Ultrafast Laser​​ Processing Modelling

Activities-challenges:​​ 

• Physical modelling of multiscale processes

• Investigation of surface modification mechanisms in​​ (sub)-ablation and sub-melting conditions in various types of materials (i.e. semiconductors, metals, dielectrics),​​ 

• Interpretation of mechanisms that account for Laser Induced Periodic Surface Structures​​ (LIPSS),​​ 

• Exploration of carrier dynamics in multilayered materials,​​ 

• Role of non-thermal electrons​​ and out-of-equilibrium excited carriers​​ 

• Density Functional Theory (DFT)-based calculations of optical properties, excitation conditions, relaxation processes

• ​​ Strain propagation​​ and​​ surface modification at different laser polarization states,​​ 

• Ultrafast dynamics at mid-IR,​​ 

• Modelling Patterning processes through Direct Laser Interference techniques,

• Machine learning based approaches,

• Role of electromagnetic modes in LIPSS formation.

• 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​​ [1,2,3].

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).​​ 

Figure 1: SEM picture,​​ and​​ Simulation results.

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. 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)​​ [4,5].​​ 

Figure​​ 3: Rippled profile with a radially (a,c)​​ and azimuthally​​ (b,d)​​ polarized​​ beam​​ 

• Tailoring​​ Sub-micrometer Periodic Surface​​ patterning​​ via Ultrashort Pulsed Direct Laser Interference​​  Patterning​​ (DLIP):​​ 

Direct laser Interference Patterning (DLIP) with ultrashort laser pulses (ULP) represents a precise and fast technique to produce tailored periodic sub-micrometer structures on various materials. An experimental and theoretical approach is​​ pursued​​ to investigate the previously unexplored fundamental mechanisms for the formation of unprecedented laser-induced topographies on stainless steel following proper combinations of DLIP with ULP​​ (Figure 4). Special emphasis is given to electron excitation, relaxation processes and hydrodynamical effects that are crucial to the production of complex morphologies. Results are expected to derive new knowledge of laser-matter interaction in combined DLIP and ULP conditions and enable enhanced fabrication capabilities of complex hierarchical sub-micrometer sized structures for a variety of applications.​​ In addition to reveal the impact of the laser beams used and their polarization, two beams are used, one Gaussian and another two- (or four-) beam DLIP of varying polarization (vertical or horizontal). Results indicate that the​​ aforementioned parameters as well as the sequence order of the two pulses play a very important role in the attained topography​​ [6,7].​​ 

Figure​​ 4:​​ Patterned profile based on DLIP technique with a two (A) and four (B) laser DLIP beam. Patterned surface is illustrated for single or two delayed pulses. Experimental results are interpreted through simulations.

Figure​​ 5:​​ Patterned profile based on a combination of two delayed (one Gaussian beam and one four beam DLIP) pulses. Experimental results are interpreted through simulations.

• 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.6)​​ [8].

Figure​​ 6:​​ (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.​​ 7)​​ [9].​​ 

​​ Figure​​ 7:​​ (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​​ [10]/fused silica​​ [11]​​ with ultrashort pulsed lasers in the mid-IR range​​ (Fig.8,9). 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​​ 8:​​ Irradiation of Silicon with mid-IR femtosecond pulses​​ 

A multiscale modelling approach is performed that correlates conditions for formation of perpendicular or parallel to the laser polarisation low spatial frequency periodic surface structures for low and high intensity mid-IR pulses (not previously explored in dielectrics at those wavelengths), respectively. Results demonstrate a remarkable domination of tunneling effects in the photoionisation rate and a strong influence of impact ionisation for long laser wavelengths. The methodology presented in this work is aimed to shed light on the fundamental mechanisms in a previously unexplored spectral area and allow a systematic novel surface engineering with strong mid-IR fields for advanced industrial laser applications.

Figure​​ 9:​​ Irradiation of Fused SilIica 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​​ towards determining the laser parameters required to pattern surfaces with morphological features (i.e. ripples, grooves, spikes) required to provide desired functionalities an properties (i.e. antireflective, antifouling, antimicrobial, wetting, etc.)​​ (Fig.10).​​ Simulated data based on physics modelling (Fig.9a) and experimental data (Fig.10b)​​ were used to​​ automate and forecast the effect of laser processing on material​​ structures. The focus is centered on the performance of representative statistical and machine learning algorithms in predicting the outcome​​ of laser processing on a range of materials. Results on experimental data showed that predictive models were able to satisfactorily learn the​​ mapping between the laser’s input variables and the observed material structure. These results are further integrated with simulation data​​ aiming to elucidate the multiscale physical processes upon laser–material interaction. As a consequence,​​ we augmented the adjusted simulated data to the experiment and substantially improved the predictive performance due to the availability of an increased number of sampling points. In parallel, an information-theoretic metric, which identifies and quantifies the regions with high predictive uncertainty, is presented, revealing that high uncertainty occurs around the transition boundaries. Our results can set the​​ basis for a systematic methodology toward reducing material design, testing, and production cost via the replacement of expensive trial-and-error based manufacturing procedures with a precise pre-fabrication predictive tool​​ [12].​​