Hongqiang Xie

Remote creation of coherent emissions in air with two-color ultrafast laser pulses

We experimentally demonstrate the generation of narrow-bandwidth emissions with excellent coherent properties at 391 and 428nm from N + (B 2 6 + (v 0 = 0) ! X 2 6 + g (v = 0,1)) inside a femtosecond filament in air by an orthogonally polarized two-color driver field (i.e. 800nm laser pulse and its second harmonic). The durations of the coherent emissions at 391 and 428nm are measured to be 2.4 and 7.8ps, respectively, both of which are much longer than the duration of the pump and its second harmonic pulses. Furthermore,

Gain dynamics of a free-space nitrogen laser pumped by circularly polarized femtosecond laser pulses

We experimentally demonstrate ultrafast dynamic of generation of the 337-nm nitrogen laser by injecting an external seed pulse into a femtosecond laser filament pumped by a circularly polarized laser pulse. In the pump-probe scheme, it is revealed that the population inversion between the C(3)Π(u) and B(3)Π(g) states of N(2) for the free-space 337-nm laser is firstly built up on the timescale of several picoseconds, followed by a relatively slow decay on the timescale of tens of picoseconds, depending on the nitrogen gas pressure. By measuring the intensities of 337-nm signal from nitrogen gas mixed with different concentrations of oxygen gas, it is also found that oxygen molecules have a significant quenching effect on the nitrogen laser signal. Our experimental observations agree with the picture of electron-impact excitation.

Identification of the physical mechanism of generation of coherent N 2 + emissions in air by femtosecond laser excitation

Recently, amplification of harmonic-seeded radiation generated through femtosecond laser filamentation in air has been observed, giving rise to coherent emissions at wavelengths corresponding to transitions between different vibrational levels of the electronic B(2)Σ(u)(+) and X(2)Σ(g)(+) states of molecular nitrogen ions [Phys. Rev. A. 84, 051802(R) (2011)]. Here, we carry out systematic investigations on its physical mechanism. Our experimental results do not support the speculation that such excellent coherent emissions could originate from nonlinear optical processes such as four-wave mixing or stimulated Raman scattering, leaving stimulated amplification of harmonic seed due to the population inversion generated in molecular nitrogen ions the most likely mechanism.

A self-induced white light seeding laser in a femtosecond laser filament

We report on the generation of remote self-seeding laser amplification by using only one 800 nm Ti:sapphire femtosecond laser pulse. The laser pulse (~40 fs) is first used to generate a filament either in pure nitrogen or in ambient air in which population inversion between the and states of is realized. Self-induced white light inside the filament covering the transition is then serving as the seed to be amplified. The self-induced narrow-band laser at 428 nm has a pulse duration of ~2.6 ps with perfect linear polarization properties. This finding opens new possibilities for remote detection in the atmosphere.

Lasing action induced by femtosecond laser filamentation in ethanol flame for combustion diagnosis

We experimentally demonstrate the generation of lasing action in the laminar ethanol/air flame on an alcohol burner array using femtosecond laser filament excitation. By probing the backward emissions of combustion species, it is found that as the interaction length of the filament in the flame increases, the signals intensity at the 388 nm band for the B2Σ−X2Σ transition of CN increases exponentially, but that at the 474 nm band for A3Πg−X′3Πu transition of C2 increases linearly. The exponential behavior of the CN emissions is ascribed to amplified spontaneous emission, which opens up a way to overcome the quenching effect of specific species in combustion diagnosis.

Alignment-Dependent Fluorescence Emission Induced by Tunnel Ionization of Carbon Dioxide from Lower-Lying Orbitals

The study of the ionization process of molecules in an intense infrared laser field is of paramount interest in strong-field physics and constitutes the foundation of imaging of molecular valence orbitals and attosecond science. We show measurement of alignment-dependent ionization probabilities of the lower-lying orbitals of the molecules by experimentally detecting the alignment dependence of fluorescence emission from tunnel ionized carbon dioxide molecules. The experimental measurements are compared with the theoretical calculations of the strong field approximation and molecular Ammosov-Delone-Krainov models. Our results demonstrate the feasibility of an all-optical approach for probing the ionization dynamics of lower-lying orbitals of molecules, which is until now still difficult to achieve by other techniques. Moreover, the deviation between the experimental and theoretical results indicates the incompleteness of current theoretical models for describing strong field ionization of molecules.

Signature of superradiance from a nitrogen-gas plasma channel produced by strong-field ionization

Recently, Yao et al. demonstrated the creation of coherent emissions in nitrogen gas with two-color (800 nm + 400 nm) ultrafast laser pulses [J. Yao, G. Li, C. Jing, B. Zeng, W. Chu, J. Ni, H. Zhang, H. Xie, C. Zhang, H. Li, H. Xu, S. L. Chin, Y. Cheng, and Z. Xu, New J. Phys. 15, 023046 (2013)]. Based on this two-color scheme, here we report on systematic investigation of temporal characteristics of the radiation emitted at 391 nm [N

Real-time observation of dynamics in rotational molecular wave packets by use of air-laser spectroscopy

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Interferometric characterization of pulse front tilt of spatiotemporally focused femtosecond laser pulses

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Generation of an air laser at extended distances by femtosecond laser filamentation with telescope optics

B{\phantom{\rule{0.16em}{0ex}}}^{2}{\ensuremath{\Sigma}}_{u}^{+}\left(\ensuremath{\nu}=0\right)\ensuremath{-}X{\phantom{\rule{0.16em}{0ex}}}^{2}{\ensuremath{\Sigma}}_{g}^{+}\left(\ensuremath{\nu}=0\right)