Nonlinear Hanle effect in Cs vapor under strong laser excitation

We report results of a theoretical and experimental study of the ground state nonlinear Hanle effect under strong laser excitation. It is shown that besides the well-known zero-magnetic field suppression of absorption on F _g = F→F _e = F - 1 transitions caused by population trapping, an optical pumping induced enhanced absorption occurs on F _g = F→F _e = F + 1 transitions for small B-fields. The latter effect becomes more pronounced for high F values. The experiment with atomic vapor of Cs (D₂ line, F _g = 4) confirms an increase of the spectrally unresolved fluorescence yield at zero magnetic field and 600 mW/cm² laser intensity by 9% or 42%, when excitation occurs with linearly or circularly polarized light, respectively. The results of the experiment agree with numerical simulation studies using equations of motion for a density matrix. Received 24 November 2001 / Received in final form 25 March 2002 Published online 24 September 2002     

Quantum memory for photons in case of many close lying exciton resonances in solids

The possibility of storage of quantum information with photons is studied in the case of resonant transitions via many close lying exciton levels in a solid with impurity -atoms. The upper levels of the impurity atom form resonant Fano states, similar to the autoionization atomic states, due to the configuration interaction with the continuum of the exciton band. In this case slowing of light pulses is shown to be realistic, in the presence of the control field, down to the group velocity much lower than that in vacuum. The possibility of storage and reconstruction of a quantum pulse is studied in the case of the instantaneous switching on/off of the control field. It is shown that the signal quantum pulse cannot be stored undistorted for differing values of Fano parameters and for non-zero two-photon detuning and decay rate between the lower levels (decoherence). However, for small difference of the Fano parameters and for small values of the two-photon detuning and the decoherence there is no distortion in the case where the length of the pulse is much longer than the linear absorption (amplification) length, so the shape and quantum state of the light pulse can be restored.     

Analytical study of four-wave mixing with large atomic coherence

Four-wave mixing in resonant atomic vapors based on maximum coherence induced by Stark-chirped rapid adiabatic passage (SCRAP) is investigated theoretically. We show the advantages of a coupling scheme involving maximum coherence and demonstrate how a large atomic coherence between a ground and an highly excited state can be prepared by SCRAP. Full analytic solutions of the field propagation problem taking into account pump field depletion are derived. The solutions are obtained with the help of an Hamiltonian approach which in the adiabatic limit permits to reduce the full set of Maxwell-Bloch equations to simple canonical equations of Hamiltonian mechanics for the field variables. It is found that the conversion efficiency reached is largely enhanced if the phase mismatch induced by linear refraction is compensated. A detailed analysis of the phase matching conditions shows, however, that the phase mismatch contribution from the Kerr effect cannot be compensated simultaneously with linear refraction contribution. Therefore, the conversion efficiency in a coupling scheme involving maximum coherence prepared by SCRAP is high, but not equal to unity. Received 16 August 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: korsunsky@physik.uni-kl.de     

Manipulating the phase of a single-mode micromaser by polarized three-level atoms

We investigate the phase probability distribution (PPD) of a single-mode micromaser pumped by atoms injected in the most general case, i.e. in the superposition of the upper, intermediate and lower states by the Monte Carlo wave function approach. The phase properties of the cavity mode are greatly influenced by the relative phases and the amplitudes of the polarized atoms, and the detunings between the atom and cavity. The cavity field has a single preferred phase if the cavity is pumped by the atoms in the superposition of the upper and intermediate states or of the intermediate and lower states. However, a double-peak feature appears in the PPD of the cavity field when the cavity is pumped by the atoms in the superposition of the upper and lower states. With appropriate detunings, the double peaks become narrower and more remarkable, which shows the better defined phase of the cavity field, as compared to the resonant case. The PPD displays complicated characteristics when the cavity is pumped by the atoms in the superposition of the upper, intermediate and lower states. The phase distribution changes from a single peak to double peaks and to another single peak when we modulate the phase of the intermediate state, which has been explained in the semi-classical radiation theory.     

Novel examples of cluster-matter produced by LUCAS, a new laser cluster source

In this paper a detailed experimental study of Coherent Population Trapping (CPT) effect on sodium induced by a dye-laser operating in a three-mode regime is presented and a detailed analysis of the role of velocity changing collisions is made. These collisions show a very small relaxation effect on the dark resonances which are visible even at high pressures. For the first time we demonstrate the persistence of the ground state coherence to pressures up to one atmosphere for a relatively “heavy” buffer gas like argon. The experimental results have been compared with theory and a very good agreement has been obtained. Preliminary results on the effect of Na-N₂ collisions on Coherent Population Trapping are presented. Received: 5 October 1998 / Received in final form: 3 December 1998     

Cross phase modulation in a five–level atomic medium: semiclassical theory

The interaction of a five-level atomic system involving electromagnetically induced transparency with four light fields is investigated. Two different light-atom configurations are considered, and their efficiency in generating large nonlinear cross-phase shifts compared. The dispersive properties of those schemes are analyzed in detail, and the conditions leading to group velocity matching for two of the light fields are identified. An analytical treatment based on amplitude equations is used in order to obtain approximate solutions for the susceptibilities, which are shown to fit well with the numerical solution of the full Bloch equations in a large parameter region.     

Absorption spectrum of a resonantly driven degenerate V-type atom with a dc-field coupling between excited states

We study the absorption spectra of a degenerate V-type atom, where a resonant driving field and a probe field drive different branches of transitions and a dc field is applied to drive the transition between two excited states. The effects of vacuum induced coherence (VIC) on the absorption spectra are investigated. It is demonstrated that in some special cases the VIC can lead to the depression of absorption and narrow resonance. The origin of these features are discussed. When the pump field and the dc field have the same intensity, it is interesting to find that the whole absorption spectrum comes mainly from the absorptions induced by the interferences among different transitions between dressed states.     

Quantum memory process with a four-level atomic ensemble

We examine in detail the quantum memory technique for photons in a double Λ atomic ensemble in this work. The novel application of the present technique to create two different quantum probe fields as well as entangled states of them is proposed. A larger zero-degeneracy class besides dark-state subspace is investigated and the adiabatic condition is confirmed in the present model. We extend the single-mode quantum memory technique to the case with multi-mode probe fields, and reveal the exact pulse matching phenomenon between two quantized pulses in the present system.     

Enhanced narrow spectral line and double electromagnetically induced two-photon transparency induced by double dark resonances

We theoretically investigate the features of two-photon absorption in a five-level atomic system with interacting dark resonances. It is found that two-photon absorption can be completely suppressed at two different frequencies due to the application of two coherent coupling fields and the atomic system exhibits double electromagnetically induced transparency windows against two-photon absorption. The position and width of the double two-photon transparency windows can be controlled via properly adjusting the frequency detuning and the intensities of the two coupling fields. In addition, one enhanced narrow central line can be observed in the two-photon absorption spectra, which may find applications in high-precision spectroscopy. Form a physical point of view, we explicitly explain these results in terms of quantum interference induced by three different two-photon excitation channels in the dressed-state picture.     

Study of width and height of EIT resonance in a Doppler broadened five-level system with varying probe power

We report the experimental observation of electromagnetically induced transparency (EIT) in a Doppler broadened rubidium vapour at room temperature for different probe intensities at a fixed pump intensity in a five-level Λ-type system formed by the D₂ transition of ⁸⁵Rb. For a constant pump intensity, we find that the EIT width and height change with the variation of probe intensity. We observe a nonlinear variation of the height of the EIT peak and a linear variation of the width (FWHM) of the EIT signal with probe intensity. In the Doppler broadened multilevel system, we also observe the velocity selective dips along with the EIT signal. A numerical simulation of the probe response signal based on density matrix representation in a five-level system is carried out to reproduce the experimentally observed spectra.     

Propagation of shape-preserving optical pulses in inhomogeneously broadened multi-level systems

We study stable propagation of multiple shape-preserving optical pulses in an inhomogeneously broadened multi-level atomic medium. By analytically solving the Maxwell-Schr?dinger equations governing the evolution of N coupled optical fields and atomic amplitudes we show that N pulsed optical waves coupling to (N+1)-levels can be automatically matched with the same soliton waveform and identical yet very slow propagation velocity. Several sets of coupled soliton solutions for two different (N+1)-level models are given and their stability is studied by using a numerical simulation.     

Absorption and dispersion by a multiple driven two-level atom

We investigate the absorption and dispersion properties of a two-level atom driven by a polychromatic field. The driving field is composed of a strong resonant (carrier) frequency component and a large number of symmetrically detuned sideband fields (modulators). A rapid increase in the absorption at the central frequency and the collapse of the response of the system from multiple frequencies to a single frequency are predicted to occur when the Rabi frequency of the modulating fields is equal to the Rabi frequency of the carrier field. These are manifestations of the undressing or a disentanglement of the atomic and driving field states, that leads to a collapse of the atom to its ground state. Our calculation permits consideration of the question of the undressing of the driven atom by a multiple-modulated field and the predicted spectra offer a method of observing undressing. Moreover, we find that the absorption and dispersion spectra split into multiplets whose structures depend on the Rabi frequency of the modulating fields. The spectral features can jump between different resonance frequencies by changing the Rabi frequency of the modulating fields or their initial phases, which can have potential applications as a quantum frequency filter. Received 23 October 2001 and Received in final form 31 January 2002     

Line-shape of dark line and maser emission profile in CPT

The paper is concerned with the line shapes of resonance phenomena observed in Coherent Population Trapping (CPT) applied to alkali atoms in a cell containing a buffer gas. Significant asymmetries and departures from a Lorentzian shape have been observed in connection with the measurement of dark lines and CPT maser emission profiles. Measurements are reported as a function of the power and frequency tuning of the laser used to create the CPT phenomenon. The paper reports on different experimental conditions and a comparison between theory and experiments is made for the cases of cesium and rubidium in a buffer gas. Received 3 March 2000 and Received in final form 10 April 2000     

Non‐electromagnetically induced transparency mechanisms for slow light

We show how the pump‐induced sharp holes in homogeneously or inhomogeneously broadened absorption line shapes can be used for the production of ultra‐slow light. We present a detailed discussion of underlying concepts and results on the pump power dependences of the group index and the bandwidth–delay product. Further, we discuss experimental results, which are in agreement with theoretical predictions.     

Phase and amplitude control of the inversionless gain in a microwave-driven <Emphasis Type="Bold">Λ</Emphasis>-type atomic system

In order to achieve the phase-sensitive probe gain without population inversion, we investigate a three-level Λ-type atomic system driven by a coherent field and a microwave field. It is shown that, by modulating the relative phase of applied fields, we can obtain quite high inversionless gain at different probe detunings and change the gain behavior of the probe correspondingly. We find that amplitudes of the coherent field and the microwave field are also important factors that can result in different gain behavior of the probe. Here, we use the microwave field to induce the quantum coherence between the two ground levels, which is necessary for phase-sensitive effects, since it can result in the interference between two different transition channels. Received 20 June 2002 / Received in final form 5 December 2002 Published online 4 March 2003     

Two-photon interaction between trapped ions and cavity fields

In this paper, we generalize the ordinary two-photon Jaynes-Cummings model (TPJCM) by considering the atom (or ion) to be trapped in a simple harmonic well. A typical setup would be an optical cavity containing a single ion in a Paul trap. Due to the inclusion of atomic vibrational motion, the atom-field coupling becomes highly nonlinear what brings out quite different behaviours for the system dynamics when compared to the ordinary TPJCM. In particular, we derive an effective two-photon Hamiltonian with dependence on the number operator of the ion's center-of-mass motion. This dependence occurs both in the cavity induced Stark-shifts and in the ion-field coupling, and its role in the dynamics is illustrated by showing the time evolution of the probability of occupation of the electronic levels for simple initial preparations of the state of the system.     

Controllable shape-matched propagation of two signal beams in a double-Λ atomic ensemble

We study a four-level double-Λ atomic ensemble interacting with two time-dependent signal fields and two stationary control fields. Though, in each Λ channel, a pair of signal and control fields couple resonantly with the two lower levels of atoms, the occurrences of electromagnetically induced transparency (EIT) is affected by the coherence of the four fields. In the discussion of atomic susceptibilities, we show that the quantum coherence between the two lower levels can be either formed or released according to the phase matching of the four fields. We analyze the propagation equation of the two signal fields, and find two characteristic solutions: the stationary transmission wave and the transient decay wave. The former corresponds to a correlated EIT effect in which two signal pulses are shape-matched. The latter is an opposite effect to the correlated EIT in which two pulses quench simultaneously, thus named as the correlated two-signal absorption (CTSA). We propose the CTSA condition in correspondence with the EIT condition. The numerical simulation shows that the double-Λ configuration is capable of manipulating synchronous optical signals and thus provides multiplicity and versatility in quantum information process.     

Spontaneous emission from a four-level system

We examine the decay dynamics of a free four-level system in the -V configuration. Quantum interference strongly manifests itself in this system, as can be seen by looking at the combined spectral distribution of the two emitted photons and at the time evolution of the intermediate-level populations, whose effective lifetimes can become very long under certain conditions for the atomic parameters. This effect is attributable to a population transfer mechanism induced in the time evolution equations by the Fano terms, also responsible for the strong modifications of the spectral correlation between the emitted photons which we analyze in detail. Finally, population trapping can also occur when the two intermediate levels are degenerate. Received: 20 October 1998     

Spontaneously generated coherence in a Doppler broadened atomic system

We investigate the spontaneously generated coherence (SGC) in a Doppler broadened four-level atomic system driven by two coherent fields. We plot the spontaneous emission spectra with different parameters and discuss how the initial atomic conditions and parameters of both fields change the number of peaks and dark lines of spontaneous emission spectra. Furthermore, we also show how the spontaneous emission spectrum is modified by Doppler effects in the viewed direction. Our results have important references to the experimental observation of SGC in hot atomic vapors.