This study discusses electromagnetically induced waveguiding in double-/spl lambda/ systems. The waveguiding effect is shown to be strongly phase dependent, indicating that it derives from the phase-dependent effective third-order susceptibility rather than the phase-independent effective first-order susceptibility, as is the case in previously studied systems. This work also shows that when the second /spl Lambda/ system initially involves only a single laser beam, the loop is completed by the efficient generation of radiation at the four-wave mixing frequency, within a propagation distance much shorter than the diffraction length.
This study discusses electromagnetically induced waveguiding in double-/spl lambda/ systems. The waveguiding effect is shown to be strongly phase dependent, indicating that it derives from the phase-dependent effective third-order susceptibility rather than the phase-independent effective first-order susceptibility, as is the case in previously studied systems. This work also shows that when the second /spl Lambda/ system initially involves only a single laser beam, the loop is completed by the efficient generation of radiation at the four-wave mixing frequency, within a propagation distance much shorter than the diffraction length.
Near the threshold for electromagnetically induced transparency (EIT) or coherent population trapping (CPT), two-photon-resonance-enhanced self-focusing of a $\ensuremath{\Lambda}$ system can be exploited to induce spatial confinement in a second, diffracting $\ensuremath{\Lambda}$ system. The diffracting $\ensuremath{\Lambda}$ system is characterized by parameters below the EIT or CPT threshold, and the two $\ensuremath{\Lambda}$ systems must be coupled to form a closed-loop double-$\ensuremath{\Lambda}$ system. The waveguiding effect is shown to be strongly phase dependent, indicating that it derives from the phase-dependent effective third-order susceptibility rather than the phase-independent effective first-order susceptibility, as is the case in previously studied systems. We also show that when the second $\ensuremath{\Lambda}$ system initially involves only a single laser beam, the loop is completed by the efficient generation of radiation at the four-wave-mixing frequency, within a propagation distance much shorter than the diffraction length. Both the applied and generated fields exhibit electromagnetically induced waveguiding.
In a Λ-type system employing a two-photon pump field, a four-wave mixing field can be generated simultaneously and, hence, a closed-loop system forms. We study theoretically the effect of the relative phase between the two incident fields on the generated four-wave mixing field and the electromagnetically induced transparency. It is found that the phase of the generated four-wave mixing field is the sum of the incident relative phase and a fixed phase that is irrelative to the incident relative phase. Hence, the total phase of the closed-loop system is independent of the incident relative phase. As a result, the incident relative phase has no effect on the electromagnetically induced transparency, which is different from the case of a Λ-type loop system closed by a third incident field.
We present a theory of a continuous-wave light propagation in a medium of atoms with a double-$\ensuremath{\Lambda}$ configuration of levels. This is a configuration with a closed cycle of radiation-induced transitions. An interference of excitation channels in such closed-loop systems leads to a strong dependence of the atomic state on the relative phase and the relative amplitudes of applied electromagnetic waves. Therefore, the medium response may be controlled by the phases. On the other hand, the phases themselves change during the propagation. Thus the state of the medium and all the field parameters are tightly coupled to each other in the present problem. We consider the propagation of four-frequency laser radiation through the double-$\ensuremath{\Lambda}$ medium for two situations. At resonant or near-resonant excitation of atoms, both the medium and the field evolve into a nonabsorbing state. This state implies specific coherent superposition for atoms (``dark state''), and particular relations for the field phases, amplitudes, and frequencies. In this way, the propagation results in the phase, amplitude, and frequency matching of the laser waves. In the second case, one $\ensuremath{\Lambda}$ system in double-$\ensuremath{\Lambda}$ atoms is excited resonantly, while the second $\ensuremath{\Lambda}$ system is far off resonance. Such an excitation scheme ensures the preparation of atoms in the nearly dark state throughout the medium. Therefore, the total light energy is dissipated very weakly, whereas each individual laser wave can vary considerably along the propagation path. We have found that the resonant fields change as much as the far-detuned ones. The intensities oscillate sinusoidally with the optical length, with the energy being transferred back and forth between two waves in each frequency pair, resonant and far detuned. This gives the possibility for an almost lossless amplification of two of the laser waves, or an even generation of one of them.
We investigate a four-state system interacting with long and short laser pulses in a weak probe beam approximation. We show that when all lasers are tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM) field is strongly absorbed. The part that is not absorbed has the exact intensity required to destructively interfere with the excitation pathway involved in producing the FWM state. We show that with this three-photon destructive interference, the conversion efficiency can still be as high as 25%. Contrary to common belief, our calculation shows that this process, where an ideal one-photon electromagnetically induced transparency is established, is not most suitable for high-efficiency conversion. With appropriate phase matching and propagation distance, and when the three-photon destructive interference does not occur, we show that the photon flux conversion efficiency is independent of probe intensity and can be close to 100%. In addition, we show clearly that the conversion efficiency is not determined by the maximum atomic coherence between two lower excited states, as commonly believed. It is the combination of phase matching and constructive interference involving the two terms arising in producing the mixing wave that is the key element for the optimized FWM generation. Indeed, in this scheme no appreciable excited state is produced, so that the atomic coherence between states |0〉 and |2〉 is always very small.
It is experimentally demonstrated that the effect of electromagnetically induced transparency (EIT) can be controlled through the radiation phases. Corresponding investigations are performed by exciting a sodium atomic vapour by four-frequency laser radiation in a double-Λ configuration. In such a system the nonabsorbing (dark) state, responsible for EIT, is created only for particular values of the relative radiation phase. This phase dependency is confirmed using three different methods of phase control. Additionally, the relative phase is set at defined values in order to adjust the medium to a fixed transmission behaviour.
Abstract We propose a method to implement electromagnetically induced grating in a phaseonium medium that has been coherently generated via atomic mechanisms. Phaseonium atoms have a Λ-type structure and three distinct energy levels; such atoms are originally generated in a coherent superposition of two lower levels. The phaseonium system is comprised of three-level atoms with a Λ-type configuration, which are initially prepared in a coherent superposition of two lower levels. To accomplish this spatial modulation based on the susceptibility of phaseonium medium, a standing-wave field is used. By looking at how an optical field diffracts at different relative phases, we find that the zeroth and first order diffraction intensities increase as the relative phase changes. We also investigate the impact of the Rabi frequency of the field on diffraction intensity and notice that an increasing strength of the Rabi frequency leads to amplification in the intensity of both central zeroth order and first-order diffraction. Furthermore, it has been observed that a significant rise in diffraction intensity occurs at longer interaction lengths between external fields and the atomic medium.