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| 2. |
- Ding, Pengji, et al.
(författare)
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Bidirectional Flame Lasing
- 2018
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Ingår i: Northern Optics & Photonics 2018 12-14 September 2018 Lund University, Lund, Sweden : Conference Proceedings - Conference Proceedings. - 9789163964886
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 3. |
- Ding, Pengji, et al.
(författare)
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Detection of atomic oxygen in a plasma-assisted flame via a backward lasing technique
- 2019
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Ingår i: Optics Letters. - 0146-9592. ; 44:22, s. 5477-5480
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Tidskriftsartikel (refereegranskat)abstract
- In this Letter, we have investigated 845 nm lasing generation in atomic oxygen, present in a lean methane-air flame, using two-photon pumping with femtosecond 226 nm laser pulses, particularly focusing on the impact of nanosecond repetitively pulsed glow discharges forcing on the backward lasing signal. Characterizations of the backward lasing pulse, in terms of its spectrum, beam profile, pump pulse energy dependence, and divergence, were conducted to establish the presence of lasing. With plasma forcing of the flame, the backward lasing signal was observed to be enhanced significantly, ∼50%. The vertical concentration profile of atomic oxygen was revealed by measuring the backward lasing signal strength as a function of height in the flame. The results are qualitatively consistent with results obtained with two-dimensional femtosecond two-photon-absorption laser-induced fluorescence, suggesting that the backward lasing technique can be a useful tool for studies of plasma-assisted combustion processes, particularly in geometries requiring single-ended standoff detection.
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| 4. |
- Ding, Pengji, et al.
(författare)
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Femtosecond laser-induced quantum-beat superfluorescence of atomic oxygen in a flame
- 2021
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Ingår i: Physical Review A. - 2469-9926. ; 104:3
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Tidskriftsartikel (refereegranskat)abstract
- Among different approaches to generate mirrorless lasing, resonant multiphoton pumping of gas constituents by deep-UV laser pulses exhibits so far the highest efficiency and produces measurable lasing energies, but the underlying mechanism was not yet fully settled. Here, we report lasing generation from atomic oxygen in a methane-air flame via femtosecond two-photon excitation. Temporal profiles of the lasing pulses were measured for varying concentrations of atomic oxygen, which shows that the peak intensity and time delay of the lasing pulse approximately scales as N and 1/N, respectively, where N represents the concentration. These scaling laws match well with the prediction of oscillatory superfluorescence (SF), indicating that the lasing we observed is essentially SF rather than amplified spontaneous emission. In addition, the quantum-beating effect was also observed in the time-resolved lasing pulse. A theoretical simulation based on nonadiabatic Maxwell-Bloch equations well reproduces the experimental observations of the temporal dynamics of the lasing pulses. These results on fundamentals should be beneficial for the better design and applications of lasing-based techniques.
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| 5. |
- Ding, Pengji, et al.
(författare)
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Femtosecond two-photon-excited backward lasing of atomic hydrogen in a flame
- 2018
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Ingår i: Optics Letters. - 0146-9592. ; 43:5, s. 1183-1186
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Tidskriftsartikel (refereegranskat)abstract
- We report on an observation of bi-directional 656 nm lasing action of atomic hydrogen in a premixed CH4∕air flame induced by resonant femtosecond 205 nm two-photon excitation. In particular, the backward-propagating lasing pulse is characterized in the spatial and temporal domains for the sake of a single-ended diagnostic. Its picosecond-scale duration and smooth temporal profile enable spatially resolved detection of hydrogen atoms in the millimeter range, which is successfully demonstrated using two narrow welding flames.
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| 6. |
- Ding, Pengji, et al.
(författare)
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Gain mechanism of femtosecond two-photon-excited lasing effect in atomic hydrogen
- 2019
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Ingår i: Optics Letters. - 1539-4794. ; 44:9, s. 2374-2377
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Tidskriftsartikel (refereegranskat)abstract
- By aiming to establish single-ended standoff combustion diagnostics, bidirectional lasing emissions of atomic hydrogen at 656 nm wavelength have been generated via two-photon resonant excitation by focusing 205 nm femtosecond laser pulses into a premixed CH4/O2 flame. The forward lasing strength is approximately one order of magnitude stronger than that of the backward one, due to the geometry of traveling wave excitation over a 2-mm-long pencil-shaped gain volume and the short gain lifetime of 3.5 ps. The gain coefficient of hydrogen lasing was determined to approximate 52/cm. As for the underlying physics of hydrogen lasing, amplified spontaneous emission (ASE) occurs simultaneously with four-wave mixing (FWM), and ASE dominates in the forward direction, whereas the backward lasing is virtually only ASE.
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| 7. |
- Ding, Pengji, et al.
(författare)
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Nonadiabaticity of cavity-free neutral nitrogen lasing
- 2017
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Ingår i: Physical Review A. - 2469-9926. ; 96:3
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Tidskriftsartikel (refereegranskat)abstract
- We report on a theoretical study of cavity-free lasing of neutral nitrogen molecules in femtosecond laser filaments with a nonadiabatic Maxwell-Bloch model, compared with recent pump-seed experiments. The nonadiabaticity of the lasing process is revealed and it is found that electron-neutral collisions dominate the dipole dephasing rate. Moreover, we show that the asymmetry between forward and backward lasing not only depends on the different amplification lengths but also on the temporal dynamics of electron-neutral collisions. The comparison of the nonadiabatic model with simulations based on the adiabatic approximation (such as radiative transfer equations) explicitly sets a bound on the validity of the latter model for cavity-free nitrogen lasing phenomenon, which holds a unique potential in optical remote sensing applications.
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| 8. |
- Ding, Pengji, et al.
(författare)
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Picosecond backward-propagating lasing of atomic hydrogen via femtosecond 2-photon-excitation in a flame
- 2018
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Ingår i: CLEO : QELS_Fundamental Science, CLEO_QELS 2018 - QELS_Fundamental Science, CLEO_QELS 2018. - 9781557528209 ; Part F93-CLEO_QELS 2018
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Konferensbidrag (refereegranskat)abstract
- We report on the observation of backward-propagating 656-nm lasing of atomic hydrogen in flame using 205-nm femtosecond laser pulses. It shows a donut-shaped spatial mode and smooth temporal profile, suggesting spatially-resolved measurements in few-millimeters resolution.
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| 9. |
- Ding, Pengji, et al.
(författare)
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Signature of femtosecond laser-induced superfluorescence from atomic hydrogen
- 2022
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Ingår i: Physical Review A. - 2469-9926. ; 105:1
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Tidskriftsartikel (refereegranskat)abstract
- Cavity-free lasing generation from gas constituents has been studied in the past decade since it promises great potentials in remote sensing and optical diagnostics techniques. Here we report on experimental investigations of temporal characteristics of H-atom lasing emission at 656 nm by examining the dependences of its durations and delays on the pump-laser-pulse energies. An indirect measurement was also performed to test the delays of the lasing pulse for varying H-atom concentrations. The results show that the lasing pulse exhibits considerable superfluorescence signatures. Analysis based on experimental parameters by using deductive expressions of superfluorescence theory shows good agreement to this conjecture. Our investigations on fundamentals could pave the way to a better understanding of the lasing generation and further applications of lasing-based optical diagnostics.
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| 10. |
- Ding, Pengji, et al.
(författare)
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Temporal dynamics of femtosecond-TALIF of atomic hydrogen and oxygen in a nanosecond repetitively pulsed discharge-assisted methane-air flame
- 2021
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 54:27
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Tidskriftsartikel (refereegranskat)abstract
- The temporal dynamics of the spatial distribution of atomic hydrogen and oxygen in a lean methane-air flame, forced by a nanosecond repetitively pulsed discharge-induced plasma, are investigated via femtosecond two-photon absorption laser-induced fluorescence technique. Plasma luminescence that interferes with the fluorescence from H and O atoms was observed to decay completely within 15 ns, which is the minimum delay required for imaging measurements with respect to the discharge occurrence. During discharge, H atoms in the excited state rather than the ground state, produced by electron-impact dissociation processes, are detected at the flame front. It was found that the temporal evolution of H and O fluorescence intensity during a cycle of 100 µs between two discharge pulses remains constant. Finally, the decay time of O-atoms produced by the discharge in the fresh methane-air mixture was about 2 µs, which suggests a faster reaction between O-atoms and methane than in air.
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