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Physical Review A

Influence of frequency exchange rate on free induction decay after saturation

Anatoly I. Burshtein, Anatoly A. Zharikov, Vladimir S. Malinovsky

Published Feb 1, 1991• 12 citations
Publisher

Abstract

The free induction decay (FID) and light-echo kinetics are calculated under the assumption that a two-level system frequency is a purely discontinuous noncorrelated Markovian process, and its statistical distribution is a bi-Lorentzian contour. The latter assumption is primarily mathematical, but useful. The model allows one to obtain an exact solution of the FID problem, which recovers the results obtained earlier with perturbation theory (the fast modulation limit) as well as the result concerning the slow modulation limit. It is shown that only in the latter case does the field dependence of the FID rate found by De Voe and Brewer [Phys. Rev. Lett. 50, 1269 (1983)] have a self-consistent explanation.

DOI
10.1103/physreva.43.1538

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The steady-state free-induction decay (FID) has been investigated in samples of crystalline and amorphous quartz containing paramagnetic two-level electron spin centers (S=1/2). We report experimental data on the dependence of the FID rate on the Rabi frequency \ensuremath{\chi} induced during the preparative stage. Our results confirm that the decay of the FID signal is much slower than expected on the basis of the conventional Bloch equations. The range of \ensuremath{\chi} investigated here is wide enough to show the whole transition from Bloch-type behavior to the high-power Redfield limit, in which the FID rate is determined only by \ensuremath{\chi}. The experimental data are compared with theoretical results recently obtained by hypothesizing Gaussian-Markovian fluctuations of the resonance frequency. Even if agreement is poor, the comparison suggests by itself the possibility that more than one source of fluctuations may be effective. Other possible origins of the failure of the Bloch equations as well as of the stochastic theories are examined.

Physical Review A•May 1, 1990•24 citations
The free induction decay (FID) and light-echo kinetics are calculated under the assumption that a two-level system frequency is a purely discontinuous noncorrelated Markovian process, and its statistical distribution is a bi-Lorentzian contour. The latter assumption is primarily mathematical, but useful. The model allows one to obtain an exact solution of the FID problem, which recovers the results obtained earlier with perturbation theory (the fast modulation limit) as well as the result concerning the slow modulation limit. It is shown that only in the latter case does the field dependence of the FID rate found by De Voe and Brewer [Phys. Rev. Lett. 50, 1269 (1983)] have a self-consistent explanation.
Sudden-modulation theory is used for the calculation of stationary saturation and polarization of two-level systems. The randomly fluctuating frequency of the two-level system is assumed to be a noncorrelated, purely discontinuous Markovian process. The stationary probability of field-induced transitions is calculated exactly. It is shown that if the frequency modulation is fast (or if the field is strong enough) the transition probability coincides with that obtained in the framework of perturbation theory when the latter is developed in its integrodifferential non-Markovian version. This probability differs from the kinetic probability calculated by Markovian perturbation theory (with fluctuating frequency) in its corresponding spectral wing shape and in the field dependence of its corresponding resonance width. This field dependence is smoother than that in Markovian theory and stabilizes when the resonance width is not equal to the natural transition width. Nevertheless, in the Lorentzian approximation the free-induction decay rate in the framework of exact (or non-Markovian) theory increases with the field in nearly the same way as that in Markovian theory. Almost the same is true of the field dependence of the free-induction decay amplitude in either theory, provided that the signal correlation before and after the field is switched off is neglected. The conditions are found under which the Lorentzian approximation of the transition probability is valid. These conditions are shown to break down when the theory (in the fast-frequency-modulation limit) leads to agreement with experiments reported by De Voe and Brewer [ Phys. Rev. Lett.50, 1269– 1272 ( 1983)].
The free-induction decay (FID) after saturation by a laser radiation pulse of finite duration is studied for systems with spectral diffusion. The exact solution of the FID signal shape has been obtained in the framework of a model of noncorrelated spectral exchange. The analysis accounts for the finite duration of the saturating field and is valid at an arbitrary value of the spectral exchange rate and amplitude of the coherent field. An exact expression for the FID signal is derived in the weak-external-field limit. A self-consistent explanation of the experimental field dependence of the FID rate [A. Szabo and T. Muramoto, Phys. Rev. A 39, 3992 (1989)] is obtained using a model of slow noncorrelated spectral diffusion.
The free-induction decay (FID) after saturation by a laser radiation pulse of finite duration is studied for systems with spectral diffusion. The exact solution of the FID signal shape has been obtained in the framework of the telegraph noise model. This solution takes into account the finite duration of the saturating field and is valid at arbitrary values of the spectral exchange rate and the amplitude of the coherent field. The exact expression of the FID signal is derived in the weak-external-field limit. It is shown that the FID signal suddenly disappears after a time equal to the pulse width. This confirms the validity of the theorem on coherent transients, which has been proved for the Bloch equations [A. Schenzle, N. C. Wong, and R. G. Brewer, Phys. Rev. A 22, 635 (1980)].
The free induction decay (FID) after saturation by a laser radiation pulse of finite duration is studied for systems with spectral diffusion. The exact solutions of the FID signal shape have been obtained in the framework of telegraph noise model as well as model of noncorrelated spectral exchange. These solutions take into account the finite duration of saturating field and are valid at arbitrary value of the spectral exchange rate and the amplitude of the coherent field. A selfconsistent explanation of the field dependence of the FID rate found by Szabo and Muramoto is obtained under the slow noncorrelated spectral diffusion.
In a previous paper [ J. Opt. Soc. Am. B3, 564 ( 1986)], the validity conditions for the optical Bloch equations were reviewed. It was shown that, even within the limits of an impact or Markovian approximation, the optical Bloch equations fail to account properly for fluctuation-induced changes in atomic transition frequencies. Such changes are properly incorporated in a quantum-mechanical transport equation (QMTE) in which the fluctuation-induced frequency shifts are totally characterized by kernels W(∊′ → ∊) that give the probability density per unit time for a fluctuation to change the frequency shift from ∊′ to ∊. The QMTE describes the interaction of atoms with both an external radiation field and the perturber bath producing the fluctuations. A general method for solving the QMTE as a perturbation series in the external field is presented. Specific calculations are carried out for strong-redistribution, difference [W(∊′ → ∊) is a function of (∊–∊′) only], and Brownian motion kernels. It is shown that, although the kernels possess fundamental differences, they can yield similar results in certain limits. As an example, a perturbation calculation is performed for the free-induction decay (FID) of atoms prepared by a cw laser field and then allowed to radiate when the field is suddenly removed. Radical departures from the predictions of the conventional Bloch equations are found in certain limits, including a first-order contribution to FID in vapors and a nonexponential FID decay for atoms in vapors or solids. The implications of these results to a consistent interpretation of a recent experiment [ Phys. Rev. Lett.50, 1269 ( 1983)] on FID in the impurity ion crystal Pr3+:LaF3 are explored.
The steady-state free-induction decay (FID) has been investigated in samples of crystalline and amorphous quartz containing paramagnetic two-level electron spin centers (S=1/2). We report experimental data on the dependence of the FID rate on the Rabi frequency \ensuremath{\chi} induced during the preparative stage. Our results confirm that the decay of the FID signal is much slower than expected on the basis of the conventional Bloch equations. The range of \ensuremath{\chi} investigated here is wide enough to show the whole transition from Bloch-type behavior to the high-power Redfield limit, in which the FID rate is determined only by \ensuremath{\chi}. The experimental data are compared with theoretical results recently obtained by hypothesizing Gaussian-Markovian fluctuations of the resonance frequency. Even if agreement is poor, the comparison suggests by itself the possibility that more than one source of fluctuations may be effective. Other possible origins of the failure of the Bloch equations as well as of the stochastic theories are examined.