Optical properties of four level medium via spontaneously generated coherence

Tianjin University of Technology and Springer-Verlag Berlin Heidelberg 2012 In this paper, we study the effect of spontaneously generated coherence (SGC) on transient evolution of gain without inversion (GWI) in a Doppler broadened quasi Λ-type four-level atomic system. It is shown that transient evolution of GWI is very sensitive to the variation of SGC strength, and the transient maximum value and steady value of GWI both increase with SGC strength increasing. The transient and steady values of GWI with SGC are much larger than those without SGC. When Doppler broadening is present, the transient maximum value and steady value of GWI first increase and then decrease with Doppler broadening width (D) increasing, and the value of D which corresponds to the maximum transient GWI is different from that corresponding to the maximum steady GWI. The time needed for reaching the steady GWI increases with D increasing. The steady GWI, which is larger than that without Doppler broadening (D = 0), can be obtained by choosing appropriate D and SGC strength.

We provide a four-level double-V type atomic system driven by two week probe fields and two strong coupling laser beams. In the condition of resonant four-wave mixing, the two probe fields can be amplified without population inversion. Due to the fact that the two excited states are close- lying upper levels of hyperfine structure, the spontaneously generated coherence (SGC) effect must be considered. Interestingly, the amplitude of gain is sufficiently enhanced with the same parameters as those in the case without considering SGC. In addition, we find that the probe gain is sensitive to the phase of two laser fields which interact with the same lower level. To be more specific, the amplitude of gain is modulated by the phase periodically but restricted by θ (the angle between two induced dipole moments). At the same time, we also analyze the influence of the coherence pumping (strong coupling fields) detuning.

We have studied the probe gain via a double-Λ atomic system with a pair of closely lying lower levels in the presence of two probe and two coherent pump fields. The inversionless gain can be realized by using nondegenerate four-wave mixing under the condition of spontaneously generated coherence (SGC) owing to near-degenerate lower levels. Note that by using SGC, two probe fields can be amplified with more remarkable amplitudes, and the gain spectra of an extremely narrow linewidth can be obtained. Last but not least, our results show that the probe gain is quite sensitive to relative phases due to the SGC presence which allows one to modulate the gain spectra periodically by phase modulation, and can also be influenced by all laser field intensities and frequencies, and the angles between dipole elements.

Owing to interference induced by spontaneous emission, the density-matrix equations in a three-level Λ-type system have an additional coherence term, which plays a critical role in modulating the inversionless gain and electromagnetically induced transparency effect. In addition, it is shown that spontaneously generated coherence (SGC) has an effect on the entanglement between an atom and a photon of the coupling laser field by calculating the degree of entanglement (DEM) of the atomic system. In this paper, we investigate the influence of the SGC effect on atom-photon entanglement in a Λ-type system, which generally remains a high entangled state. When an incoherent pump source is introduced, we find that the SGC effect could exert considerable influence on the atom reduced entropy under certain conditions for both transient and steady states. More interestingly, such an incoherent pump field could actively affect the short-time dynamic behaviors of the transient quantum entangled state at a certain range of pump rate as a typical coherent case.

A double cascade-type four-level multiple-quantum-well-based exciton—biexciton transitions are proposed. The study is carried out on a 4.8-nm ZnSe single-quantum well which is embedded into ZnMgSSe cladding layers and pseudomorphically grown by molecular beam epitaxy on a (0 0 1) GaAs substrate. It is displayed that the exciton spin relaxation and relative phases between applied fields can influence the transient and steady-state behaviors of absorption, dispersion, and group velocity of two weak probe and signal fields. Also, transient behaviors of electron population of different levels are discussed. It is found that the probe or signal amplification occurs in the absence of population inversion.

High-precision three-dimensional atom localization via three-wave mixing in V-type three-level atoms

We propose an efficient scheme to manipulate the Goos-Hänchen (GH) shift of a reflected beam from a metal-clad waveguide, where a coherent atomic medium with a Λ-type configuration is employed as the substrate. Using experimentally achievable parameters, we identify the conditions under which spontaneously generated coherence (SGC) allows us to enhance the spatial and angular GH shifts of the reflected beam. With the help of SGC, the relative phases of the probe and control fields can alter the absorption gain and refractive index of the atomic medium, thereby manipulating the magnitudes, signs, and positions of the spatial and angular shifts. Furthermore, the spatial and angular GH shifts can be coherently controlled via adjusting the incoherent pumping rate and the intensity of the control field. Our proposal provides an avenue for the manipulation of spatial and angular GH shifts and potential applications in optical switching and optical steering.

In this work, the effects of quantum interference and spontaneously generated coherence (SGC) are theoretically analyzed in a four level system of a $^{87}\mathrm{Rb}$ atom. For the effects of SGC, we find that a new kind of EIT channel can be induced due to destructive interference, and the nonlinear Kerr absorption can be coherently narrowed or eliminated under different strengths of the coupling and switching fields.

The effect of Er3+ ion concentration on transient and steady-state behavior in 45-nm Er3+:YAG crystal is investigated. It is shown that by changing the signal field, the coherent field and the concentration of Er3+ ions in the crystal, the absorption, dispersion, and group index of the weak probe field can be adjusted. Also, it is found that the probe absorption occurs in the presence of population inversion and probe amplification is obtained in the absence of population inversion.

We investigate the steady optical response of a coherently driven five-level M-type atomic system in three different situations. When all three coupling fields have the same zero detuning, we just find one deep transparency window accompanied by a steep normal dispersion in the probe absorption and dispersion spectra. When two coupling fields are detuned from the relevant transitions to the same extent, however, a second deep transparency window may be observed in the presence of a narrow absorption line of linewidth ∼ 50 kHz. In this case, two single-photon far-detuned transitions can be replaced by a two-photon resonant transition, so the five-level M system in fact reduces into a four-level quasi-Λ system. Finally, we note that no deep transparency windows and no narrow absorption lines can be found when all three coupling fields have unequal detunings.

We show the steady-state optical multistability (OM) behavior in a four-level atomic system inside a unidirectional ring cavity. We find that the intensity and the detunings of the fields can affect the optical multistability behavior dramatically, which can be used to control the transition from OM to OB or vice versa without the need to resort to the effect of spontaneously generated coherence (SGC). The effect of the atomic cooperation parameter on OM is also studied. Our scheme may be used for building more efficient all-optical switches and logic-gate devices for optical computing and quantum information processing.

Localization of cold [formula omitted] atom simulated by a three-level quantum system within half-wavelength domain

The fluorescence photon correlations in a driven four-level atom consisting of three upper closely lying excited states and a single lower state are investigated. It is found that in the presence of triple spontaneously generated coherence (SGC) effects, which are created by three degenerate levels coupling to the common ground state via the same vacuum mode, the correlation properties of the three fluorescence fields can be switched from strong correlation to anticorrelation or vice versa by modifying the detunings. In addition, such a system is very sensitive to the relative phases of the three fields, which can provide an effective way to control the photon correlation. Physically, these interesting phenomena can be interpreted in terms of dressed state analysis.

Comparison of steady and transient optical responses between a four-level Tripod system and a three-level Lambda system

In this paper we study the coherent transient property of a &Lambda;-three-level system (&Omega;_<i>d</i> = 0) and a quasi- &Lambda; -four-level system (&Omega;_<i>d</i>&gt;0). Optical switching of the probe field can be achieved by applying a pulsed coupling field or rf field. In &Lambda; -shaped three-level system, when the coupling field was switched on, there is a almost total transparency of the probe field and the time required for the absorption changing from 90% to 10% of the maximum absorption is 2.9_&Gamma;0 (_&Gamma;0 is spontaneous emission lifetime). When the coupling field was switched off, there is an initial increase of the probe field absorption and then gradually evolves to the maximum of absorption of the two-level absorption, the time required for the absorption of the system changing from 10% to 90% is 4.2_&Gamma;0. In four-level system, where rf driving field is used as switching field, to achieve the same depth of the optical switching, the time of the optical switching is 2.5_&Gamma;0 and 6.1_&Gamma;0, respectively. The results show that with the same depth of the optical switching, the switch-on time of the four-level system is shorter than that of the three-level system, while the switch-off time of the four-level system is longer. The depth of the optical switching of the four-level system was much larger than that of the three-level system, where the depth of the optical switching of the latter is merely 14.8% of that of the former. The speed of optical switching of the two systems can be increased by the increase of Rabi frequency of coupling field or rf field.