Absorption-free superluminal light propagation in a quantum-dot molecule

The propagation of a weak probe field in a three-level quantum-dot molecule is investigated by employing the tunnel coupling. It is shown that in the absence of tunnel coupling, the propagation of light pulse is superluminal, similar to a simple two-level system. A high-resolution dip appears in optical spectra due to the presence of tunnel coupling. We show that this narrow dip leads to the subluminal light propagation with doublet absorption, so the group velocity of a light pulse can be controlled by interdot tunnel coupling. It is also demonstrated that by applying an indirect incoherent pumping field to the probe transition, the absorption doublet switches to the gain doublet and then the absorption-free superluminal light propagation is appeared.     

Upper level and phase controlling group velocity in V-type system

With the external field coupling the two upper levels, we investigate the light pulse propagation properties with weak probe field in a V-type system. Due to the external upper level (UL) coupling field, the dispersion of the system has been influenced by the relative phase. It is shown that the UL field and the relative phase can be regarded as switches to manipulate light propagation between subluminal and superluminal.     

Subluminal to superluminal light propagation in a V-type three-level atomic system interacting with squeezed vacuum

We study the light propagation properties in a V-type three-level atomic system interacting with two independent broadband squeezed baths. Using the graphic method, the dependences of the absorption and the dispersion of the system on the coupling field strength, the incoherent pumping field strength and the intensity of the squeezed vacuum are analysed. The result shows that the probe pulse propagation can be changed from subluminal to superluminal speed.     

Transient dispersion and absorption in a V-shaped atomic system

We investigate the dynamical behavior of the dispersion and the absorption in a V-type three level atomic system. It is shown that in the presence of decay-induced interference the probe dispersion and absorption are phase dependent. We find that an incoherent pumping field provides an additional control parameter for switching the group velocity of a light pulse. The required switching times for switching the group velocity of a probe field from subluminal to superluminal pulse propagation is then discussed.     

Phase-Controlled Transparent Superluminal Light Propagation in a Doppler-Broadened Four-Level N-Type System

The propagation of a weak probe field in a four-level N-type quantum system in the presence of spontaneously generated coherence(SGC) is theoretically investigated. The optical properties of the system are studied and it is shown that the group velocity of light pulse can be controlled by relative phase of applied fields. By changing the relative phase of applied fields, the group velocity of light pulse changes from transparent subluminal to the transparent superluminal light propagation. Thus, the phase-controlled absorption-free superluminal light propagation is obtained without applying an incoherent laser fields to the system. The propagation of a weak probe light pulse is studied by solving the Maxwell's wave equation on numerical grid in space and time. Moreover, we study the third order self- and cross-Kerr susceptibility of probe field and calculate the nonlinear cross-phase shift for different values of intensity of applied fields. In addition, we take into account the effect of Doppler broadening on the light pulse propagation and it is found that a suitable choice of laser propagation directions allows us to preserve our results even in the presence of Doppler effect. It is demonstrated that by increasing the Doppler width of distribution to the room temperature,the dispersion changes from transparent subluminal to transparent superluminal light propagation which is our major motivation for this work.     

Group velocity of the light pulse in an open V-type system

We investigate the group velocity of the probe light pulse in an open V-type system with spontaneously generated coherence. We find that, not only varying the relative phase between the probe and driving pulses can but varying the atomic exit rate or incoherent pumping rate also can manipulate dramatically the group velocity, even make the pulse propagation switching from subluminal to superluminal; the subliminal propagation can be companied with gain or absorption, but the superluminal propagation is always companied with absorption.     

Control of light in a quantized four level graphene atomic system via self and cross-Kerr nonlinearity

We investigate self and cross-Kerr nonlinearity in a four level quantized graphene atomic medium. The absorption, dispersion, transmission and subluminal/superluminal behaviors of a probe light field are studied. An amplification of the probe light field is observed in the absorption spectrum. The normal and anomalous slope of dispersion is also investigated at the positive/negative absorption region. It is shown that Kerr nonlinearity invert and enhance the subluminal/superluminal behaviors of the pulse and self-Kerr effect is found to be more subluminal/superluminal as compared to cross-Kerr effect. The results show significant applications in information storage, self and cross phase modulation and lasing without inversion.     

On coherently driven group index of probe pulse at double-frequency regimes: An analytical formulation

The group index and its lineshape of a relatively weak double-resonance probe pulse (signal) propagates through an open doubly driven N-type five-level atomic system are analytically formulated. It is shown that the signal can be controlled by manipulating the driving contribution of the two coupling fields, so that both superluminal and subluminal propagation can take place simultaneously at two different (probe field induced transition) frequency regimes with reduced absorption/gain. Hence the proposed double-control scheme may be exploited to observe the double-switching effect (i.e. from superluminal to subluminal and vice versa) with negligible distortion. In our realization this double-controlling mechanism may be regarded as simultaneous tuning of two knobs, to regulate the group index of the signal at double-frequency regimes.     

Dispersive and absorptive properties of a <Emphasis Type="Italic">Λ</Emphasis>-type atomic system with two fold lower-levels

The dispersion and the absorption properties of a driven four-level Λ-type atomic system is investigated. It is found that the interaction of double-dark states lead to controllable group velocity of the weak probe field by the intensity of driving field and the relative phase between applied fields. Moreover, the transient dispersion, absorption and the group index are also discussed. The required switching time for switching the group velocity of a weak probe field from subluminal to superluminal pulse propagation is then discussed.     

Transient and steady-state properties of asymmetric semiconductor quantum wells at telecom wavelength bands

The transient and steady-state dispersion and absorption properties of a three-subband asymmetric semiconductor quantum well system are investigated. In the steady-state regime, it is shown that by increasing the strength of Fano-interference as well as enhancement of energy splitting of two excited states the slope of dispersion changes from negative to positive which is corresponding to a switch between superluminal to subluminal light propagation. At the same time, the probe absorption reduces at telecommunication wavelength λ = 1550 nm. The influence of incoherent pumping fields on time-dependent susceptibility is then discussed. It is found that due to more transfer of population to the upper levels, increasing the rate of incoherent pump field leads to the reduction of probe absorption. In addition, it is realized that incoherent pumping has a major role in converting fast to slow propagation of light at long wavelength. We also introduce an extra controllability for the light pulse to be slow downed at Telecom wavelength just through the quantum interference arising from incoherent pumping fields. The obtained results may be practical in telecommunication applications.     

Absorption free superluminal light propagation in a three-level pump-probe system

We investigate the dispersion and the absorption properties of a weak probe field in a three-level pump-probe atomic system. It is shown that the slope of dispersion changes from positive to negative just with the intensity of the coherent or indirect incoherent pumping fields. It is demonstrated that the absorption free superluminal light propagation is appeared in this system.     

Roles of Atomic Injection Rate and External Magnetic Field on Optical Properties of Elliptical Polarized Probe Light

In this paper we investigate the optical properties of an open four-level tripod atomic system driven by an elliptically polarized probe field in the presence of the external magnetic field and compare its properties with the corresponding closed system.Our result reveals that absorption,dispersion and group velocity of probe field can be manipulated by adjusting the phase difference between the two circularly polarized components of a single coherent field,magnetic field and cavity parameters i.e.the atomic exit rate from cavity and atomic injection rates.We show that the system can exhibit multiple electromagnetically induced transparency windows in the presence of the external magnetic field.The numerical result shows that the probe field in the open system can be amplified by appropriate choice of cavity parameters,while in the closed system with introduce appropriate phase difference between fields the probe field can be enhanced.Also it is shown that the group velocity of light pulse can be controlled by external magnetic field,relative phase of applied fields and cavity parameters.By changing the parameters the group velocity of light pulse changes from subluminal to superluminal light propagation and vice versa.     

Anomalies of radiation absorption and superluminal propagation of light: I. An isotropic layer

The features of superluminal propagation of light through an isotropic layer are investigated and the group velocity is calculated. Multilayer systems providing superluminal propagation of light over large distances with compensation of losses upon light transmission through the system are considered. The situations in which the propagation speed of a light pulse decreases or in which it is equal to zero are also investigated. The features of radiation absorption in a thin isotropic layer are considered. The effects of anomalously high and anomalously low absorption are found. It is shown that these effects are caused by an increase (decrease) in the density of light energy in the layer and by changes in the group velocity. The possibility of experimental observation of the effects discovered is discussed.     

倒Y型四能级量子系统中亚光速和超光速传播现象的转换研究

研究了探测场、耦合场和驱动场三场作用下倒Y型四能级量子系统的光学特性,利用数值模拟的方法探讨了外加相干驱动场的拉比频率和失谐量变化时系统对探测光场吸收特性的影响. 通过绘制三维立体图,发现了探测光场的群速度在电磁诱导透明窗口处的变化规律,并且选择合适的驱动场拉比频率和失谐量,可以在理论上实现亚光速和真空光速以及超光速传播之间的转换. 关键词： 倒Y型四能级 群速度 超光速传播 亚光速传播     

Coherent Control of the Hartman Effect through a Photonic Crystal with Four-Level Defect Layer

In this paper, we examine the transmission of a probe field through a one dimensional photonic crystal (1DPC) when the sixth layer of the crystal is doped with four level atoms. We analyze effects of the external driving field on the passage of weak probe field across the photonic crystal. It is found that for the phase time delay of the probe photons, intensity of the driving field switches the Hartman effect from sub to superluminal character. It is interesting to note that in our model, the superluminal transmission of the probe pulse is accompanied by a negligibly small absorption of the incident beam. It ensures that the probe field does not attenuate while passing through the photonic crystal. A similar switching of the Hartman effect may be obtained by adjusting detuning of the probe field related to the excited states of the four-level doping atoms.     

The manipulation of light pulse from subluminal to superluminal propagation in a degenerate two-level Cs atomic system

We experimentally demonstrate the propagation of light pulse from subluminal to superluminal light based on quantum coherence in a degenerate two-level atomic system in a Cs vapor cell.It is shown that the group velocity of light pulse can be switched from subluminal to superluminal propagation via changing the coupling field from a traveling wave to a standing wave,while can also be continuously manipulated by varying the intensity of two waves superposed to form a standing wave.The observed maximum delay and advance times are about 0.45 and 0.54μs,corresponding to the group velocity of g=168km/s and g=138 km/s,respectively.This investigation may have the practical applications of devices for optical tunable delay lines,optical switching and optical buffering.     

Role of incoherent pumping field on absorption–dispersion properties of probe pulse in a graphene nanostructure under external magnetic field

The optical properties of weak probe light based on quantum coherence and interference in Landau-quantized graphene nanostructure driven by two coherent fields and incoherent pumping field is investigated. The linear dynamical properties of the graphene by means of density matrix method and perturbation theory are discussed. It is found that under certain condition and for appropriate choosing the parameters of the medium, the absorption, dispersion, group index of the weak probe light can be controlled. Moreover, it is shown that by means of incoherent pumping field the superluminal light propagation in the system is accompanied by amplification to make sure that the probe field is amplified as it passes through the system. Moreover, it is observed that the probe amplification can be obtained in the presence or absence of population inversion by properly choosing of system’s parameters. We hope that these results may have useful in the future quantum communicational system and networks.     

Gain-assisted superluminal light propagation via incoherent pumping field

We investigate the dispersion and the absorption properties of a weak field in a three-level Λ and V-type atomic system. We use just an incoherent pumping field to control the group velocity of a light pulse. The slope of dispersion is shown to be changed from positive to negative just by varying the intensity of the indirect incoherent pumping field. Gain-assisted superluminal light propagation appears in this system. No laser field is used in the pumping processes.     

Subluminal and Superluminal Phenomena in a Four-Level Atom

In a four-level atomic system, we investigate the light pulse propagation properties interacting with only one laser field. It is shown that in the steady state, the group velocity of the light pulse can be changed from subluminal to superluminal by varying the field detuning. Meanwhile, the effects of the field intensity on the group velocity are also shown. At last, with special parameters, the analytical solution for the group index is also obtained.     

Theoretical Investigation of Lasing without Inversion in a Multiple Quantum Well System

In this study, the influence of exciton spins relaxation (ESR) on spatial evolution of amplification without population inversion in a four-level GsAs multiple quantum wells is discussed theoretically. The role of propagation distance on absorption, dispersion and group index of weak probe light in the presence and absence of ESR is then analyzed. It is found that exciton spin relaxation (spin decoherence) which resulted from many-particle interaction in semiconductors have essential roles to adjusting the absorption and dispersion properties of weak probe light. Moreover, it is realized that the propagation distance in the presence of ESR can lead to switching between subluminal and superluminal light propagation. Our study provides a suitable treatment for the next generation of all-optical systems in semiconductor nanostructures.