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Non-Linear Narrow Resonances of the Density of Excited Molecules in a Standing-Light Wave (Nelineinie Uzkie Rezonansy Plotnosti Vozbuzhdennykh Molekul v Stoyachei Svetovoi Volne),

Vladilen S Letokhov, B. D. Pavlik

Published Sep 18, 1974• 0 citations
OpenAlex

Abstract

The possibility of the resonant variation in the total number of particles on the excitation level with the transmission of the field prequency through the center of a Doppler-widened absorption line is examined. The article examines the effect of the formation of the resonance of the total number of particles in the simplest mode of the moving two-layer particles with a varied degree of saturation of absorption, and also the effect of rotational relaxation on the value of the resonance in the case of molecular transitions.

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The possibility of the resonant variation in the total number of particles on the excitation level with the transmission of the field prequency through the center of a Doppler-widened absorption line is examined. The article examines the effect of the formation of the resonance of the total number of particles in the simplest mode of the moving two-layer particles with a varied degree of saturation of absorption, and also the effect of rotational relaxation on the value of the resonance in the case of molecular transitions.
The review deals with resonance phenomena resulting from saturation of low-pressure-gas absorption lines by a coherent optical field, when the spectral lines are inhomogeneously broadened by the Doppler effect. Principal attention is paid to narrow resonances of width much smaller than the Doppler widths, which are produced upon saturation of the absorption by light waves under various experimental conditions (inside and outside the laser resonator, in traveling and standing waves, in spatially separated waves, etc.), owing to different effects (absorption saturation, mode interaction in the laser). Each of the phenomena is considered from both the theoretical and the experimental points of view. The phenomena considered serve as a basis of a new branch of optical spectroscopy, namely nonlinear laser spectroscopy of ultrahigh resolution.
A theoretical investigation is carried out through the interaction of the plane running monochromatic light wave, having an arbitrary intensity, with atoms (molecules) of a rarefied gas in the plane cell (at the normal incidence of the wave). Cases of closed and open resonance transitions from the non-degenerate ground (or metastable) quantum level are considered. Possible sub-Doppler resonances are analysed in the wave absorption, caused by the transient establishment of the optical coherence on the transition, Rabi oscillations between its levels, and optical pumping during the free flights of particles between the walls of the cell. Results of the previous works on given problems are generalized, which were obtained at definite restrictions on the wave intensity and cell length. Moreover, non-trivial sub-Doppler spectral structures, resulting as a consequence of the dependence of the absorption saturation on the transit relaxation of particles, have been established and investigated. Such structures may consist of a number of peaks and dips caused by Rabi oscillations between the transition levels. The results obtained can be used in sub-Doppler spectroscopy and for the stabilization of laser frequencies in thin gas cells.
The effect of collisions on the resonance fluorescence spectrum of three-level atoms with a V-configuration of levels when excited by two monochromatic resonance fields has been theoretically investigated. The analysis has been carried out for the systems with a small Doppler broadening compared to the collision frequency (high buffer gas pressures) and for the general case of an arbitrary change (from complete failure to complete preservation) of the phase memory at any of the three transitions in the V-system. It has been shown that in the three-level V-type atoms excited by two monochromatic fields, the resonant fluorescence spectrum (the number of spectral lines and their widths) at one of the transitions strongly depends on the presence of resonant radiation at the adjacent transition. Under certain conditions, the line widths will be completely determined by the relaxation characteristics of the adjacent transition and, in particular, can be repeatedly narrowed with an increase in the radiation intensity at the adjacent transition. It has been shown that despite the homogeneous nature of the broadening of the absorption lines, the resonance fluorescence spectrum has a pronounced anisotropy with respect to the mutual orientation of the wave vectors of spontaneous and exciting radiation. It has been established that the line widths in the resonance fluorescence spectrum are proportional to the diffusion coefficient of the atoms interacting with radiation under certain conditions. This fact can serve as the basis for a spectroscopic method for measuring the transport frequencies of the collisions of the radiation-absorbing particles with the buffer ones. It has been shown that the effects of phase memory in the collisions lead to a strong qualitative and quantitative modification of the resonance fluorescence spectrum.
We describe the influence of the intensity variation of the light in a standing wave on the lineshape of a Doppler-free multi-photon transition. We show that as long as the light shifts and the saturation are small compared to the Doppler width, a motional narrowing effect occurs and the line remains Lorentzian, the centre being shifted by an amount equal to the mean value of the dynamic Stark effect. We consider also the case of transients. We show that at resonance, it is possible to observe different responses according to the light intensity. In particular, it is possible to observe Rabi oscillations only if the Rabi frequency is larger than the natural width and smaller than the Doppler width of the transition.
An exact expression of the gain constant of two-level molecules (or atoms) in a standing-wave optical field is obtained for a special case where the optical frequency is in exact resonance with the molecular transition and the longitudinal and transverse relaxations have equal decay constants. Secondly, an exact form of gain constant as a function of frequency detuning which decribes the shape of Lamb dip or saturated absorption is shown for another case where the transverse relaxation is much faster than the longitudinal relaxation. A flat velocity distribution of molecules at Doppler limit is assumed in either case.