Zero-Area Single-Photon Pulses.

Broadband single photons are usually considered not to couple efficiently to atomic gases because of the large mismatch in bandwidth. Contrary to this intuitive picture, here we demonstrate that the interaction of ultrashort single photons with a dense resonant atomic sample deeply modifies the temporal shape of their wave packet mode without degrading their nonclassical character, and effectively generates zero-area single-photon pulses. This is a clear signature of strong transient coupling between single broadband (THz-level) light quanta and atoms, with intriguing fundamental implications and possible new applications to the storage of quantum information.

One- and two-photon quantum interferences in parametric four-wave mixing.

We report the observation of the interferometric response in parametric four-wave mixing in rubidium atoms excited by a pair of ultrashort pulses. For pulses with orthogonal polarizations and in the presence of the two-photon transition 5S-5D reveals a one-photon quantum interference, in contrast to the optical interference observed when the pulses have parallel polarization. For opposite circular polarizations no interference is observed in the fluorescence of the two-photon transition, in contrast to the one- and two-photon interferences exhibited in the nonlinear process.

Influence of propagation and external phase in sequential two-photon absorption of femtosecond pulses.

We study the influence of shaping the phase of the optical field when time-delayed femtosecond pulse pairs excite a sequential two-photon transition in rubidium atoms. Propagation through this optically dense medium modifies the pulse profile. When an external phase is introduced into one of the pulses, partial cancellation of the two-photon absorption signal is observed for temporal delays much greater than the pulse duration. This behavior is qualitatively explained in terms of a negative group delay.

Quantum interference in atomic vapor observed by four-wave mixing with incoherent light.

Pairs of pulses from an incoherent source are used to investigate the time-resolved four-wave mixing response of atomic rubidium when a two-photon resonance is involved in the nonlinear process. By varying the relative polarization of the pulse pairs, we are able to select the quantum pathways and clearly distinguish optical and quantum interferences.

Temporal coherent control of a sequential transition in rubidium atoms.

An experimental investigation of the coherent interaction of femtosecond pulses with two resonant sequential transitions of Rb atoms is presented. Fluorescence from the atomic system exhibits beating at a frequency given by difference in the sequential atomic transitions. The results are interpreted in terms of quantum interference in the induced coherence and its interaction with the field that results from a cooperative emission process.

Self-focusing-induced saturable loss for laser mode locking.

Self-focusing has recently been used for fast saturable absorber mode locking and femtosecond pulse generation in Ti:Al(2)O(3) lasers. We describe the operation, design, and optimization of self-focusing loss modulators for mode locking. A new formalism is developed that simplifies the treatment of self-focusing in thick nonlinear media by using a simple transform of the complex Gaussian beam parameter. Theoretical predictions are confirmed by experimental measurements.

Method to determine the phase dispersion of the third-order susceptibility.

A new method for measuring the phase of the third-order susceptibility chi((3)) that is based on the phenomenon of polarization beats is proposed and demonstrated. As a first application, the method was employed to study the phase dispersion of chi((3)) in Cd(S, Se)-doped glasses.

Femtosecond temporal encoding in barium titanate.

We describe two-beam coupling and temporal encoding experiments in barium titanate. Volume gratings are created in the photorefractive material by 50-fs optical pulses. Information in the writing pulses may be encoded as spatially distributed volume gratings in the crystal. Femtosecond temporal waveform reconstruction is demonstrated.