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.     

Effect of spontaneously generated coherence on probe response in an open system

The effect of spontaneously generated coherence on the probe response in an open ladder atomic system with equispaced levels is studied. The result shows that by adjusting the strength of spontaneously generated coherence (SGC), electromagnetically induced transparency (EIT) and high dispersion (index of refraction) with zero absorption can be realized, a much larger gain without inversion (GWI) than that without SGC can be obtained. Moreover, in the open system and for some strength of SGC, GWI without the incoherent pumping is much larger than that with the incoherent pumping; however, in the corresponding closed system, when the incoherent pumping is absent, we cannot obtain any gain (with or without inversion) for any strength of SGC. In addition, the manipulation role of the atomic exit and injection rates on SGC-dependent absorption property is analyzed for the case when the incoherence pumping does not exist.     

Effect of spontaneously generated coherence on Kerr nonlinearity in a four-level atomic system

We propose a scheme for giant enhancement of the Kerr nonlinearity in a four-level atomic system in which spontaneously generated coherence is present. The physics mechanism of the enhancement of Kerr nonlinearity is mainly based on the presence of an extra atomic coherence induced by the spontaneously generated coherence. Numerical values obtained by solving the density matrix equations agree well with these exact analytical values.     

Electromagnetically induced negative refraction in an atomic system with spontaneously generated coherence

We propose a new scheme for realizing left-handed electromagnetic properties in a coherently driven four-level system. At the presence of spontaneously generated coherence and the local field effect, both dielectric permittivity and magnetic permeability may have negative real parts of the same order in a small frequency band. Thus, one can achieve the negative refractive index and then manipulate it by modulating the coherent field or changing the atomic density. With incoherent pumping applied to have more remarkable spontaneously generated coherence, one can further obtain a wider frequency band of negative refraction and simultaneously reduce the accompanied absorption.     

Control of group velocity of light via magnetic field

We have theoretically studied the effects of quantum coherence in a driven quasi-degenerate two-level atomic system. We have shown that the quantum interference, which can be destructive or constructive, can be controlled by an externally applied magnetic field allowing one to implement both electromagnetically induced transparency and electromagnetically induced absorption in the same atomic system. Determined by frequency dispersion of the index of refraction of the system, the group velocity of light pulses ranges from ultra-slow to superluminal with changing of the magnitude of the magnetic field.     

Effects of relative phase in an open ladder system without incoherent pumping

We studied effects of the relative phase between the probe and driving fields on the absorption and dispersion properties in an open three-level ladder system with spontaneously generated coherence but without incoherent pumping. It is shown that by the phase controlling, switching from absorption to lasing without inversion (LWI) and enhancing remarkablely LWI gain can be realized; large index of refraction with zero absorption and the electromagnetically induced transparency can be obtained. We also find that varying the atomic injection and exit rates has a considerable influence on the phase dependent-absorption property of the probe field, existent of the atomic injection and exit rates gives the necessary condition of the realization of LWI, getting LWI is impossible in the corresponding closed system without incoherent pumping.     

Phase-dependent gain and absorption properties of mid- to far-infrared lights in three-coupled-quantum-wells

We investigate the phase-dependent gain and absorption behaviors of mid- to far-infrared lights under a cascade configuration in an asymmetric semiconductor three-coupled-quantum-well (TCQW) structure based on intersubband transitions (ISBTs). In the proposed TCQW scheme, the amplification of the probe and signal fields, i.e., the far- and mid-infrared waves, can be realized with realistic experimental parameters. The gain-absorption spectra are strongly dependent on the relative phase of the applied laser fields. The influences of the probe detuning and the two pump Rabi energies on the gain-absorption responses of two weak far- and mid-infrared light fields are also discussed in details. The results show that amplifications of the far- and mid- infrared waves can be achieved by adjusting the relative phase, the probe detuning, and the two pump Rabi energies appropriately. Such a semiconductor system is much more practical than its atomic counterpart because of its flexible design and the wide adjustable parameters. For practical applications, this investigation may provide a new possibility for realizing the efficient gain of far- and mid-infrared waves in the solid-state materials.     

Coherence induced by incoherent pumping field and decay process in three-level Λ type atomic system

Following the method proposed by Kozlov et al. [Victor V. Kozlov, Yuri Rostovtsev, Marlan O. Scully, Phys. Rev. A 74 (2006) 063829], we have investigated the atomic coherence induced by incoherent pump and vacuum spontaneous decay process in a Λ type three-level atomic system. The system can be in a coherent population trapping state and multi-steady states in different conditions. Interestingly, two kinds of new states are derived from the system with different pumping rate and decaying rate. They are the “robust” steady state and the “weak” steady state. Under the action of pump field and vacuum reservoir, these two kinds of states exhibit stable or unstable characteristics, respectively. Moreover, by investigating the difference between these states, we reveal the mechanism of coherence excitation and level-population transition. The special feature of the Λ atomic system will promise fruitful applications in quantum optics.     

Transient gain property of a weak probe field in an asymmetric semiconductor coupled double quantum well structure

The transient gain property of a weak probe field in an asymmetric semiconductor coupled double quantum well structure is reported. The transient process of the system, which is induced by the external coherent coupling field, shows the property of no inverse gain. We find that the transient behavior of the probe field can be tuned by the change of tunneling barrier. Both the amplitude of the transient gain and the frequency of the oscillation can be affected by the lifetime broadening.     

Six-Fold Degenerate Dark States in a Four-Level Atomic System

With all driving fields on Raman resonance, a tripod-type atomic system quickly evolves into a dark state decoupled from the lossy excited level. The dark state depends strongly on field Rabi frequencies, spontaneous decay rates, and the initial atomic population in a complicated way. Analytical results reveal that it is a sixfold degenerate dark state with its three components superposed both coherently and incoherently due to population redistribution from spontaneous emission.     

Amplification without population inversion in molecules

We have studied the feasibility of amplification without population inversion (AWI) in LiH molecule for three-level ladder, V, and Λ schemes. We have shown the salient features of AWI in a molecular system in comparison to that in an atomic system, viz. the dependence of gain profile on the choice of different rotational and vibrational transitions. Under different three-level schemes, the effect of homogeneous and inhomogeneous broadening on the gain profile has been studied. We have also studied the dependence of gain on different external parameters. The temporal evolution of gain has been analyzed and it was found that AWI is achievable in both the transient and the steady state regime. For all the three-level schemes and in particular for the ladder scheme, amplification of the weak probe was obtained using a coupling laser of smaller frequency.     

Slow light propagation via quantum coherence in a semiconductor quantum well

With the Schrödinger equations, we investigate the low-intensity light pulse propagation through a semiconductor quantum wells. Through studying the dispersion and absorption properties of the weak probe field, it is shown that slow light propagation is observed in this system. From the view point of practical purpose, it is more advantageous than its corresponding atomic system. Such investigation of slow light propagation may lead to important practical applications in semiconductor quantum information.     

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.     

Evolution of population in an equispaced three-level ladder-type atomic system

Transient response of nearly equispaced three-level ladder-type atomic system with a broad-band squeezed vacuum (SV) is investigated. We focus our attention in the interplay between the quantum interference and the squeezed field on the population distribution. It is shown that an atomic population inversion can be attained on one of the optical transitions due to the SV. Additionally, we show, with the proper value of the relative phase, the SV can also lead to unexpected population inversion on the transition between two different levels.     

Local Modulation of Double Electromagnetically Induced Transparency Spectrum

The double electromagnetically induced transparency induced by two coupling fields can be realized in a fourlevel tripod-type atom. Such double transparency spectra can be locally modulated by using the weak coherent fields to perturb the coupling transitions. These investigations within this scheme can be independent of Doppler broadening by properly orienting these fieds.     

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.     

Coherent laser-induced optical behaviors in three-coupled-quantum wells and their application to terahertz signal detection

We investigate quantum optical behaviors of a weak-probe laser field in an asymmetric semiconductor three-coupled-quantum wells (TCQW) structure based on intersubband transitions (ISBTs) via switch-on/off of terahertz (TH) signal radiation under the application of a control laser field. A scheme for TH signal detection and its strength measurement based on this probe absorption characteristic also are put forward, where TH signal field does not interact directly with electron, but significantly affects the coherent optical absorption properties of such a weak-probe laser field. Consequently, the proposed TCQW nanostructure may be used for reducing and cancelling out the important thermionic dark current component in the process of TH signal detection, measurement and photodetector design.     

Investigation of atomic coherence in multilevel systems with embedded tunable dark state superposition

The coherent superposition of two-atomic levels induced by coherent population trapping is employed in the two-level system, the standard three-level Λ type scheme and the four-level N-type systems and a weak probe pulse scans across the system. A theoretical analysis about the response of medium to the probe field is given. It is shown that under different initial conditions, the coherent superposition of the dark state exhibits abundant optical phenomena response to the probe field. It can change the absorption or gain and the dispersion relationship in the medium experienced by the probe. In the embedded three-level scheme, the probe experiences a crossover from absorption to transparent and then to amplification. Consequently the group velocity of the probe pulse can be controlled to propagate either as a subluminal, a standard, a superluminal or even a negative speed. In the embedded four-level N-type system, it can give an enhancement to the Giant Kerr effect and overcome the limitation of two-photon absorption, then make the nonlinear properties of the medium richer than the traditional N-type scheme.     

Coherent control of a light field with electromagnetically induced transparency in a dark state Raman coherent tripod system

A dark state superposition is employed and formed a tripod-like system. A weak probe pulse propagates in it can experience a crossover from absorption to transparent and then to amplification, and its group velocity can be controlled in any desired speed by determining the initial states of the dark state superposition.     

Coherent control of laser pulse temporal duration: An experimental proposal

We present a study of temporal compression resulting from the coherent control peculiarities of electromagnetically induced transparency propagation dynamics. We discuss the crucial conditions required to accomplish temporal compression in an experiment with a sample of hot atoms.