Gain without population inversion in V-type systems driven by a frequency-modulated field

We obtain gain of the probe field at multiple frequencies in a closed three-level V-type system using frequency modulated pump field. There is no associated population inversion among the atomic states of the probe transition. We describe both the steady-state and transient dynamics of this system. Under suitable conditions, the system exhibits large gain simultaneously at series of frequencies far removed from resonance. Moreover, the system can be tailored to exhibit multiple frequency regimes where the probe experiences anomalous dispersion accompanied by negligible gain-absorption over a large bandwidth, a desirable feature for obtaining superluminal propagation of pulses with negligible distortion.Comment: 10 pages + 8 figures; To appear in Physical Review

Fluorescence spectrum of a two-level atom driven by a multiple modulated field

We investigate the fluorescence spectrum of a two-level atom driven by a multiple amplitude-modulated field. The driving held is modeled as a polychromatic field composed of a strong central (resonant) component and a large number of symmetrically detuned sideband fields displaced from the central component by integer multiples of a constant detuning. Spectra obtained here differ qualitatively from those observed for a single pair of modulating fields [B. Blind, P.R. Fontana, and P. Thomann, J. Phys. B 13, 2717 (1980)]. In the case of a small number of the modulating fields, a multipeaked spectrum is obtained with the spectral features located at fixed frequencies that are independent of the number of modulating fields and their Rabi frequencies. As the number of the modulating fields increases, the spectrum ultimately evolves to the well-known Mellow triplet with the sidebands shifted from the central component by an effective Rabi frequency whose magnitude depends on the initial relative phases of the components of the driving held. For equal relative phases, the effective Rabi frequency of the driving field can be reduced to zero resulting in the disappearance of fluorescence spectrum, i.e., the atom can stop interacting with the field. When the central component and the modulating fields are 180 degrees out of phase, the spectrum retains its triplet structure with the sidebands located at frequencies equal to the sum of the Rabi frequencies of the component of the driving field. Moreover, we shaw that the frequency of spontaneous emission can be controlled and switched from one frequency to another when the Rabi frequency or initial phase of the modulating fields are varied

Controlling optical bistability using electromagnetic-field-induced transparency and quantum interferences

We demonstrate the application of electromagnetic-field-induced transparency and quantum interference effects in the cooperative phenomenon, such as optical bistability. The control field used in tandem with the usual electromagnetic field of the two-level scheme results in a considerable lowering of the threshold intensity. We discuss the transient response of the system in the mean-field limit and describe the regression to the steady state when perturbed away from it; the regression exponent is itself dependent on the control field. We also demonstrate the possibility of control-field-induced multistability in two-level systems

Enhancement of nonlinear-optical signals under coherent-population-trapping conditions

We develop a nonperturbative approach to study the generation of nonlinear signals in a coherently prepared medium. Specifically, we study the process ω<SUB>1</SUB>-ω<SUB>2</SUB>+(ω<SUB>2 </SUB>+Ω), in a system prepared in a coherent-population-trapping state. We calculate enhancement factors of the order of ∼102 in the generated signal, under a variety of conditions thus supplementing the work of Jain et al. [Phys. Rev. Lett. 77, 4326 (1996)]. Our approach also enables us to study other related issues. We demonstrate the existence of pulse matching at higher probe powers

Multiple Landau-Zener crossings and quantum interference in atoms driven by phase modulated fields

The excitation amplitude after multiple crossings is not a mere product of Landau-Zener transition probabilities at each crossing, due to coherent evolution of the system in between crossings. In the three-level ladder system, the trapping of population by frequency modulated fields ensures coherent evolution, and inclusion of phase effects for population redistribution after multiple crossings becomes necessary. The relative phase accumulated by various adiabatic states as they evolve along different paths is tailored to show the existence of quantum interference effects. We present a method of inverting the population in a three-level system, without affecting the population in the intermediate state. We also present an all optical implementation of the three-level ladder system, where these effects can be realized