Ultrafast Laser Biomimetic Processing of Materials
Ultrafast Laser Processing of Materials
Activities-challenges: Bio-inspired surface modification and functionalization of solid surfaces via ultrashort laser pulses in various types of materials (i.e. semiconductors, metals, dielectrics, polymers), investigation of surface micro/nano structure role on wetting, optical, and tribological applications. Control of surface morphology with shaped ultrashort double pulses. Development of laser induced metasurfaces and investigation of the physical mechanisms that lead to laser induced surface structure formation.
Biomimetic laser processing
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
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 [1].
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).
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 [2,3].
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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 4, left). At Δτ = 20 ps 2D-HSFL were obtained and a structure morphology inversion was observed (Figure 4, 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 4, 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.
Investigation of LIPSS formation on pre-patterned surfaces
To illustrate the role of pre-patterrned surfaces and impact of laser polarisation in the periodic pattern formation,
Figure 5a: Laser-induced ripples on a pre-patterned surface. A comparison of non-irradiated pre-pattern structures and laser-induced ripples (upper and lower SEM micrographs respectively).
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Figure 5b: Electromagnetic modes that are excited which are used to explain the LIPSS formation on prepatterned surfaces.
Nickel surfaces are irradiated with femtosecond laser pulses of polarisation perpendicular or parallel to the orientation of the pre-pattern ridges. 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. The detailed analysis of the LIPSS formation in different conditions and ridge distances can set the basis for control of the parameters to fabricate patterns with desired properties for a wide range of optoelectronic applications. To understand the LIPSS formation on prepatterned surfaces a theoretical model has been developed that discusses how the induced electromagnetic modes can account for the orientation and periodicity of the periodic structures [6].
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
Complex structure generation with advanced spatiotemporal femtosecond beam shaping
Α novel approach for tailoring the laser induced surface topography upon femtosecond (fs) pulsed laser irradiation is followed. The method employs spatially controlled double fs laser pulses to actively regulate the hydrodynamic microfluidic motion of the melted layer that gives rise to the structures formation. The pulse train used, in particular, consists of a previously unexplored spatiotemporal intensity combination including one pulse with Gaussian and another with periodically modulated intensity distribution created by Direct Laser Interference Patterning (DLIP). The interpulse delay is appropriately chosen to reveal the contribution of the microfluidic melt flow, while it is found that the sequence of the Gaussian and DLIP pulses remarkably influences the surface profile attained (Figure 7).
Figure 7: A Gaussian and a DLIP pulse are combined in DLIP irradiation. The delay between the double pulses is 500 ps. After repetitive irradiation a variety of complex morphologies were obtained.
Figure 8: A complex morphology comprising structures in multiple length scales.
Apart from the order of the pulses and the shape of the DLIP pulse, tailoring of secondary process parameters impact the induce morphology. After studying the effects of the fluence and the number of irradiations, distinct complex morphologies were obtained comprising features in multiple length scales. An example of such a surface is presented in Figure 8 and analyzed in components of different size by means of inversed furrier transformation [4,5].
Fabrication of Biomimetic 2D Nanostructures through Irradiation of Stainless Steel Surfaces with Double Femtosecond Pulses
Femtosecond laser induced changes on the topography of stainless steel with double pulses is investigated to reveal the role of parameters such as the fluence, the energy dose and the interpulse delay on the features of the produced patterns. Our results indicate that short pulse separation (Δτ = 5 ps) favors the formation of 2D Low Spatially Frequency Laser Induced Periodic Surface Structures (LSFL) while longer interpulse delays (Δτ = 20 ps) lead to 2D High Spatially Frequency LIPSS (HSFL). The detailed investigation is complemented with an analysis of the produced surface patterns and characterization of their wetting and cell-adhesion properties. A correlation between the surface roughness and the contact angle is presented which confirms that topographies of variable roughness and complexity exhibit different wetting properties. Furthermore, our analysis indicates that patterns with different spatial characteristics demonstrate variable cell adhesion response which suggests that the methodology can be used as a strategy towards the fabrication of tailored surfaces for the development of functional implants (Figure 9) [7].
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Figure 9: Impact of topography produced with double pulses on stainless steel on (a) wetting and (b) cell adhesion properties.
Tailoring surface topographies on solids with Mid-IR femtosecond laser pulses
Irradiation of solids with ultrashort pulses using laser sources in the mid-infrared (mid-IR) spectral region is a yet predominantly unexplored field that opens broad possibilities for efficient and precise surface texturing for a wide range of applications. In the present work, a detailed combined experimental and theoretical investigation of the impact of laser sources both on the generation of surface modification related effects and patterning on metallic and semiconducting materials is performed. A series of experiments were surface irradiation upon mid-IR ultrafast laser pulses is performed to allow a parametric study and correlate the laser parameters with the onset of material damage and the formation of a variety of periodic surface structures for laser wavelengths λL=3200 nm of pulse duration τp=45 fs. Results for nickel and silicon indicate that the produced topographies comprise high/low spatial frequency laser induced periodic structures similar to those observed at lower wavelengths while groove formation is absent. The evaluation of the damage thresholds entails the incorporation of nonlinear effects generated from three-photon-assisted excitation (for silicon) and the consideration of the role of the non-thermal excited electron population (for nickel) at very short pulse durations. Results demonstrate the potential of surface structuring with mid-IR pulses that can constitute a systematic novel engineering approach with strong fields at long wavelength spectral regions that can be used for advanced industrial laser applications.
Project Members
Theory
Dr George.D.Tsibidis (Modelling of laser-matter Interactions)
Dr Panos Lingos (Electromagnetic Simulatrions)
Dr Leonidas Mouchliadis (Density Functional Theory calculations)
Maria-Christina.Velli (Modelling of laser-matter Interactions+Machine learning-based approaches)
Experiment
Dr Fotis Fraggelakis (Experiment)
Dr Stella Maragkaki (Experiment)
Dr Ioanna Sakelari (Experiment)
Dr Dimitris Mansour (Experiment)
Matina Vlahou (Experiment)
Dr Emmanuel Stratakis (Experiment-Group Leader)
Representative publications
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).
Skoulas E., Manousaki A., Fotakis C, Stratakis E., ‘Biomimetic surface structuring using cylindrical vector femtosecond laser beams’, Scientific Reports 7,1 (2017).
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).
Fraggelakis F., Tsibidis G.D., Stratakis E., ‘Tailoring Sub-micrometer Periodic Surface Structures via Ultrashort Pulsed Direct Laser Interference Patterning’, Physical Review B 103, 054105 (2021).
Fraggelakis F., Tsibidis G.D., Stratakis E., ‘Ultrashort pulsed laser induced complex surface structures generated by tailoring the melt hydrodynamics’, Opto-Electronic Advances, 5 210052 (2022), (Front Cover of Issue).
Maragkaki S., Lingos P., Tsibidis G.D., Deligeorgis P., Stratakis E., ‘Impact of pre-patterned structures on features of Laser Induced Periodic Surface Structures’, Molecules 26 (3) 7330 (2021).
Vlahou M., Fraggelakis F., Manganas P., Tsibidis G.D., Ranella A., and Stratakis E., ‘Fabrication of biomimetic 2D nanostructures through irradiation of stainless steel surfaces with double femtosecond pulses’, , Nanomaterials 12 (4) 623 (2022).