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Physical Review A

Stimulated-Raman-scattering-assisted superfluorescence enhancement from ionized nitrogen molecules in 800-nm femtosecond laser fields

Zhiming Miao, Xunqi Zhong, Linlin Zhang, Wei Zheng, Yunan Gao, Yunquan Liu, Hongbing Jiang, Qihuang Gong, Chengyin Wu

Published Sep 4, 2018• 16 citations
Publisher

Abstract

We have observed collective spontaneous emission (superfluorescence) from ionized nitrogen molecules generated by 800-nm laser pulses. The superfluorescence intensity around 391 nm was found to be greatly enhanced when injecting an external seed with various wavelengths. Our finding provides insights into controlling the coherent emission from air filamentation induced by intense femtosecond laser fields, which has great potential application in remote atmospheric sensing.

DOI
10.1103/physreva.98.033402

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arXiv:1705.02635•May 7, 2017•0 citations
We have observed collective spontaneous emission (superfluorescence) from ionized nitrogen molecules generated by 800-nm laser pulses. The superfluorescence intensity around 391 nm was found to be greatly enhanced when injecting an external seed with various wavelengths. Our finding provides insights into controlling the coherent emission from air filamentation induced by intense femtosecond laser fields, which has great potential application in remote atmospheric sensing.
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.
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 ions pumped by intense femtosecond laser pulses present an optical gain at 391.4 nm, evident by energy amplification of an injected resonant seeding pulse. We report a time-resolved measurement of the amplification process with seeding pulses having varying intensities. It is found that the amplification factor depends on the intensity of the seeding pulse and the effective temporal window for the optical gain becomes longer by applying more intense seeding pulses. These two features are in sharp contrast with classic pump-probe experiments, pinpointing the crucial role of macroscopic coherence and its dynamics during the lasing process. We further measure the temporal profile of the amplified emission for seeding pulse injected at different time delays. A complicated temporal behavior is observed, which highlights the nature of the superfluorescence.
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.
We report on an investigation of simultaneous generation of several narrow-bandwidth laser-like coherent emissions from nitrogen molecular ions ( ) produced in intense mid-infrared laser fields. With systematic examinations on the dependences of coherent emissions on gas pressure as well as several laser parameters including laser intensity, polarization and wavelength of the pump laser pulses, we reveal that the multiple coherent emission lines generated in originate from a series of nonlinear processes beginning with four-wave mixing, followed with stimulated Raman scattering. Our analyses further show that the observed nonlinear processes are greatly enhanced at the resonant wavelengths, giving rise to high conversion efficiencies from the infrared pump laser pulses to the coherent emission lines near the transition wavelengths between the different vibrational energy levels of ground X and that of the excited B states.