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Soviet Journal of Quantum Electronics

Observation of the recoil effect in saturated dispersion resonances in methane

В. А. Алексеев, Nikolay G. Basov, M A Gubin, V V Nikitin, A. V. Nikulchin, В. Н. Петровский, Evgeniy D. Protsenko, D A Tyurikov

Published Apr 30, 1984• 3 citations
Publisher

Abstract

The results are given of an experimental investigation of the structure of the λ= 3.39 μ methane line by recording saturated dispersion resonances. A method is described for reducing the intensity of the laser field in the absorbing medium to a value of < 10-6 W/cm2 and thus ensuring that the resolving power of the method is determined mainly by the collisional and field broadening of the lines, and exceeds considerably the transit limit. A 2.2 kHz doublet line splitting, resulting from the recoil effect, was recorded using a laser having a 3-cm diameter beam.

DOI
10.1070/qe1984v014n04abeh004992

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Absorption of a Gaussian laser mode in a molecular beam

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This paper presents a calculation of the depth of the saturation dip observed when laser radiation of a Gaussian ${\mathrm{TEM}}_{00}$ mode is absorbed in a thermal molecular beam or in a very-low-pressure gas. Transit-time broadening is assumed to be the only broadening mechanism, with neglect of collisions and other relaxations. There is no restriction to small saturation parameters. The absorption experienced by one molecule has been calculated via the optical Bloch equations. Averaging over all relevant parameters of the molecular trajectory and the laser field followed. To get a reference for the saturation, from which to measure the dip depth, simply twice the absorption from a running-wave field has been taken, which therefore also had to be calculated in addition to the calculation of the absorption in a standing wave. Preliminary experiments have been carried out for comparison, and the data agree very well with the calculated shape of the curve and the absolute values of the calculated dip depth. From the laser-power scaling factor, necessary to fit the experimental data to the universal theoretical curve, the dipole moment can be derived without having to know such hard-to-determine quantities as absorber density, partition function, collision relaxation rate, or even the diameter of the laser beam.

Physical Review A•Jan 1, 1987•3 citations
The results are given of an experimental investigation of the structure of the λ= 3.39 μ methane line by recording saturated dispersion resonances. A method is described for reducing the intensity of the laser field in the absorbing medium to a value of < 10-6 W/cm2 and thus ensuring that the resolving power of the method is determined mainly by the collisional and field broadening of the lines, and exceeds considerably the transit limit. A 2.2 kHz doublet line splitting, resulting from the recoil effect, was recorded using a laser having a 3-cm diameter beam.
We present a new scheme of laser spectrometer based on an active optical cavity containing saturated absorbing and amplifying media. Such an active cavity may ensure enhancement in the amplitude of nonlinear optical resonances of saturated absorption at least by one order of magnitude as compared with similar passive system. The efficiency of the spectrometer was demonstrated by the observation of saturated absorption resonances in methane at a wavelength of 3.39 /spl mu/m under transit-time conditions. In these conditions, the contribution of fast molecules can be greatly reduced because of a finite interaction time of particles with the radiation field. In our work with a light beam 0.6 cm in diameter, we eliminated the transit-time broadening limit by using optical selection of slow methane molecules with a transverse velocity 20 times less than the mean thermal one, and obtained sub-DoppIer resonances with a homogeneous width less than 5 kHz.
The general method developed in a previous paper (see ibid., vol.10, p.345 (1977)) is applied to the problem of resonance Raman scattering in very intense laser fields. At very low intensities the spectrum consists of three monochromatic lines (Rayleigh, Raman Stokes and anti-Stokes) which split into a triplet and two doublets when the laser light, tuned on one of the two transitions connecting the upper state to the two lower sublevels, has a sufficiently high intensity to saturate this transition but not the second one. The Raman and Rayleigh lines are completely mixed when the intensity becomes so high that both transitions are saturated. The corresponding evolution of the spectrum is determined. Optical-pumping effects leading to asymmetries in the fluorescence spectrum are taken into account.
In conventional saturated absorption experiments utilizing I 2 absorption cells, the acccuracy in determining the line center is ultimately limited by : unresolved recoil structure /1/ ; residual transit-time broadening ; second-order Doppler broadening and shift ; and collisions (nonlinear pres­ sure broadening and shift). These limitations can be greatly reduced by applying the method of separated field excitation /2/ in a supersonic I 2 beam. The experiments were carried out using a high-resolution I 2 -stabilized Ar'-laser spectrometer at 514.5 nm and a collimated supersonic beam. The spectral width of the Ar'-laser was estimated to be in the range of 10 kHz. From the high density and the narrow velocity distribution of the super­ sonic beam we expect to achieve good signal-to-noise ratio, reduced second order Doppler broadening and improved knowledge of the second order Doppler shift. The experimental setup is shown in Fig. 1. The I 2 molecules of the supersonic be am are excited sequentially by two pairs of travelling waves propagating parallel but in opposite directions. A coherence between the initial and the excited states is built up in the first interaction zone /4/. It then precesses freely through the field-free region. In the second zone, two of the initial state and the excited state are created in the Vz -space, depending on the phase difference between the coherence and the exciting field. The two gratings are slightly shifted relatively to each other due to the recoil splitting. The distance a bet­ ween the first two interaction zones is limited to about 3 mm to avoid excessive decay of the upper state of the selected absorption line. During the free flight between the second and third interaction zones, separated by a distance d = 35 mm, the upper state population decays completely into many ro-vibrational levels of the electronic ground state, leaving the population grating of the initial state practically unchanged. Correspon­ dingly, only the initial state population grating will contribute to the Ramsey signal generated after the final two interactions and only the recoil component associated with the initial state will contribute to the Ramsey fringes /1,3,4/.
For pt.I see ibid., vol.7, p.1235 (1974). Uses the results of an earlier work to consider recoil effects on spectroscopic features observed with a saturable absorber. Systematic approximation schemes are presented, both the straightforward perturbation expansion in the field amplitudes and a non-perturbative approach, which neglects pulsations in the populations. This method generalizes the REA of recoil-free strong-signal theory and is expected to give good results for velocity-averaged observables. Its validity is discussed and the consequences are compared with those of other calculations. The results of numerical evaluations of spectra are shown, in particular the limit of poorly resolved recoil features is explored. Parameter regions where recoil-induced splittings and asymmetries occur are presented.
We report the first observation of the recoil shift of an optical resonance in the visible part of the spectrum. The method rests on the fact that the recoil shifts have opposite signs for Doppler-generated level crossings with a common upper level and for those having a common lower level. The experiment has been performed with the argon laser at 5145 \AA{} as the source and iodine as the absorber. The measured values are within 12% of the theoretical recoil shift. We give a detailed discussion of the possible influence of shifts of various origins and especially light shifts on the measured frequency differences. We display saturated absorption and saturated dispersion spectra and give formulas for the intensities of the various resonances derived from the corresponding density-matrix diagrams.
This paper presents a calculation of the depth of the saturation dip observed when laser radiation of a Gaussian TEM00{\mathrm{TEM}}_{00}TEM00​ mode is absorbed in a thermal molecular beam or in a very-low-pressure gas. Transit-time broadening is assumed to be the only broadening mechanism, with neglect of collisions and other relaxations. There is no restriction to small saturation parameters. The absorption experienced by one molecule has been calculated via the optical Bloch equations. Averaging over all relevant parameters of the molecular trajectory and the laser field followed. To get a reference for the saturation, from which to measure the dip depth, simply twice the absorption from a running-wave field has been taken, which therefore also had to be calculated in addition to the calculation of the absorption in a standing wave. Preliminary experiments have been carried out for comparison, and the data agree very well with the calculated shape of the curve and the absolute values of the calculated dip depth. From the laser-power scaling factor, necessary to fit the experimental data to the universal theoretical curve, the dipole moment can be derived without having to know such hard-to-determine quantities as absorber density, partition function, collision relaxation rate, or even the diameter of the laser beam.