intensity, the resonance fluorescence spectrum is shown to depend on whether the two-level system is closed or open. This is especially true for the narrow unshifted Rayleigh scattering line. It is shown that, although the absorption line is homogeneously broadened, the resonance fluorescence spectrum exhibits a clearly pronounced anisotropy. In a direction close to the direction of propagation of the excitation radiation, the Rayleigh scattering line is maximally narrowed. Under certain conditions (that can easily be created in experiments), the width of this line is proportional to the diffusion coefficient of atoms interacting with the radiation.