ULTRAFAST LASER MICRO- AND NANO- PROCESSING GROUP

In the Ultrafast Laser Micro- and Nano- processing group (ULMNP) of IESL research is focused on the development of novel ultrafast pulsed laser processing schemes for controlled biomimetic structuring at micro- and nano- scales of a variety of materials, including biopolymers. By applying ultrafast laser pulses novel surface structures with sub-micron sized features are produced while the physical properties of semiconductor, dielectric and metallic surfaces are significantly modified. The biomimetic surfaces developed  exhibit controlled dual-scale morphology, that mimics the hierarchical structuring of natural surfaces with exciting properties (i.e. the Lotus leaf, the Shark Skin, the Butterfly wings). As a result, the biomimetic morphology attained gives rise to notable multifunctional properties including water repellence, self-cleaning, antibacterial, anti-sticking, anti-fogging, anti-reflection and combination of those (b) smart, i.e show the ability to change their functionality in response to different external stimuli. The ability to tailor the morphology and chemistry is an important advantage for the use of the biomimetic structures as models to study the dependence of growth, division and differentiation of cells on the surface energy of the culture substrat, as well as 3D scaffolds for tissue regeneration. At the same time, novel ultrafast non-linear imaging tools are employed to characterise the biological processes taking place during the development of tissue into 3D scaffolds. At the same time, ULMNP focuses on the ultrafast laser-based development of various types of nanomaterials, nanolayers and processes applied in photovoltaic, gas sensing and energy storage applications. The exploitation of ultrashort pulses  for the doping, functionalization, spectroscopic diagnosis and quality control of graphene and other 2D materials is additionally explored, placing emphasis on the understanding of the fundamental physical properties of such materials.

 

Research Topics

 

Ultrafast Laser Processing of Materials

Abstract

We focus on the development of novel ultrafast pulsed laser processing schemes for controlled structuring at micro- and nano- scales of a variety of solid materials. By applying single or temporally shaped ultrashort UV, VIS and IR laser pulses, novel surface structures with features ranging from a few hundreds of nanometers to micron-sized ones are produced, while the physical properties of the processed semiconductor, dielectric and metallic surfaces are significantly modified. Developed methods include laser micro/nano surface structuring performed in different media, direct laser writing with variable laser polarization states and combination of those. Further control over the surface topology is achieved by proper functionalization of the 3D structures obtained with well-defined nanostructures.

The artificial surfaces developed by processing under ambient or controlled gaseous environments exhibit controlled roughness that mimics the complexity and hierarchical morphology of natural surfaces with exciting functional properties, i.e. the water repellence of a Lotus leaf, the low underwater friction properties of shark skin, the coloration and anti-reflection properties of butterfly wings. The biomimetic morphology attained gives rise to notable multifunctional properties when combined with methods of tailoring the surface chemistry.

Research results indicate that appropriate combination of topography and chemistry can lead to artificial surfaces that: (a) are of extremely low surface energy, thus water repellent and self-cleaned, (b) exhibit reduced friction under specific lubrication conditions, (c) are anti-reflective and iridescent, (d) exhibit the exceptional abilities of meta-surfaces in light manipulation, (e) are smart, i.e show the ability to change their surface properties in response to different external stimuli and (f) arevmulti-functional in the sense that exhibit multiple functionalities at the same time.

Activities

a) Wetting properties: By applying ultrashort UV, VIS and IR laser pulses novel surface structures with sub-micron sized features are produced while the physical properties of semiconductor, dielectric and metallic surfaces are significantly modified. Developed methods include laser micro/nano surface structuring performed in different media, direct laser writing with variable laser polarization states and combination of those. Further control over the surface topology is achieved by proper functionalization of the 3D structures obtained with well-defined nanostructures. The artificial surfaces developed by processing under ambient controlled gaseous environments or in ambient environment exhibit controlled dual-scale roughness, that mimics the complexity of hierarchical morphology of natural surfaces with exciting wetting properties (i.e. the Lotus leaf, Texas horned Lizard), comprising micro-conical structures decorated with nanometer sized protrusions. The biomimetic morphology attained gives rise to notable wetting properties when combined with methods of tailoring the surface chemistry.

Figure 1: Wetting response and SEM pictures of actual lotus leaf (left) and fs treated silicon (right) surfaces

b) Optical properties Based on the concepts and underlying principles discovered in nature, an interdisciplinary field has been developed, aiming to design and fabricate photonic biomimetic structures. This capability comes as the outcome of the optimal combination of the ultrafast laser field and material properties that enable the production of features with sizes beyond the diffraction limit (i.e., nanoscale) that can mimic the functionalities of cicada and butterfly wings. A prominent example is the formation of self‐organized subwavelength, laser‐induced periodic surface structures (LIPSS), which have been proven an important asset for the fabrication of nanostructures with a plethora of geometrical features. With precise ultrafast laser processing we can produced high anti-reflective artificial glass surfaces and high absorbing metal and semiconductor materials.

Figure 2: SEM images of actual cicada Cretensis wing (left) and of an fs treated glass surface (right). Photograph of half treated glass SiO2 with reduced light reflection (below).

c) Tribological properties: A prominent aspect of the fs laser material interaction is that the spatial features of the surface structures attained are strongly correlated with the laser beam polarization. However, to date, laser fabrication of biomimetic structures has been demonstrated using laser beams with a Gaussian intensity spatial profile and spatially homogeneous linear polarization. In this context and based on the sensitivity of laser induced structures on laser polarization, it is possible to further advance the complexity of the fabricated structures via utilizing laser beams with a spatially inhomogeneous state of polarization. Therefore we can mimic the skin of elasmobranches like shark for in water drag reduction and reduced friction sliding friction under the presence of oil lubricance.

 

Figure 3: SEM images of actual shark skin (left) and of an fs treated metal surface (right)

Controlling 2D LIPSS formation with double pulses

Figure 4: SEM images of 2D structures induced by double pulses on stainless steel surface.

Employing DPI enables us to intervene into the evolution of the structure formation in a non-deterministic way. The interpulse delay (Δτ) is considered the main parameter in DPI, since it defines the stage of structure formation process, which is targeted by the second pulse. Depending on the Δτ value several effects have been observed on 2D LIPSS formation on stainless steel (4). When 1 ps < Δτ < 10 ps the hierarchical morphology of triangular 2D-LIPSS was tailored via tuning the high spatial frequency LIPSS (HSFL) formation (Figure 1, left). At Δτ = 20 ps 2D-HSFL were obtained and a structure morphology inversion was observed (Figure 1, center). When Δτ ranges in the nanosecond timeframe the microfluidic motion of the melt reaches its maximum amplitude. Then the second pulse intervenes to the existing temperature profile and impacts Marangoni flow. We showed that at Δτ = 0.5 ns a variety of 2D subwavelength structures were obtained (Figure 1, right), assuming the development of convection flow (CF) on the surface. According to CF theory the pattern formation apart from the amplitude and temporal profile of the excitation depends on the excitation profile, i.e. the spot profile in the case of laser irradiation (Process I). Therefore, by means of DPI we can manipulate the CF dynamics, while upon tuning of the spot profile we could define the CF pattern that will be developed.

(d) Investigation of LIPSS formation of prepatterned surfaces

Intensive experimental observations of LIPSS over a wide range of materials have been accompanied by theoretical studies. However, the underlying physical mechanisms are still under debate. We investigate the impact of the surface morphology on the spatial period and orientation of ripples. The LIPSS formation on pre-patterned surfaces aims to reveal more information regarding to the physical information of the ripples formation induced by ultrashort laser pulses. Understanding the underlying physical mechanisms is the key to control their feature characteristics in different materials.

Figure 5: SEM micrograph of laser-induced ripples on a pre-patterned surface

(e) Ultrafast laser induced metasurfaces

Metasurfaces are two-dimensional metamaterials with planar and ultrathin nanostructures that have shown exceptional abilities in light manipulation and versatility in optical applications. The core research strategy here is to exploit ultrafast laser fabrication technology combined with the state-of-the art planar metamaterial designs in order to engineer the geometric parameters of artificially constructed subwavelength meta-atoms and, thus generate a variety of metasurfaces that will enable the manipulation of optical waves in a prescribed manner.

Figure 6: Planar and ultrathin nanostructures fabricated by ultrafast laser processing of silicon

Representative publications

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 98224103 (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).

Submitted manuscripts

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)

Project Members

  • F. Fraggelakis, S. Maragkaki, A. Mimidis, I. Sakelari, E. Skoulas, A. Lemonis (Experiment)
  • 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)

Collaborators

J. Bonse

J. Siegel

J. Solis

J. Heitz

S. Amoruso

S. Petrovic

W. Baumgartner

P. Commans

Ultrafast Laser Processing Modelling

 

G.D.Tsibidis, and E.Stratakis

 

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,
  • 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,
  • Machine learning based approaches,

 

 

  1. 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).

 

Figure 1: SEM picture, and Simulation results.

Figure 1: SEM picture, and Simulation results.

 

 

 

 

Figure 2: (a) hydrothermal waves, (b) convection rolls

 

 

  1. 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).

 

 

 

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

 

 

  1. 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. 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.


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.

 

 

  1. 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.5).

 

 

Figure 5: (a) Electronic and lattice temperature profile using the classical TTM and revised TTM, (b) Spatial strain profile simulated TTM and rTTM.

 

 

  1. 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. 6).

 


 Figure 6: (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.

 

 

 

  1. 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.7,8). 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 7: Irradiation of Silicon with mid-IR femtosecond pulses [2]

 

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 8: Irradiation of Fused SilIica with mid-IR femtosecond pulses

 

 

  1. 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.9). Simulated data based on physics modelling (Fig.9a) and experimental data (Fig.9b) 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.

 

 

 

 

 


 

 

 

Figure 9: (a) Simulated data based on physics modelling, (b) experimental data, (c-d) Machine learning based approaches results. Stainless steel is used as a test material.

 

 

 

Representative publications

 

  1. Fraggelakis F., Tsibidis G.D., Stratakis E., ‘Sub-micrometer periodic surface structure formation with ultrashort pulsed Direct Laser Interference Patterning of solids’, (Arxiv, under review)
  2. Velli MC, Tsibidis G.D. Mimidis A., Skoulas E., Pantazis Y., Stratakis E., ‘Predictive modeling approaches in laser-based material processing’, Special Issue on Machine Learning for Materials Design and Discovery), Journal of Applied Physics, 28 18 (2020). (DOI: 10.1063/5.0018235)
  3. Skoulas E., Mimidis A., Demeridou I., Tsibidis G.D., Stratakis E., ‘Polarization dependent spike formation on black silicon via ultrafast laser structuring’ Journal of Optoelectronics and Advanced Materials 22, 501 (2020).
  4. Allahyari,E., Nivas J.JJ, Skoulas E., Bruzzese R., Tsibidis G.D., Stratakis E., and Amoruso S. ‘On the formation and the features of the supra-wavelength grooves generated during femtosecond laser surface structuring of silicon’ Applied Surface Science, 528 146607 (2020).
  5. Stratakis E., Bonse J., Heitz J., Siegel J., Tsibidis G.D., Skoulas E. Papadopoulos A., Mimidis A., Joel A.-C., Comanns P., Kruger J., Florian C., Fuentes-Edfuf Y., Solis J., Baumgartner W., ‘Laser Engineering of Biomimetic Surfaces’ (Review Article), Materials Science and Engineering: R: Reports, 141, 100562 (2020).
  6. Tsibidis G.D., Stratakis E., ‘Ionization processes and laser induced periodic surface structures in dielectrics with mid-infrared femtosecond laser pulses’ 'Invitation for Special Collection: Intense ultra-short pulses from femtosecond to attosecond', Scientific Reports 10, 8675 (2020).
  7. Tsibidis G.D., Mouchliadis L., Pedio M., Stratakis E., ‘Modelling ultrafast out-of-equilibrium carrier dynamics and relaxation processes upon irradiation of hexagonal Silicon-Carbide with femtosecond laser pulses’, Physical Review B 101, 075207 (2020).
  8. 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).
  9. 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).
  10. 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).
  11. 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).
  12. 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).
  13. 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).

 

  1. Tsibidis G.D., ‘Ultrafast dynamics of non-equilibrium electrons and strain generation under femtosecond laser irradiation of Nickel’, Applied Physics A, 124,311 (2018).

 

  1. 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).

 

  1. 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).

 

  1. 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.

 

  1. 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).

 

  1. 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).

 

  1. 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).

 

  1. Tsibidis G.D., Skoulas E., and Stratakis E. “Ripple formation on Nickel irradiated with radially polarized femtosecond beams’, Optics Letters, 40 (22), 5172 (2015).

 

  1. 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).

 

  1. 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).

 

  1. 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).

 

  1. 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).

 

  1. 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).

 

  1. 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).

 

 

Project Members

 

  • Dr G.D.Tsibidis (Modelling of laser-matter Interactions)
  • Dr Leonidas Mouchliadis (Density Functional Theory calculations)
  • M-C.Velli (Modelling of laser-matter Interactions+Machine learning-based approaches)
  • Dr F.Fraggelakis (Experiment)
  • Dr S.Maragkaki (Experiment)
  • Dr I.Sakelari (Experiment)
  • M.Vlachou (Experiment)
  • Dr E.Skoulas (Experiment)
  • A.Mimidis (Experiment)
  • Dr E.Stratakis (Experiment-Group Leader)

 

 

Direct Laser Fabrication of Biomimetic, 3D Scaffolds for Tissue Regeneration

 

Short Description and Main Findings of the Research​​ Topics:

 

Research Topic​​ 1:​​ Direct Laser Micro/Nano Fabrication of Biomimetic Scaffolds

 

Short description

The​​ aim​​ is to investigate the biocompatibility of​​ laser-engineered biomimetic 3D scaffolds​​ fabricated on​​ hard​​ metallic and​​ soft polymeric materials,​​ exhibiting​​ different micro/nano topographies and surface energies.

 

Abstract

The​​ extracellular matrix provides the necessary cues at micro and nano-scale for cell adhesion, alignment, proliferation and differentiation. In this context,​​ the surface topography of biomaterials can have an important impact on cellular adhesion, growth and proliferation. Apart from the overall roughness, the detailed morphological features, at all length scales, significantly affect the cell-biomaterial interactions in a plethora of applications including structural implants, tissue engineering scaffolds and biosensors. 

 

The main objective is to investigate the biocompatibility of​​ laser-engineered biomimetic 3D scaffolds​​ fabricated on​​ hard​​ metallic and​​ soft polymeric materials,​​ exhibiting​​ different micro/nano topographies and surface energies.​​ ​​ Ultrafast pulsed laser irradiation is considered as a simple, precise​​ and effective microfabrication method to produce structures​​ of​​ controlled​​ geometry and pattern regularity. The variation of​​ irradiation parameters,​​ such as​​ fluence​​ and irradiation environment gives rise to​​ significant​​ changes​​ of the​​ surface morphology​​ attained​​ (i.e. geometry, dimensions and density of the structures).​​ As a consequence,​​ morphologies​​ ranging from microcones​​ to​​ nanoripples (Figure 1),​​ as well​​ as hierarchical micro/nano structures (Figure 2) can be​​ fabricated​​ and further used as cell culture platforms.​​ 

 

The laser fabricated scaffolds with controlled surface roughness,​​ wettability​​ and surface energy​​ can be used as model​​ platforms to study the influence of topography on cell response.​​ It is demonstrated​​ that, depending on the laser processing conditions, distinct cell-philic or cell-repellant patterned areas can be attained with a desired motif​​ (Figure​​ 3).​​ Laser processing​​ could​​ thus​​ enable spatial patterning of cells in a controllable manner, giving rise to advanced capabilities in cell biology research.

 

Research has​​ shown​​ that​​ cell​​ adhesion​​ and migration​​ could be tuned​​ via the laser-patterned substrates. It was also shown that microconical substrates could influence​​ sympathetic and sensory neuronal alignment​​ as well as​​ NGF-induced PC12 cell differentiation​​ (see also​​ Research Topic 2).​​ 

 

 

 

Scaffolds​​ on​​ hard​​ materials

Figure 1:​​ Various​​ types of​​ femtosecond​​ laser fabricated​​ scaffolds​​ on hard materials​​ with feature sizes ranging from​​ a few hundreds of nanometers​​ to tens of​​ microns​​ (C Simitzi, P Efstathopoulos, A Kourgiantaki, A Ranella, I Charalampopoulos, C Fotakis, I Athanassakis, E Stratakis, A Gravanis, Biomaterials, 2015, 67: 115-128, doi.org/10.1016/j.biomaterials.2015.07.008).

 

Figure 2:​​ Laser fabricated arrays of​​ biomimetic​​ hierarchical micro/nano conical structures(Chara Simitzi, Pascal Harimech, Syrago Spanou, Christina Lanara, Amelie Heuer-Jungemann, Aleka Manousaki, Costas Fotakis, Anthi Ranella, Antonios G Kanaras, Emmanuel Stratakis, Biomater. Sci., 2018,6,1469, doi: 10.1039/c7bm00904f)

 

 

Figure​​ 3:​​ Patterning of Schwann (SW10) cells cultured on laser fabricated substrates exhibiting cell-philic and cell-repellant areas​​ (Ch Yiannakou, Ch Simitzi, A Manousaki, C Fotakis, A Ranella, E Stratakis,​​ 2017​​ Biofabrication​​ 9​​ 025024,​​ https://doi.org/10.1088/1758-5090/aa71c6)

 

Scaffolds​​ on​​ soft materials

a)​​ Replicated Scaffolds​​ 

Soft lithography has been successfully used to transfer well-defined micro-sized patterns from​​ hard materials to soft​​ (bio)materials.​​ The replication of​​ micro/nano topographies​​ is​​ realised​​ on polymeric systems,​​ such as poly (lactic-co-glycolic acid) (PLGA), polycaprolactone (PCL)​​ (Figure​​ 4)​​ and polydimethylsiloxane (PDMS),​​ in order to investigate the effect of material​​ surface energy​​ and​​ stiffness on cellular responses (adhesion, proliferation, survival, growth and differentiation).

Figure 4a:​​ SEM images (tilted view) of​​ PLGA replicas​​ with the hierarchical (microcone-spikes and nano-ripples) topographies​​ (Babaliari, E., Kavatzikidou, P., Angelaki, D., Chaniotaki, L., Manousaki, A., Siakouli-Galanopoulou, A., Ranella, A., & Stratakis, E. (2018). Engineering Cell Adhesion and Orientation via Ultrafast Laser Fabricated Microstructured Substrates. International journal of molecular sciences, 19(7), 2053,​​ doi: 10.3390/ijms19072053).

 

 

Figure​​ 4b:​​ SEM images (tilted view) of​​ PCL replicas​​ exhibiting various​​ microcone​​ topographies.

 

b)​​ 3D​​ scaffolds​​ of porous collagen​​ via subtractive laser manufacturing:

Laser​​ micromachining​​ provides a novel​​ CAD/CAM rapid prototyping​​ microfabrication process that can provide complex implant designs based on an established biomaterial utilized in clinical practice.​​ It is shown that​​ fs laser​​ micromachining​​ of porous collagen​​ (Figure​​ 5), in particular,​​ can fabricate high-precision micron-sized features (e.g. canals, wells)​​ and​​ provides novel ways to modulate the microenvironment felt by interacting cells, tailor implants to the needs of individual patients,​​ or tools to meet the current major challenges of regenerative medicine.

 

Figure​​ 5:​​ fs laser micromachining of porous collagen scaffolds

 

Research​​ Topic​​ 2:​​ 3D Scaffolds Hosting Neurons and Neural Stem Cells

 

Short Description

 

The aim is to develop laser-engineered micro/nano scaffolds (3DLS) for hosting 3D cultures of neural stem cells.

 

Abstract

 

Neural stem cells (NSCs) are intrinsically capable of differentiating into different neural cell types: neurons, oligodendrocytes and astrocytes, and have emerged as important players in the generation and maintenance of neural tissue as well as in treating neurodegenerative diseases and neurological injuries. However, successful development of NSC-based therapies requires more sophisticated technologies from the ones that are already available and deeper understanding of NSCs’ functions. The NSCs reside in a complex three-dimensional (3D) niche in vivo where they are exposed to a plethora of signals, including physical signals such as tensile, compressive and shear stresses, discontinuities and differences in roughness of the ECM molecules. Topography is capable of inducing different effects on NSCs, such as changes in cell morphology, alignment​​ (Figures​​ 1​​ and​​ 2), adhesion, migration, proliferation, cytoskeleton organization and also differentiation​​ (Figure​​ 3). However, simulating this 3D environment for NSC culture and subsequent development of 3D neuronal networks that maintain functional neuronal properties (synaptogenesis and neurotrophic performance) remains a challenge.

 

To respond to this challenge we have fabricated 3D laser-engineered micro/nano scaffolds (3DLS) featuring different micro/nano topographies for hosting neurons, glia and NSCs. These are advantageous platforms to study the biology of NSC proliferation, differentiation, neuritogenesis and synaptogenesis.​​ Patterning of neuronal outgrowth in vitro is important in tissue engineering as well as for the development of neuronal interfaces with desirable characteristics. ​​ Laser-patterned​​ biomimetic​​ scaffolds​​ could potentially be a useful platform for patterning neurons into artificial networks, allowing the study of neuronal cells interactions under 3D ex-vivo conditions.

 

Figure​​ 1:​​ SCGs​​ neurons​​ orientation on laser fabricated discontinuous anisotropic microconical substrates​​ (C Simitzi, P Efstathopoulos, A Kourgiantaki, A Ranella, I Charalampopoulos, C Fotakis, I Athanassakis, E Stratakis, A Gravanis, Biomaterials, 2015, 67: 115-128, doi.org/10.1016/j.biomaterials.2015.07.008).

 

Figure​​ 2:​​ DRG cultures​​ on laser fabricated discontinuous anisotropic microconical substrates​​ (C Simitzi, P Efstathopoulos, A Kourgiantaki, A Ranella, I Charalampopoulos, C Fotakis, I Athanassakis, E Stratakis, A Gravanis, Biomaterials, 2015, 67: 115-128, doi.org/10.1016/j.biomaterials.2015.07.008).

 

 

Figure​​ 3:​​ Effect of surface roughness on PC12 cell differentiation.​​ (C. Simitzi, E. Stratakis, C. Fotakis, I. Athanassakis and A. Ranella, Microconical silicon structures influence NGF-induced PC12 cell morphology, J Tissue Eng Regen Med2015;9: 424–434, DOI:10.1002/term.1853)

 

 

 

 

 

 

 

 

 

Research​​ Topic​​ 3:​​ Development of microfluidic systems for cell studies under dynamic culture conditions  

Understanding the cell-biomaterial interaction​​ under dynamic culture conditions,​​ in vitro,​​ is potentially​​ useful​​ in the fields of tissue engineering and regenerative medicine. ​​ 

 

A​​ precise flow controlled microfluidic system with specific custom-designed chambers, incorporating laser-microstructured​​ polyethylene terephthalate (PET) substrates comprising microgrooves,​​ is developed​​ to assess the combined effect of shear stress and topography on cells’ behavior​​ (Fig.1).​​ Specifically the​​ dynamic cultures are performed for the study of the cytoskeleton, directionality and proliferation of cells on micro-nano patterns.​​ A​​ comparison between static and dynamic cultures is​​ always​​ performed​​ in combination with​​ computational flow simulations to calculate accurately the shear stress values. The main findings demonstrate​​ that​​ wall shear stress gradients may be acting either synergistic or antagonistic​​ depending on the​​ substrates​​ groove orientation​​ relative to the flow direction (Fig.2).

 

Figure​​ 1:​​ Custom-designed Microfluidic System

 

 

Figure​​ 2:​​ Confocal images of SW10 cells cultured on the PET-Flat (a, b, c) or inside the MG of the PET-MG substrates (d-i), under static (a, d, g) or dynamic conditions, applying 200 (c, f, i) μL/min, on the third day of culture. The cytoskeleton of the cells is visualized with red color (Alexa Fluor® 680 Phalloidin) while the nuclei with blue color (DAPI). The direction of the​​ flow was parallel (f) or perpendicular (i) to the microgrooves. The inset SEM images, framed by a yellow box, depict the geometry of microgrooves (paper under submission).

 

Figure 3:​​ Directional polar plots of cells’ cytoskeleton: ​​ a) No flow, PET-Flat (black line) 200​​ μL/min, PET-Flat (blue line), b) No flow, PET-MG (green line)​​ - 200​​ μL/min parallel to MG, PET-MG (turquoise line), c) No flow, PET-MG (green line) -​​ 200​​ μL/min perpendicular to MG, PET-MG (dark blue line). The black and red arrows represent the direction of the flow and the microgrooves, respectively.​​ 

 

Selected Publications:

 

  • C. Simitzi, E. Stratakis, C. Fotakis, I. Athanassakis and A. Ranella, J Tissue Eng Regen Med,​​ 2015;9: 424–434, DOI:10.1002/term.1853

  • Ch Yiannakou, Ch Simitzi, A Manousaki, C Fotakis, A Ranella, E Stratakis,​​ 2017​​ Biofabrication​​ 9​​ 025024,​​ doi: 10.1088/1758-5090/aa71c6

  • C.​​ Simitzi, A.​​ Ranella, E.​​ Stratakis,​​ Acta Biomaterialia,​​ Volume 51, 15 March 2017, Pages 21-52, doi:​​ 10.1016/j.actbio.2017.01.023.

  • Chara Simitzi, Pascal Harimech, Syrago Spanou, Christina Lanara, Amelie Heuer-Jungemann, Aleka Manousaki, Costas Fotakis, Anthi Ranella, Antonios G Kanaras, Emmanuel Stratakis,​​ Biomater. Sci., 2018,6,1469, doi:​​ 10.1039/c7bm00904f

  • Babaliari, E., Kavatzikidou, P., Angelaki, D., Chaniotaki, L., Manousaki, A., Siakouli-Galanopoulou, A., Ranella, A., & Stratakis, E. (2018). International journal of molecular sciences, 19(7), 2053,​​ doi: 10.3390/ijms19072053

 

 

Project Members:

Dr Paraskevi​​ (Evi)​​ Kavatzikidou

Dr Phanee Manganas

Dr Lambrini (Lina) Papadimitriou

Mrs​​ Despina Angelaki​​ (PhD Candidate)

Mrs​​ Eleftheria Babaliari​​ (PhD Candidate)

Mr Evangelos Skoulas (PhD Candidate)

Mrs​​ Lida Evmorfia Vagiaki​​ (PhD Candidate)

Mr Dionysios Xydias (PhD Candidate)

Ms. Christina Lanara, M.Sc.

Dr. Anthi Ranella

Dr. Emmanuel Stratakis

Prof. Costas Fotakis

 

Former Group Members:​​ 

Dr Chara Simitzi

Dr Kanelina Karali

Mrs Christina Yannakou

Mrs Syrago Spanou

 

2D Materials

Abstract:

The research topic aims at the investigation and manipulation of the optoelectronic properties two-dimensional (2D) materials.

 

Two-dimensional Transition Metal Dichalcogenides MX2 (M=Mo, W and X=S, Se, Te) are of keen interest for emerging optoelectronic and valleytronic applications. The main activities in our lab focus on the optical response of 2D layered materials and their heterostructures. Spectroscopic techniques such as μ-PL, μ-Reflectance, and Raman are used to characterize the samples from 4K to 300K. We investigate how the degree of valley polarization (VP)-under resonant and non-resonant conditions-is affected by the dielectric environment in different heterostructures such as MX2/h-BN, MX2/graphene, as well as their corresponding encapsulated structures. Besides, a fast-photochemical doping technique was developed in our lab to sufficiently control the carrier density of a single MX2 layer by incorporating chlorine atoms on the surface. Photochlorination leads to a controllable reduction of VP that is directly related to the decrease of the active defect sites and consequently to the increase of the non-radiative exciton lifetime. We also study the PL emission and VP of suspended MX2 in different configurations with respect to strain induced by patterned substrates. Suspended MX2 monolayers (near zero strain) display enhanced PL and absorption due to the absence of charged excitons.

 

 

Spectroscopy of 2D materials provide the principal knowledge towards the development of state-of-the-art optoelectronic and valleytronic devices.

 

Research topics:

  • Fabrication of graphene and other 2D materials
  • Spectroscopy of graphene and other 2D materials
  • Photochemical doping of graphene and other 2D materials

 

 

Highlights:

 

  1.  Spatial Non-Uniformity in Exfoliated WS2 Single Layers