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Journal of Physics B Atomic Molecular and Optical Physics

Mechanism of population inversion in laser-driven N 2 +

Youyuan Zhang, Erik Lötstedt, Kaoru Yamanouchi

Published Jan 3, 2019• 17 citations
Publisher

Abstract

Abstract On the basis of our theoretical model of the population transfer between two levels interacting suddenly with an intense laser pulse (Zhang et al 2017 J. Phys. B: At. Mol. Opt. Phys. 50 185603), we investigate the mechanism of the population inversion between the electronically excited B 2 Σ u + state and the electronic ground X 2 Σ g + state of N 2 + achieved by the irradiation of a near-IR (800 nm) intense laser pulse, leading to the generation of the coherent emission at 391 nm. We show that one of the Floquet states generated at the moment when N 2 + starts interacting with an intense laser pulse is adiabatically connected to the ground vibrational state of the electronic B 2 Σ u + state and keeps a constant population throughout the period of the interaction with the light field, indicating that the efficient off-resonance B 2 Σ u + – X 2 Σ g + transition proceeds.

DOI
10.1088/1361-6455/aafbdc

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Abstract On the basis of our theoretical model of the population transfer between two levels interacting suddenly with an intense laser pulse (Zhang et al 2017 J. Phys. B: At. Mol. Opt. Phys. 50 185603), we investigate the mechanism of the population inversion between the electronically excited B 2 Σ u + state and the electronic ground X 2 Σ g + state of N 2 + achieved by the irradiation of a near-IR (800 nm) intense laser pulse, leading to the generation of the coherent emission at 391 nm. We show that one of the Floquet states generated at the moment when N 2 + starts interacting with an intense laser pulse is adiabatically connected to the ground vibrational state of the electronic B 2 Σ u + state and keeps a constant population throughout the period of the interaction with the light field, indicating that the efficient off-resonance B 2 Σ u + – X 2 Σ g + transition proceeds.
The time-dependent population transfer process of N2+ generated in an intense laser pulse has been investigated using the quasi-stationary Floquet theory by assuming that N2+ experiences an intense laser pulse with the sudden turn-on. A light-dressed B state is formed with a significant amount of population when pulse is suddenly turned on and is adiabatically transformed to the vibrational ground state (v = 0) of the field-free B state when the pulse vanishes. In addition, a part of the population is transferred to the electronically excited A state through one-photon resonance, which also contributes to decreasing the final population in the X state, facilitating the population inversion between the B state and the X state.
Abstract The population transfer dynamics in a strongly driven two-level system is studied. It is shown that population inversion can be efficiently achieved when a laser pulse whose photon energy is about half of the energy separation of the two levels is turned on suddenly. This situation is realized when a neutral atom or molecule is ionized at the peak of the laser field, followed by the laser-driven population transfer among two states of an atomic or molecular ion. The mechanism of the efficient population transfer for an ion effectively exposed to a laser field with a sudden turn-on is examined by numerical calculations based on the quasi-stationary Floquet theory.
We experimentally demonstrate the existence of long-lived coherent polarizations coupling simultaneously ground state X (X2Σg+) to excited states A (A2Πu) and B (B2Σu+) of N2+ inside a plasma created by a short intense laser pulse at 800 nm. This three-level V-scheme arrangement is responsible for a strong optical gain without population inversion at the B–X transition at 391.4 nm. Simulations based on Maxwell-Bloch equations reproduce well the kinetics and the pressure dependence of the gain.
We prepare N2+{{\mathrm{N}}_{2}}^{+}N2​+ 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 N2+{{\mathrm{N}}_{2}}^{+}N2​+ lasing. Our results shed light on the long-term controversy over the mechanism of strong-field-induced N2+{{\mathrm{N}}_{2}}^{+}N2​+ lasing and reveal a general route to achieving amplification of light resonantly interacting with molecular ions without population inversion.
We investigate the pump wavelength dependence of population inversion in a nitrogen ion when nitrogen gas is irradiated by a femtosecond laser pulse in the wavelength range from ultraviolet to near-infrared. It is found that the population inversions corresponding to the transitions at 391 and 428 nm are very sensitive to the laser wavelength, with extreme values near 400 and 1000 nm. For the ultraviolet pump, the population inversions for 391 and 428 nm transitions occur at different wavelengths: a maximum inversion for the first corresponds to a minimum for the second. This is attributed to the near-resonance single-photon transition combined with the Raman process. A significantly stronger inversion is observed for the near-infrared laser, with the maximum depending on the laser intensity, ascribed to the ac Stark effect. The Floquet theory confirms this conclusion and reveals that the three-photon transition assisted by the strong laser field leads to this enhanced population inversion. This study provides the optimal conditions for air lasing based on ionic transitions of nitrogen molecules.
It has been widely believed that short-pulse multiphoton ionization is dominated by a resonance process: excited states of the atom are shifted by the radiation field into multiphoton resonance with the ground state, are populated, and then rapidly ionized. A recent experiment, shows significant population remaining in excited states at the end of the laser pulse, and this result has been interpreted as indicating that ionization proceeds slowly out of these states. We investigate this issue in the framework of Floquet theory, supplemented by direct numerical solution of the time-dependent Schrodinger equation. In this framework, the standard picture is portrayed as an adiabatic transition between Floquet eigenstates, which occurs as the amplitude of the laser field is ramped through the resonance value (i.e. through an avoided crossing of Floquet quasienergies). Thus it follows that the same type of transition must occur as the field is turned off. This mechanism can yield significant populations of excited states that do not suffer appreciable ionization. We exhibit this effect by numerical studies of a model atom with two bound states and two continua.