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Trans Black Sea Region Symposium on Applied Electromagnetism

Coherent effects in He I under laser interaction

Emilia A. Alipieva, S.I. Karabasheva, Elena T. Taskova

Published Nov 11, 2002• 0 citations
Publisher

Abstract

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.

DOI
10.1109/aem.2000.943296

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Abstract A theoretical and experimental study has been made of the intensity of forward scattered light in conditions of radio-frequency double resonance and level-crossing. The main conclusions are (i) that the magnetic resonance and level-crossing effects which are familiar in the laterally scattered light are to be found also in the forward scattered light, but (ii) that the resonance curves are Doppler-broadened, and the strength of the oscillatory field necessary to generate them is correspondingly increased, and (iii) that coherence narrowing is much more pronounced in forward than in lateral scattering. The differences derive from the coherence between the radiation from different atoms which is present in forward, but not in lateral scattering, and which is implicitly recognized in the classical theory of the propagation of light in polarizable media. The equations of electromagnetism form the basis of the present analysis. In the presence of the oscillatory magnetic field the polarization tensor is complex and time-dependent. The time dependence leads to the result that the number of eigenwaves necessary to describe propagated waves in the medium is four, as against the usual two. An arbitrary wave is resolved into a superposition of the four eigenwaves, all of which have different propagation constants. The analysis is very general, and is applicable to experiments with lasers. The components of the polarization tensor are derived by finding first the polarizability of individual atoms from a semi-classical theory of their interaction with light and magnetic fields. The macroscopic polarization, obtained by summing over individual atoms, is then given by folding the individual polarizabilities with the Doppler distribution of resonance frequencies. In the particular case studied in detail (the inter-combination resonance line, 2537 A, in mercury), the Doppler width is much greater than the radiation width, and the imaginary (absorptive) part of the polarizability is Gaussian. Expressions are derived in a ‘weak scattering’ approximation for the intensity of the forward scattered light. They are magnetic resonance functions which are found to be the Doppler-broadened equivalents of the familiar functions, based on Lorentzians, characteristic of scattering by a single atom. In the case of double resonance the forward scattered light is strongly modulated, as is the laterally scattered light. To investigate coherence narrowing, the zero-field level-crossing has been studied with the restriction of ‘weak scattering’ removed. The result is obtained that the width of levelcrossing curves is inversely proportional to the vapour density for magnetic fields such that the Zeeman splitting is small in relation to the Doppler width. This analytical result has been verified experimentally.
We report magneto-optical rotation due to the interaction of atoms with a resonance laser beam. Our experiment is performed on the excited He states, 23P-33D transition, (lambda) equals 587,56nm. Two processes are responsible for the magneto-optical activity: the Zeeman coherences, induced in the absorbing level, and the optical pumping processes, each of them due to the laser beam excitation. The Zeeman coherence creates peaks with a width determined by the coherence relaxation rate, while the optical pumping creates a structure with a width in the order of the natural width of the transition. Due to the difference between the relaxation rates of the 2P and 3D He levels, the two parts of signal are well resolved and depend on the He pressure in different ways. The narrow coherence signal disappears at high He pressure, unless the amplitude of the optical pumping signal increases. The optical pumping signals saturates as He pressure increased and changes its sign. At high gas pressures, such that only the optical pumping signal exists, an increase of the He pressure leads to the narrowing of both the 'classical parabolic dependence' and the optical pumping signal.
We have experimentally and theoretically investigated the behavior of excited Na atoms under the combined influence of resonant laser light and a weak magnetic field. The use of weak magnetic fields gives another possibility to vary the polarization of the atoms, additional to variation of the polarization of the exciting laser. We calculated in detail the influence of weak magnetic fields on the density matrix describing the excited atoms, using generalized rate equations. We checked the results of our model experimentally, at the Na(3 2 S1/2, Fl=2) →(3 2 P3/2, Fu =3) transition, excited with circularly polarized light. We determined the polarization of the excited atoms, by measuring the fluorescence light at various angles, as a function of both magnetic field strength and laser irradiance. Model and experiment agreed very well. We applied our model to the associative ionization of the excited Na atoms. We measured the ion signal as a function of magnetic field. Analyzing the results, we could determine a specific associative ionization cross section, which is not accessible in our experimental geometry without the use of magnetic fields. Thereby we showed that the application of magnetic fields in polarization dependence studies can provide us with additional information about the process.
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In the introduced paper the results of theoretical and experimental investigations in a branch of probe-field spectroscopy of degenerated metastable atoms in a laser radiation field of arbitrary intensity with taking into account of its anisotropical excitation and the presence of additional channels of radiant decay of upper level are presented. It is shown that for the degenerated levels which are excited anisotropically the absorption spectrum of the probe field contains, besides resonances caused by occupancy effects, resonances associated with the magnetic coherence and manifested in the form of peaks and dips with substantially different amplitudes and widths. A fundamentally new property of these resonances is that their widths are proportional to the intensity of the orienting field, and the amplitude of the narrowest resonance becomes appreciable when the orienting field is very strong.
Theoretical and experimental studies of a gas laser amplifier are presented, assuming the amplifier is operating with a saturating optical frequency signal. The analysis is primarily concerned with the effects of the gas pressure and the presence of an axial magnetic field on the characteristics of the amplifying medium. Semiclassical radiation theory is used, along with a density matrix description of the atomic medium which relates the motion of single atoms to the macroscopic observables. A two-level description of the atom, using phenomenological source rates and decay rates, forms the basis of our analysis of the gas laser medium. Pressure effects are taken into account to a large extent through suitable choices of decay rate parameters. Two methods for calculating the induced polarization of the atomic medium are used. The first method utilizes a perturbation expansion which is valid for signal intensities which barely reach saturation strength, and it is quite general in applicability. The second method is valid for arbitrarily strong signals, but it yields tractable solutions only for zero magnetic field or for axial magnetic fields large enough such that the Zeeman splitting is much larger than the power broadened homogeneous linewidth of the laser transition. The effects of pressure broadening of the homogeneous spectral linewidth are included in both the weak-signal and strong-signal theories; however the effects of Zeeman sublevel-mixing collisions are taken into account only in the weak-signal theory. The behavior of a He-Ne gas laser amplifier in the presence of an axial magnetic field has been studied experimentally by measuring gain and Faraday rotation of linearly polarized resonant laser signals for various values of input signal intensity, and by measuring nonlinearity - induced anisotropy for elliptically polarized resonant laser signals of various input intensities. Two high-gain transitions in the 3.39-μ region were used for study: a J = 1 to J = 2 (3s2 → 3p4) transition and a J = 1 to J = 1 (3s2 → 3p2) transition. The input signals were tuned to the centers of their respective resonant gain lines. The experimental results agree quite well with corresponding theoretical expressions which have been developed to include the nonlinear effects of saturation strength signals. The experimental results clearly show saturation of Faraday rotation, and for the J = 1 t o J = 1 transition a Faraday rotation reversal and a traveling wave gain dip are seen for small values of axial magnetic field. The nonlinearity induced anisotropy shows a marked dependence on the gas pressure in the amplifier tube for the J = 1 to J = 2 transition; this dependence agrees with the predictions of the general perturbational or weak signal theory when allowances are made for the effects of Zeeman sublevel-mixing collisions. The results provide a method for measuring the upper (neon 3s2) level quadrupole moment decay rate, the dipole moment decay rates for the 3s2 → 3p4 and 3s2 → 3p2 transitions, and the effects of various types of collision processes on these decay rates.