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Physical Review A

Transient nutation signal locking

R. N. Shakhmuratov

Published May 1, 1999• 4 citations
Publisher

Abstract

A method of making the dephasing time ${T}_{2u}$ measurement in an ensemble of inhomogeneously broadened two-level systems is proposed. This method has its origin in the spin-locking technique developed in NMR. It is shown that in spite of inhomogeneous broadening it is possible to lock the induced polarization in phase with the driving field. The amplitude of the locked polarization can be as large as half of the first maximum of the nutation signal, i.e., of the polarization bump just after the pulse switch on. Decay of the locked polarization is caused by the ${T}_{2u}$ dephasing or by the dephasing of the in-phase component of polarization.

DOI
10.1103/physreva.59.3788

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We study experimentally the dynamical and decay properties of the stimulated nutation echo (SNE) in a two-level spin system, the signal of which allows the observation timescale of the driven coherence relaxation to be extended. This signal appears in the transient response of the system to the second pulse at time τ 1 from its start and coinciding with the duration of the first pulse. The information about the first pulse duration is imprinted into the population difference of the inhomogeneously broadened ensemble of the two-level absorbers. The decay of the SNE signal has two contributions. One originates from the population decay during the time τ between the two pulses. Another is caused by the coherence loss during the excitation by the first pulse and the reading time of the second pulse. Experimental results on the decay properties induced by these mechanisms are presented for the first time. We investigate the dependence of these decay rates on the pulse intensity and we examine its relationship with the anomalous (non-Bloch) decay of other coherent transients in solids.
We study experimentally the dynamical and decay properties of the Stimulated Nutation Echo (SNE) in a two-level spin system. This is the signal which appears in the transient response of the system to the second pulse at time tau_1 elapsed from its beginning and coinciding with the duration of the first pulse. The information about the first pulse duration is imprinted into the population difference of the inhomogeneously broadened ensemble of the two-level absorbers. The decay of the SNE signal has two contributions. One originates from the population decay during the time tau between the pulses. Another is caused by the coherence loss during the excitation by the first pulse and the reading time of the second pulse. Experimental results on the decay properties induced by these mechanisms are presented. The dependence of these decay rates on the pulse intensity is discussed, and its relationship with the anomalous (non-Bloch) decay of other coherent transients in solids is examined.
Qualitative aspects of the polarization locking in phase with the driving field are discussed. It is shown that particles, with a resonant detuning smaller than Rabi frequency, can be preferentially oriented along the driving field amplitude. This direction determines a new quantization axis of the two-level system (TLS) driven by the resonant field. So, the behavior of the driven TLS can be described by a new nondriven TLS which reaches the equilibrium by the interaction with a reservoir or its environment. Eigenstates of the new TLS are symmetric and asymmetric superpositions of the original TLS states.
Microscopic dynamics. of materials can be studied by time delayed four-wave mixing spectroscopy. Dephasing constants can be measured by such experiments. Experimental results are easily analysed when the correlation time r of the exciting electric field is much shorter than the dephasing time ''2 However when these quantities are on the same time scale numerical fits are necessary. In the present work we analysed theoretically a time delayed four-wave mixing experiment. The material is described by an ensemble of two-level systems and a large inhomogeneous broadening is assumed. Using the third order term of the density matrix we establish a relation between 1''2 -r and ''Tm the time delay for which the maximum of the diffracted light intensity is observed. The conditions to obtain an estimate of T2 from the measurement of Tm are discussed.
The phase relaxation of optically excited electronic states can be studied in fourwave mixing experiments where three excitation pulses at t = 0, t =t21, and t = T, respectively, interact via the third-order polarization of the medium to generate a nonlinear signal whose intensity depends on the dephasing time T2. For inhomogeneously broadened optical transitions the signal will be emitted as a time delayed photon echo pulse at t = T + t21 provided the pulse duration is short compared with T2. The echo amplitude decays as exp(−4t21/T2).1