First-principles calculations of heat capacities of ultrafast laser-excited electrons in metals

Ultrafast laser excitation can induce fast increases of the electronic subsystem temperature. The subsequent electronic evolutions in terms of band structure and energy distribution can determine the change of several thermodynamic properties, including one essential for energy deposition; the electronic heat capacity. Using density functional calculations performed at finite electronic temperatures, the electronic heat capacities dependent on electronic temperatures are obtained for a series of metals, including free electron like, transition and noble metals. The effect of exchange and correlation functionals and the presence of semicore electrons on electronic heat capacities are first evaluated and found to be negligible in most cases. Then, we tested the validity of the free electron approaches, varying the number of free electrons per atom. This shows that only simple metals can be correctly fitted with these approaches. For transition metals, the presence of localized d electrons produces a strong deviation toward high energies of the electronic heat capacities, implying that more energy is needed to thermally excite them, compared to free sp electrons. This is attributed to collective excitation effects strengthened by a change of the electronic screening at high temperature

Excitation électronique et relaxation de matériaux soumis à une irradiation laser ultrabrève

La synthèse de mes travaux de recherche est présentée dans la suite de manière à refléter lanature multi-échelle et multi-applicative du travail réalisé, se détachant ainsi du sens chronologiquede sa réalisation. En guise d’introduction, je présenterai le contexte de l’étude puis la secondepartie sera dévolue aux aspects fondamentaux de l’interaction laser ultracourt-solide, mettant enlumière les remarquables degrés d’excitation atteignables et les états de matière exotiques associés.L’excitation d’ondes de surface et les structures périodiques en résultant, est un des thèmes majeursde mon travail actuel et constitue logiquement la troisième partie de ce document de synthèse. Laquatrième partie est consacrée à l’étude des produits d’ablation, avec pour fil conducteur la miseen forme temporelle des impulsions afin d’optimiser l’excitation des espèces en phase plasma. Unecinquième partie se focalise sur l’interaction en volume d’un matériau, en particulier pour le casde diélectriques transitoirement métallisés. La dernière partie dresse une synthèse des travaux etdétaille les actions émergentes pour augmenter la synergie entre le rayonnement et la réponsesouhaitée du matériau irradié

Femtosecond laser irradiation of dielectric materials containing randomly-arranged nanoparticles

International audienceWe investigate femtosecond laser irradiation of dielectric materials containing randomly-arranged nanoparticles. For this, numerical modeling is performed based on three different methods: Mie theory, static solution of linear Maxwell's equations and a solution of nonlinear Maxwell's equations together with kinetic equations for free electron excitation/relaxation processes. First two approaches are used to define the static intensity distribution and to analyze the electromagnetic interaction between the nanoparticles. The third method allows us to investigate the complex dynamics of the laser-matter interaction. Multiphoton absorption is shown to be responsible for electron plasma generation in the regions of strong intensity enhancements in the vicinity of nanoparticles. The irradiation of the dielectric material leads to the elongation of nanoplasmas by the near-field enhancement perpendicular to the laser polarization and to their strong interaction resulting in periodic arrangement. Numerical results shed light on such effects as femtosecond laser-induced nanograting formation

From random inhomogeneities to periodic nanostructures induced in bulk silica by ultrashort laser

International audienceFemtosecond laser-induced volume nanograting formation is numerically investigated. The developed model solves nonlinear Maxwell's equations coupled with multiple rate free carrier density equations in the presence of randomly distributed inhomogeneities in fused silica. As a result of the performed calculations, conduction band electron density is shown to form nanoplanes elongated perpendicular to the laser polarization. Two types of nanoplanes are identified. The structures of the first type have a characteristic period of the laser wavelength in glass and are attributed to the interference of the incident and the inhomogeneity-scattered light waves. Field components induced by coherent multiple scattering in directions perpendicular to the laser polarization are shown to be responsible for the formation of the second type of structures with a subwavelength periodicity. In this case, the influence of the inhomogeneity concentration on the period of nanoplanes is shown. The calculation results not only help to identify the physical origin of the self-organized nanogratings, but also explain their period and orientation

Hydrodynamic simulations of metal ablation by femtosecond laser irradiation

Ablation of Cu and Al targets has been performed with 170 fs laser pulses in the intensity range of 10^12-10^14 W/cm^2. We compare the measured removal depth with 1D hydrodynamic simulations. The electron-ion temperature decoupling is taken into account using the standard "two-temperature model". The influence of the early heat transfer by electronic thermal conduction on hydrodynamic material expansion and mechanical behavior is investigated. A good agreement between experimental and numerical matter ablation rates shows the importance of including solid-to-vapor evolution of the metal in the current modeling of the laser matter interaction

Plasmonic and Hydrodynamic Effects in Ultrafast Laser-Induced Periodic Surface Structures on Metals

International audienceWe report results on the development of laser-induced periodic surface structures produced by ultrashort laser pulses irradiating metallic surfaces. The surface topology features are discussed in terms of periodicity and amplitude contrast of the pattern formation, and in relation to the chrono-logical sequence of laser-induced events. Resonant excitation of Surface Plasmons in metallic grat-ings show that the surface wave excited during the femtosecond laser pulse can initiate the observed patterning. Metallic behavior under nonequilibrium conditions on the picosecond timescale is then investigated to correlate the amount of material experiencing solid-to-liquid transitions and the sub-sequent structure amplitude. With the derived observation, the calculation of the transient nonequi-librium thermodynamic characteristics of excited nickel is performed, allowing to define character-istic timescales of thermocapillary processes which may occur under multi-pulse irradiation

Laser-induced periodic alignment of Ag nanoparticles in soda-lime glass

International audienceOne-, two- or three-dimensional arrays of closely spaced silver nanoparticles may lead to new optical properties, due to short or long range coupling between their resonant surface plasmons, so that the spatially controlled growth of silver nanoparticles provides an efficient way to tune their optical properties. Towards this way, we present here the periodic pattern of a glass surface with silver nanoparticles by continuous ultraviolet laser exposure. The formation of the 160 nm period pattern is well described by an interference-based model which agrees with the experimental conclusions, mainly obtained by various forms of microscopy. Statistical approach based on the autocorrelation function gives quantitative description about the quality of the order in the periodic structure and about the nanoparticles averaged diameter (80 nm). We also present the optical extinction spectrum of the Laser Induced Periodic Surface Structure (LIPSS)-containing area of the glass, which unusually shows several bands in the visible range. The period of 160 nm of the periodic structure is short enough to allow coupling between nanoparticles, which makes it a possible candidate for plasmon-based optical applications

Experimental study of radiative shocks at PALS facility

We report on the investigation of strong radiative shocks generated with the high energy, sub-nanosecond iodine laser at PALS. These shock waves are characterized by a developed radiative precursor and their dynamics is analyzed over long time scales (~50 ns), approaching a quasi-stationary limit. We present the first preliminary results on the rear side XUV spectroscopy. These studies are relevant to the understanding of the spectroscopic signatures of accretion shocks in Classical T Tauri Stars.Comment: 21 pages, 1 table, 7 figure

Ultrafast laser-induced surface complexity patterns at the nanoscale

International audienceUltrashort laser sources (100 fs pulse duration) concentrate a large number of photons in time and space, enabling them to transform and sculpt any solid surface achieving ultimate scales of structuring down to 100 nm [1]. Upon multi-shot irradiation the self-organization and growth of periodic patterns arise from small, localized perturbations of the underlying optical coupling on random surface nanoreliefs. Brought far from equilibrium under successive ultrafast laser pulse photoexcitation, the material progressively exhibits periodic pattern structure that reveal signatures of complexity. Some laser-induced surface structures have periods typic from plasmonic and nonlinear optics origin while others show symmetry breaking characteristics of fluid dynamics, interrogating on the main mechanism that drives the organization nature. In particular, a flat surface turns into a forest of nanopeaks with a remarkably high aspect-ratio (5:1) and a sub-100 nm periodicity. Finally, the one-step approach to fabricate high density of nanocavities or nanopeaks offers a promising way to design and engineer surface properties with nanofeatures surpassing those of any naturally-occurring surfaces with expected innovative applications in biomedecine, metaphotonics and nanocatalysis [2].From the fundamental aspect, one of the challenges is to develop a general model that inherit relevant symmetry and scale invariance properties and that contain the nonlinear dynamics able to reproduce dissipative structures in spatially extended systems. A nonlinear dynamics modelling is proposed to reproduce hydrodynamic fluctuations at the onset of convective instability that we have recently demonstrated as the very nature of the laser-induced self-organized nanopatterns. I will show that the complexity of surface 2D patterns emergence can be finally learned by a deep convolutional network to connect the model coefficients to the experimental irradiation conditions, providing key laser process parameters to design a specific pattern [3].[1] R. Stoian and J.P. Colombier, “Advances in ultrafast laser structuring of materials at the nanoscale”, Nanophotonics 9(16), 4665-4688 (2020). [2] A. Nakhoul, A. Rudenko, C. Maurice, S. Reynaud, F. Garrelie, F. Pigeon, and J.P. Colombier, “Boosted Spontaneous Formation of High-Aspect Ratio Nanopeaks on Ultrafast Laser-Irradiated Ni Surface”, Advanced Science, 9 (21) 2200761 (2022).[3] E. Brandao, A. Nakhoul S. Duffner, R. Emonet, F. Garrelie, A. Habrard, F. Jacquenet, F. Pigeon, M. Sebban. & J.P. Colombier “Learning complexity to guide light-induced self-organized nanopatterns”, Physical Review Letters, in press (2023)

Advancing Laser-Induced Nanoscale Surface Self-Organization with Machine Learning Guidance

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