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Physical Review A

Squeezing and dressed-state polarization of driven atoms coupled to a frequency-dependent vacuum reservoir

Maciej Lewenstein, THOMAS W. MOSSBERG

Published Jul 1, 1988• 14 citations
Publisher

Abstract

We consider the response of an isolated two-level atom to strong-field excitation in the special case where the atom decays via coupling to a frequency-dependent photon-mode density. It is found that atomic squeezing substantially in excess of that found in the case of free-space atoms arises. The enhanced squeezing is related to a polarization of the atom-field dressed states that is also found to occur.

DOI
10.1103/physreva.38.1075

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Journal of Physics B Atomic Molecular and Optical Physics•Feb 28, 1994•21 citations
We consider the response of an isolated two-level atom to strong-field excitation in the special case where the atom decays via coupling to a frequency-dependent photon-mode density. It is found that atomic squeezing substantially in excess of that found in the case of free-space atoms arises. The enhanced squeezing is related to a polarization of the atom-field dressed states that is also found to occur.
Interesting new aspects of atomic behavior in the presence of strong driving fields appear when the driven atom resides not in free space, but in a region (such as an optical cavity) that displays a frequency-dependent photon-mode density. Under such conditions, it is found that a strong driving field can modify the spontaneous decay properties of an atom, and thereby give rise to interesting new features in the spectrum of strong-field resonance fluorescence. It is also found that a high level of dressed-state polarization can be maintained in a sample of resonantly or nonresonantly driven atoms by appropriate tuning of an enclosing cavity. Furthermore, for appropriate laser and cavity tunings, it is found that the atomic states become highly squeezed. In the course of analyzing these effects, a set of modified Bloch equations is derived that explicitly accounts for the finite response time associated with a frequency-dependent photon-mode density.
The relaxation process of a single two-level atom driven by an intense resonant coherent field is studied in the presence of a broadband squeezed vacuum field. Generalized forms for the self-field operator and the field-dependent damping coefficients are derived. In the steady state, positive atomic inversion (~5%) is shown for some range of the phase of the squeezed vacuum field. The squeezing-induced enhanced and asymmetric coherence (i.e.?non-zero dispersive atomic polarization) induces profound asymmetry in the side-bands of the Mollow fluorescent spectrum and the absorptive?dispersive spectra near the Rabi side-band frequencies. The case of two and three cooperative atoms, instead of a single atom, shows (i)?positive atomic inversion in the steady state for larger interval of the squeeze phase parameter and more enhanced extrema for the atomic polarization components and (ii) the extra side peaks in the fluorescent spectrum become asymmetric and of dispersive-like profile. Dressed-state analysis in the single-atom case with the field-dependent decay process in the presence of the squeezed vacuum field is presented and shows that the inequality of the (field-dependent) decay rates of the dressed states leads to more positive inversion in the dressed states as compared with the normal vacuum case.
The steady-state behavior of two-level atoms, driven by a strong field and relaxing into a frequency-dependent vacuum reservoir, has been studied in an experiment involving Ba atoms in a confocal cavity. Enhancement or suppression of the atomic excitation is observed when the cavity is tuned to either of the strong-field fluorescence sidebands. These effects result from a redistribution of the population among the dressed atom-field states induced by the cavity-modified vacuum field. The enhancement effect may be useful in achieving significant steady-state inversion.
The quadrature squeezing spectra in the resonance fluorescence of a V-type three-level atom driven by a coherent field and coupled to a single-mode cavity is investigated. For weak excitation, the fluorescence field exhibit squeezing in the out-of-phase quadrature. The coupling between the atom and the cavity mode can greatly enhance the squeezing centred at the laser frequency. More importantly, for strong excitation, under the effect of the cavity-atom coupling, the in-phase quadrature of fluorescence can exhibit two-mode squeezing at the two inner sideband frequencies. By working in the dressed-state representation and hiring secular approximation, we give an analytical explanation for the effect. The result shows, under appropriate conditions, the squeezing can be greatly enhanced by appropriately tuning the cavity resonant frequency.
The authors present analytical and numerical results for resonance fluorescence produced by a two-level atom damped by a broad-band squeezed vacuum. Previous theoretical analyses of this model have assumed that the laser frequency and the carrier frequency of the squeezed vacuum are equal. The authors introduce a non-zero detuning between the laser frequency and the carrier frequency while maintaining resonance between the laser frequency and atomic resonance. In the absence of the driving field there is a squeezing threshold: above threshold the decay rate of the atomic polarization can be reduced, and below threshold a squeezing-induced two peak spectrum and frequency shift, which is analogous to the Bloch-Siegert shift, are predicted. In the presence of the driving field, the spectrum is asymmetric for Rabi frequencies smaller or comparable to the detuning of the squeezed vacuum. A symmetric spectrum is obtained for large Rabi frequencies. An interpretation is predicted by means of a dressed-state analysis. The authors find that for not too large Rabi frequencies a remarkably large population inversion can be induced in the dressed states, even at exact resonance of the driving field, provided that the laser frequency and carrier frequency are detuned. Moreover, for detection times larger than the inverse detuning, the spectrum is insensitive to the phase between the driving field and the squeezed vacuum, in marked contrast to the case of no detuning.