Modelling of Ultrafast Laser Processing of Materials
Ultrafast Laser Processing Modelling
Activities-challenges: Investigation of surface modification mechanisms in sub-ablation and sub-melting conditions in various types of materials (i.e. semiconductors, metals, dielectrics), exploration of carrier dynamics in multilayered materials, simulation of incubation effects, role of nonthermal electrons, strain propagation, surface modification at different laser polarization states, ultrafast dynamics at mid-IR, machine learning based approaches.
Surface modification: A desirable effect in the laser-mater processing applications is to control and influence the morphology of the material surface by regulating the way of energy delivery from the laser into the various degrees of freedom of the system. Femtosecond pulsed laser interaction with matter triggers a variety of timescale-dependent processes, influenced by the fluence and pulse duration. A multiscale theoretical investigation is pursued to describe the physical fundamentals and mechanisms that account for the associated experimental observations after single and multiple-pulse ultrashort pulse irradiation and provide a systematic and controllable way of linking the observed surface modification with the applied conditions.
Although surface patterning has been previously investigated upon irradiation with ultrashort pulses in ablation conditions, physics fundamentals of surface modification and a novel surface patterning mechanism for ultrashort pulses have never been addressed in conditions near evaporation (sub-ablation). More specifically, we suggested a new physical mechanism that governs surface patterning formation (i.e. ripples) based on a combination of interference effects (and development of surface plasmon waves) coupled with hydrodynamics capillary induced effects and the dynamics of a superheated liquid layer. The ripple periodicity and morphological changes appear to agree satisfactorily with experimental observations. The model has been revised to allow the description of supra-wavelength structures (grooves) that result from the formation of hydrothermal convection rolls (Fig.1,2). Experimental results supported with theoretical simulations of the underlying physical processes manifest the universality of the mechanisms regardless of the type of the material (Fig.1). [1.3-5,8-20].
Figure 1: SEM picture, and Simulation results [9,13,15].
Figure 2: (a) hydrothermal waves, (b) convection rolls 
Surface modification for complex polarization states: An extension of the model has been performed to explore the role of laser polarization [8,12,14]. More specifically, radial and azimuthal polarisation were considered to elaborate on the effect on the ripple periodicity in various materials (Fig.3 shows subwavelength structure formation in fused silica ).
Figure 3: Rippled profile with a radially (a,c) and azimuthally (b,d) polarized beam 
Out-of-equilibrium electron dynamics and impact to mechanical effects: The significant influence of the contribution of the dynamics of produced nonthermalised electrons to electron thermalisation and electron-phonon interaction is also thoroughly investigated within a range of values of the pulse duration. The consideration of the role of the nonthermal electrons in the thermalisation of the lattice leads to thermomechanical changes compared to the results the traditional Two Temperature Model (TTM) provides (Fig.4) [7,17].
Figure 4: (a) Electronic and lattice temperature profile using the classical TTM and revised TTM, (b) Spatial strain profile simulated TTM and rTTM .
- Out-of-equilibrium electron dynamics: a unification of a DFT approach+ TTM model: To highlight the role of out-of-equilibrium processes for very short pulses a coupling of results from DFTcalculations (evaluation of optical properties) and the classical TTM has been performed to assess the influence of nonthermal electrons in surface damage in 6H-SiC (Fig. 5) .
Figure 5: (a) Reflectivity as a function of the photon energy through DFT calculations, (b) coupling of DFT calculations with TTM to compute carrier density evolution for 6H-SiC.
Ultrafast dynamics and surface modification related effects for mid-IR femtosecond pulses: A detailed theoretical framework was also presented to describes both the ultrafast dynamics and thermal response following irradiation of Silicon/fused silica with ultrashort pulsed lasers in the mid-IR range (Fig.6) . Results for Silicon demonstrated that the Kerr effect is important at lower wavelengths (~2.2 μm) while it leads to substantially large deviations to the maximum lattice temperature reached that it affects the damage threshold. A systematic analysis of the Surface Plasmon dispersion relation for mid-IR revealed that irradiation in the mid-IR region yielded SP that are weakly confined on the surface, exhibit longer lifetimes, and propagate on larger areas. These features can be potentially exploited to promote mid-IR-based technology to produce sensors, detectors or to present new capabilities in laser-based manufacturing.
Figure 6: Irradiation of Silicon with mid-IR femtosecond pulses 
Machine learning-based approaches: Recently, a new activity has been initiated in which machine learning based approaches and predictive modelling are followed to reduce the number of simulated and real experiments (Fig.7).
Figure 7: Machine-learning based approach.
Fuentes-Edfuf Y., Sánchez-Gil J.A., Garcia-Pardo MG., Serna R., Tsibidis G.D., Giannini V., Solis J. and Siegel J., ‘Tuning the period of femtosecond laser induced surface structures in steel: from angled incidence to quill writing ’ Applied Surface Science 493, 948 (2019).
Petrakakis E., Tsibidis G.D., and Stratakis E., ‘Modelling of the ultrafast dynamics and surface plasmon properties of silicon upon irradiation with mid-IR femtosecond laser pulses’ Physical Review B 99, 195201 (2019).
Papadopoulos A., Skoulas E., Mimidis A., Perrakis G., Kenanakis G., Tsibidis G.D.,, and Stratakis E., ‘Biomimetic omnidirectional anti-reflective glass via ultrafast laser nanostructuring’, Advanced Materials 31, (32), 1901123 (2019).
Margiolakis A., Tsibidis G.D., Dani K.M. and Tsironis G.P, ‘Ultrafast dynamics and sub-wavelength periodic structure formation following irradiation of GaAs with femtosecond laser pulses’ Physical Review B 98, 224103 (2018).
Museur L., Tsibidis G.D. Manousaki A., Anglos D., and Kanaev A. ‘Surface structuring of rutile TiO2 (100) and (001) single crystals with femtosecond pulsed laser irradiation’, Journal of Optical Society of America B, 35, 10, 2600 (2018).
Tsibidis G.D., ‘The influence of dynamical change of optical properties on the thermomechanical response and damage threshold of noble metals under femtosecond laser irradiation’, Journal of Applied Physics 123, 085903 (2018).
Tsibidis G.D., ‘Ultrafast dynamics of non-equilibrium electrons and strain generation under femtosecond laser irradiation of Nickel’, Applied Physics A, 124,311 (2018).
Papadopoulos A., Skoulas E., Tsibidis G.D., and Emmanuel Stratakis E., ‘Formation of periodic surface structures on dielectrics after irradiation with laser beams of spatially variant polarisation: a comparative study’, Applied Physics A 124, 146 (2018).
Tsibidis G.D., Mimidis A, Skoulas E., Kirner S.V, Krüger J, Bonse J and Stratakis E., ‘Modelling periodic structure formation on 100Cr6 steel after irradiation with femtosecond-pulsed laser beams’, Applied Physics A 124, 27 (2018).
Zuhlke C., Tsibidis G.D., Anderson T., Stratakis E., Gogos G., and Alexander R.D. (2018), ‘Investigation of femtosecond laser induced ripple formation on copper for varying incident angle’, AIP Advances 8(1):015212.
Gaković B., Tsibidis G.D, Skoulas E., Petrović S.,Vasić B. and Stratakis E., ‘Selective ablation of Ti/Al nano-layer thin film by single femtosecond laser pulse’, Journal of Applied Physics 122, 223106 (2017).
Tsibidis G.D., and Stratakis E., ‘Ripple formation on silver after irradiation with radially polarized ultrashort-pulsed lasers’, Journal of Applied Physics 121, 163106 (2017).
Tsibidis G.D., Skoulas E., A.Papadopoulos, and Stratakis E., ‘Convection roll-driven generation of supra-wavelength periodic surface structures on dielectrics upon irradiation with femtosecond pulsed lasers’, Physical Review B (Rapid Communications) 94, 081305 (2016).
Tsibidis G.D., Skoulas E., and Stratakis E. “Ripple formation on Nickel irradiated with radially polarized femtosecond beams’, Optics Letters, 40 (22), 5172 (2015).
Tsibidis G.D., Fotakis C., and Stratakis E., ‘From ripples to spikes: a hydro-dynamical physical mechanism to interpret femtosecond laser induced self-assembled structures’, Physical Review B (Rapid Communications), 92 ,041405 (2015).
Tsibidis G.D., Stratakis E., Loukakos P.A., and Fotakis C., ‘Controlled ultrashort pulse laser induced ripple formation on semiconductors’, Applied Physics A (Invited Paper), 114:57–68 (2014).
Tsibidis G.D. ‘Thermal response of double-layered metal films after ultrashort-pulsed laser irradiations: the role of nonthermal electron dynamics’, Applied Physics Letters 104, 051603 (2014).
Barberoglou M., Tsibidis G.D., Grey D., Magoulakis M., Fotakis C., Stratakis E., and Loukakos P.A., ‘The influence of ultrafast temporal energy regulation on the morphology of Si surfaces through femtosecond double pulse laser irradiation’, Applied Physics A (Rapid Communications), 113, 273-283 (2013).
Tsibidis G.D., Barberoglou M., Loukakos P.A., Stratakis E., and Fotakis C. ‘Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in subablation conditions’, Physical Review B, 86, 115316 (2012).
Tsibidis G.D., Stratakis E., Aifantis K.E. ‘Thermoplastic deformation of silicon surfaces induced by ultrashort pulsed lasers in submelting conditions’, Journal of Applied Physics, 111, 053502 (2012).
Tsibidis G.D., Mouchliadis L., Pedio M., Stratakis E., ‘Modelling ultrafast non-equilibrium carrier dynamics and relaxation processes upon irradiation of hexagonal Silicon-Carbiade with femtosecond laser pulses’ (Under Review http://arxiv.org/abs/1910.14501)
G.D.Tsibidis, and E.Stratakis
- G.D.Tsibidis (Modelling of laser-matter Interactions)
- E.Petrakakis (Modelling of laser-matter Interactions)
- M-C.Velli (Modelling of laser-matter Interactions+Machine learning-based approaches)
- F.Fraggelakis, S.Maragkaki, A.Mimidis, I.Sakelari, E.Skoulas, E.Stratakis (Experiment)