On the possibility of obtaining stimulated emission without inversion

George C. Baldwin, J.P. Neissel, J. H. Terhune +1 more

The possibility of a stimulated emission device, analogous to the laser, which can generate radiation at gammaray wavelengths is examined. Lacking reflectors, its operation would consist of multiplication during single passage of photon avalanches initiated by spontaneous emission. The required high concentration of excited states may be obtainable in many nuclides by radiochemical procedures. The maximum possible resonant cross section exceeds ordinary absorption by several orders of magnitude. Only a fraction of the resonant cross-section can be effective, because of line-broadening interactions with the nuclear environment. Singular effects in solids (optical branch excitation, localization of lattice disturbance, or Bragg-angle effects) are suggested that may allow operation at a recoil-displaced energy without population inversion, as alternatives to the Mossbauer effect. An experimental technique for directly measuring the stimulated-emission cross-section by detection of a coherent component in the total radiation is described in some detail. It is concluded that the question of gammamaser feasibility can be conclusively answered only by experiment. (auth)

A. F. Harvey

Maria Richter, Marianna Lytova, Felipe Morales +4 more

In standard lasers, light amplification requires population inversion between an upper and a lower state to break the reciprocity between absorption and stimulated emission. However, in a medium prepared in a specific superposition state, quantum interference may fully suppress absorption while leaving stimulated emission intact, opening the possibility of lasing without inversion. Here we show that lasing without inversion arises naturally during propagation of intense femtosecond laser pulses in air. It is triggered by the combination of molecular ionization and molecular alignment, both unavoidable in intense light fields. The effect could enable inversionless amplification of broadband radiation in many molecular gases, opening unusual opportunities for remote sensing.

W. Brunner, Henryk Paul, G. Richter +2 more

Mikhail A. Noginov, J. Novak, S. N. Williams

The dynamics of stimulated emission in random lasers is studied using the system of rate equations for population inversion and density of emitted photons. In order to model the behavior of random lasers with nonresonant feedback, no coherence effects are intentionally taken into account. It has been shown that the feedback in the system is necessary for the realization of the regime of relaxation oscillations and spatial confinement of the stimulated emission to the interior regions of the pumped volume. The model also predicts the possibility of localization of stimulated emission in several spatially separated subvolumes of the random laser medium. Finally, it has been demonstrated that two adjacent random laser volumes can strongly enhance the stimulated emission in each other (the ``critical mass'' effect in random lasers).

H.-G. Unger