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Optics Communications

Amplitude-squared squeezing in two-photon resonance fluorescence

N. A. Enaki, Mihai Macovei

Published Dec 1, 1998• 4 citations
Publisher

Abstract

Interaction of single-mode coherent light with dipole-forbidden two-level atoms via non-resonant virtual level is studied in this paper. It is shown that, in the case of two-photon resonance fluorescence , the atomic system generates three bands of correlated photon pairs with the carrier frequencies ω k 0 , ω k 0 ± Γ , where Γ=[Ω 2 +(Δ/2) 2 ] 1/2 , 2Ω is the two-photon Rabi frequency and Δ is the detuning from the two-photon resonance. It is established that amplitude-squared squeezing occurs from the sidebands of the two-photon resonance fluorescence spectrum . When the atom number increases, the amplitude-squared squeezing also increases.

DOI
10.1016/s0030-4018(98)00534-3

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Physical Review A•May 1, 1995•62 citations
Interaction of single-mode coherent light with dipole-forbidden two-level atoms via non-resonant virtual level is studied in this paper. It is shown that, in the case of two-photon resonance fluorescence , the atomic system generates three bands of correlated photon pairs with the carrier frequencies ω k 0 , ω k 0 ± Γ , where Γ=[Ω 2 +(Δ/2) 2 ] 1/2 , 2Ω is the two-photon Rabi frequency and Δ is the detuning from the two-photon resonance. It is established that amplitude-squared squeezing occurs from the sidebands of the two-photon resonance fluorescence spectrum . When the atom number increases, the amplitude-squared squeezing also increases.
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 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.
A new possibility of generating one-mode squeezed light by coupling a system of two-level atom with a bichromatic pump field in an optical cavity is suggested. The non-classical character of light in this process is caused by two-photon radiation at the central frequency of the spectrum of resonance fluorescence in the bichromatic field. The intensity of the resonance fluorescence cavity output field as well as the variance of the quadrature amplitudes and the second-order correlation function of intensity are calculated.
The dynamics of a strongly driven two-level atom are examined for the case that the atom is coupled to a ``two-mode'' squeezed vacuum. The source of squeezed light is taken to be a (frequency) nondegenerate parametric oscillator. We consider the situation in which the Rabi sidebands produced by the coherent driving field coincide with the spectral peaks of the parametric oscillator. Approximate analytical results are presented and checked against numerical simulations of the exact equations. For a suitable choice of relative phase, all three spectral lines in the fluorescence triplet are found to exhibit subnatural linewidths. Significant modifications to the fluorescence spectrum are also possible with a nonzero laser-atom detuning.
A theoretical study is made of the steady-state intensity and squeezing properties of the fluorescent light from a three-level-atom system in a ladder configuration, which is subject to spontaneous-emission decay to the electromagnetic-field vacuum. Two cases are examined: in the equispaced (ES) level case, the two atomic transition frequencies are nearly equal and two photon absorption processes occur from a single coherent laser field that couples both transitions and has a small detuning from each; in the non- equispaced (NES) level case, the atomic transition frequencies are rather different and two photon absorption processes occur from two coherent laser fields, each coupled to a single transition and in near resonance with it. In both cases, a situation of small two-photon detuning occurs. Optical Bloch equations for the atomic density matrix in rotating frames are given and matrix expressions for determining the steady-state populations and coherences are obtained. Analytic expressions are also given for special cases. The fluorescent intensity is obtained from the populations of the intermediate and upper states, with the normally ordered variance (NOV) for quadrature components of the fluorescent field involving, in addition, the atomic coherences. The fluorescent intensities are shown graphically as a function of two-photon detuning for a variety of one-photon detunings and Rabi frequencies for the ES and NES cases.In both cases, the fluorescent intensities show the well known resonances from the upper state at zero two-photon detuning and from the intermediate state at zero one-photon detuning for the lower transition. However, further resonances are also found. The intermediate-state intensity displays a resonance at zero two-photon detuning. Also, the upper-state intensity shows a resonance at zero one-photon detuning for the upper transition, but only for the NES case; evidently, the transfer rate from upper to lower transition coherences destroys this resonance for the ES case. The time-averaged NOV is also shown graphically as a function of two-photon detuning for a variety of one-photon detunings and Rabi frequencies for the ES and NES cases. For the ES case, the quadrature frequency is chosen as the single laser frequency and for the NES case, it is chosen as either the average of the two laser frequencies or the lower transition laser frequency. Squeezing occurs near zero two-photon detuning for both ES and NES cases, though only for the quadrature frequency equal to the average laser frequency in the latter case. It also occurs near zero detuning for the lower transition for both cases, though only for the quadrature frequency equal to the lower transition laser frequency in the NES case. The squeezing minima show a splitting effect for large Rabi frequencies corresponding to two-photon or one-photon Rabi splitting of the dressed atom levels for the situation near two-photon or one-photon resonance, respectively. The large squeezing near two-photon resonance for moderate Rabi frequencies and large one-photon detuning corresponds to essentially pure three-level squeezing, while that near one-photon resonance for weak Rabi frequencies involves two-level squeezing.