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Multilevel and multimode laser physics and non local hydrodynamic transport and fluctuations

A number of novel atomic coherence effects in strongly driven two-level and three-level systems were predicted and analysed. These effects are caused by the dependence of the relaxation rates in the dresses frequencies and therefore on the amplitude and frequency of the driving field. This dependence becomes important in case of sufficiently strong driving field. These effects cannot be described by the traditional master equation with the field-independent relaxation rates (Bloch equation). Hence we based our analysis on the generalised master equation (O. Kocharovskaya et al., Phys. Rev. A 49 (1994) 4928) and derived the limits in which the traditional master equation becomes invalid.

Emphasised, was the important role of the frequency selective environment for the clear appearance of field-dependent relaxation effects and made the connection for this domain with cavity QED.

Also studied was the tight connection between the phenomena of field-dependent relaxation and amplification without inversion in strongly driven three-level atomic systems. The novel mechanism of amplification without inversion entirely based on field-dependent relaxation was predicted and established some fundamental thermodynamic restrictions inherent to the process of amplification without inversion.

The major results are the following:

1. Field-dependent relaxation effects appear and may lead to a drastic modification of the atomic response both to a strong driving field and weak probe fieldeven within the Markoff approximation where a finite memory time of the reservoir is completely negligible.

2. The origin of the field-dependent relaxation can be easily understood in the dressed state picture. Namely the usual Wigner-Weisskopf formula for decay rates is applicable but with one important modification: frequencies of the bare transitions have to be replaced by the dressed frequencies.

3. In general the dependence of the relaxation rates on the dressed frequencies breaks a symmetry of the atomic response with respect to the case of theresonant driving. This symmetry breaking is weak but nevertheless measurable even for two-level atoms outside of the cavity.

4. Crossing between a dynamic Stark sublevel and a neighbouring bare atomic state results under certain condition in a coherent trapping of atoms to this Starklevel. This in turn leads to the excitation of the maximal atomic coherence at the resonant transition, vanishing absorption and maximal refraction index.

5. Large population inversion is achievable in a steady state regime at the driven transition both between bare and dressed states of the atomic system in case ofthe appropriate detuning of the driving field from the resonance with a driven transition.

6. Cavity QED enhancement of the relaxation rates via the density of modes plays a crucial role for the observation and applications of these effects. Exploitingthe dependence of the density of modes on the generalised Rabi frequency and detuning a high Q cavity to one of the side-band, one can control the majorcharacteristics of the atomic response via manipulation by the magnitude and sign of the dressed states population inversion.

7. Field-dependent spontaneous relaxation provides a new mechanism for electromagnetically induced transparency and amplification without inversion which isbased on population trapping of atoms into the dynamic Stark level. This mechanism requires fast relaxation between lower sublevels contrary to the traditionalmechanism of inversionless amplification based on population trapping of atoms onto coherent superposition of the lower sublevels.

8. Amplification without inversion by any means requires the presence of a nonequilibrium incoherent pumping. Independently on the magnitude of the excitedatomic coherence (and hence intensity of the driving field) and amplification with extraction of energy from the medium is impossible if atoms are coupled with a reservoir which is in a thermally equilibrium state.

Reported by

Université Libre de Bruxelles
Campus Plaine - Boulevard du Triomphe
1050 Bruxelles
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