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Journal of Anhui University

Optical free induction decay in an almost degenerate V-Type Three-Level system

Zhuo‐Liang Cao

Published Jan 1, 2002• 0 citations
OpenAlex

Abstract

A optical free induction decay is studied semiclassically in the V-type three-level system with two upper levels being almost degenerate in Bloch form. It is discussed that the systematic relaxation has effect on the signal field of the free-induction decay. The decay behavior has three contributions: a homogeneous part with transverse relaxation time and two inhomogeneous parts with different time constants that reflect the velocity bandwidth excited.

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Journal of the Optical Society of America B•Apr 1, 1986•56 citations
A optical free induction decay is studied semiclassically in the V-type three-level system with two upper levels being almost degenerate in Bloch form. It is discussed that the systematic relaxation has effect on the signal field of the free-induction decay. The decay behavior has three contributions: a homogeneous part with transverse relaxation time and two inhomogeneous parts with different time constants that reflect the velocity bandwidth excited.
A theory of optical free-induction decay (FID) induced by two exciting light pulses in three-level atomic systems subjected to a constant magnetic field is presented. The theory takes into account the hyperfine interaction, the level degeneracy, and the dependence of the FID signal on the polarizations of the exciting pulses. An analysis of the FID signal in a magnetic field suggests new ways to measure the g factors of the levels, the hyperfine structure constants, and the relaxation rates. The results obtained are extended to include the echelon excitation of the multilevel system by a definite set of light pulses with specially chosen frequencies and polarizations.
A V-type three level system with two closely spaced upper levels interacting with a single mode of the electromagnetic field is studied for the absorptive and dispersive lineshapes. A semiclassical description for the atom-field interaction is being adopted to obtain the usual optical Bloch equations. The approximate analytical solutions for the density matrix elements are obtained by solving coupled optical Bloch equations. Through the off-diagonal density matrix elements, the introduction of phase angles between the levels participating in dipole allowed transitions are automatic. It is shown that these phases are appreciable if the applied electromagnetic field is strong. These field dependent phases (coherence) are responsible for the reduction of the inversionless gain and the absorptionless dispersion. For large decay constants, it is found that the energy gap between the upper two levels has no role to play on the laser without inversion and on the absorptionless dispersion. However, for large population per unit decay, a significant increase of the absorptionless dispersion and the inversionless gain is obtained. The condition for large population decay and a possible experimental realization of the field dependent phases are discussed.
Free induction decay and transient nutations are investigated in a system of two-level particles with an inhomogeneously broadened spectrum. The influence of the x (real) and y (imaginary) parts of the complex amplitude of the noise, induced by the driving field in the sample, is considered. It is shown that both noises affect T1{T}_{1}T1​ and T2{T}_{2}T2​ relaxation processes. When field-noise contributions to 1/T2{1/T}_{2}1/T2​ and 1/T1{1/T}_{1}1/T1​ relaxation rates are dominant and equal to each other, Bloch's saturation is changed to Redfield's saturation. This effect also accelerates transient nutation decay. Comparison with experimentally observed free induction decay and transient nutation in color centers in quartz is discussed.
Photon echo and free induction decay, i.e., representative coherent optical transients, are described for two level system whose transition frequency is modulated by the Gaussian-Markoffian process. Photon echo signal is shown to decay biexponentially and to be effective in fixing the magnitude and the correlation time of the frequency modulation. Then we present the transition of the free induction decay rate against laser intensity from the Bloch type at the weak laser field into the Redfield one at the strong laser field.
In a recent experiment [ DevoeR. G.BrewerR. G., Phys. Rev. Lett.50, 1269 ( 1983)], it was found that the optical Bloch equations could not satisfactorily explain the signal that was observed for free-induction decay in the impurity ion cyrstal Pr3+:LaF3. Several theories have been proposed to explain this failure of the Bloch equations. In this paper, the general validity conditions for the optical Bloch equations are examined within the limits of a Markovian relaxation model. The specific problem to be considered is the interaction of an optical field with two-level atoms. The atoms undergo relaxation as a result of coupling to a perturber bath that, itself, is negligibly affected by the relaxation process. First, a simple decay-parameter model is assumed for the relaxation of atomic density-matrix elements. Such a model is found to lead to a set of generalized Bloch equations of which the conventional Bloch equations form a subset. Subsequently, more-realistic models for relaxation in both vapors and solids are considered within the limits of the impact approximation (i.e., the duration of a fluctuation can be viewed as instantaneous with respect to all relevant time scales in the problem). It is found that, even in the impact (Markovian) approximation, the generalized Bloch equations cannot be expected to provide an adequate description of relaxation, owing to effects in which a fluctuation-induced change in the atomic transition frequency persists between fluctuations. In vapors this frequency shift is produced by velocity-changing collisions that change the atomic resonance frequency (as seen in the laboratory frame), whereas in solids it is produced by local-field fluctuations. The limiting conditions under which one can expect both the generalized and the conventional Bloch equations to retain their validity are explored.