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
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].
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).