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arXiv:1508.05516 · Physical Review Letters

Recollision-Induced Superradiance of Ionized Nitrogen Molecules

Yi Liu, Pengji Ding, G. Lambert, Aurélien Houard, Vladimir T Tikhonchuk, A. Mysyrowicz

Published Sep 24, 2015• 163 citations
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Abstract

We propose a new mechanism to explain the origin of optical gain in the transitions between the excited and ground states of the ionized nitrogen molecule following irradiation of neutral nitrogen molecules with an intense ultrashort laser pulse. An efficient transfer of population to the excited state is achieved via field-induced multiple recollisions. We show that the proposed excitation mechanism must lead to a superradiant emission, a feature that we confirm experimentally.

DOI
10.1103/physrevlett.115.133203

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We propose a new mechanism to explain the origin of optical gain in the transitions between the excited and ground states of the ionized nitrogen molecule following irradiation of neutral nitrogen molecules with an intense ultrashort laser pulse. An efficient transfer of population to the excited state is achieved via field-induced multiple recollisions. We show that the proposed excitation mechanism must lead to a superradiant emission, a feature that we confirm experimentally.
Nitrogen molecules are ionized when an intense 800-nm femtosecond laser propagates through a nitrogen gas cell. We experimentally probe the relative population in different vibrational states and electronic states of ionized nitrogen molecules by using an external seed with various wavelengths. It is found that the seed is amplified or attenuated depending on its wavelength when the seed passes through the ionized nitrogen gas. Through measuring the temporal profile, the amplified emission is attributed to the seed-triggered superradiance. Combining with theoretical calculation, our results demonstrate that both vibrational and electronic excitation occurs for ionized nitrogen molecules in 800-nm femtosecond laser fields. The population inversion is built between some quantum states of ionized nitrogen molecules, through which the seed-triggered superradiance is achieved.
Nitrogen molecules in ambient air exposed to an intense near-infrared femtosecond laser pulse give rise to cavity-free superradiant emission at 391.4 and 427.8 nm. An unexpected pulse duration-dependent cyclic variation of the superradiance intensity is observed when the central wavelength of the femtosecond pump laser pulse is finely tuned between 780 and 820 nm, and no signal occurs at the resonant wavelength of 782.8 nm (2ω_{782.8 nm}=ω_{391.4 nm}). On the basis of a semiclassical recollision model, we show that an interference of dipolar moments of excited ions created by electron recollisions explains this behavior.
Singly ionized nitrogen molecules in ambient air pumped by 800 nm femtosecond laser give rise to superradiant emission. Here, we study this superradiance by injecting a pair of resonant seeding pulses at different intensity ratios inside the nitrogen gas plasma. Strong modulation of the 391.4 nm superradiant emission with a period of 1.3 fs is observed when the delay between the two seeding pulses is finely tuned. The modulation contrast is increased and then decreased with the delay time when the second seed pulse is stronger than the first one, and the maximum modulation contrast occurs at longer delay time when the second seeding pulse is stronger. This reveals the increase of the macroscopic polarization with time after the seeding pulse. Moreover, these observations provide a new level of control on the "air lasing" based on nitrogen ions, which can find potential applications in optical remote sensing.
A cavity-free laser in the sky could lead to revolutionary improvements in optical remote sensing for atmospheric science. Abundant in air, nitrogen molecules are prime candidates as an active medium for such a laser. Nitrogen molecules, either neutral or singly ionized, exhibit a significant optical gain in the UV regime when pumped by an intense femtosecond laser. Despite numerous works, the physical origin of this lasing is still not understood. Here we offer a consistent interpretation of all experimental facts by introducing a concept drawn from the field of quantum optics: transient lasing without inversion. Our experiments and simulations identify the crucial role of macroscopic coherence, instead of population inversion, in the nitrogen ions lasing system. Manipulating the macroscopic coherence provides a new level of control over nitrogen air lasing and offers new perspectives for the generation of very short wavelength laser where population inversion is difficult to achieve.
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.