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Journal of Modern Optics

Four-level ‘N-scheme’ in bare and quasi-dressed states pictures

Tony Abi-Salloum, Seth J. Meiselman, J.P. Davis, Frank A. Narducci

Published Oct 20, 2009• 8 citations
Publisher

Abstract

In this work, we consider a four-level ‘N-scheme’ (three radiation fields coupling four atomic levels) that has previously been shown to feature different group velocity regimes (sub-luminal, super-luminal, and negative) controlled by the strength of the third interacting field, which is referred to as the control field. Another interesting aspect of the N-scheme is the ‘appearance’ and ‘disappearance’ of an electromagnetically induced transparency (EIT) window at resonance. In this paper, based on a study in the bare states picture, we point out differences and similarities between two settings (different strengths of the acting fields) of this four-level N-scheme. Conclusions are drawn based on a work in a simplified quasi-dressed states picture. We find that the four-level N-scheme under study can be approached as either a one or two three-level lambda-like system(s) that display the same physics regardless of the relative strength between the two fields that form the EIT portion of the system.

DOI
10.1080/09500340903199939

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In this work, we consider a four-level ‘N-scheme’ (three radiation fields coupling four atomic levels) that has previously been shown to feature different group velocity regimes (sub-luminal, super-luminal, and negative) controlled by the strength of the third interacting field, which is referred to as the control field. Another interesting aspect of the N-scheme is the ‘appearance’ and ‘disappearance’ of an electromagnetically induced transparency (EIT) window at resonance. In this paper, based on a study in the bare states picture, we point out differences and similarities between two settings (different strengths of the acting fields) of this four-level N-scheme. Conclusions are drawn based on a work in a simplified quasi-dressed states picture. We find that the four-level N-scheme under study can be approached as either a one or two three-level lambda-like system(s) that display the same physics regardless of the relative strength between the two fields that form the EIT portion of the system.
Using density matrix eqution, the absorption and dispersion properties of a A-type four-level system driven by coherent fields are investigated. The result obtained by means of numerical method indicates that by adjusting the intensity of Rabi frequencies of the coherent field in the steady state, three, double or single electromagnetically induced transparency (EIT) can appear explicitly in this system. And the explanations of these phenomena are given by using the dressed states. In the three EIT, the location of transparent windows can be manipulated by controlling the detuning of the driving field. What's more, it is also found that the medium shows a large refractive index at each transparency window, and the group velocity of the probe laser can be enhanced.
The optical properties of a five-level atomic system composed of a 'Lambda'-type four-level atomic and a tripod four-level atomic systems are investigated. It is found that the behaviors of electromagnetically induced transparency (EIT) and group velocity can be controlled by choosing appropriate parameters with the interacting dark resonances. In particular, when all the fields are on resonance, the slow light at the symmetric transparency windows with a much broader EIT width is obtained by tuning the intensity of the coupling field in comparison with its sub-system, which provides potential applications in quantum storage and retrieval of light.
Electromagnetically induced transparency (EIT) resonance is obtained in a well-studied lambda three level system where an optical coupling field and a probing field interact with two optical transitions. However, both electromagnetically induced absorption (EIA) and EIT can be obtained in a modified quasi-lambda four level system consisting of an optical-radio two-photon coupling field and a probing field. By solving the density matrix equation of motion, a physical account of EIA and EIT is given in terms of a dressed state picture. It can be seen that the optical coupling field in this quasi-lambda four level system has a crucial effect on the forming of EIA and EIT. An EIA is observed under a resonant optical coupling and it evolves into an EIT when there is a detuning. Furthermore, the width of EIA or EIT increases as the optical coupling Rabi frequency increases.
Abstract We theoretically study the two- and three-photon coherence in a Ξ−Λ type four-level system for stationary as well moving atoms at the room temperature using density matrix formalism. We discuss the role of dressed states to elucidate the electromagnetically induced transparency and electromagnetically induced absorption phenomena. The presence of the third field induces absorption at the line centre. A negative dispersion slope owing to the enhanced absorption results in super-luminal light propagation and the group index variation with the coupling field is switched from sub- to super-luminal. Group index with probe detuning shows super-luminal light propagation behaviour at the dressed state positions. The three optical fields trigger four-wave mixing as a result of the third order nonlinearity. The transient evolution is also discussed for optimum strengths of the coupling and drive fields to realize the optical switching in the system.
Electromagnetically induced transparency (EIT) resonance in a $\ensuremath{\Lambda}$ configuration is obtained when the frequencies of two fields are close to resonance with two of the transitions and their frequency difference matches the frequency of the third transition. In this situation the spectrum of one swept field as a probe gives a simple transparency feature. However, when an additional field drives the third transition the EIT feature associated with the probe is split. This perturbed EIT is illustrated for both single and bichromatic driving fields. In the single-driving-field case a density matrix treatment is shown to be in reasonable agreement with experiment, and in both single and bichromatic cases the structure in the spectrum can be explained using the dressed-state formalism. The dressed states can also be used to account for subharmonic resonances observed in the strong-probe regime.
The A-type three-level system coupled by two coherent fields is a basic model to study the electromagnetically induced transparency(EIT).Based on that,the cascade quasi- A-four-level configuration is formed when a radio-frequency(RF)field drives the transition between the excited state and an additional excited state level.The absorption property is studied along with the parameters of the coupling field and the RF driving field by numerically solving the relevant equations of motion of the density matrix.It is shown that under the applying of the coupling field and the RF driving field,both the Autler-Townes doublet and the EIT are observed in the absorptive spectra and it can be explained properly by the theory of dressed state.An important conclusion is that the Autler-Townes doublet originates from the atom system interaction with the strong RF field,while the EIT originates from atom system quantum interference interaction with the coupling field.