Title of seminar: TALIF diagnostics coupled to (ultra)fast detectors for probing atomic species in reactive plasmas
Abstract:
Recent advances in laser-based diagnostics have opened new avenues for the quantification of fundamental parameters in plasmas (electron density, electric field magnitude, species densities, etc.). Particularly, two-photon absorption laser induced fluorescence (TALIF) is among the most used diagnostics [1, 2]. This is because it allows determining reactive atom densities in situ with excellent spatiotemporal resolution and sensitivity.
Recently, the growing accessibility to ultrafast lasers (namely, picosecond (ps) and femtosecond (fs) lasers) has triggered a wider implementation of ps– and fs–TALIF in reactive plasmas [1, 3–5]. Their pulse duration is significantly smaller than the decaying phases of common TALIF signals (from ns to sub-ns timescales), allowing them to efficiently resolve the decay times of laser-excited states. Thus, they overcome the limits of traditional ns–TALIF [1, 6]. A factor that determines the reliable use of ultrafast lasers is the laser intensity (IL; units: W/cm2). In most cases, this must be kept small enough to avoid saturated fluorescence regimes due to substantial depletion of the species ground state by the laser, and the laser-excited states by photoionization (PIN) and stimulated emission (SE) [1, 2]. To avoid saturation, the TALIF intensity should scale quadratically versus IL (quadratic regime). Intrinsically, ps–TALIF is conducted in the quadratic regime (IL<GW/cm2) being ideal for quenching rate measurements, while fs–TALIF operates at much higher IL (few TW/cm2), entering saturated regimes where classic calibration schemes of ns– and ps–TALIF are not valid [1–5].
At pressures between 0.1–1 bar (or higher), the effective lifetime of excited states can fall to 100 ps or lower [3, 4]. Ideally, directly capturing ultrafast decays requires detectors with exceptional temporal resolution. A streak camera is a representative case, reaching highest time resolutions of a few ps [4]. However, its use in ps-TALIF studies requires careful consideration of their peculiarities [4]. In fs–TALIF its use seems not mandatory since fs–TALIF reaches the quench free regime where PIN and SE dominate the depletion of the excited state [2, 5].
Finally, in the classic quadratic regime approach, large systematic errors (up to about 60%) may obscure the density measurements [1]. These essentially originate in the ratio of two-photon absorption cross sections of the reactive (H, N, O, etc.) and the calibrating (Kr, Xe) species; these have been initially measured with ns lasers and are not sufficiently accurate [6]. Recent studies have tried to tackle this issue, but the discussion is still ongoing whether the initially published values are valid and accurate for the ps and fs regimes as well. An accurate and independent determination of the cross-section ratio in ps– and fs–TALIF regimes would be ideal and could significantly improve the accuracy of the corresponding TALIF density measurements.
Overall, this talk will contain recent developments in TALIF techniques for determining absolute densities of essential reactive species (N–, O–, H–atoms, …) in laboratory plasmas. It outlines fundamental principles and essential requirements for proper implementation of ns–, ps– and fs–T ALIF, drawing on representative studies from literature. The discussion also addresses factors that limit accurate density measurements, such as gas pressure in the probed discharge (which enhances quenching of excited states and limits their effective lifetime), and effects related to high instantaneous power density of the lasers used, including photodissociation, SE, and PIN. The promises of ultrafast laser regimes for overcoming the limits of classic ns–TALIF will be also pointed out. Finally, the necessity and peculiarities of ultrafast detectors for capturing actual fluorescence signals and enhancing the detection limits of conventional detectors will be discussed.
References:
[1] K. Gazeli, G. Lombardi, X. Aubert, C.Y. Duluard, S. Prasanna, K. Hassouni 2022 Plasma 4 1 pp 145
[2] G.B. Stancu 2020 Plasma Sources Sci. Technol. 29 pp 054001
[3] A.C. Siby, D. Stefas, Y. Agha, L. Invernizzi, K. Gazeli, G. Lombardi, K. Hassouni, S. Prasanna, Phys. Plasmas 31 pp 033506
[4] L. Invernizzi, C.Y. Duluard, H. Höft, K. Hassouni, G. Lombardi, K. Gazeli, S. Prasanna 2023 Meas. Sci. Technol. 34 pp 095203
[5] A. Brisset, C. Pascual-Fort, N.Q. Minesi, N. De Oliveira, G.D. Stancu 2025 Front. Phys. 13
[6] K. Niemi, V.S. der Gathen, H.F. Döbele 2001 J. Phys. D.: Appl. Phys. 34 pp 2330–5
