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Journal of Physics C Solid State Physics

Two-photon spectroscopy of a resonant three-level spin system. II. Resonance shift

R. Boscaino, A Tripo

Published Jun 14, 1979• 0 citations
Publisher

Abstract

For pt.I see ibid., vol.11, p.365 (1978). A resonance shift, orders of magnitude larger than the Bloch-Siegert shift, has been measured in the second-harmonic microwave emission spectrum of a dilute ruby sample (Cr 3+ :Al 2 O 3 ) near a three-level resonance. The observed shift is ascribed to the interference between the direct and the resonant second-harmonic generating processes.

DOI
10.1088/0022-3719/12/11/025

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For pt.I see ibid., vol.11, p.365 (1978). A resonance shift, orders of magnitude larger than the Bloch-Siegert shift, has been measured in the second-harmonic microwave emission spectrum of a dilute ruby sample (Cr 3+ :Al 2 O 3 ) near a three-level resonance. The observed shift is ascribed to the interference between the direct and the resonant second-harmonic generating processes.
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.
We report experimental observation of multiphoton resonances and Bloch-Siegert shifts in a strongly driven classical two-level system. The system is an optical ring resonator, and the levels are two orthogonal linear polarizations. The Rabi frequency and the Bloch-Siegert shift were measured for both the one- and three-photon cases. Satisfactory agreement was achieved with theory developed for a quantum two-level system. The experiment demonstrates that these coherence phenomena can be observed in a purely classical system.
For pt.II see ibid., vol.9, p.1221 (1976). Discusses the steady-state response of a three-level atom in two-photon absorption. The applied laser fields can have arbitrary intensities (within the rotating-wave approximation (RWA)). A general expression is derived for the population excited on the uppermost energy level. Two special cases which allow a trivial Doppler averaging are treated in detail. For an off-resonant intermediate state the two-photon spectrum is a simple Lorentzian showing a power shift and modest cross power broadening. The same features are retained when several off-resonance intermediate levels are assumed. For very strong laser fields the Doppler shifts can be neglected in the first approximation. The corresponding spectra are analysed.
The present paper applies the method of high-intensity quantum electrodynamics developed earlier to several problems of current experimental interest. In this paper the intensitydependent shifts and widths of atomic levels are calculated and applied to a given transition near resonance. The technique is demonstrated with three examples: the multiphoton ionization of atomic cesium and hydrogen in atomic beams and third-harmonic production in an atomic vapor. It is found that the multiphoton-ionization rate in a relatively weak laser beam (of order 108{10}^{8}108 W/cm2{\mathrm{cm}}^{2}cm2) is determined by the intensity-dependent level width (6f6f6f configuration) as well as the interference of the fine-structure states, and that the rates for hydrogen, in a much more intense beam (of order 1012{10}^{12}1012 W/cm2{\mathrm{cm}}^{2}cm2) are determined by both the intensity-dependent widths and shifts as‘\stackrel{\mathrm{`}}{\mathrm{s}}s‘ well as the wave-function renormalization constants. The agreement of these two examples of calculations with the experiments done recently is satisfactory. For third-harmonic production the role of two-photon resonance is emphasized, and the dependence of the generated third-harmonic intensity on the intensity and frequency of the fundamental is discussed. Near resonance, the ratio of third-harmonic intensity to fundamental intensity saturates.
Observation of Bloch--Siegert shifts in the 2sup2sup 2sup2S/sub 1/2/ - 2sup2sup 2sup2P/ sub 1/2/ microwave resonance in atomic hydrogen
The Bloch-Siegert-type shifts and multiple-quantum resonance shifts of a spin-one system subjected to an oscillating radio-frequency applied at right angles to a DC field have been calculated to sixth order in V, the amplitude of the oscillating field, for the single-quantum resonance, and to fourth order in V for the higher resonances. The level crossing criterion appropriate to two-level systems cannot be used here, and a resonance condition based on a previously derived analogue of Shirley's equation for spin-one systems is used instead. The calculation uses degenerate continued-fraction perturbation theory. Numerical calculations were performed in order to obtain the shape of the resonances, and partially check the analytic results. The atomic levels were assumed to be completely stable; the corrections due to finite atomic lifetimes are expected to be small.