Self-portrait of Van Gogh obtained by a nanostructured surface with a femtosecond laser (Dusser et al. Opt. Express 2010).
Postdoctoral Research Project 2: Determination of Transient Thermodynamic and Transport Properties of Femtosecond Laser Irradiated Materials
2012-: Université Jean Monnet - Laboratoire Hubert Curien
Supervisors: Dr J.-P. Colombier, Dr R. Stoian
Femtosecond lasers consist in a specific type of laser with ultrashort irradiations of few tens of femtoseconds. The corresponding technologies are used for material ablation and for material surface nanostructuring. They are now involved in a growing number of applications in the fields of wettability or surface colorized information. Due to the ultrashort timescales involved with these devices, the physic beyond the material response is specific and can approximately be described by three steps. 1) First, a significant part of the incident laser beam is absorbed by the electrons of the material, leading to their massive inhomogeneous excitation. 2) In a second step, the excited electrons exchange energies between themselves with electron-electron collisions and reach a so-called thermalized state, i.e. a Fermi-Dirac distribution of the electrons within the electronic structure. For intense irradiations, the thermalization starts during the laser irradiation and can be reached in few tens of additional femtoseconds after the end of the irradiation. At this stage, the concept of electronic temperature is effective, and the corresponding electronic temperature largely exceeds the lattice temperature, defining a nonequilibrium state. 3) To further timescales, the hot electrons release their energy towards the lattice through the electron-phonon coupling, progressively leading to a decrease of the electronic temperature while the lattice temperature increases. This last step lasts up to the return of the equilibrium and up to the total dissipation of the energy due to electronic or thermal conductivities. The surface nanostructuring is the result of the conjugation of these three steps, and accordingly one needs an accurate understanding and description of them in order to better control the structuring. Due to timescales and lengthscales involved, the theoretical methods have proven to be efficient and suitable to quantify the elementary mechanisms describing these stages. Accordingly, we determined thermodynamic and transport properties for materials in nonequilibrium conditions with nonequilibrium density functional theory calculations with the Abinit code.
The two temperature model is generally employed to describe the conditions undergone by the material during the irradiation, it is based on the approximation that electrons are rapidly thermalized. This model relies on a source term describing the quantity of energy absorbed by the material from the laser beam. This part is conditioned by the material intrinsic transport properties, i.e. its optical properties. The following increase of the electronic temperature as well as energy exchange are described by electronic temperature dependent parameters like the electronic heat capacity and the electron-phonon coupling. They give access to the corresponding electronic and ionic pressure. These parameters were available for a series of metals, but with a constant electronic structure, i.e. assuming it is not impacted by the massive excitation process. For a series of metals, as well as for a stainless structure, we determined electronic temperature dependent density of states and from these calculations we derived the following electronic temperature dependent thermodynamic parameters: 1) electronic chemical potentials, 2) electronic heat capacities, 3) electronic pressures, 4) electron-phonon coupling constants, and 5) free electrons numbers [1-4]. These parameters can differ significantly from the non-dependent ones, specially at high temperatures. Indeed, while determining these parameters, a dependence of the d-block energy location on the electronic temperature was also noticed, with consequences on the determined thermodynamic parameters [1]. The shift of the d-block toward lower or higher energy was then explained by changes of the electronic screening induced by modification of the electronic localization during the excitation [2].
2012-: Université Jean Monnet - Laboratoire Hubert Curien
Supervisors: Dr J.-P. Colombier, Dr R. Stoian
Femtosecond lasers consist in a specific type of laser with ultrashort irradiations of few tens of femtoseconds. The corresponding technologies are used for material ablation and for material surface nanostructuring. They are now involved in a growing number of applications in the fields of wettability or surface colorized information. Due to the ultrashort timescales involved with these devices, the physic beyond the material response is specific and can approximately be described by three steps. 1) First, a significant part of the incident laser beam is absorbed by the electrons of the material, leading to their massive inhomogeneous excitation. 2) In a second step, the excited electrons exchange energies between themselves with electron-electron collisions and reach a so-called thermalized state, i.e. a Fermi-Dirac distribution of the electrons within the electronic structure. For intense irradiations, the thermalization starts during the laser irradiation and can be reached in few tens of additional femtoseconds after the end of the irradiation. At this stage, the concept of electronic temperature is effective, and the corresponding electronic temperature largely exceeds the lattice temperature, defining a nonequilibrium state. 3) To further timescales, the hot electrons release their energy towards the lattice through the electron-phonon coupling, progressively leading to a decrease of the electronic temperature while the lattice temperature increases. This last step lasts up to the return of the equilibrium and up to the total dissipation of the energy due to electronic or thermal conductivities. The surface nanostructuring is the result of the conjugation of these three steps, and accordingly one needs an accurate understanding and description of them in order to better control the structuring. Due to timescales and lengthscales involved, the theoretical methods have proven to be efficient and suitable to quantify the elementary mechanisms describing these stages. Accordingly, we determined thermodynamic and transport properties for materials in nonequilibrium conditions with nonequilibrium density functional theory calculations with the Abinit code.
The two temperature model is generally employed to describe the conditions undergone by the material during the irradiation, it is based on the approximation that electrons are rapidly thermalized. This model relies on a source term describing the quantity of energy absorbed by the material from the laser beam. This part is conditioned by the material intrinsic transport properties, i.e. its optical properties. The following increase of the electronic temperature as well as energy exchange are described by electronic temperature dependent parameters like the electronic heat capacity and the electron-phonon coupling. They give access to the corresponding electronic and ionic pressure. These parameters were available for a series of metals, but with a constant electronic structure, i.e. assuming it is not impacted by the massive excitation process. For a series of metals, as well as for a stainless structure, we determined electronic temperature dependent density of states and from these calculations we derived the following electronic temperature dependent thermodynamic parameters: 1) electronic chemical potentials, 2) electronic heat capacities, 3) electronic pressures, 4) electron-phonon coupling constants, and 5) free electrons numbers [1-4]. These parameters can differ significantly from the non-dependent ones, specially at high temperatures. Indeed, while determining these parameters, a dependence of the d-block energy location on the electronic temperature was also noticed, with consequences on the determined thermodynamic parameters [1]. The shift of the d-block toward lower or higher energy was then explained by changes of the electronic screening induced by modification of the electronic localization during the excitation [2].
Electronic temperature dependent density of states, and in insert the difference of electronic densities between the 25000 and 300K calculations. Blue stands for electronic density losses while red corresponds to electronic density gains.
Since the transport properties are dependent on the ionic temperature a different approach is necessary to compute them and to show a possible additional dependence on electronic temperatures. With the Density Functional Theory, the ionic temperature is taken into account by Molecular Dynamic simulations and these calculations requires a significant number of atoms and a significant timesteps in order to reach a thermodynamic pseudo-equilibrium. Once obtained, precise electronic structure calculations are performed on well-chosen ionic configurations from the previous MD run. These calculations are coupled to the Kubo-Greenwood formalism allowing the determination of the optical properties. As for the thermodynamic parameters, the dependence of optical properties on the laser excitation is evaluated by the application of an electronic temperature. Depending of the laser wavelength and the electronic temperature, the optical properties can significantly vary [4,5], modifying in turn the quantity of absorbed energy during the laser pulse, and thus the source term in the two temperature model.
Example of molecular dynamic run (Cu - 3000K).
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Transient optical properties of W.
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This change of optical properties can also have interesting consequences, as for the generation of surface plasmons. Surface plasmons are collective oscillations of charges at the surface of the material that are activated under condition of specific optical properties. For tungsten, a material known for its non-plasmonic characteristic, the evolution of optical properties during the irradiation conditions lead to an activation of its plasmonic condition [5]. These can have interesting applications in the field of opto-electronic for example.
An other interesting phenomenon is the potential ultrafast change of the material structure induced by the charge redistribution originating in the ultrashort laser irradiation. This emerging research field has been studied for Ni with possible phase transitions [6], and for W in the frame of ultrafast destructuring [7].
References:
[1] Manutech-sise web page: http://manutech-sise.universite-lyon.fr/axe-scientifique-4/dynopt-resultats-313294.kjsp
[2] Free-electron properties of metals under ultrafast laser-induced electron-phonon nonequilibrium: A first-principles study, E. Bévillon, J.-P. Colombier, V. Recoules, R. Stoian, Physical Review B, 89, 115117 (2014). DOI: 10.1103/PhysRevB.89.115117
[3] First-principles calculations of heat capacities of ultrafast laser-excited electrons in metals, E. Bévillon, J.-P. Colombier, V. Recoules, R. Stoian, Applied Surface Science, 336, 79-84 (2014). DOI: 10.1016/j.apsusc.2014.09.146
[4] Ab Initio Nonequilibrium Thermodynamic and Transport Properties of Ultrafast Laser Irradiated 316L Stainless Steel, E. Bévillon, J.-P. Colombier, B. Dutta, R. Stoian, The Journal of Physical Chemistry C, 119, 11438-11446 (2015). DOI: 10.1021/acs.jpcc.5b02085
[5] Ultrafast surface plasmonic switch in non-plasmonic metals, E. Bévillon, J.-P. Colombier, V. Recoules, R. Stoian, Physical Review B, 93, 165416 (2016). DOI: 10.1103/PhysRevB.93.165416
[6] First-principles assessment of potential ultrafast laser-induced structural transition in Ni, E. Bévillon, J.-P. Colombier, R. Stoian, Applied Surface Science, 374, 365-369 (2016). DOI: 10.1016/j.apsusc.2015.12.139
[7] Ultrafast destructuring of laser-irradiated tungsten: Thermal or nonthermal process, H. Zhang, C. Li, E. Bevillon, G. Cheng, J. P. Colombier, and R. Stoian, Physical Review B, 94, 224103 (2016). DOI: 10.1103/PhysRevB.94.224103
