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Optics and Spectroscopy

Effect of energy level degeneracy on the state of a gaseous ensemble in a strong field in the presence of radiation capture

S. G. Zeiger

Published Sep 1, 1984• 0 citations
OpenAlex

Abstract

An equation is derived for the density matrix of a gaseous ensemble in the field of a strong wave of circular or linear polarization. The competition of polarizations in a laser is discussed, and it is shown that for the 1--0 and 1--1 transitions, it is independent of the magnitude of the field, so that circular polarization will be generated at an arbitrary value of the gain. An equation is derived for the lasing intensity in a laser as a function of pumping, account being taken of the degeneracy of the energy levels. It is shown that for the 1--0 and 1--1 transitions, this equation reduces to a third-order equation for the quantity GAMMA/..gamma../sub 10/, where GAMMA is the width of the Bennett dip.

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An equation is derived for the density matrix of a gaseous ensemble in the field of a strong wave of circular or linear polarization. The competition of polarizations in a laser is discussed, and it is shown that for the 1--0 and 1--1 transitions, it is independent of the magnitude of the field, so that circular polarization will be generated at an arbitrary value of the gain. An equation is derived for the lasing intensity in a laser as a function of pumping, account being taken of the degeneracy of the energy levels. It is shown that for the 1--0 and 1--1 transitions, this equation reduces to a third-order equation for the quantity GAMMA/..gamma../sub 10/, where GAMMA is the width of the Bennett dip.
An analysis is made of competition between modes emitted due to adjacent transitions having a common upper level. Conditions are obtained for the onset of lasing due to one transition in the presence of lasing of arbitrary intensity due to the other transition in standing- and traveling-wave lasers. Both transitions were assumed to be homogeneously broadened. Allowance is made for level degeneracy, collisional broadening of the dipole-forbidden transition between the lower active levels, and collisions with the buffer gas that equalize the populations of the lower active levels. The ranges of single-mode and two-mode lasing are determined as a function of the radiation intensity, the spectroscopic parameters of the transitions, and their collisional broadening.
Summary form only given. The resonance laser interaction with an atomic system with degenerate levels creates coherent properties in the medium. Depending on the laser polarization and the experimental geometry it could be alignment, orientation or optical pumping. This coherence could be destroyed by application of a magnetic field, which removes the level degeneracy. This process has an effect on the optical characteristics of the medium, such as absorption and fluorescence from the levels involved. They show the resonance dependence on the applied magnetic field with widths, depending on /spl gamma//sub a/, /spl gamma//sub b/, and /spl gamma//sub ab/, where /spl gamma//sub I/ is the width of the upper or lower state of the transition, resonant to the laser beam, /spl gamma//sub ab/ is the width of the optical transition a/spl rarr/b. Using different experimental geometry, /spl gamma//sub a/, /spl gamma//sub b/, and /spl gamma//sub ab/ could be measured, as well as some rate constants of these levels. We investigated the coherent properties of the 2/sup 3/ P/spl rarr/3/sup 3/ D transition of He I under resonant laser radiation. Due to the peculiarities of the coherent signals, registered in the lateral and forward direction the cross-section for velocity selective collisions for the 3/sup 3/ D He I level were obtained. The influence of the strong laser field on the widths and shapes of the signals was determined. The cross-section, for depolarizing collisions of the 3/sup 3/ D level was measured, too.
Expressions are developed in this paper which describe the behavior of a gas laser having generalized polarization characteristics. It is found that degeneracies of the atomic energy levels play an important part in determining the behavior of such a laser since significant terms occur in the nonlinear polarization which are attributable to an oscillatory mixing of these levels. As a result, it is found, for example, that for single-mode operation the field intensity is greatest for either plane or circular polarization depending upon whether a $\ensuremath{\Delta}j=\ifmmode\pm\else\textpm\fi{}1$ or $\ensuremath{\Delta}j=0$ atomic transition is involved in the laser action. For two-mode operation, on the other hand, the behavior depends in a complicated way both on the polarization states of the oscillations and on the degree of degeneracy of the energy levels. This behavior is discussed in a number of special cases.
A theoretical analysis is made of the stimulated Raman scattering in gases in the presence of spatial degeneracy of the energy levels. Saturation of the population difference and influence of anti-Stokes radiation are allowed for. Various polarizations of the pump fields and of the Stokes and anti-Stokes radiation are considered. It is shown that the profile of the stimulated Raman line is generally a sum of three Lorentzian lines and the scattering includes scalar, symmetric, and antisymmetric components. The use of circularly polarized pump radiation suppresses the influence of anti-Stokes radiation on the gain of the Stokes wave.
A theoretical calculation of the polarization changes of a weak probe laser beam as it traverses a strongly pumped gaseous atomic medium is presented. The medium is chosen to be a three-level cascade (J = 0–J = 1–J = 0) system. The polarization of the probe changes from linear to elliptical owing to the asymmetrical population transfer induced by a strong, circularly polarized pumping laser beam. We study this phenomenon as a function of laser intensities, laser detunings, and the relaxation parameters of the medium. We compare the predictions of the theory with results obtained from experimental studies of the neon 1s4(J = 1)–2p4(J = 2)–3s2(J = 1) system.
Saturation effects in gaseous media in the presence of a high-intensity resonant laser field and in the field of incoherent radiation caused by the intrinsic spontaneous emission are considered. A simple nonlinear set of kinetic equations is proposed for describing the saturation effect in the case of optically thin media and nondegenerate levels. The Bennett structure and the band of homogeneous saturation, whose parameters are interdependent, exist in the velocity distribution. The regularities in the behavior of the band and the Bennett structure under variation in the transition parameters and the intensity of laser radiation are elucidated.