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Physical Review Letters

Unexpected Sensitivity of Nitrogen Ions Superradiant Emission on Pump Laser Wavelength and Duration

Yi Liu, Pengji Ding, Neven Ibrakovic, Samuel Bengtsson, Shihua Chen, Rostyslav Danylo, Emma R. Simpson, Esben Witting Larsen, Xiang Zhang, Zhengquan Fan, Aurélien Houard, J. Mauritsson, Anne L’Huillier, Cord L. Arnold, Songlin Zhuang, Vladimir T Tikhonchuk, A. Mysyrowicz

Published Nov 17, 2017• 63 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.119.203205

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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.
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.
Theoretical and experimental study of nonresonance generation of laser radiation by atoms, molecules and ions of matter - superradiation - is an important area of research in the field of femtosecond optics. Narrowly directed (forward and reverse), with an intensity proportional to the square of the number of inverted atoms, molecules and ions, radiation can be used to probe the atmosphere using femtosecond lidars. This paper presents the results of comparative experimental studies of the appearance of superradiance at transitions of molecular nitrogen ion in the atmosphere and at transitions of atomic strontium ion in a mixture with inert gases (He, Ne) in a heated cuvette. For the first time, superradiance was observed in the transition from the resonant electronic state 2Р1/2 / to the ground state 2S1/2 (407.8 nm), when Sr pairs were pumped by radiation at a wavelength of 268.1 nm.
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.
Coherent control of the multiple wavelength lasing of N2+{\rm N}_2^ +N2+​: coherence transfer and beyond
Nitrogen molecules pumped by intense femtosecond laser pulses give rise to coherent emission in the forward direction at a series of wavelengths, coined “air lasing.” We demonstrate the coherent control of these emissions via a pair of seeding pulses at two different frequencies at low pressures, revealing a coherence transfer through vibrational motion. It is found that the injection of a 427.8 nm (391.4 nm) seeding pulse results in its amplification at the expense of the 391.4 nm (427.8 nm) signal, demonstrating a competition between the two spectral components of the emission from the upper level population. Moreover, when the delay between the seeding and pump pulses is finely tuned, the coherent control of both transitions is observed via the coherence transfer. A microscopic molecular relaxation model reproduces these observations, highlighting the crucial role of electronic and vibrational coherences, as well as their coupling, during the lasing action.