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Journal of Physics C Solid State Physics

Two-photon transitions in spin systems: Effect of a resonant intermediate level

R. Boscaino, A Tripo

Published Mar 28, 1976• 2 citations
Publisher

Abstract

The effect of a resonant intermediate state on a two-photon transition is experimentally investigated in a three-level paramagnetic spin system. It is shown that, together with transitions which involve all three levels, a distinct Raman-like process is always present, to which the intermediate does not take part.
DOI
10.1088/0022-3719/9/6/005

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Two-photon transitions in a nearly resonant three-level spin system are experimentally investigated at microwave frequencies by detecting the second harmonic (SH) wave emitted by a resonant paramagnetic spin sample. The dependence of the SH intensity and lineshape on the detuning of the intermediate level from the resonance is measured in a dilute ruby sample (Cr 3+ :Al 2 O 3 ). It is shown that the SH output is the coherent superposition of two different contributions which come, respectively, from processes which involve all three levels and from Raman-like processes to which the intermediate level does not take part. The dependence on the intensity and polarisation of the input radiation is measured for each one of the two kinds of processes and interference effects between them are detected.
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We have investigated the steady-state cavity-field properties of a single-mode two-photon micromaser when the atoms in a cascade three-level configuration are initially prepared in a mixture of the upper and intermediate states. The mean photon number, trapping state and sub-Poissonian effect are discussed with upper (intermediate)-state population changing from 1(0) to 0(1). These properties are very different from those in a pure two- or one-photon transition process, due to the competition among different transition processes. In particular, the trapping states of nonzero photons are discovered in this system under some conditions, which is contrary to the previous findings.
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Rabi oscillations reflect the existence of a coherent superposition of two molecular eigenstates during the interaction of molecules with a strong, resonant electromagnetic field. This phenomenon is not restricted to two-level systems. We demonstrate its occurrence in a three-level system where the radiation induces a two-photon absorption. The intermediate level is (slightly) detuned from resonance. For a theoretical description the dressed-state picture is adopted. The two-photon excitation is performed either by using one laser, yielding a one-color two-photon process, or by using two counterpropagating, spatially overlapping laser beams, yielding a two-color two-photon process. Rabi oscillations are produced by varying the fluence, i.e., by changing either the laser power or the molecule-laser interaction time. For the one-color case the two-photon transition dipole moment for an SF6{\mathrm{SF}}_{6}SF6​ transition has been determined from the relation between laser fluence and number of Rabi oscillations. For the two-color cases the detuning with the intermediate level is varied by tuning the respective laser frequencies while keeping the sum frequency constant. A dramatic change in the degree of excitation as a function of the intermediate detuning has been observed. \textcopyright{} 1996 The American Physical Society.