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arXiv:1910.13596 · Physical Review A

Extremely nonlinear Raman interaction of an ultrashort nitrogen ion laser with an impulsively excited molecular wave packet

Zhaoxiang Liu, Jinping Yao, Haisu Zhang, Bo Xu, Jinming Chen, Fangbo Zhang, Zhihao Zhang, Yuexin Wan, Wei Chu, Zhenhua Wang, Ya Cheng

Published Apr 13, 2020• 23 citations
arXivHTMLPDF

Abstract

We report on the generation of cascaded rotational Raman scattering up to 58th order in coherently excited $\mathrm{C}{\mathrm{O}}_{2}$ molecules. Raman scattering with more than 600 sidebands is obtained using an intense femtosecond laser to impulsively excite rotational coherence and femtosecond-laser-induced ${{\mathrm{N}}_{2}}^{+}$ lasing to generate cascaded Raman signals. This configuration allows this experiment to be performed with a single femtosecond laser beam at free-space standoff locations. It is revealed that the efficient spectral extension of Raman signals is attributed to the specific structures of ${{\mathrm{N}}_{2}}^{+}$ lasing in the spectral and temporal domains and the ideal spatial overlap of the femtosecond laser and ${{\mathrm{N}}_{2}}^{+}$ lasing. A Raman spectrum extending above $2000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wave packets.

DOI
10.1103/physreva.101.043404

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We report on the generation of cascaded rotational Raman scattering up to 58th order in coherently excited CO2\mathrm{C}{\mathrm{O}}_{2}CO2​ molecules. Raman scattering with more than 600 sidebands is obtained using an intense femtosecond laser to impulsively excite rotational coherence and femtosecond-laser-induced N2+{{\mathrm{N}}_{2}}^{+}N2​+ lasing to generate cascaded Raman signals. This configuration allows this experiment to be performed with a single femtosecond laser beam at free-space standoff locations. It is revealed that the efficient spectral extension of Raman signals is attributed to the specific structures of N2+{{\mathrm{N}}_{2}}^{+}N2​+ lasing in the spectral and temporal domains and the ideal spatial overlap of the femtosecond laser and N2+{{\mathrm{N}}_{2}}^{+}N2​+ lasing. A Raman spectrum extending above 2000cm\ensuremath−12000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}2000cm\ensuremath−1 naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wave packets.
When exposed to intense infrared laser pulses of femtosecond duration, molecules such as N2{}_{2}2​, CO2{}_{2}2​, and H2{}_{2}2​O not only become ionized but can also produce lasers as a result of simultaneous population inversion. Scientists look deep into this ``air lasing'' phenomenon in N2{}_{2}2​ gas and uncover both quantum coherence in the rotational wave packets of the lasing molecular ions and its footprint in the air-laser spectrum.
N2+{\mathrm{N}}_{2}^{+}N2+​ lasing action involves the creation, evolution, and radiation of ions in intense laser fields. In this paper, we demonstrate that N2+{\mathrm{N}}_{2}^{+}N2+​ lasing intensity can be enhanced by several times or totally suppressed by controlling the ionic creation and radiation processes through manipulation of the coherent molecular rotational wave packets of N2{\mathrm{N}}_{2}N2​ and N2+{\mathrm{N}}_{2}^{+}N2+​ with a pump-control scheme. Our results show that if a weak control pulse arrives at the revival moments of the ionic rotational wave packet, the lasing signal can be notably promoted, which is in contrast to observations in previous studies. Meanwhile, the modulation profiles of the lasing signals caused by molecular alignments of N2{\mathrm{N}}_{2}N2​ and N2+{\mathrm{N}}_{2}^{+}N2+​ are analyzed at length, revealing the double roles of the weak control pulse in inducing molecular alignment of N2{\mathrm{N}}_{2}N2​ or monitoring the amplification of the N2+{\mathrm{N}}_{2}^{+}N2+​ system by controlling electronic and rotational coherence. Moreover, by performing a Fourier transformation of the delay-dependent N2+{\mathrm{N}}_{2}^{+}N2+​ lasing, the rotational wave-packet information of N2{\mathrm{N}}_{2}N2​ and N2+{\mathrm{N}}_{2}^{+}N2+​ is simultaneously retrieved.
Subfemtosecond pulses can be generated in a Raman generator by using electromagnetically induced transparency if ultrabroad Raman sidebands are generated and then propagate in such a way that the generated sidebands at higher frequencies are temporally advanced with regard to those at lower frequencies. However, it was recently demonstrated by experiments [1] that above-resonance Raman excitations ensured the establishment of anti-phased Raman coherence for pulse compression but only very limited number of Raman sidebands could be generated due to Raman self-defocusing [2], while broadband sidebands could be generated under below-resonance Raman excitations, which supported no pulse compression in the same Raman media. In order to establish anti-phased Raman coherence and simultaneously overcome Raman self-defocusing to benefit efficient generation of broadband sidebands, we suggest in this report a new technique to generate sub-fs pulse train in a Raman generator driven by three sufficiently intense single-mode laser beams whose carrier frequencies are tuned off the Raman resonance with two different two-photon detunings. The Raman interaction establishes a molecular modulation through nonadibatic processes. The simulation on the basis of the fundamental rotational Raman transitions in H2 shows that sub-fs pulse can be generated by above-resonance or below-resonance Raman excitations, or even Raman excitations with opposite detunings, as is shown in Fig.1. This opens a new way for experiments to observe enhanced generation of broadband Raman sidebands, together with compression of the phase-locked Raman sidebands into sub-fs pulse trains in the same Raman media
Subfemtosecond (sub-fs) pulses can be generated by cascade-stimulated Raman scattering in a Raman medium with modulated Raman excitations, driven by two sufficiently intense laser beams, one of which is amplitude modulated. The nonadiabatic Raman interaction establishes a strong modulated Raman coherence, which supports compression of the generated broadband Raman sidebands to a train of sub-fs pulses regardless of whether the carrier frequencies of the driving lasers are tuned above, below or on two-photon Raman resonance.
Laser control of molecular rotation is an area of active research. A number of recent studies has aimed at expanding the reach of rotational control to extreme, previously inaccessible rotational states, as well as controlling the directionality of molecular rotation. Dense ensembles of molecules undergoing ultrafast uni-directional rotation, known as molecular superrotors, are anticipated to exhibit unique properties, from spatially anisotropic diffusion and vortex formation to the creation of powerful acoustic waves and tuneable THz radiation. Here we describe our recent progress in controlling molecular rotation in the regime of high rotational excitation. We review two experimental techniques of producing uni-directional rotational wave packets with a "chiral train" of femtosecond pulses and an "optical centrifuge". Three complementary detection methods, enabling the direct observation, characterization and control of the superrotor states, are outlined: the one based on coherent Raman scattering, and two other methods employing both resonant and non-resonant multi-photon ionization. The capabilities of the described excitation and detection techniques are demonstrated with a few examples. The paper is concluded with an outlook for future developments.