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International audience; We examine how the interference of a coherent light-pulse with its slightly time-delayed copy may generate a pulse nearly identical to the original one and ahead of it. The simplicity of this 2-pulse system enabled us to obtain exact analytic expressions of the pulse distortion, valid for a wide class of pulse shapes. Explicit results are given for the pulses usually considered (gaussian, hyperbolic secant) but also for more realistic pulses of strictly limited duration. We finally show that the efficiency of the 2-pulse system is comparable to that of the other superluminal systems, at least for the pulse advancements actually demonstrated in the optical experiments.

International audience; The long-distance transmission of an oversized metallic waveguide filled with a gas of two-level atoms and excited in its lowest order mode is calculated within the coupled-mode theory in the general situation where the absorption and the nonlinearity are both strong. Although the gas polarization then involves numerous modes, it is shown that the modal dispersion can prevent any significant excitation of the higher order modes of the field. The propagation of stepwise light pulses is studied in this approximation.

A passive optical bistable system exhibits various phenomena when the input field is frequency modulated. The resonances are shifted towards high or low frequencies depending on the experimental conditions. The subharmonic resonance presents a similar effect. Hysteretic and chaotic behaviour have also been observed. All these effects are in excellent agreement with numerical simulations on a single-mode plane wave model of the experiments.

The calculation of the gas bulk polarization in Doppler free two photon interaction is recast within a simple model. Two photon coherence and pumping effects are clearly distinguished. Some new features of absorption and refractive index are pointed out for strong two photon saturation.

The optical notation is theoretically examined in the case of an optically thin gas suddenly switched on exact resonance with a standing wave. An analytical solution is given in the infinite saturation limit. The Rabi oscillations are shown to be strongly damped by the molecular motions through the standing wave. However the first Rabi oscillation is still extant even in the infinite Doppler limit.

We have experimentally studied the dynamics of a bistable system prepared in an initial state, close to the unstable intermediate branch of the bistability curve but far from the turning points. Our observations are indicative of a divergence of the switching times in —logϵ, ϵ characterizing the departure of the initial state from the unstable branch. This result, contrasting with the ϵ−12 standard law in critical slowing down, is well interpreted by a simple model involving a single dynamical variable.

The switching process in optical bistability is experimentally investigated on a model system made of a passive resonator containing an absorbing gas. The (up or down) switching is achieved by superimposing rectangular pulses on a cw power holding the system near a limit point. The observations are in good agreement with the predictions derived from the asymptotic theory of Mandel and support in particular his pulse area scaling law.

International audience; We present the first direct experimental evidence of the stimulated inelastic resonance fluorescence (SIRF) in a dilute, optically thick gas of 2-level systems. In particular, the shortening and modulation of the SIRF pulse and the rise time effects predicted by an analytical calculation are clearly pointed out. Previously computed shapes of the SIRF pulse are also interpreted.