Strong-field-induced <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:msub><mml:mi mathvariant="normal">N</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow><mml:mo>+</mml:mo></mml:msup></mml:math> lasing by phase control of free induction decay
Siqi Wang, Yao Fu, Erik Lötstedt, Jincheng Cao, Hongwei Zang, Helong Li, Kaoru Yamanouchi, Huailiang Xu
Abstract
We prepare ${{\mathrm{N}}_{2}}^{+}$ mostly in the vibrational ground ${X}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{g}}^{+}$ state by an ultraviolet 267-nm pulse and create the free induction decay (FID) of the $B{\phantom{\rule{0.16em}{0ex}}}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{u}}^{+}\ensuremath{-}{X}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{g}}^{+}$(0, 0) transition at 391 nm by a resonant excitation pulse. By controlling the phase shift of the FID using a near-IR control pulse through the dynamical Stark effect and rotational coherence, we reveal that the absorption spectrum of this transition, exhibiting a Lorentzian absorption profile, can be changed into a Fano-type profile, and then, a Lorentzian emission profile, resulting in ${{\mathrm{N}}_{2}}^{+}$ lasing. Our results shed light on the long-term controversy over the mechanism of strong-field-induced ${{\mathrm{N}}_{2}}^{+}$ lasing and reveal a general route to achieving amplification of light resonantly interacting with molecular ions without population inversion.