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Physical Review Letters

Phase Effects in Bichromatic Field Interactions with a Two-Level Atom

Roger E. Silverans, Gustaaf Borghs, Peter De Bisschop, Marleen Van Hove

Published Sep 2, 1985• 16 citations
Publisher

Abstract

Phase-dependent resonance transitions induced in two-level atoms by a bichromatic field are reported. Strong phase effects are observed in the interaction of a fast atomic beam with a standing-wave rf field. In the atomic rest frame, this field consists of two components with different frequencies and a controlled initial phase difference. Numerical calculations including well-defined phase factors are performed in the time-dependent density matrix formalism. The observed effects are in excellent agreement with this theory.

DOI
10.1103/physrevlett.55.1070

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Chinese Physics Letters•Jan 30, 2006•10 citations
Phase-dependent resonance transitions induced in two-level atoms by a bichromatic field are reported. Strong phase effects are observed in the interaction of a fast atomic beam with a standing-wave rf field. In the atomic rest frame, this field consists of two components with different frequencies and a controlled initial phase difference. Numerical calculations including well-defined phase factors are performed in the time-dependent density matrix formalism. The observed effects are in excellent agreement with this theory.
The influence of the phase difference of the components of a bichromatic field on the dynamics of a two-level atom is studied in the framework of an analytical approach. We give a new, more adequate explanation of an experiment [R. E. Silverano et al., Phys. Rev. Lett. 55, 1070 (1985)] where strong phase effects were observed in the interaction of a fast atomic beam with a standing-wave field. It was shown that these effects are caused by interference of contributions of components of the bichromatic field in the amplitude of the upper state. There is excellent agreement between our analytical results and both experimental data and the results of numerical calculation.
The interaction of a strong bichromatic field with a two-level atom has been the subject of numerous experimental and theoretical investigations. We performed an experimental study of the Interaction of an intense 100% amplitude-modulated (AM) field with an ensemble of homogeneously broadened two-level atoms. An optically pumped sodium atomic beam is used as the atomic medium. The field is created by interfering two intense frequency-stabilized laser fields. The frequency of one field is held fixed while another field of equal amplitude is scanned in frequency. The beat frequency produced by interfering the two laser fields is equal to twice the modulation frequency of a 100% AM field. We monitor the time-averaged fluorescent intensity of the atoms as the frequency separation between the two fields is varied. A similar experiment was done in the past in the rf regime.1 For the first time, we report the observation of multiple, well-resolved resonances in the absorption spectrum of the field in the optical regime. We present data and theory for several values of detuning between the fixed field and atomic resonance. The experimental results are in good agreement with theory.
Theoretical studies have shown that the transient dynamics of an undamped two-level atom, driven by a bichromatic laser field consisting of a strong resonant pump and a moderate strength tunable perturber, are complex. Among the interesting results, it has been found that the strong field tends to stabilize the atom against the influence of the perturber field.1 Exceptions occur when the perturber is detuned from the strong field by the Rabi frequency of the pump or its subharmonics. In addition, it has been found that the atomic response depends strongly on the initial relative phase of the two excitation-field components.1,2 We have constructed an experimental apparatus to study the transient response of two-level like Yb atoms to bichromatic field excitations. The transient bichromatic laser field is generated by sending a dye laser beam double pass through an AOM driven by a bichromatic rf field. All the optical field parameters, amplitudes, frequencies, and phases, are controlled by corresponding rf field parameters.
We have studied the transient response of undamped two-level atoms to excitation by a classical bichromatic field consisting of a strong resonant component and a moderate-strength, tunable, ``perturber'' component. It is found that the strong field tends to stabilize the atom against the influence of the perturber field. Exceptions to this general result occur when the perturber is detuned from the strong field by the Rabi frequency of the latter or its subharmonics. We believe this stabilization phenomenon is a semiclassical analog of the effect that leads to the dynamical modification of spontaneous emission [M. Lewenstein and T. W. Mossberg, Phys. Rev. Lett. 59, 775 (1987)], and the presence of detunings at which the stabilization fails provides a simple motivation for the Rabi subharmonic resonances found in certain cw bichromatic spectra. Interestingly, when the perturber field is relatively weak, detuned from the atomic resonance by the strong-field Rabi frequency, and turned on with a specific initial phase relative to the strong field, one finds that the bichromatic field can be employed to dressed-state polarize initially ground-state atoms. In a related vein, we find that perturbations at the subharmonic detunings lead to interesting phase-dependent atomic dynamics. Results were obtained using both numerical and approximate analytical techniques entirely within the context of the rotating-wave approximation. The validity of the latter is ensured by demanding that all detunings and Rabi frequencies are vanishingly small compared to the atomic transition frequency.
We have investigated the interaction between bichromatic optical fields and atomic transitions. We have considered both the interaction of a saturating bichromatic field in the form of a 100% amplitude-modulated field resonantly interacting with a single atomic transition as well as the interaction of a pair of 100% amplitude-modulated fields with two coupled atomic transitions. In our studies of a single atomic transition resonantly excited by a 100% amplitude-modulated field we have used a fully quantum-electrodynamical analysis to model the Autler-Townes effect induced by a bichromatic optical field. We find that the effect induced by a bichromatic field is markedly different from the effect produced by a monochromatic exciting field. We have derived analytic expressions for the bichromatic Autler-Townes absorption spectrum. The absorption spectrum consists of equally spaced lines spaced by the modulation frequency. The linewidths of neighboring lines alternate as functions of the field strength. Experimentally, we observed this spectrum for both resonant and detuned 100% amplitude-modulated driving fields by monitoring the absorption of a weak monochromatic field as it was tuned about the resonance frequency of the 3²P₃/₂(F = 3, mF = 3) ↔ 4²D₅/₂(F = 4, mF = 4) transition while driving the 3²S₁/₂(F = 2, mF = 2) ↔ 3²P₃/₂(F = 3, mF = 3) transition with a saturating 100% amplitude-modulated field. We found excellent agreement between our predicted and measured spectra. We then investigated the resonant interaction between a pair of 100% amplitude-modulated fields and two atomic transitions sharing a common atomic level. We considered both a three-level atomic cascade system and a three-level atomic lambda system. We examined the effect that the relative phase between the two modulations has on the rate at which the modulated fields are absorbed. We find that the saturated time-averaged field absorption exhibits parametric resonances in both the cascade and lambda systems. The time-averaged absorption is strongly dependent upon the relative phase of modulation between the two fields and for certain phase differences the time-averaged absorption can be enhanced over the maximum saturated absorption experienced by cw fields of the same time-averaged intensities. We also find that the stationary atomic response exhibits periodic oscillations and for the cascade system periodic two-photon inversions of up to 40% can be achieved. We further find dark absorption resonances as a function of phase in the lambda system. We analyze the results from the point of view of semi-classical adiabatic dressed states.