Feasibility of Producing an Inverted Medium by Photoionization of the Inner Electrons in Atoms

Andrei Benediktovitch, Vinay Pramod Majety, Nina Rohringer

Irradiation of a medium by short intense pulses from x-ray (XUV) free-electron lasers can result in saturated photoionization of inner electronic shells. As a result an inversion of populations between core levels appears. The resulting fluorescent radiation can be amplified during its propagation through the inverted medium and results in intense, quasi-transform-limited radiation bursts. While the optical counterpart of this phenomena, known as superfluorescence, was intensively investigated, a generalized treatment is needed in the x-ray (XUV) domain, where the dynamics of pumping and evolution due to fast decay processes play a crucial role. To provide a general theoretical approach, we start from the fundamental, quantized minimal coupling Hamiltonian of light-matter interaction and after a series of approximations arrive at a closed system of equations for the two-point correlation function of atomic coherences and the two-time correlation function of the emitted field. The obtained formalism enables us to investigate collective spontaneous emission in various regimes. It is extended consistently to include incoherent processes that are relevant in the x-ray (XUV) domain. These processes are introduced into the formalism by corresponding Lindblad superoperators. The connection to other approaches is discussed and numerical examples related to recent experiments are presented.

Hans Kleinpoppen, Bernd Lohmann, Alexei N. Grum-Grzhimailo

Vladislav B Rozanov

It is shown that the photoionization of the inner-shell electrons can yield metastable ions. Therefore, the accumulation of excited particles becomes possible at practically realizable optical pumping powers. This method of population inversion may be effective because a large part of the emission spectrum of an optically transparent plasma in a radiation source is in the form of "hard" photons, which are emitted when electrons are captured and atoms or ions go over to their ground state.

Philip George Burke

A. S. Choe, Yongjoo Rhee, Jongmin Lee +4 more

A principle for the efficient excitation of a three-level medium in a selective photoionization method is examined. It is shown that with special detunings for a given set of the laser-atom interaction the third-level population can be maximized. The expression of the optimal detunings is derived near the two-photon resonance region. It is shown that if the optimal detuning method is applied to three-level systems of Yb and Ca, the first-level populations are inverted to their corresponding third levels by about 100% and 86%, respectively. Application of this method to a Doppler-broadened medium is discussed and it is found that the optimal detuning method with counterpropagating waves will be very effective for selective ionization of wanted atoms.

James A. R. Samson, Anthony F. Starace