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XXVIII Международный симпозиум «Оптика атмосферы и океана. Физика атмосферы»

Superradiance in resonant electron transitions of molecular nitrogen ions in air and atomic strontium ions at optical pumping of a FS laser with a wavelength of 800 nm

Gennady G. Matvienko, В. Е. Прокопьев, Viktor K. Oshlakov, A. N. Iglakova

Published Dec 18, 2019• 0 citations

Abstract

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.

DOI: 10.1117/12.2540765

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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.

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 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 perform a combined theoretical and experimental investigation of the superradiance in the quantum coherent system generated by strong laser fields. The semiclassical theory of superradiance that includes the superradiant temporal profile, character duration, time delay, intensity is derived. The experimental data and theoretical predictions of 391-nm forward emission as a function of nitrogen gas pressure are compared and show good agreement. Our results not only demonstrate that the time-delayed optical amplification inside the molecular nitrogen ions is superradiance, but also reveal the quantum optical properties of strong-field physics.

We perform a combined theoretical and experimental investigation of the superradiance in the quantum coherent system generated by strong laser fields. The semiclassical theory of superradiance that includes the superradiant temporal profile, character duration, time delay, intensity is derived. The experimental data and theoretical predictions of 391-nm forward emission as a function of nitrogen gas pressure are compared and show good agreement. Our results not only demonstrate that the time-delayed optical amplification inside the molecular nitrogen ions is superradiance, but also reveal the quantum optical properties of strong-field physics.

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.