Interference stabilization, middle IR, visible and UV lasing in a plasma channel formed in gas by a high intensity laser field

Recent progress in creation of ionized channels in gases by high intensity short laser pulses makes it possible to study a lot of new physical processes and their practical applications. Among them is the generation of extreme ultraviolet and soft X-ray radiation and, in particular, pulses of attosecond duration produced in the process of recombination of photoelectron with the parent ion when it comes back in the electric field of laser pulse [1,2]. In this talk the new approach of generation VUV, XUV or middle IR radiation from a plasma channel formed in a gas media by high-intensity laser pulse is proposed. It is based on the interference stabilization (IS) phenomenon [3-5] and population trapping in high-lying excited (Rydberg) states near the continuum boundary [6,7] resulting in population inversion between the set of Rydberg states and low-lying excited states as well as the ground state [8]. The analysis is based on direct numerical solution of the time-dependent nonstationary Schroedinger equation for a model argon-like single electron atom irradiated by a high-intensity laser pulse of femtosecond duration. Obtained data clearly demonstrate IS of Rydberg states resulting from the multiphoton resonance between the ground state and the set of excited (Rydberg) states [5]. The performed analysis gives rise to the conclusion that population inversion between different Rydberg states appears to exist in the intensity range above 2*10¹⁴ W/cm², and amplification process is possible during transitions nl --> n0 l ± 1 with n0 < n corresponding to the mid IR frequency range. As about population inversion between the set of excited np states and the ground state it appears to exist at higher values of laser intensity, 7*10¹⁴ – 10¹⁵ W/cm². Estimations for a possible gain factor are performed and it is demonstrated that for typical conditions of IS a gain factor for the mid IR, VUV or XUV frequency range can reach ~0.1–1 cm^-1 that makes it possible to obtain lasing emission in plasma channels produced by a high intensity laser pulse.