Population inversion in the rotational levels of the superradiant N_2 ^+ pumped by femtosecond laser pulses

Abstract

Nitrogen molecular ions (N₂ ⁺) in air plasma pumped by femtosecond laser pulses give rise to superradiant emission at 391.4 nm in the presence of an external seed pulse at proper wavelength. Due to the transient alignment of the nitrogen molecular ions, the superradiance signal presents a strong modulation as a function of the temporal delay between the pump and the seed pulses. Through Fourier transformation with high frequency resolution, we distinguished the contribution of the finely separated rotation levels of the upper and lower states. It was found that the population density of certain rotational levels in the upper state is higher than that in the lower one, indicating that population inversion of the rotation levels of the two involved states is a key enabling factor for this superradiant emission.

Generation of Raman lasers from nitrogen molecular ions driven by ultraintense laser fields

“…More interestingly, at low gas pressures in the range from 10 to 30 mbar, pronounced supercontinuum spectra with a bandwidth of ∼80 nm have been observed which fill up the gap between ∼331 and ∼358 nm as well as that between ∼358 and ∼391 nm. In previous reports [5,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], + N 2 coherent emissions from different vibrational transitions between ground and excited states show an extremely narrow spectral bandwidth. To our knowledge, this is the first observation of coherent supercontinuum generated in + N 2 ions at low gas pressures.…”

“…With the tunnel ionization as the initiative process, highly nonlinear processes such as high-order harmonic generation [2], above threshold ionization [3], and non-sequential double ionization [4] have been observed, which further provide the means to access the dynamics in atomic and molecular systems on attosecond time scale. Recently, lasing actions induced by tunnel ionization of nitrogen molecules have been observed which come as a major surprise to those who have been investigating strong field physics over the past three decades [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. These observations were made with either a pump laser at 800 nm wavelength [6,[8][9][10][11][12][13][14][15][16][17][18][19][20][21] or that at longer wavelengths in the range between 1 and 4 μm [5,7,23].…”

Testing the Role of Recollision in N2+ Air Lasing

“…It encodes information about the rotational wave packets launched by the pump in the B 2 Σ + u (ν = 0) and X 2 Σ + g (ν = 0) states [34]. These modulations will be the subject of a future publication and are similar to those reported in a gas cell [35,36]. In the absence of the seed, no ion emission is measured.…”

Recent Advances in Air Lasing: A Perspective from Quantum Coherence

“…For instance, the dynamics of rotational quantum wavepackets in photo-ionized nitrogen molecules has been demonstrated by the amplified lasing signal. [78][79][80][81]…”