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Quantum Optics Journal of the European Optical Society Part B

Asymmetric two-level two-level resonance fluorescence spectra and dressed state populations in a squeezed vacuum

S Smart, S. Swain

Published Apr 1, 1993• 27 citations
Publisher

Abstract

The authors calculate the resonance fluorescence spectrum of a two-level atom interacting with a resonant classical applied field and a squeezed vacuum for general values of the squeezing phase. They find that the spectra are asymmetric under the following conditions: (i) the Rabi frequency must not be too large and (ii) the squeezing phase should be of athe order pi /2. Large asymmetries are obtainable, even if the three peaks of the spectrum are clearly resolved. An interpretation is provided by means of a dressed state analysis. Under conditions (i) and (ii), they also find that very large differences in the dressed state populations can occur.

DOI
10.1088/0954-8998/5/2/003

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The authors calculate the resonance fluorescence spectrum of a two-level atom interacting with a resonant classical applied field and a squeezed vacuum for general values of the squeezing phase. They find that the spectra are asymmetric under the following conditions: (i) the Rabi frequency must not be too large and (ii) the squeezing phase should be of athe order pi /2. Large asymmetries are obtainable, even if the three peaks of the spectrum are clearly resolved. An interpretation is provided by means of a dressed state analysis. Under conditions (i) and (ii), they also find that very large differences in the dressed state populations can occur.
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.
The steady-state resonance fluorescence spectrum of a two-level atom driven by a bichromatic field in a broadband squeezed vacuum is studied. When the carrier frequency of the squeezed vacuum is tuned to the frequency of the central spectral line, anomalous spectral features, such as hole burning and dispersive profiles, can occur at the central line. We show that these features appear for wider, and experimentally more convenient, ranges of the parameters than in the case of monochromatic excitation. The absence of a coherent spectral component at the central line makes any experimental attempt to observe these features much easier. We also discuss the general features of the spectrum. When the carrier frequency of the squeezed vacuum is tuned to the first odd or even sidebands, the spectrum is asymmetric and only the sidebands are sensitive to phase. For appropriate choices of the phase the linewidths of only the odd or even sidebands can be reduced. A dressed-state interpretation is provided.
The fluorescence field from one of the two allowed transitions in a three-level atom can sense squeezed fluctuations of a vacuum field coupled to the other transition. We examine the fluorescence spectra of strongly driven three-level atoms in \ensuremath{\Lambda}, V, and cascade configurations in which one of the two one-photon transitions is coupled to a finite-bandwidth squeezed vacuum field, when the bandwidth is much smaller than the difference in the atomic transition frequencies, though much larger than atomic decay rates and Rabi frequencies of the driving fields. The driving fields are on one-photon resonance, and the squeezed vacuum field is generated by a degenerate parameter oscillator. Details are only given for the \ensuremath{\Lambda} configuration. The extension to the V and cascade configurations is straightforward. We find that in all configurations the fluorescence spectra of the transition not coupled to the squeezed vacuum field are composed of five lines, one central and two pairs of sidebands, with intensities and widths strongly influenced by the squeezed vacuum field. However, only the central component and the outer sidebands exhibit a dependence on the squeezing phase. We also examine the fluorescence spectrum for the cascade configuration with a squeezed vacuum field on resonance with the two-photon transition between the ground and the most excited states and now generated by a nondegenerate parametric oscillator. In this case, where the squeezed vacuum field can be made coupled to both transitions, all spectral lines depend on the squeezing phase. The spectral features are explained in terms of the dressed-atom model of the system. We show that the coherent mixing of the atomic states by the strong driving fields modifies transition rates between the dressed states, which results in the selective phase dependence of the spectral features. \textcopyright{} 1996 The American Physical Society.
The authors investigate the dipole-dipole interaction role in the resonance fluorescence of two identical two-level atoms, which are confined to a region much smaller than the resonant wavelength and which are damped by a squeezed vacuum. The spectrum consists of seven or five lines, depending on the relative phase between the driving field and the squeezed vacuum, whereas the spectrum without the dipole-dipole interaction displays three lines. The widths of these lines depend on the parameters of the squeezed vacuum and can be subnatural or supernatural depending on the phase. The central line and additional sidebands at double the Rabi frequency can have intensities larger than in the normal vacuum but the intensities of lines at one Rabi frequency are always less than in the normal vacuum.
We study the system of a single two-level atom interacting with a braodband squeezed vacuum and a classical applied field. The atom is subject to collisions, and the applied field may possess a finite linewidth. The atomic populations, coherences, resonance fluorescence spectrum, and intensity fluctuation spectrum are calculated. Wherever practicable, analytic expressions are obtained that show the dependence of these quantities on the damping parameters. Numerical plots are presented. Two qualitative results that we obtain are the following: (a) we show that the dependence of the populations on the squeezing phase is increased by increasing the damping parameters, contrary to what might at first have been expected, and (b) we show that, in the absence of external damping, the squeezing induces strong asymmetries in the off-resonant resonance fluorescence spectra.
The resonance fluorescence of a three-level system is studied in each of its three possible configurations—ladder, lambda and vee—when it also interacts with broad band squeezed vacua and the applied fields are resonant. We obtain general expressions which apply for any configuration and do not make use of the secular approximation. Detailed results are presented which depend upon the degree of squeezing and the squeezing phase, in addition to the intensities of the applied fields. It is shown that asymmetric spectra may be obtained in certain circumstances. These are interpreted in terms of coherent and incoherent transitions induced by the squeezed fields. We also take the opportunity to assess the accuracy of the secular approximation.