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Physical Review A

Atomic correlations and line shape in microwave second-harmonic generation

R. Boscaino

Published Nov 1, 1976• 0 citations
Publisher

Abstract

Second-harmonic (SH) generation by paramagnetic samples at microwave frequency is considered, and the dependence of its intensity on the total number $N$ of active centers is experimentally investigated. The reported results show a ${N}^{2}$ dependence of the SH intensity near the resonance condition and a higher $N$ dependence in the far wings of the SH line. A qualitative explanation is suggested in which the observed effect is related to the internal correlations established among the spins during the SH generation process, as theoretically investigated in the following paper by Persico.

DOI
10.1103/physreva.14.1689

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Physical review. B, Condensed matter•Sep 1, 1994•0 citations
Second-harmonic (SH) generation by paramagnetic samples at microwave frequency is considered, and the dependence of its intensity on the total number NNN of active centers is experimentally investigated. The reported results show a N2{N}^{2}N2 dependence of the SH intensity near the resonance condition and a higher NNN dependence in the far wings of the SH line. A qualitative explanation is suggested in which the observed effect is related to the internal correlations established among the spins during the SH generation process, as theoretically investigated in the following paper by Persico.
Absorption of a microwave photon by a system of NNN two-level atoms followed by reemission, and absorption of two microwave photons followed by emission of a single photon of double frequency are considered. Interatomic correlations and dephasing effects are taken into account, and it is shown that they have different relative importance in different parts of the cross-section curve: On resonance, dephasing effects are expected to prevail, while far from resonance the opposite is true. This behavior is shown to be reflected in a different angular and NNN dependence of the emission curve in the center and in wings of the line. These predictions are compared with the experimental results on second-harmonic generation of the preceding paper by Boscaino, and shown to be in at least qualitative agreement.
It is suggested that the combined effect of a resonant magnetic field (dynamic Zeeman effect) and of the fluctuations due to spin-spin and spin-lattice interactions could give rise to inelastic components in the emission spectrum of a paramagnetic system. It is also shown that a similar effect can be expected in the second harmonic emission. It is found that if the Rabi frequency is much larger than the relaxation rates, both the resonant and the second harmonic spectra show the two side lines typical of the atomic resonant fluorescence. The results are illustrated by means of a semiclassical model. Plots of expected spectra as a function of the frequency are given for some realistic parameters of paramagnetic samples.
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.
It is theoretically and experimentally demonstrated that optical second-harmonic generation (SHG) is possible to be observed in atomic media irradiated with an intense laser pulse. The SHG is induced by a static electric field spontaneously generated in the interaction, and any special condition is not required for the SHG. The experiment to observe the SHG has been made using atomic sodium vapor and a mode-locked Nd-doped yttrium aluminum garnet laser system (pulse duration, 28 psec; output energy \ensuremath{\lesssim}50 mJ). The second harmonic (SH) polarization, spatial beam pattern, and dependences of SH signal on the sodium density and on the fundamental intensity have been examined. The experimental results agree well with the theoretical predictions and numerical results.