We use the phase difference of two lasers with equal frequencies for the control of spontaneous emission in a four-level system. Effects such as extreme spectral narrowing and selective and total cancellation of fluorescence decay are shown as the relative phase is varied. [S0031-9007(98)

The phenomenon of electromagnetically induced quantum coherence is demonstrated between three confined electron subband levels in a quantum well which are almost equally spaced in energy. Applying a strong coupling field, two-photon resonant with the 1-3 intersubband transition, produces a pronounced narrow transparency feature in the 1-2 absorption line. This result can be understood in terms of all three states being simultaneously driven into "phase-locked" quantum coherence by a single coupling field. We describe the effect theoretically with a density matrix method and an adapted linear response theory.

We show that a three-level ⌳-type atom interacting with a classical standing-wave field resonantly coupling one transition and a weak probe laser field resonantly coupling the second transition can be localized provided the population of the upper state is observed. DOI: 10.1103/PhysRevA.63.065802 PACS number͑s͒: 42.50.Gy The subwavelength localization of an atom using laserinduced schemes has been actively studied ͓1-9͔. Several models have been proposed using, for example, the measurement of the phase shift due to an off-resonant standing-wave field ͓1-3͔, the entanglement between the atom's position to its internal state ͓4͔, and others ͓5,6͔. Recently, Zubairy and co-workers ͓7-9͔ have proposed two simple localization schemes using either the measurement of Autler-Townes split spontaneous emission in a three-level system ͓7,8͔ or the resonant fluorescence in a two-level system ͓9͔. The main advantage of these schemes is that the localization of the atom occurs immediately in the subwavelength domain of the standing-wave field as spontaneous emission is recorded during the atom's motion in the standing-wave field.In this article we describe a related method for localizing an atom in a standing-wave field. We use a three-level ⌳-type atom that interacts with two fields, a probe laser field and a classical standing-wave coupling field. If the probe field is weak then the measurement of the population in the upper level can lead to subwavelength localization of the atom during its motion in the standing wave. The degree of localization is dependent on the parameters of interaction, especially on the detunings and the Rabi frequencies of the atom-field interactions.The atomic system under consideration is shown in Fig. 1. It consists of three atomic levels in a ⌳-type configuration. The atom is assumed to be initially in state ͉0͘. The transition ͉1͘↔͉2͘ is taken to be nearly resonant with a classical standing-wave field aligned along the x direction. In addition, the atom interacts with a probe laser field near resonant with the ͉0͘↔͉2͘ transition. We assume that the center-ofmass position of the atom is nearly constant along the direction of the standing wave. Hence, we apply the Raman-Nath approximation ͓10͔ and neglect the kinetic part of the atom from the Hamiltonian. Then, the Hamiltonian of the laserdriven part of the system in the interaction picture and the rotating wave approximation reads Hϭ⍀͉0͗͘2͉eϪi⌬ 0 t ϩg͑x ͉͒1͗͘2͉e Ϫi⌬ 1 t ϩH.c. ͑1͒Here ⍀ϭϪ ជ 02 • a E a , g(x)ϭG sin(kx)(GϭϪ ជ 12 • b E b ) are the Rabi frequencies of the probe and coupling fields, respectively, with ជ nm (n,mϭ0Ϫ2) being the dipole matrix element of the ͉n͘↔͉m͘ transition. The unit polarization vector and the amplitude of the probe ͑coupling͒ field are denoted by a ( b ) and E a (E b ), respectively. The Rabi frequency g(x) is position dependent with G being its constant part. The Rabi frequencies are taken to be real. Also, ⌬ 0 ϭ 20 Ϫ a (⌬ 1 ϭ 21 Ϫ b ) is the field detuning from resonance with the ͉0͘↔͉2͘ (͉1͘↔͉2͘) transition, ...

No abstract

The electronic structure of a spherical quantum dot with parabolic confinement that contains a hydrogenic impurity and is subjected to a DC electric field is studied. In our calculations we vary the position of the impurity and the electric field strength. The calculated electronic structure is further used for determining the nonlinear optical rectification coefficient of the quantum dot structure. We show that both the position of the impurity and the strength of the electric field influence the nonlinear optical rectification process.

We analyze the interaction of N laser fields with a (Nϩ1)-level quantum system. A general analytic expression for the steady-state linear susceptibility for a probe-laser field is obtained and we show that the system can exhibit multiple electromagnetically induced transparency, with at most NϪ1 transparency windows occurring in the system. The group velocity of the probe-laser pulse can also be controlled. DOI: 10.1103/PhysRevA.66.015802 PACS number͑s͒: 42.50.Gy, 42.50.Md For more than a decade there has been intensive interest in the phenomenon of electromagnetically induced transparency ͑EIT͒ ͓1-5͔ in three-level systems. In this phenomenon, an otherwise opaque medium is rendered transparent to a resonant probe-laser field that couples one of the transitions by the application of a strong, coupling laser field to the other transition. EIT has been observed in atoms ͓6͔, rareearth-ion-doped crystals ͓7͔, and semiconductor quantum wells ͓8͔. Potential applications of EIT range from lasing without inversion and enhanced nonlinear optics to quantum computation and communication ͓1-5͔. EIT has also been shown to occur in four-level systems of various configurations ͓9-14͔ and some experimental results already exist for these systems ͓15-19͔. Quite recently, McGloin et al. ͓20͔ have shown how EIT can also be extended to five-and sixlevel cascade systems.In this paper, we analyze the interaction of a (Nϩ1)-level quantum system in the configuration illustrated in Fig. 1 with N coherent laser fields. We assume that the system is initially prepared in a particular lower level and study the absorption and dispersion properties of a probe-laser field coupling this level to the upper level. To achieve this we use a densitymatrix formalism and obtain a general analytical expression for the linear susceptibility of the probe-laser field. We then use this result to show that the system can become transparent to the probe-laser field at NϪ1 different frequencies. In addition, the group velocity of the probe-laser pulse is analyzed. We show that the group velocity can obtain NϪ1 different values at transparency and can be controlled by the coupling laser fields.Denoting the excited state by ͉0͘ and the lower levels by ͉1͘, ͉2͘, . . . ,͉N͘ and assuming that each laser pulse drives only one transition, the Hamiltonian of this system in the interaction picture and in the rotating wave and dipole approximations is given by ͑we use units such that បϭ1)Ϫi␦ n t ͉n͗͘0͉ϩH.c. ͑1͒Here, ⍀ n ϭϪ ជ n0 • n E n is the Rabi frequency of the transition ͉n͘↔͉0͘, with ជ n0 being the associated dipole transition-matrix element. Also, n is the polarization vector and E n the electric-field amplitude of each laser pulse. Finally, ␦ n ϭ 0 Ϫ n Ϫ n is the laser field detuning from resonance with the transition ͉0͘↔͉n͘, with the energies of the nth lower level and upper level, respectively, being n and 0 and the angular frequency of the laser field being n . We will analyze the system using a density-matrix approach. From the Liouville equation we ob...

No abstract

No abstract