Robust and ultralow-energy-threshold ignition of a lean mixture by an ultrashort-pulsed laser in the filamentation regime

Abstract: Ultrashort laser pulse filamentation in air can extend the delivery of focused laser energy to distances greatly exceeding the Rayleigh length. In this way, remote measurements can be conducted using many standard methods of analytical spectroscopy. The performance of spectroscopic techniques can be enhanced by temporal gating, which rejects the unwanted noise and background. In the present work, we investigate the thermal relaxation of air in the wake of single-filament plasmas using shadowgraphy. We demonstrate that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas. This method can augment temporal gating of simple optical detectors.

Abstract: Unstable random multi-filament distribution by free propagation of high power femtosecond laser in transparent medium is unfavorable to the filamentation applications and controllable regular multi-filament array is desired. Here, we present a novel method to modulate the spatial distribution of multi-filament by a phase plate-assisted interference of two delayable femtosecond laser pulses. The experimental results show that regular femtosecond laser multifilament array can be realized and the array can be conveniently converted from one patten to another with the varying of the delay of the two laser pulses. It is found that the spatiotemporal phase variation of the interference beam is responsible for this control process. This work provides a new way to regulate the spatial distribution of multi-filament by simply varying relative phase of the two laser beams and holds a great flexibility for a filament array in many applications.

Abstract: Association reactions by femtosecond laser filamentation in gaseous C2H4 were studied by time-of-flight mass spectrometry of neutral reaction products. Direct sampling from the reaction cell to a mass spectrometer via a differential pumping stage allowed the identification of various hydrocarbon molecules C n H m with n = 3-7 and m = 4-7, which includes species not observed in the previous studies. It was found that products containing three and four carbon atoms dominate the mass spectrum with smaller yields for higher-mass species, suggesting that carbon chain growth proceeds through the reaction with C2H4 in the reaction cell. The product distribution showed a clear dependence on the laser pulse energy for filamentation.

Abstract: Laser ablation ignition is an efficient approach in igniting lean-fuels with significantly reduced minimum pulse energy (MPE), but it requires a precise position control between the laser focal spot and the target surface, and is hardly for long-term use due to laser-induced surface damage. Here we adopt femtosecond filamentating laser to ablate a tantalum target for igniting a premixed methane-air mixture, and demonstrate the experimental realization of lean-fuel ignition over 1-m distance with 100 % success rate and ultralow MPE at ∼1 mJ. We further show that this technique is insensitive to the ablator position and can operate at a durable time with a little amount (∼80 ng/pulse) of sample removal from the target surface. By characterizing the evolution of flame kernel using time-resolved spectroscopy, we clarify the ultralow MPE ignition mechanism, in which the exothermal chemical reactions occurring inside the plasma filament effectively extends the kernel reaction zone, resulting in a plasma-channel-assisted ablation ignition.

Abstract: We investigate lasing of a nitrogen gas induced by intense femtosecond laser pulses at around 400 nm. By examining both the self-induced and externally seeded forward emission spectra, we unambiguously identify the ${{\mathrm{N}}_{2}}^{+}$ lasing actions at 427.8 and 423.6 nm assigned respectively to the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}\ensuremath{-}X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ (0, 1) and (1, 2) emissions and show that the lasing mechanism is totally different from the lasing induced by near-infrared (800 nm) laser pulses, in which the population transfer from $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ to $A^{2}\mathrm{\ensuremath{\Pi}}_{u}$ proceeds through the resonant $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}\ensuremath{-}A^{2}\mathrm{\ensuremath{\Pi}}_{u}$ transition, facilitating the population inversion between the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}\ensuremath{-}X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ states [Phys. Rev. Lett. 123, 203201 (2019)]. We simulate the population distributions among the vibrational levels in the three lowest electronic states of ${{\mathrm{N}}_{2}}^{+}$ by the 400-nm laser pumping and find that the population is efficiently transferred between the $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ state and the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}$ state by a Rabi oscillation combined with a Raman-type transition, leading to the population inversion so that the lasing occurs at the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}\ensuremath{-}X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ (0, 1) and (1, 2) emissions.

Abstract: This paper introduces and discusses the main aspects of ultrashort laser pulse filamentation in various transparent media such as air (gases), transparent solids and liquids. The properties of femtosecond filaments and their applications are presented. Theoretical models developed to explain filaments and the main predictions inferred from these models are reviewed. The various techniques to observe filaments and to measure their characteristics are described. The main measurements of filament features performed so far are reviewed.

Abstract: We review the theoretical and experimental status of intense laser alignment---a field at the interface between intense laser physics and chemical dynamics with potential applications ranging from high harmonic generation and nanoscale processing to stereodynamics and control of chemical reactions After placing the intense laser approach in context with other alignment techniques, we proceed with a discussion of the physics underlying this technique and a description of methods of observing it in the laboratory The roles played by the laser frequency, the pulse duration, and the system temperature are illustrated numerically and experimentally Alignment is extended to three-dimensional orientational control, a method of hindering the rotation about all three axes of polyatomic molecules We conclude with a discussion of potential applications of intense laser alignment

Abstract: When a powerful femtosecond laser pulse propagates in an optical medium, self-focusing occurs. Normally, it is the most powerful part (slice) of the pulse that self-focuses first during its propaga...

Abstract: Laser ignition has become an active research topic in recent years because it has the potential to replace the conventional electric spark plugs in engines that are required to operate under much higher compression ratios, faster compression rates, and much leaner fuel-to-air ratios than gas engines today. It is anticipated that the igniter in these engines will face with pressures as high as 50 MPa and temperatures as high as 4000 K. Using the conventional ignition system, the required voltage and energy must be greatly increased (voltages in excess of 40 kV) to reliably ignite the air and fuel mixture under these conditions. Increasing the voltage and energy does not always improve ignitability but it does create greater reliability problem. The objective of this paper is to review past work to identify some fundamental issues underlying the physics of the laser spark ignition process and research needs in order to bring the laser ignition concept into the realm of reality.

Abstract: Experimental valence ionisation energies of 143 hydrocarbons, CnHm, have been determined from their He (IIα) (40.80 eV) and He (Iα) (21.22 eV) excited photoelectron spectra. Ionization energies, usually up to 26 eV are given in tabular form for 108 hydrocarbons with n≤6, together with their (tentative) assignment. The ionization energies up to approximately 25 eV, of 35 selected hydrocarbons with 7≤n≤10, are presented by means of their He (IIα) photoelectron spectra.