Abstract
Recently both amplification and lasing without inversion, which had been predicted about 10 years ago,1 were realized experimentally.2 It is attractive to extend the concept of inversionless amplification to gamma-ray nuclear transitions where it is difficult to achieve population inversion. This implies the search for the nuclear analogues of those schemes that were realized in atomic systems in optical range. Such Schemes involve splitting of one of two operating atomic levels and coherent driving at the transition between splitted sublevels. In principal, one could use microwave driving of hyperfine or Zeeman sublevels of the nuclear levels. But the large difference in the frequencies of the driven and amplifying transitions leads to a number of difficulties,1 which could be essentially reduced in case of optical driving. However the absence of optical transitions in the structure of nuclear levels does not allow direct driving of nuclear transitions by laser radiation. We show that coherent optical driving of the electronic transitions can provide amplification without inversion (AWI) at gamma-ray nuclear transition in cases when the structure of the electronic levels is essentially modified by excitation of nucleus. In the absence of interaction between the nuclear and electronic degrees of freedom, the structure of electronic levels in the ground and excited states of nucleus is exactly the same (Fig. la). Apparently optical driving of the electronic transitions in this case does not influence polarization of the gamma-ray nuclear transition. Formally, in the density matrix calculations it is reflected by compensation of contributions from the electronic transitions of the upper and lower nuclear states. Coupling between nucleus and electron leads to the effective and/or splitting some of electronic levels in the excited state with respect to the electronic levels in the ground state. When the magnitude of the corresponding frequency shift exceeds the linewidth of corresponding optical transition by tuning of the driving field to the resonance with the corresponding transition, a realization becomes possible of the close nuclear analogues of the basic atomic optical schemes (so-called p-, h-, and ladder schemes).
© 1998 Optical Society of America
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