Theory of femtosecond strong field ion excitation and subsequent lasing in N$_2^+$

Abstract

Delayed cavity-free forward lasing at the wavelengths of 391 and 428 nm was observed in recent experiments in air or pure nitrogen pumped with an intense femtosecond laser pulse at wavelength of 800 nm. The mechanism responsible for the lasing is highly controversial. In this article we explain the delayed emission by the presence of long-lived polarizations coupling simultaneously ground state X 2 Σ + g to states A 2 Π u and B 2 Σ + u of singly ionized nitrogen molecules N + 2 . Ionization of neutral nitrogen molecules in a strong laser field and subsequent ion excitation are described by a system of Bloch equations providing a distribution of ions in the ground and excited states A and B at the end of the laser pulse. The delayed signal amplification at the B-X transition wavelength is described by a system of Maxwell-Bloch equations with polarization coupling maintained by a weak laser post-pulse. Two regimes of signal amplification are identified: a signal of a few ps duration at low gas pressures and a short (sub-picosecond) signal at high gas pressures. The theoretical model compares favorably with experimental results.

Superradiance of molecular nitrogen ions in strong laser fields

“…Over the past decade, numerous articles reported and/or discussed the 391-nm "lasing" action of molecular nitrogen ions in strong femtosecond laser fields [1][2][3][4][5][6][7][8][9][10][11][12][13]. The forward emission inside the underdense plasma of pure nitrogen, experiences an increase of energy by several orders of magnitude compared to the seed pulse at 391 nm [7,8].…”