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Physical Review A

Experimental spectra of oscillatory free-induction decay

R. Boscaino, F. M. Gelardi

Published Apr 1, 1987• 6 citations
Publisher

Abstract

The free-induction decay (FID) signals emitted by a two-level spin system (${E}_{1}^{\mathcal{'}}$ centers in glassy silica), coherently prepared by a short pulse of resonant radiation, have been experimentally investigated in the frequency domain. Our results show that, under the same conditions in which FID signals are oscillatory in the time domain, the spectra are characterized by a regular series of emission zeros. A vectorial model accounts for the occurrence of the zeros and for their spectral positions.

DOI
10.1103/physreva.35.3561

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Physical Review A•Dec 1, 1995•8 citations
The free-induction decay (FID) signals emitted by a two-level spin system (E1′{E}_{1}^{\mathcal{'}}E1′​ centers in glassy silica), coherently prepared by a short pulse of resonant radiation, have been experimentally investigated in the frequency domain. Our results show that, under the same conditions in which FID signals are oscillatory in the time domain, the spectra are characterized by a regular series of emission zeros. A vectorial model accounts for the occurrence of the zeros and for their spectral positions.
Large-amplitude oscillations are predicted in the free-induction decay (FID) of atomic or spin two-level quantum systems that are coherently prepared by a resonant electromagnetic field in the form of a square amplitude pulse of duration TTT. The effect should be detectable in nuclear magnetic resonance (NMR) or in optical resonance experiments when the transition line shape is inhomogeneously broadened (${T}_{2}^{*}\ensuremath{\ll}{T}_{2}$) and the applied field is intense enough that the pulse area $\ensuremath{\chi}T\ensuremath{\ge}2\ensuremath{\pi}$, where $\ensuremath{\chi}$ is the Rabi frequency. Physically, the effect is due to Rabi oscillations which are generated in the preparative stage and are then reproduced because of atomic memory in the radiative period which follows. The atomic polarization derived from the Bloch equations assumes the form of an integral over the inhomogeneous line shape, and can be evaluated numerically for arbitrary linewidth $\ensuremath{\sigma}$ or analytically when $\ensuremath{\sigma}\ensuremath{\rightarrow}\ensuremath{\infty}$. The characteristics of the oscillation are unusual since the oscillation frequency, which is of order $\ensuremath{\chi}$, increases with time while the oscillation envelope vanishes identically for times $t\ensuremath{\ge}2T$. Coherent emission is therefore confined to one pulse width TTT immediately following the pulse. Previous NMR and infrared FID experiments with small-area pulses ($\ensuremath{\chi}T\ensuremath{\le}4\ensuremath{\pi}$), interpreted erroneously in terms of an edge echo effect, support the oscillatory FID behavior described here. For the case $\ensuremath{\chi}T>4\ensuremath{\pi}$, an experimental test would be even more decisive, as the oscillations are more pronounced.
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 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.
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.
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)].