The Stratakis Lab

The Stratakis Lab

Ultrafast Laser Micro and Nano Processing Laboratory

Modelling of Ultrafast Laser Processing of Materials

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).​​ 

 

all materials

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].​​