Spontaneously Generated Coherence Research Articles

Gain spectrum of a laser-driven tripod-type atom dynamically induced in the presence of spontaneously generated coherence

We examine the absorption of a weak probe beam in a laser-driven tripod-type atom with three closely lying ground levels, where both the driving and probe lasers interact simultaneously with the three transitions. The effects of spontaneously generated coherence (SGC) are taken into account. We introduce dipole moments in the dressed-state picture and the Hamiltonian in terms of the dressed states describing the interaction between the probe and the atom. Gain spectrum under various conditions are presented and analyzed. We show that the spectral structure and the gain amplitude of the probe are strongly influenced by the effect of SGC and the frequency separation of the three closely lying ground levels. The tripod-type atomic system with quantum interference in spontaneous emission can be simulated in the dressed-state picture of a coherently driven four-level N-type system where no SGC effect exists.

Spatial evolutions of inversionless gain and lasing field intensity in an open V-type system with SGC

This paper studies manly spatial evolution of gain without inversion (GWI) and the Rabi frequency E (intensity ɛp) of the probe field in an open V-type three-level inversionless lasing system with spontaneously generated coherence (SGC) for both cases with and without Doppler broadening. We found that: (1) Varying sizes of SGC strength (measured by angle θ), atomic exit rate (r0) and ratio (S) of the atomic injection rates has remarkable effect on spatial evolutions of GWI and E (ɛp). This effect in the case with Doppler broadening is similar to but weaker than that in the case without Doppler broadening. (2) Regardless of that Doppler broadening is present or not, GWI and E (ɛp) increase with increase of θ, r0 and S in certain value ranges of θ, r0 and S; in the case with SGC we can obtain GWI and E (ɛp) much larger than those in the case without SGC, while by choosing values of γ0 and S, in the open system we can obtain LWI gain and E (ɛp) much larger than those in the corresponding closed system. (3) The propagation distance in which GWI exists in the case with Doppler broadening is longer than that in the case without Doppler broadening; in the case without Doppler broadening, we can obtain larger GWI than that in the case with Doppler broadening; but in the case with Doppler broadening, we can obtain larger E (ɛp) than that in the case without Doppler broadening.

Storage and Retrieval of a Weak Optical Signal Improved by Spontaneously Generated Coherence in an Atomic Assemble

We investigate the reversible storage of a weak single-mode light signal in a Λ-type three-level atomic system with spontaneously generated coherence (SGC) and pumped by an incoherent field. The scheme is phase-dependent. If the phase of the controlling field is set oppositely to that of the signal field, the combination of SGC with weak pump overcomes the drawback of incompletely retrieval in the conventional scheme, leading to 100% retrieval fidelity or even to the amplification of the retrieved signal with respect to its initial one.

Left-handedness in K-type multilevel system in the presence of spontaneously generated coherence

The left-handedness in atomic vapor medium consisting of five energy levels in K-type configuration is studied in this work. The theoretical modeling has been done using density matrix approach in which the spontaneously generated coherence (SGC) is also included. The system shows negative electrical permittivity (ϵ) as well as negative magnetic permeability (μ) and thus displays the negative refractive index (n) for certain values of system parameters in the presence of SGC. The parameters (ϵ, μ, n) quantifying left-handedness in the system become more negative if the SGC is strong. Under the off-resonant conditions these parameters become less negative and the left-handedness of system degrades.

Influence of relative phase on propagation effect in open V-type three-level system

In this paper we study the influence of the relative phase between the probe and driving fields on propagation effect in an open Doppler broadening V-type three-level atomic system with spontaneously generated coherence (SGC) by using the calculation result of the density matrix motion equations and the propagation equations of the driving and probe fields. It is shown that the relative phase (Φ) has remarkable periodical influence on the propagation effect, and the period is 2π. By selecting appropriate value of Φ, we can get larger lasing without inversion (LWI) gain and longer propagation distance in which gain exists, and hence obtain higher probe field (i.e. LWI) intensity. When Φ=π/2, the largest LWI gain and probe field intensity can be got. In addition, the atomic exit rate (γ0) and ratio (S) of the atomic injection rates also have a considerable modulation role on the phase-dependent propagation effect. In certain value range of γ0 (S), LWI gain and probe field intensity increase with γ0 (S) increasing. In the open system, LWI gain and probe field intensity much larger than those in the corresponding closed system can be obtained.

Effects of Spontaneously Generated Coherence on Intensity-Intensity Correlation in a Driven Y-Type Atom

In this paper, we investigate the spontaneously generated coherence (SGC) effects on the fluorescence fields intensity-intensity correlation in a resonant driven four-level Y-type atomic system. By using a strong control field, in the absense of SGC effects, strong correlation and anticorrelation fluorescence photons can be produced from two ladder transitions in this system respectively. However, in the presence of SGC effects, the fluorescence fields correlation properties are reversed for the two transitions. The above phenomena can be traced to the quantum destructive or constructive interference in terms of dressed states.

Effect of spontaneously generated coherence on left-handedness in a degeneracy atomic system

A theoretical investigation is carried out into the effect of spontaneously generated coherence (SGC) on the left-handedness in a four-level Y-type atomic system with two highest nearly degenerate lying levels. It is found, with the spontaneously generated coherence intensity enhancing, the atomic system gradually displays left-handedness with simultaneous negative permittivity and permeability. And the refractive index enhances with the increasing intensity of SGC. However, the absorption is suppressed by the SGC effect when the SGC has a large intensity. When the probe field is near-resonant coupled to the atomic system, the appearance of SGC doesn’t always change the permeability from positive to negative and allow for lefthanded behavior, unless the SGC reaches a large intensity.

Electromagnetically induced phase grating controlled by spontaneous emission

Electromagnetically induced phase grating controlled by spontaneous emission is studied. The results show that the diffraction efficiency of phase grating is strikingly enhanced due to the existence of spontaneously generated coherence (SGC), and the diffraction efficiency of 35% can be obtained in optimal SGC parameter case. Furthermore, at large SGC parameter, the high efficiency of phase grating can be maintained by the application of weak coupled and relatively strong probe fields, and choosing the proper length of atomic sample. At the same time, in such an atomic system, the SGC effect depends on incoherent pump.

Controlling the optical bistability via quantum interference in a four-level N-type atomic system

We investigate the optical bistability (OB) and optical multi-stability (OM) in a four-level N-type atomic system. The effect of spontaneously generated coherence (SGC) on OB and OM is then discussed. It is found that SGC makes the medium phase dependent, so the optical bistability and multi-stability threshold can be controlled via relative phase between applied fields. We realize that the frequency detuning of probe and coupling fields with the corresponding atomic transition plays an important role in creation OB and OM. Moreover, the effect of laser coupling fields and an incoherent pumping field on reduction of OB and OM threshold is then discussed.

Electromagnetically induced grating via enhanced nonlinear modulation by spontaneously generated coherence

By investigating the third-order nonlinearity of a four-level ladder-type atomic system, it is found that, with spontaneously generated coherence (SGC) present, the nonlinear absorption or refraction can be significantly enhanced with vanishing linear absorption. We attribute the enhancement of nonlinearity mainly to quantum interference in the two decay pathways from the two upper closely lying levels. With a standing-wave trigger field, absorption or phase grating, which effectively diffracts a weak probe into high-order direction, can be induced by the SGC-enhanced absorptive or refractive nonlinear modulation. In contrast to the schemes for enhancing nonlinearity, no additional coupling field is required. Moreover, the present gratings result from the nonlinear modulation, which differs from the recent investigations based on linear modulation.

Quantum interference in a four-level system of aRb87atom: Effects of spontaneously generated coherence

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 incoherent pump rate: an optical tool for controlling the probe response and dispersion in a three-level Λ system in the presence of spontaneously generated coherence

The probe response and dispersion of a three-level closed Λ system considering spontaneously generated coherence (SGC) have been investigated with and without an incoherent pumping. One can get electromagnetically induced transparency (EIT) whatever be the value of the SGC parameter and phase difference when an incoherent pump is absent. But by turning the incoherent pump rate on, one can control the light propagation from subluminal to superluminal and also the nonlinear response from EIT to electromagnetically induced absorption (EIA) by controlling the SGC parameter and phase difference. Using the incoherent pump rate as an optical tool, the desired configuration around the resonance can be realized. Exact analytical expressions for the coherences in the steady-state limit have been derived in the absence of an incoherent pump rate (keeping all orders of the system parameters) to generalize the analysis, and any restriction on the system parameters has been avoided to make it applicable to various atomic and molecular systems.

ANALYSIS OF THE OPTICAL MULTISTABILITY BEHAVIOR IN A FOUR-LEVEL ATOMIC SYSTEM WITH AN ASSISTING MICROWAVE-DRIVEN FIELD

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.

High refractive index without absorption via spontaneously generated coherence in a three-level ladder system

A large refractive index without absorption in a closed three-level ladder system with incoherent pumping in the presence of spontaneously generated coherence (SGC) is investigated. It has been found that a large refractive index without absorption can be obtained by choosing proper system parameters. The maximum value of refractive index at vanishing absorption depends on the strength of the SGC parameter.

Sub- and superluminance with gain in a three-level ladder-type atomic system with spontaneously generated coherence

The effects of spontaneously generated coherence (SGC) on the propagation of a weak probe pulse are investigated in a ladder-type atomic system. Due to SGC, the properties of the dispersion and the absorption of a three-level ladder-type system are changed greatly. With proper parameters, the system can produce normal and anomalous dispersion regions in the gain region. The effects of the incoherent pumping and the relative phase on the group velocity are also studied. They all can be used as switches to manipulate the group velocity of the weak probe field from gain-assisted sub- to superluminance.

The Kerr Nonlinearity in an N-Type Four-Level System via Spontaneously Generated Coherence

The Kerr nonlinearity is investigated in an N-type four-level atomic system whose both supper levels are degenerated or nearly degenerate. It is found that the spontaneously generated coherence as a result of the degenerate levels can change the Kerr nonlinearity dramatically depending on the angle between two spontaneous channels. In addition, we discover that the probe field makes its own contribution to the nonlinearity in comparison to indifference in general three-level systems.

Effects of spontaneously generated coherence on the resonance fluorescence spectrum of a laser-driven four-level diamond-shape atomic system

The resonance fluorescence spectra of a laser-driven four-level closed-loop diamond-shape atomic system are investigated in this paper. The four decay channels in the system interfere with each other in different ways according to several distinct arrangements of the two intermediate atomic levels, and hence produce the spontaneously generated coherence (SGC) effects. The numerical calculation shows that the amplitude, the linewidth and the number of sidebands of the resonance fluorescence spectrum are deeply modified by the SGC effects; accordingly, interesting effects such as the spectral-line narrowing, spectral-line suppression, spectral-line enhancement and even the complete quenching of fluorescence are generated under certain conditions. Moreover, we propose a new kind of level structure, the seesaw structure, in which the SGC effect can also be attained without the rigorous conditions of degenerate levels. In the end, we discuss the possibility of demonstrating our scheme in real atomic systems and summarize our results.

Effect of spontaneously generated coherence on EIT and its refractive properties in four- and five-levels systems

The effects of spontaneously generated coherence (SGC) and phases of optical fields on the phenomenon of electromagnetically induced transparency (EIT) are investigated in a four-level inverted-Y system and in a five-level K-type system under various parametric conditions in order to demonstrate controllability of the EIT, dispersion properties, and group velocity in such systems. Non-zero second-order susceptibility in both systems is due to the SGC effect. The experimental viability of the model in semiconductor quantum well systems is also discussed.

Comparison of transient process between open and closed ladder-type systems with spontaneously generated coherence and incoherent pumping

We theoretically investigated transient response of open and closed three-level ladder-type atomic system with or without the spontaneously generated coherence (SGC) which could be satisfied with the help of an incoherent pumping. The existence of the SGC effect makes the open and closed system to be distinguished. We compared transient response of weak probe between open and closed system and found that transient properties exhibit different features by adjusting some related parameters, such as the relative phase between probe and coupling fields, the angle between two dipole moments.

Group Velocity of Transmitted Light and Reflected Light from a Dispersive Dielectric Layer

The reflected light and transmitted light from a slab doped with a three-level Λ-type atoms are theoretically investigated. The effect of the relative phase between coupling and probe fields on the group velocity of the transmitted light and reflected light has been discussed. We show that in the presence of a spontaneously generated coherence (SGC), the group velocity of the transmitted light and reflected light becomes phase dependent.