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Abstract
Motivated by the hot debate on the mechanism of laser-like emission at 391 nm from ${{\rm N}_2}$ gas irradiated by a strong 800 nm pump laser and a weak 400 nm seed laser, we theoretically study the temporal profile, optical gain, and modulation of the 391 nm signal from ${\rm N}_2^ +$. Our calculation sheds light on the long standing controversy on whether population inversion is indispensable for optical gain and show the Ramsey fringes of the emission intensity at 391 nm formed by additionally injecting another 800 nm pump or 400 nm seed, which provides strong evidence for the coherence driven modulation of transition dipole moment and population transfer between the ${A^2}{\Pi _u}(\nu = 2) - {X^2}\Sigma _g^ + $ states and the ${B^2}\Sigma _u^ + (\nu = 0) - {X^2}\Sigma _g^ +$ states. Our results show that the 391 nm optical gain is susceptible to the population inversion within ${\rm N}_2^ +$ states manipulated by the Ramsey technique and thus clearly reveal their symbiosis. This study reveals not only the physical picture of producing ${\rm N}_2^ +$ population inversion but also versatile control of the ${\rm N}_2^ +$ air laser.
© 2020 Optical Society of America
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