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Physical Review A

Light-force-induced fluorescence line-center shifts in high-precision optical spectroscopy: Simple model and experiment

M. Artoni, Iacopo Carusotto, F. Minardi

Published Jul 17, 2000• 14 citations
Publisher

Abstract

We calculate the effect of light-induced forces on the fluorescence line shape of a two-level atom crossing at right angles two counterpropagating light beams of parallel linear polarizations $(\mathrm{lin}\ensuremath{\Vert}\mathrm{lin})$ in a common configuration for ultrahigh-precision optical spectroscopy. For an incident atomic beam with a narrow spread of transverse velocities the dipole force induces a redshift of the fluorescence maximum, while in the reverse case of a wide spread of transverse velocities the radiation-pressure force induces a blueshift of the saturation dip minimum. We then use our theory to explain the blueshift of the saturation line-center dip occurring for the closed transition $2{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}$ of a ${}^{4}\mathrm{He}$ beam. The observed shift, which is in quite good agreement with the theory, can be of the order of 1/10 of the transition natural linewidth and hence quite important for ultrahigh-precision spectroscopy measurements.

DOI
10.1103/physreva.62.023402

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We calculate the effect of light-induced forces on the fluorescence line shape of a two-level atom crossing at right angles two counterpropagating light beams of parallel linear polarizations (lin\ensuremath∥lin)(\mathrm{lin}\ensuremath{\Vert}\mathrm{lin})(lin\ensuremath∥lin) in a common configuration for ultrahigh-precision optical spectroscopy. For an incident atomic beam with a narrow spread of transverse velocities the dipole force induces a redshift of the fluorescence maximum, while in the reverse case of a wide spread of transverse velocities the radiation-pressure force induces a blueshift of the saturation dip minimum. We then use our theory to explain the blueshift of the saturation line-center dip occurring for the closed transition 23S1\ensuremath→23P22{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}23S1​\ensuremath→23P2​ of a 4He{}^{4}\mathrm{He}4He beam. The observed shift, which is in quite good agreement with the theory, can be of the order of 1/10 of the transition natural linewidth and hence quite important for ultrahigh-precision spectroscopy measurements.
We report on the observation of a light-pressure-induced asymmetry of the well-known absorption dip occurring in optical saturation spectroscopy. This asymmetry is due to the small modification of the atomic velocity distribution that is caused by the spontaneous scattering force of the saturating light field. Our measurements were performed on the \ensuremath{\lambda}=555.65 nm 1S0^{1}\mathrm{S}_{0}1S0​${\mathrm{\ensuremath{-}}}^{3}$P1{\mathrm{P}}_{1}P1​ transition in atomic ytterbium vapor by the use of a frequency-modulated test field propagating collinearly with the scattering field. The experimental results clearly confirm theoretical predictions and demonstrate the significance of the phenomenon for high-resolution laser spectroscopy.
This paper has been temporarily withdrawn. Highly nonlinear dependences of 4He 2^3S - 2^3P lines on pressure were observed in vapor cell saturation spectroscopy. Line shifts agree with those expected from light-pressure-induced perturbation of atomic velocity distribution. In the lin-perp-lin pump and probe polarization configuration, multilevel atomic structure must be taken into account. For a two-level atom, the shift magnitude is determined by the collisional time constant, but when the system has dark states there is an additional time constant due to optical pumping. The collisional and optical time constants compete, and become equal at a certain pressure p_0. Saturation signal vanishes at p_0 whereas signal due to light pressure remains. This leads to a dependence of line centers on pressure that exhibits apparently singular behavior at p_0. The 2S - 2P(J=1) line shift reaches the natural half width. Vapor cells may be more suitable for investigation of this effect than atomic beams since the shifts vanish at sufficiently high pressures. Two ways of controlling the shifts for the helium 2P fine structure measurement are discussed.
We report the results of studying the effect of the resonant light pressure force on the amplitude and frequency properties of the nonlinear susceptibility and well known absorption resonance dip occurring in optical saturation spectroscopy of two level atomic gas. On the basis of numerical solutions of the Liouville and Fokker-Plank equations the behaviour pattern of the velocity distribution function of a two-level atomic gas in relation to the saturation intensity and time of interaction with a strong resonant optical field was obtained. Time course of the velocity distribution function due to the action of spontaneous light pressure force leads to modification of the nonlinear susceptibility, absorption saturation resonant line form and to change of frequency dip relative to the atomic resonant frequency transition. The represented numerical modeling results explained qualitatively a number ofexperimental facts, that could not be explained before.
In this paper we analyse the effect of coherent saturation in the laser-induced fluorescence (LIF) of molecules. We develop an earlier model introduced by Billy et al. in 1987 of this coherent effect and propose a simple formula predicting the line shape and the line intensity for a laser-molecule interaction time considerably longer than the excited-state radiative lifetime. The predictions of this formula are tested on a LIF experiment in which a Na2 molecular beam is excited with a continuous-wave single-frequency dye laser. Considerable spectral broadening, up to two hundred times the natural linewidth is observed, in good agreement with the model.
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The optical Hanle effect (light-shift-induced zero-field level crossing) has been investigated theoretically and experimentally. In the case of a $J=0\ensuremath{\rightarrow}1\ensuremath{\rightarrow}0$ three-level system and of a $J=0\ensuremath{\rightarrow}1$ two-level system, the resonance line shape has been obtained using a nonlinear-response-function method. Approximate expressions, valid under the conditions for the observation of light shifts, are obtained and compared with exact calculations as well as with the corresponding expression for the magnetic Hanle effect. As concerns the weak laser beam which produces the coherent excitation of atomic substates, we have calculated the linear response in the limiting case of monochromatic and of broad-band excitation. The experimental investigation has been performed for the Ba resonance line using two cw dye lasers at $\ensuremath{\lambda}=5535$ \AA{} and an atomic beam. The absorption- and dispersion-shaped level crossing resonances have been observed, for narrow- and broad-band excitation, by varying the power of the strong nonresonant laser beam. We have checked that the line shapes are only dependent upon the reduced variable $\frac{{P}_{L}}{\ensuremath{\delta}}$ (PL{P}_{L}PL​; beam power; $\ensuremath{\delta}$: frequency detuning). Using the theoretical expressions, we have tried to fit the experimental curves. Provided that the Gaussian distribution of the beams is taken into account, we have obtained a good agreement between theory and experiment for the whole set of experimental data.