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Annual Meeting Optical Society of America

Observation of subharmonic resonances of the Rabi frequency in the absorption spectrum of a two-level atom in a bichromatic field

Stephanos Papademetriou, Stephen H. Chakmakjian, Carlos R. Stroud

Published Jan 1, 1990• 0 citations
Publisher

Abstract

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.

DOI
10.1364/oam.1990.mtt2

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We report our results on measurement of the total fluorescent power radiated by two-level atoms excited by a 100% amplitude modulated or bichromatic laser field. For the case of single-frequency excitation, it is well known that the total fluorescent power monotonically saturates as the intensity of the field is increased. This occurs because the inversion of the atoms approaches zero at high field strengths. In contrast, when the field is bichromatic the total fluorescent power displays resonant peaks as the strength of the driving field is increased. This occurs because there is enhanced absorption when the modulation frequency coincides with the Rabi freqeuncy or one of its subharmonics.

Annual Meeting Optical Society of America•Jan 1, 1987•0 citations
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.
We report the observation of the first six subharmonic resonances of the Rabi frequency in the absorption of a 100% amplitude-modulated laser beam by a two-level atom. The amplitude-modulated field is formed by combining two separate cw lasers; one is held fixed in frequency, and the other, of equal amplitude, is scanned in frequency. We present data and theory for several values of the detuning between the fixed field and the atomic resonance. The experimental results are in good agreement with theory.
We report the observation of the Autler-Townes absorption spectrum of a two-level atom in a 100% amplitude-modulated optical field. Two frequency-stabilized dye lasers interact with a three-level atomic cascade system. One of the two lasers is used to create the bichromatic field that saturates the lower transition, while the second one is tuned in the frequency between the second and third levels to probe the absorption line shape. We investigate the dependence of the spectrum on the detuning between the average frequency of the bichromatic field and the lower atomic resonance. In order to study the effects of saturation, for every value of the detuning we take a series of data for different intensity levels of the bichromatic field. There is very good agreement between theory and experiment and profound differences from the conventional Autler-Townes results with a monochromatic pump field. \textcopyright{} 1996 The American Physical Society.
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.
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 report our results on measurement of the total fluorescent power radiated by two-level atoms excited by a 100% amplitude modulated or bichromatic laser field. For the case of single-frequency excitation, it is well known that the total fluorescent power monotonically saturates as the intensity of the field is increased. This occurs because the inversion of the atoms approaches zero at high field strengths. In contrast, when the field is bichromatic the total fluorescent power displays resonant peaks as the strength of the driving field is increased. This occurs because there is enhanced absorption when the modulation frequency coincides with the Rabi freqeuncy or one of its subharmonics.