Relativistic effects in superradiant-emission processes

O. N. Gadomskiĭ, V. R. Nagibarov, N. K. Solovarov

Luqi Yuan

This work is focused on quantum coherence and superradiant emission in a dense pencil-like multi-level medium where many novel effects appear such as transient lasing without inversion, coherence-brightened sky laser, and quantum amplification by superradiant emission of radiation. \n\nWe start from an interesting cascade model where quantum coherence effects can lead to surprising phenomena, gain without population inversion and gain suppression, under different parameters. We further show superradiant emission inside helium plasma. The population evolution shows the decay is significantly faster than collisional decoherence and spontaneous decay rates. This indicates superradiant coherent behavior of the atomic system inside the plasma.\n\nBased on these results, we demonstrate lasing without inversion on a time scale shorter than the decoherence time. The possibility of transient lasing without inversion holds promise for lasing in the extreme-ultraviolet/x-ray regime. We propose experiments to demonstrate this in helium or helium-like ions plasma. \n\nWe also study coherent emission from ambient air and demonstrate efficient generation of laser-like beams directed both forward and backward with respect to a nanosecond ultraviolet pumping laser beam. The emission process exhibits nonadiabatic quantum coherence, which is similar in nature to Dicke superradiance. This coherence-brightened backward light source in air provides possibility for atmospheric remote sensing through a phase-matched coherent Raman scattering process.\n\nFinally, we present a new kind of quantum amplifier, the QASER (quantum amplification by superradiant emission of radiation), based on collective superradiant emission which does not require initial population in the excited state. We show that parametric resonance between the driving field and collective superradiant oscillations of the atomic polarization can yield light amplification at high frequencies. The resulting superradiant amplifier is many orders of magnitude more efficient than nonlinear multiphoton excitation and holds promise as a new way to generate high-frequency coherent radiation.

F. De Martini, Ottavia Jedrkiewicz, Paolo Mataloni

The process of generation of non-classical radiation in an active Fabry-Perot dye microcavity, under femtosecond excitation, has been investigated. Single photon states, with a non-classical sub-Poissonian distribution have been generated after the excitation of a small number of active molecules, located between the two mirrors. By multiple excitation of the active medium, collective emission phenomena are expected because of the strong superradiant coupling occurring within the transverse coherence area of the microcavity. In particular, we have experimentally verified the principle of relativistic causality within the process of two-dipole superradiance by transverse interaction, in condition of strong microcavity confinement.

Joseph H. Eberly, Nicholas E. Rehler

Christoph M. Greiner, THOMAS W. MOSSBERG

Summary form only given. The radiative behavior of an ensemble of correlated atoms can be dramatically different from that of a single atom. Correlation, for example, can give rise to sub- or superradiant emission. Experimental studies of the fascinating correlative effects expected have been severely constrained by difficulties in creating suitably correlated media and by propagation effects that perturb correlation during emission. We have found that optical cavities of moderate reflectivity and very short ring-down time can be employed to "accelerate" the superradiant process and bring it to dominance in media sufficiently dilute so that propagation effects both in excitation and emission are absent. For the first time, it is possible to observe strong optical superradiance under the pure and simple conditions that were originally envisioned. Note that superradiance here does not refer to the emission of initially inverted atomic samples (i.e. superfluorescence), but to cooperative emission from samples of atoms prepared in correlated superposition states. Moreover, the cavity accelerated superradiant system is found to display heretofore unknown nonlinear behavior in terms of its effect on the "area" of injected optical pulses.

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