Title of seminar: Emission of colloidal quantum dots in dielectric and plasmonic environements

Abstract:

Under weak optical excitation, CdSe/CdS colloidal quantum dots are excellent sources of single photons at room temperature and can be likened to a two-level system. However with stronger excitation, their spectrum broaden, the emission intensity varies non-linearly with power, so that such a single quantum dot can no longer be assimilated to a two-level system.We modelize the emission of an isolated quantum dot, demonstrating that its spectrum remains discrete at lower energies and presents a continuum at higher energies. We used a statistical description of energy level occupancy giving  the chemical potential and a temperature T describing the carriers population  in the linear and non linear regimes of emission.

Moreover when one single quantum dot is placed into a plasmonic cavity, like a patch nano-antenna (a gold substrate below a gold patch and separated by a thin dielectric layer),  the emission of such quantum dots changes dramatically. Features as emission acceleration or directionality are signature of coupling between quantum dot emission and plasmons. We will here analyse this coupling in the case of one single patch antenna and discuss the case of an array of antennas, exhibiting narrow lines surface lattice resonance.

I will shortly present  the realisation of whispering gallery mode laser, based on the interaction between dielectric poymeric microcavities et CdS/CdSe/CdS quantum shells

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

Title of seminar: A more rational microscope for quantum gases

Abstract:

Since its first demonstration in 2009-2010, the detection of individual atoms in quantum gases by means of in-situ fluorescence microscopy has been established as a sensitive probe of quantum correlations in these systems, enabling an impressive series of experimental successes. After more than 15 years of service, however, it is fair to say that this detection method is still not an option for most teams because it imposes stringent constraints on the experimental setup, and requires several expensive pieces of equipment, such as powerful monomode CW lasers, a high-NA and aberration-free objective, a sensitive camera, etc.
In this seminar, I will introduce our implementation of a significantly `cheaper’ fluorescence microscope. This advance was made possible by choosing a favorable atomic species (strontium 84), simplifying the imaging system, and improving the algorithm used to detect the atoms in the images. Whether used separately or together, these new developments should contribute to democratising the use of fluorescence microscopy in quantum gases.

Title of seminar: The Pseudopotential Approximation

Abstract:

The treatment of atoms and molecules containing a large number of electrons remains a major challenge in quantum chemistry. Conventional Hartree–Fock (HF) and density functional theory (DFT) calculations generally exhibit steep computational scaling with the number of basis functions. Despite the exponential increase in computer power over recent decades – which had already reached the teraflop regime by the turn of the century – and the development of sophisticated algorithms and approximations for HF, post-HF, and DFT methods, the accurate treatment of large and heavy molecules remains computationally demanding. In some cases, advanced approaches can achieve nearly linear scaling with the number of atoms, but further approximations are still required when large systems are to be treated efficiently without significant loss of accuracy for the properties of interest.

One such approximation is the pseudopotential approximation (PPA), also known as the effective core potential (ECP) method. It was originally introduced by Hans G. A. Hellmann in 1934 as the kombiniertes Näherungsverfahren (combined approximation method). The PPA is based on the fundamental chemical concept that, for many purposes, the valence electrons are primarily responsible for chemical bonding and reactivity, while the chemically inert core electrons can be treated implicitly. An important additional advantage is that relativistic effects can be incorporated into pseudopotentials in a computationally economical manner.

This talk provides a critical overview of modern pseudopotential schemes, focusing on their accuracy, their limitations, and the circumstances under which they should – or should not – be employed.

References:

M. Dolg and X. Cao, “Relativistic Pseudopotentials – Their Development and Scope of Applications“, Chem. Rev. 112, 403-480 (2012).
P. Schwerdtfeger, “The Pseudopotential Approximation in Electronic Structure Theory”, Chem. Phys. Chem. 12, 3143-3155 (2011).
P. Schwerdtfeger, T. Fischer, M. Dolg, G. Igel-Mann, A. Nicklass, H. Stoll, A. Haaland, « The Accuracy of the Pseudopotential Approximation. I. An Analysis of the Spectroscopic Constants for the Electronic Ground States of InCl and InCl3 », J. Chem. Phys. 102, 2050-2062 (1995).
P. Schwerdtfeger, B. Assadollahzadeh, J. R. Cheeseman, U. Rohrmann, and R. Schäfer, “Breakdown of the pseudopotential approximation for magnetizabilities and electric multipole moments – Test calculations for Au, AuF and Snn cluster (n ≤ 20)”, J. Chem. Phys. 134, 204102-1-11 (2011).