Lasing action in air induced by ultra-fast laser filamentation

Natural population inversion in a gaseous molecular filament

“…According to Becker's calculation [10] of the transition between the excited and ground states of the N transitions are weak lines. Experimentally, the strongest lines are from the (0 → 0) and (0 → 1) transitions and they exhibit gain [3,7,8] ; i.e. population inversion.…”

“…Lasing action in gaseous filaments of intense femtosecond laser pulses has attracted a lot of attention in recent years [1−6] since the first observation in air in 2003 [7] . In the latter work, amplified spontaneous emission from N 2 and N + 2 was measured.…”

“…These two electronic states of N + 2 are prepared by the excitation from the electronic ground state of neutral N 2 through multiphoton/tunnel ionization inside the filament. The first qualitative explanation of the population inversion was given by Luo et al [7] as being due to the faster decay time of the ground state as compared to that of the excited state of N + 2 through recombination with electrons. Such decay times would have been rather slow, of the order of hundreds of picosecond or nanosecond.…”

Optical gain in rotationally excited nitrogen molecular ions

“…Intensity inside the core of the femtosecond laser filament is of the order of 100 TW/cm 2 , which is sufficient to partially ionize oxygen and nitrogen molecules, producing, in particular, singly ionized molecular nitrogen ions N + 2 . It has been suggested that air filamentation may result in the optical gain on the electronic transitions of N + 2 at 391 and 428 nm emission wavelengths, as well as on several weaker transitions [8]. However, the gain mechanism responsible for those emissions has been controversial.…”

Spectroscopic analysis of high electric field enhanced ionization in laser filaments in air for corona guiding

“…During femtosecond laser FIL in air, many nonlinear effects occur simultaneously, such as intensity clamping, molecular alignment, harmonic generation, self-phase modulation and so on [3][4][5] . These complex and interesting nonlinear phenomena accompanying femtosecond laser FIL lead to promising applications in different fields, such as remote sensing [6,7] , few-cycle pulse compression [8] , atmospheric condensation and precipitation [9][10][11] , secondary light source generation ranging from air lasing to terahertz emission [12][13][14][15][16][17][18] , and triggering and guiding high-voltage discharge with possible applications in lightning control [19][20][21][22][23][24] .…”