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Journal of Experimental and Theoretical Physics

Squeezed harmonic generation in cooperative resonance scattering of intense radiation by dipole molecules

Viktor A Kovarskii, O. B. Prepelitsa

Published Apr 1, 1998• 1 citation
Publisher

Abstract

We examine the processes of resonance Raman scattering of intense electromagnetic radiation by a lumped system of two-level atoms with a constant dipole moment. We calculate the intensity of the s th generated harmonic in a saturating electromagnetic field and the statistical characteristics of this harmonic. Finally, we show that the s th harmonic is squeezed at saturating field intensities.

DOI
10.1134/1.558522

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We examine the processes of resonance Raman scattering of intense electromagnetic radiation by a lumped system of two-level atoms with a constant dipole moment. We calculate the intensity of the s th generated harmonic in a saturating electromagnetic field and the statistical characteristics of this harmonic. Finally, we show that the s th harmonic is squeezed at saturating field intensities.
An analysis is made of the collective resonant generation of higher harmonics by a spatially extended system of two-level molecules possessing an intrinsic electron dipole moment in the excited state. Frequency and angular dependences of the scattered radiation are studied. It is shown that for moderately small numbers of harmonics their intensity depends comparatively weakly on the number (plateau) and the intensity of the emitted harmonics then falls sharply as the number increases. The angular distribution of the harmonics is strongly anisotropic. It is also shown that collective effects significantly change the time profile of the generated harmonic pulse. In addition, as a result of cooperative effects harmonics of a certain parity are suppressed in the low-frequency part of the radiation spectrum.
The influence of a strong field resonant to the Raman transition on the behavior of the Stokes harmonic in saturated two-wave stimulated Raman scattering is theoretically considered based on the description of a gas of three-level atoms by medium density matrix equations. The vibrational mode of energy redistribution between laser and scattered radiation, threshold with respect to the resonant field intensity, is predicted. The action of optical fields on the absorption and amplification of the resonant field in a Raman medium is described.
The resonance two-photon interaction of the broadband squeezed field (without coherent displacement) with the dipole-forbidden two-level atoms via nonresonant virtual levels is investigated. It is demonstrated that the broadband squeezed field excites coherently the system of radiators (atoms, molecules) that have dipole-forbidden transitions. The two-photon Rabi frequency, in this case, is amplified due to the large number of couples of correlated modes of the broadband squeezed field involved. It should be mentioned that this coherent excitation is not specific for optical allowed dipole transitions. In the high-field limit, the quantum statistics of the photon pairs generated in the process of resonance fluorescence is analysed. As the fluorescence spectrum consists of three bands of the correlated photon pairs we analysed the second-order fluctuations in the central and sideband peaks of the triplet. It is shown that the amplitude-squared squeezing that is presented in the sidebands arises from the interaction of the spontaneously radiated photon pairs with the vacuum of the electromagnetic field.
The resonance two-photon interaction of the broadband squeezed vacuum with the dipole-forbidden two-level atoms via nonresonant virtual levels is investigated. It is demonstrated that the broadband squeezed vacuum excites coherently the system of radiators (atoms, molecules) that have dipole-forbidden transitions. The quantum statistics of the generated photon pairs in the process of resonance fluorescence is analyzed. As the broadband squeezed field is amplitude-squared squeezed the interaction of this field with the dipole-forbidden atoms can contribute in further amplitude-squared squeezing. In the high-field limit this squeezing increases when the atom number also increases.
The interaction between radiation and a two-level molecular system in the presence of permanent dipole moments is analyzed. It is found that the nonzero difference between permanent dipoles induces a polarization that oscillates at frequencies /spl omega/, 2/spl omega/, 3/spl omega/, etc., where /spl omega/ is the frequency of the electromagnetic field resonant with the two-level system. An analysis of the propagation equation of the harmonics shows that even harmonics are amplified while odd harmonics are absorbed.
The dynamics of high-intense laser fields at coherent Raman self-scattering are investigated by the use of numerical simulation. The processes under investigation take place at the Raman scattering on electronic states in atoms with transition to be forbidden in electro-dipole approximation, on vibrational or rotational states in molecules, on states connected with the spin flip and other. It is found that the threshold intensities exist provided that the pulse duration is smaller than dephasing time of the Raman transition in the medium. Under this, the following processes occur: an effective generation of the high-order harmonics comes up; the plateau forms in the harmonic spectrum. The maximum which arises on this plateau shifts to the higher-harmonic numbers in case of the higher threshold intensities of the pulse. It is shown how the transformation of the initial pulse intensity into high-order harmonics intensity can be considerably increased with the different initial parameters of the pulse and the medium.