Fundamental and applicative research in laser-material interactions

Resume : Bio-inspired micro- and nanopatterning of materials by means of laser radiation is a rapidly growing field to tailor special industrial, medical, and scientific applications. This is significantly driven by the exciting properties of micro- and nanopatterned materials found in natural biological species, including pronounced adhesive and anti-adhesive properties, wetting and directional fluid transport, and control of cell growth. The structuring techniques addressed here focus on developments in laser processing using short and ultrashort laser pulses. This includes the self-organized formation of micro- and nanopatterns at surfaces induced by exposure to laser radiation as well as direct writing techniques such as two-photon polymerization. Acknowledgement: This study was funded by the European Union’s Horizon 2020 research and innovation programme under the agreements No. 862016 (BioCombs4Nanofibers) and No. 951730 (LaserImplant).

Resume : The interaction between ultrashort laser beam pulses with a material can induce the formation of periodic surface micro/nano structures commonly referred to as LIPSS (Laser Induced Periodic Surface Structures). Controlling such a process can pave the way for the tuning of the physico-chemical properties of the material’s surface. In the case of electrochemical cells made of assembly of thin films incorporating Gadolinium-Doped Ceria (GDC), LIPSS formation can enhance the performance of the electrode by increasing its surface area and thus enhancing the reactions of the active species at the electrode/electrolyte interface. In this work, a Nd: YAG laser beam operating at the third harmonic (355 nm) and emitting 40 ps laser pulses is employed to irradiate a 4x4 mm2 surface of a GDC thin layer, that is deposited by magnetron sputtering on yttria-stabilized zirconia (YSZ) substrate. Using high resolution scanning electron microscopy (HR-SEM), it is found that LIPSS are produced at a low fluence laser multi-pulse regime close to the ablation threshold. In agreement with the literature, it is found that these periodic structures can be distinguished depending on their spatial period and can be classified as low and high spatial frequency LIPSS, LSFL and HSFL, respectively. However, under the static mode (irradiation of the same area of 500 µm diameter) and under appropriate values of laser fluence (50 to 250 mJ/cm2) with a number of pulses varying from 1 to 70, we have also identified two types of LSFLs that are distinct in their direction and spatial period. LSFL#1 are parallel to the beam polarization, with a typical period of 238 nm and found in the center of the irradiated zone, whereas LSFL#2 are oriented perpendicular to beam polarization with a spatial period of 296 nm and found on the rim of the irradiated zone. Our results suggest that the appearance of the two types of LSFL within the irradiated spot can be attributed to different metallic and dielectric behaviors of the inner and outer zones of the GDC film, respectively. These differences are attributed to increased oxygen losses under the higher beam intensity region. We have also optimized the process parameters to generate well resolved LIPSS under beam scanning conditions. Using numerical tools for SEM/AFM images and thanks to a simple geometric model developed on such structures, the enhancement of the specific surface following laser structuring is estimated to be in the range from 50 to 80%.

Resume : Using nanofiber-like cell appendages, secreted proteins and sugars, bacteria can establish initial surface contact followed by irreversible adhesion and the formation of multicellular biofilms. Here, the stabilizing extracellular biofilm matrix together with physiological changes on the single cell level leads to an increased resilience towards harsh environmental conditions, antimicrobials, the host immune response and established cleaning procedures. Persistent microbial adhesion on e.g., medical implants, in water supply networks or food-processing industry is often associated with chronic inflammation, nosocomial and foodborne infections, enhanced biofouling and product contamination. To prevent persistent microbial colonization, antibacterial surface strategies often target the initial steps of biofilm formation and impede adhesion of single cells before a mature biofilm is being formed. While chemical coatings have been widely used, their restricted biocompatibility for eukaryotic cells and attenuated antibacterial-effects due to compound release limit their areas of application and alternative strategies focus on modified surfaces topographies to impede bacterial adhesion. In this work, we used ns-UV laser treatment (wavelength 248 nm and a pulse duration of 20 ns) to generate laser-induced periodic surface structures (LIPSS) with different submicrometric periods ranging from ~210 to ~610 nm on commercial poly(ethylene terephthalate) (PET) foils. Following structurally and chemically analyses, PET samples were subjected to bacterial colonization studies with Escherichia coli TG1, a bacterial test strain with a strong biofilm formation capacity due to the formation of nanofiber-like cell-appendages (pili). Bacterial adhesion tests revealed that E. coli repellence decisively depends on the spatial periods of the LIPSS with the strongest reduction (~91%) in cell adhesion observed for LIPSS periods of 214 nm. Scanning electron microscopy and additional biofilm studies using a pili-deficient E. coli TG1 strain revealed the importance of extracellular appendages in the bacterial repellence observed here, thus, pointing out new antibiotics-free strategies for antibacterial surfaces by impeding nanofiber-mediated bacterial adhesion.

Resume : We present a study on the formation of Laser Induced Periodic Surface Structures (LIPSS), by means of nanosecond laser vector beams, at a wavelength of 532nm, on the surface of thin films of either a conductive polymer: poly(3-hexylthiophene) (P3HT) or a fullerene derivative: [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) deposited over n-silicon (1 0 0) doped with arsenic. These materials have raised interest in the field of organic electronics and photovoltaics due to their electronic properties, the possibility of thin film preparation and their optical absorption in the visible range. Previous studies [1,2] in both materials have shown the possibility of forming LIPSS using standard beams. The electrical conductivity of these structures is higher in the valleys of LIPSS as compared to the peaks. In this work, we try to extend these investigations by using vector beams. The vector beams were generated using a commercially available vortex half-wave plate which, when irradiated with linearly polarized light, transforms it into a vector beam with radial, azimuthal, or spiral polarization depending on the angle between the polarization vector of the input beam and the fast axis of the plate. The topographical changes of the samples after irradiation were studied by confocal microscopy in order to check the orientation of the LIPSS and by Atomic Force Microscopy (AFM) to characterize both their period and depth. Regarding the conductivity of the samples, it is measured by Conducting AFM (C-AFM). We were able to generate LIPSS, which appear parallel to the polarization of the beam, that is radial, concentrical or spiral. They appear after irradiation from 1200 to 4800 pulses, in a fluence from 11 to 36 mJ/cm2 for P3HT and from 17 to 19 mJ/cm2 for PC71BM. Periods around 500 nm and 360 nm and depths of 120 nm and 85 nm are observed for P3HT for PC71BM respectively. In summary, by following a single step top-down approach we were capable of generating concentrical, radial and spiral periodical structures by using nanosecond laser vector beams in thin films of PH3T and PC71BM. [1] A. Rodríguez-Rodríguez, E. Rebollar, M. Soccio, T.A. Ezquerra, D.R. Rueda, J.V. Garcia-Ramos, M. Castillejo, M.C. Garcia-Gutierrez. Macromolecules 2015, 48, 12, 4024–4031. [2] E. Gutiérrez-Fernández, A.Rodríguez-Rodríguez, M.C. García-Gutiérrez, A. Nogales, T.A. Ezquerra, E. Rebollar. Applied Surface Science 2019, (476), 668-675.

Resume : We report on the process of laser surface structuring with a sequence of femtosecond laser pulses for repetition rates varying from 0.01 to 200 kHz. The investigation is carried out both in vacuum and in air by irradiating a crystalline (100) silicon (intrinsic - resistivity > 200 Ω cm - thickness ≈400 μm) target in static conditions. The sequence with a fixed number of pulses N is provided by a diode pumped CPA Yb:KGW system. The wavelength and duration of the laser pulses are ≈1030 nm and ≈180 fs, respectively. The sequence of N laser pulses at a given repetition rate is selected by using a pulse picker acting as pulse divider, while the system runs at 200 kHz. This operational mode allows keeping constant, within their typical statistical fluctuations, the features of the output laser beam during the whole experiment, meanwhile limiting the maximum laser pulse energy at the value achievable at 200 kHz. Both the threshold fluence for shallow crater formation on the irradiated target surface and the morphological features of the surface structures produced inside the crater were analyzed as a function of the repetition rate, for a fixed sequence of N laser pulses. Our experimental findings evidence an interesting variation of the threshold fluence between vacuum and air irradiation above 10 kHz, suggesting a change in the laser-target energy coupling at the higher repetition rates. This observation is ascribed to a plume shielding effect occurring at higher repetition rates in air but negligible in vacuum. In addition, there seems to be a complete absence of heat accumulation. We also observe that the threshold fluence in air is three times lower than vacuum. Such an effect is associate to the diverse efficiency of nanoparticulate debris coverage of the target surface due to material back-deposition in air with respect to vacuum. As for the features of the surface structures, our experimental findings show that the ripples period is independent of the repetition rate over the investigated range of 0.01-200 kHz. However, ripples with a larger period are formed in vacuum, an aspect ascribed to both the different modulation of the surface depth and nanoparticles coverage in the two different experimental conditions. Instead, irradiation in vacuum hampers the formation of grooves, an intriguing aspect that suggests a possible role of nanoparticles debris in their formation mechanisms. All the aspects illustrated above are particularly important in view of industrial laser surface processing requiring target irradiation at high repetition rates.

Resume : The processing of laser-induced periodic surface structures (LIPSS) represents a simple and robust way for the nanostructuring of solids that allows creating a wide range of surface functionalities featuring applications in optics, tribology, medicine, energy technologies, etc. While the currently available laser and scanner technology already allows surface processing rates at the m2/min level, industrial applications of LIPSS are sometimes hampered by the complex interplay between the nanoscale surface topography and the specific surface chemistry. This typically manifests in difficulties to control the processing of LIPSS and in limitations to ensure the long-term stability of the created surface functions. This presentation aims to identify some unsolved scientific problems related to LIPSS, discusses the pending technological limitations, and sketches the current state of theoretical modelling. Hereby, it is intended to stimulate further research and developments in the field of LIPSS for overcoming these limitations and for supporting the transfer of the LIPSS technology into industry.

Resume : The fabrication of artificial biomimetic surfaces via femtosecond laser processing is presented. Metallic, semiconductor and dielectric surfaces were irradiated and Laser Induced Surface Structures (LIPSS) were observed in each type of material. In particular, femtosecond laser pulses with linear, circular, radial or azimuthal polarization states were used for structuring steel, silicon and fused silica surfaces. Experimental results showed that the direction of LIPSS in each case proved to be dependent on the laser beam polarisation. A complete study was carried out for the investigation of LIPSS dependence on fluence and the number of pulses per spot for variable beam polarization states and irradiation strategies, allowing the production of new and more complex surface structures. Furthermore, we present a novel way to control and modulate the different LIPSS morphologies and geometries. Moreover, large area surfaces were fabricated, tailored with various micro-and/or nano- structures bearing great structural resemblance to surfaces found in nature such as the lotus leaf, shark skin and butterfly Greta Oto wing. Those bioinspired surfaces manifest remarkable optical and wetting properties, which were attributed to the specific surface morphology. Thus, femtosecond laser processing can be a novel and one single-step method for the fabrication of functional surfaces on almost all classes of solid materials.

Resume : Topological insulators are interesting materials both for their peculiar properties when reduced at low dimension and as thermoelectric materials. Investigation on the effects of irradiation of topological insulator with femtosecond laser pulses is particularly interesting to exploit the possibilities offered by laser patterning and direct writing approaches for the modification of target surface and development of functional materials. However, this topic remains still scarcely investigated. Here we report on the process of femtosecond laser irradiation and surface structuring of a crystal of bismuth telluride (Bi2Te3) with different sequences of N laser pulses (1≤ N ≤1000) at different pulse energies Ep (3 J < Ep < 60 J). The sequence of N laser pulses was generated by a Ti:Sa femtosecond laser source providing 800 nm pulse with a duration of about 35 fs at a repetition rate of 100 Hz. The laser beam was focused on the target surface, at normal incidence in air, by means of a plano-convex lens with a nominal focal length of 75 mm. The target was a piece of Bi2Te3 cleaved from a single crystal grown in a floating zone image furnace. The sample was held on a XYZ translation stage and different spots were produced by changing the laser energy and the number of pulses. The corresponding morphological features of the target surface were analyzed by using a field emission scanning electron microscope (FE-SEM). We analyzed the threshold fluence for the formation of shallow ablation crater ensuing the irradiation of the target surface. Interestingly, the shallow crater is decorated with laser induced periodic surface structures (LIPSS) on the peripheral region or in the tail of the gaussian spot but does not present the development of any surface feature in the central region of the spot irradiated by the higher energy part of the beam. Such an aspect of peculiar crater formation on the values of laser fluence and number of pulses are investigated. Remarkably, at the best of our knowledge, the formation of femtosecond LIPSS on a topological insulator material is reported for the first time. Interestingly, the presence of a two-dimensional array of periodic bumps is evidenced outside the main crater in a region when irradiated with high number of pulses at much lower values of the local laser fluence. In addition, we will also discuss the effects of the number of laser pulses, pulse energy and laser polarization on the morphological features of irradiated target surface. The possible effects of material phase change or surface oxidation on irradiation with femtosecond pulses are also considered to find the correlation with the annular shaped crater formation in Bi2Te3.

Resume : Laser-induced Forward Transfer (LIFT) is an additive manufacturing technique, in which short laser pulses (typically fs, ps or ns) are employed to deposit donor material onto a substrate (receiver). In this technique, the donor material is coated on the bottom of a substrate (carrier), which is transparent to the wavelength of the laser beam. Upon absorption of the laser energy by the donor, at the carrier-donor interface, a small volume of the donor material is transferred to the receiver. Silver particle-based (AgNP) inks are widely used as conductive material in the field of printed electronics. LIFT of these inks has been successfully demonstrated. Unfortunately, the processing window is narrow, due to the non-Newtonian behavior of these inks. That is, small variations of the processing parameters can lead to undesirable effects like turbulent jets, off-angle ejection, satellite depositions etc. Therefore, fundamental understanding of the physical phenomena and dynamics, which drive the ejection of AgNP inks, is needed to improve the robustness of LIFT. This is needed to increase the robustness of the process, the “cleanliness” of deposits, as well as to reduce the volume of the depositions—i.e. to increase deposition resolution. We present the results of an experimental study in which the effect of the main laser parameters (laser pulse energy, pulse duration, laser spot diameter, donor layer thickness and the distance between the donor layer and the receiver) on the size of the AgNP ink depositions is assessed. The diameter of the smallest reproducible deposit was about 40 um, which was obtained at a spot diameter of 10 um, a donor layer thickness of 3 um and at a distance between the donor and the receiver of 13 um. Next, we employed time-resolved shadowgraphy imaging of jets, in order to study the dynamics of the jet formation during the ejection of the donor. In this study, the focus position of the laser beam was varied w.r.t. carrier-donor interface at a fixed pulse energy. This effectively implies a variation of the laser spot size and fluence level at the interface. This allowed us to study a jet-on-jet ejection mechanism, which was observed when comparing the jet formation induced by ns pulses to ps pulses. That is, a smaller secondary jet was found to form on top of the “common” main, larger jet; hence the naming jet-on-jet. The formation of the smaller jet is attributed to the formation of plasma at the carrier-donor interface. As this smaller jet has the potential to result in smaller depositions, we studied the application of two laser beams during LIFT to trigger jet-on-jet. This (also) has the potential to increase the processing window of LIFT of AgNP inks. It was found that, when the two laser spots are coaxially aligned, also a jet-on-jet forms. And that, the resolution of the deposits was maintained, as compared to a single beam, but at the same time the donor-receiver distance for successful printing was increased.

Resume : Printing techniques applied to biology have begun to develop since the 2000s and hold great promise in a near future. They are based on interdisciplinary approaches and use a combination of cells, chemistry, engineering and sophisticated protocols to create artificial tissues. In that scope, it has been more than a decade since Laser-Induced Forward Transfer (LIFT) is studied in lab scale for its ability to print biomaterials and more specifically living cells [1], [2]. This method uses a short laser pulse to transfer tiny amounts of material from a thin film donor to a receptor substrate. Under appropriate conditions, the pulse induces the formation of a jet propagating perpendicularly to the donor substrate. The targeted material is then deposited as a droplet on the collector. It is a nozzle free printing technique, and it is considered as a suitable method to print three dimensional cellular structures with a very high spatial resolution. Combined with stem cells technology this innovative printing process opens new perspectives for the creation of complex bio-models strongly mimicking the in-vivo environment with numerous applications ranging from regenerative medicine to pharmaceutic study and drugs screening. However, the optimization of its performance and its use with living cells require a deep understanding of the ejection dynamic depending on several laser parameters (laser fluence, repetition rate, laser spot size, laser wavelength, pulse duration, absorption mechanism…) and of the effects of living cells on the bio-ink printability. At LP3 (Marseille-France) and in close collaboration with MMG (Marseille-France), we took advantages of our expertise in LIFT process [3] to master the printing by LIFT of living cells in good condition (high spatial resolution, high printing resolution and high cellular viability after printing). Here, we will present the LIFT process and its optimization allowing to master the bio-ink deposition in order to create reliable ordered patterns of bio-ink micro-droplets containing stem cells with a high spatial resolution. By changing the film concentration in cells and the laser pulse energy, we can control the droplet size and the number of cells in each droplet (from tens of cells down to the single cell level). Then, we will present a viability study of the printed cells after transfer to prove the ability of this process to create relevant bio-models. [1] J. A. Barron, et al. , « Biological laser printing of three dimensional cellular structures », Appl. Phys. A, vol. 79, no 4‑6, p. 1027‑1030, sept. 2004. [2] M. Duocastella et al., « Study of the laser-induced forward transfer of liquids for laser bioprinting », Appl. Surf. Sci., vol. 253, no 19, p. 7855‑7859, juill. 2007. [3] P. Delaporte et A.P. Alloncle, «Laser-induced forward transfer: A high resolution additive manufacturing technology », Opt. Laser Technol., vol. 78, p. 33‑41, avr. 2016.

Resume : A strategy to achieve high-resolution µm-sized conductive tracks without sintering temperatures is to implement laser technologies. We present here a laser-induced forward transfer process (LIFT) to transfer metallic particles on textiles. The process was optimized (1) to be applied on different types of porous and rough substrates, such as woven and knitted fabrics, and (2) to deposit silver and copper nanoparticles on localized areas of the fabrics. The deposited particles are used afterwards as seeds for the generation of copper conductive lines on fabrics via electroless copper deposition. Here we discuss the laser parameters and mechanisms of the LIFT process required for the generation of metallic nanoparticles on fabrics, as well as optimizations on the LIFT process for the formation of localized conductive lines via electroless deposition.

Resume : The laser digital processing of metal nanoparticle inks has contributed significantly to the rapid advancement of flexible electronics over the past 10 years. Despite these advancements in processing, many challenges related to complex surface morphologies, non – planar form factors and multi-material interfaces with diversified thermal and optical properties, remain unmet. In this work, we explore the boundaries of conformal laser processing of Ag nanoparticle inks employing the laser induced forward transfer (LIFT) technique, for micro-patterns formed on particularly sensitive substrates and multilayered structures within an OTFT architecture. The laser sintering process parameters are tuned so as to minimize the damage induced on the neighboring interfaces. The latter involve challenging surface morphologies, such as patterns and micro-components with periodicity and aspect ratio in the nano to 100-micron scale. We investigate the effect of a plethora of essential for the laser sintering technique parameters, such as the laser repetition rate, the laser pulse duration, the pulse to pulse spatial and temporal overlap and the laser wavelength to the overall result of the process. Thorough characterization comprising optical microscopy, profilometry and SEM observation sheds light to the heat affected zone of the substrates, as well as the morphology and structure of the laser sintered patterns. The electrical performance of the laser printed and sintered patterns is assessed by means of electrical measurements in a 4-point probe IV station. The demonstrated results validate the versatility and flexibility of LIFT combined with laser sintering, which can offer a digital solution to particularly challenging use cases and applications in flexible electronics, in particular for OTFTs for the next generation of flexible displays.

Resume : Increased selectivity, response speed, and sensitivity in the chemical and biological determinations of gases and liquids are arguably an important step towards future micro and nano sized sensing systems. This work surveys our latest progress in engineered complex materials, i.e. graphene functionalized with monoclonal antibody anti-alpha-fetoprotein, for applications as recognizing elements in miniaturized surface acoustic wave sensor design and application. Graphene functionalized with monoclonal antibody anti-alpha-fetoprotein have been printed by laser induced forward transfer (LIFT) into 2-dimensional pixels onto the active surface of a SAW platform. First, a parametric study (i.e. laser fluence, donor film morphology and thickness as well as single versus multiple pixel deposition) was carried out to determine the optimum experimental conditions under which sensitive pixels are obtained. Following the morphological and structural characterization of the laser printed material, the responses of the coated SAW resonators are measured. The sensitivity of the monoclonal antibody anti-alpha-fetoprotein functionalized graphene coated SAW devices gives an indication these devices represent an enabling technology for monitoring liver damage. The obtained sensor showed a good signal for detection of AFP in range 1,2-145,1 micro g/L with potential application in early detection of liver cancer forms. Acknowledgements: This work was supported by a grant of the Ministry of Education and Research, CNCS UEFISCDI, project number PN-III-P2-2.1-PED-2019-1603 “Surface acoustic wave biosensor based on functionalized graphene with monoclonal anti-alpha-fetoprotein antibody for hepatic cancer diagnostic” within PNCDI III.

Resume : Months before the first experiments using a 10 PW laser at ELI-NP, we explore the avenues opened by this unprecedented level of light-mater interaction. Already at the edge of material science technology, the complex laser and experimental architecture poses numerous challenges on critical aspects of laser-based acceleration, diagnostic and control of the laser-plasma interaction. We will present our strategies to generate spatially and temporally landscaped electromagnetic fields for acceleration of particles and applications. Not only they will enhance the performance of the most powerful laser in the world, but it will also impact the acceleration mechanisms if coupled with a careful target design. We are already in an age where the desired particle beam properties are saturating with laser intensity. New designs and technologies will help scaling laws to reach the extreme level of intensities and pressures required for new physics and applications ranging from radiation generation to advanced QED experiments.

Resume : Laser marking has gained a lot of interest in the past two decades for anti-counterfeiting applications, such as medicinal product identification [1]. To improve the writing resolution, UV femtosecond laser can be used: Concentrating the energy in time makes it possible to benefit from athermal multiphoton absorption to achieve smaller ablation. However, these lasers are still expensive. An alternative method is to concentrate the energy in space using a Bessel beam [2] or photonic nanojet [3]. A photonic nanojet is a highly focused beam at a mesoscale of a dielectric object. Optical fiber tips are a good candidate to achieve photonic nanojet for laser marking applications. The main limitation was the difficulty to fabricate single-mode optical fiber-shaped tips with high curvature. Throughout the years, various fiber micro-lens fabrication techniques have been developed such as laser or electric arc heating processes, wet etching, micro-bead splicing, mechanical polishing, etc. A method, we have recently developed, using polymer-based techniques, offers a low cost and ease fabrication of Hemi-aspherical micro-lenses with high curvatures that cover only the fiber core. Such micro-lenses are the best for light focusing especially for single-mode fibers. In addition, the flexibility of the polymer-based techniques [4], gives precise control over the shape and size of the micro-lens without putting any stress on the fiber. With our technique, a variety of polymers are suitable for the fabrication of fiber micro-lenses such as PDMS, NOA, SU8, etc. Although a lot of information about these polymers is readily available, for instance, the refractive index, ablation fluence thresholds, operating temperature [5], the possibility to use them as microlens for high power applications is not known. Our work aims to study the focusing behavior or the change in the focal spot intensity as a function of the laser fluence, exposure time, and micro-lens temperature. For that we use a direct imaging technique of the focused beam [6], to monitor the potential changes in light cartography generated by the micro-lens. The study will also be accompanied by thermodynamic simulations carried out on COMSOL Multiphysics. The power range of application of these lenses is determined as a function of the polymer choice. High power laser writing application is demonstrated. [1] Krisztina Ludasi et al. “Anti-counterfeiting protection, personalized medicines − Development of 2D identification methods using laser technology”, International Journal of Pharmaceutics, vol. 605, no. 10, 120793, 2021, [2] R. Stoian, MK. Bhuyan, G. Zhang, G. Cheng, R. Meyer, F. Courvoisier, "Ultrafast Bessel beams: advanced tools for laser materials processing”, Adv. Opt. Technol. 7, 165–174 (2018). [3] R. Pierron, J. Zelgowski, P. Pfeiffer, J. Fontaine, and S. Lecler, “Photonic jet: key role of injection for etchings with a shaped optical fiber tip,” Opt. Lett. 42, 1–3 (2017). [4] M. Zaboub et al., ”Fabrication of polymer microlenses on single-mode optical fibers for light coupling”, Optics Communications, vol. 366, pp. 122–126, 2016, doi: 10.1016/j.optcom.2015.12.010. [5] N. E. Stankova et al., “Optical properties of polydimethylsiloxane (PDMS) during nanosecond laser processing,” Applied Surface Science, vol. 374, pp. 96–103, Jun. 2016, doi: 10.1016/j.apsusc.2015.10.016. [6] D. Bouaziz et al., “Direct imaging of a photonic jet at shaped fiber tips,” Opt. Lett., vol. 46, no. 20, p. 5125, 2021, doi: 10.1364/ol.435867.

Resume : Smart terminals, wearable devices and increasingly integrated sensors and analysers on chips or in fibres are requiring smaller and smaller optical components. Therefore, finding suitable methods and materials for implementing photonic functions at the microscale, has always been a direction for researchers to explore. In our research, we focus on the generation of luminescence properties, as this is an important part of optical devices, especially in the display field, sensors, labelling or data storage. Luminescent organic molecules with less than eight conjugated double bond structures can usually only be excited by UV light. Their application is limited, due to the tendency of UV light to alter the properties of other neighbouring molecules. Thus, luminescent materials that can be excited by visible light, such as quantum dots (QDs), nanoparticles (NPs), dyes, etc., have been considered for fabrication or insertion into solids (or fibres), but are not easy to design in three dimensions. We discovered that direct fs laser writing (DLW) in a new type of cyclo olefin polymer (Zeonex® glass) fulfils these requirements. The process through multiphoton absorption with an infrared femtosecond laser makes it possible to perform flexible three-dimensional transformation inside transparent materials. Recently, Kallepalli et al. created luminescence in other more complex polymers (in PMMA), showing data storage applications. However, Zeonex has several advantages over PMMA, especially a low water absorption, interesting for biological applications. On the other hand, water absorption changes the refractive index to a large extent, which restricts application as an optical substrate. On the contrary, Zeonex has good performance in both fields. Zeonex is thus widely used in the fields of optics, life sciences and electronics. It is used in lens and display, however, it hasn’t been considered as a substrate for photonics. In our study, we demonstrate that using a femtosecond laser with a dedicated parameter range, guided by analytical thermal models, luminescence can be induced in Zeonex glass. The luminescent volume is localized at the focus (ca. a few micron3) of the laser beam and a few microns away from it according to laser parameters, as the thermal condition induced by the laser pulse energy seems to define the effect. On the other hand, by regulating the pulse frequency (repetition rate, RR) and pulse energy (Ep), thermal effects are controllable. We found that at least 2 luminophores are created with all luminescence in the visible range between 500 and 585 nm. These luminophores show different ratios depending on the laser parameters and positioning. For instance, with low power irradiation (Ep=100nJ, RR=200kHz), luminescence spectroscopy, located at the beam focus, shows 2 created luminophores: one(L1) is excited at 476nm and the other(L2) is excited at 510nm, they emit light at 530nm and 580nm, respectively. With higher power irradiation (Ep=100nJ, RR=500kHz), L1 is still presents, but L2/L1 ratio is larger, especially in the region a few microns away from the focus. Interpretation was discussed based on a 2-level system (S0, S1). The luminescence at these wavelengths (green-yellow) is very attractive. Besides that, we are even more excited by the fact that DLW can efficiently create new functions in organic material locally, although the mechanism needs to be further investigated.

Resume : Photoluminescence decay characterization in solar cells and light emitting diodes by time-resolved photoluminescence is of high importance for material quality and operation speed aspects. Conventional photoelectron streak cameras are very costly due to a very complicated and specialized production process in vacuum. That indicates, that a cost-efficient and simple streak camera suitable for such tests would be highly demanded. In this presentation, a quadrupole electro-optic deflecting device is presented to be extended to full functionality streak camera operating in wide spectral range (200-1600 nm) by using electro-optic DKDP crystal. The experimental setup consisted of a laser with 10 picosecond pulse duration for sample excitation, electro-optic deflector with high voltage switch and imaging spectrograph with silicon and InGaAs cameras. This device allows spectral and temporal imaging of time-resolved luminescence dynamics in semiconductor materials, with variable bandgap and composition. The device achieves temporal resolution up to 100 ps and further can be improved by using faster high voltage switches. The research was supported by the Research Council of Lithuania under the project No. S-MIP-19-34, LT 2020 536A.

Resume : Coupling molecules or nanomaterials to plasmonic nanocavities with extreme light confinement allows for largely boosted light-matter interaction at the nanoscale and can reveal atomic-scale dynamics affecting the local optical response. Recently, building on the theory of molecular cavity optomechanics, it has been proposed that surface-enhanced Raman scattering (SERS) and infrared absorption (SEIRA) of molecules could be coherently linked to realize THz frequency upconversion toward the single-photon level in nanoscale pixels, opening new possibilities for spectroscopy, imaging, and sensing. Here, we develop a plasmonic nanoparticle-in-groove nanocavity coupled with a few hundred molecules and demonstrate optomechanical transduction of sub-microwatt continuous wave signals from the mid-infrared (32 THz) onto the visible domain (~400 THz) at ambient conditions. To achieve that, both IR and visible plasmonic modes (with electric field enhancement >100) are tightly confined in the ~1-nm-thick BPT molecular monolayer, with a target molecule vibration mode of 1080 cm-1 that are both active for IR absorption and Raman scattering. The incoming IR field resonantly drives the collective molecular vibration via SEIRA, which is probed by a visible pump laser via parametrically enhanced sum- and difference-frequency generation, and results in upconverted SERS signal on both the anti-Stokes and Stokes bands, respectively. High-resolution Raman spectra show sub-natural linewidth of the upconverted signals with respect to their underlying broad spontaneous Raman emissions (Fig. 2D, E), suggesting the coherent nature of the frequency upconversion process with a measured detection limit down to a few nW μW-2. Our dual resonant nanocavity offers an estimated 13 orders of magnitude enhancement in upconversion efficiency per molecule, which gives the external and internal IR-to-VIS photon conversion efficiency to be on the order of 10−7/W and 10-1/W of pump power, respectively. Our results establish molecular cavity optomechanics as a new paradigm for coherent frequency conversion free of phase-matching constraints.

Resume : The well known phenomenon of laser ablation recently attracted interest also in space applications as possible propulsion technique for micro and nano satellites, whose number in earth orbit is rapidly increasing. During laser ablation a small quantity of mass leaves the irradiated surface with very high exhaust velocity, so that a recoil impulse of the order of μN s is generated on the target material. In order to exploit this potential application of laser ablation it becomes then of fundamental importance the of connection between thermodynamic and optical properties of the target and the efficiency in generating the impulse to optimize fuel materials. Polymers are generally considered to be good candidates for laser ablation propulsion, with respect to metals, thanks to their lower mass density and lower ablation threshold. Among commercially available polymers poly(vinyl chloride) (PVC) shows interesting impulse generation performances[1]. However because of its low absorption it also owns poor ablation efficiency. A commonly used technique to increase polymer absorption consists in doping it with an absorber, that can be another polymer, a dye molecule, or nanoparticles. In this work impulse generated by PVC doped with carbon nanoparticles (CNP) and by PVC blended with poly(styrene-sulfonate) (PSS) is compared, by using a specifically designed ballistic pendulum[2]. Optical and thermodynamical characterizations of these materials allowed to show that the localized absorption of laser radiation by CNP results in a more efficient laser-material interaction for the generation of a mechanical impulse[3]. [1] Urech, L., et al. "Polymer ablation: From fundamentals of polymer design to laser plasma thruster." Applied Surface Science 253.15 (2007): 6409-6415. [2] Battocchio, P., et al. "Ballistic measurements of laser ablation generated impulse." Measurement Science and Technology 32.1 (2021): 015901. [3] Battocchio, P., et al. “Poly(vinyl chloride) Coupling with UV Laser Radiation: Comparison between Polymer Absorbers and Nanoparticles to Increase Efficiency for Laser Ablation Propulsion.” J. Phys. Chem. C 2021, 125, 51, 28088–28099

Resume : Laser processing of material surfaces is one of the key technologies for state-of-the-art applications in micro- and nanoelectronics, photonics, sensing technologies, photovoltaics, and in other fields. Silicon as one of the most abundant materials whose importance in scientific and industrial applications is not diminishing with time. Laser treatment of this material is a complicated phenomenon, which proceeds through triggering of a wealth of linear and nonlinear, thermal and nonthermal processes. Many numerical models have been developed targeting on prediction of optimal regimes of silicon laser processing. However, most of the models are usually limited to an only laser wavelength or one pulse duration with the use of a number of fitting parameters and their direct application for other laser irradiation parameters often does not provide agreement with experimental data. In this report, we will analyze the processes involved in silicon excitation and heating by ultrashort laser pulses at 1030 nm wavelength. This involves single- and multiphoton absorption, avalanche ionization, free electron recombination, dynamic optical response of the excited surface layer, melting threshold and melting dynamics at over-threshold laser fluences. The results of analysis will be directly compared with experimental data on the ablation thresholds and crater depths obtained by using a PHAROS laser (Light Conversion, 1030 nm) at pulse durations of 260 fs and 7 ps. A critical assessment is given of the concepts of plasma screening in Auger recombination [1], electron density dependence of energy coupling to the lattice [2], applicability of the multi-layer reflectivity model [3,4], ambipolar diffusion [5-7], and two-photon absorption coefficient as an effective value for several nonlinear absorption processes [7,8]. The Time Dependent Density Functional Theory (TDDTF) [9] is involved to directly derive the rate of the photo-stimulated transition through the direct Si band gap that accounts for the band gap evolution during irradiation. Finally, a set of Si parameters is outlined, which yields a good agreement between modeling and experimental data for 1030 nm, a wavelength widely used for material processing. [1] J. Bok and M. Combescot, Phys. Rev. Lett. 47, 1564 (1981). [2] T. Sjodin, H. Petek, and H-L. Dai, Phys. Rev. Lett. 81, 5664 (1998). [3] K. Sokolowski-Tinten and D. von der Linde, Phys. Rev. B 61, 2643 (2000). [4] N.M. Bulgakova, R. Stoian, A. Rosenfeld, I.V. Hertel, and E.E.B. Campbell, Phys. Rev. B 69, 054102 (2004). [5] A. Rämer, O. Osmani, and B. Rethfeld, J. Appl. Phys. 116, 053508 (2014). [6] Tao Feng, Gong Chen, Hainian Han, and Jie Qiao, Micromachines 13,14 (2022). [7] H.M. van Driel, Phys. Rev. B 35, 8166 (1987). [8] A.D. Bristow, N. Rotenberg, and H.M. van Driel, Appl. Phys. Lett. 90, 191104 (2007). [9] T.J.-Y. Derrien, N. Tancogne-Dejean, V.P. Zhukov, H. Appel, A. Rubio, and N.M. Bulgakova, Phys. Rev. B 104, L241201 (2021).

Resume : Heating and melting solid materials over small space- and time-scales is a way to access the early stages of the melting phenomenon. Nanosecond-pulse laser annealing (LA) is a powerful processing tool for both fundamental investigations of molten phase ultra-rapid kinetics and technological applications, such as nanoscale reshaping, alloy fraction and defects manipulation, dopant redistribution and activation, which are crucial for the fabrication of unconventional 3D sequentially integrated devices. LA processes need to be developed concurrently with the device design, optimizing topography and materials’ choices in complex 3D structures of nm-sized elements with different shapes and phases. This requires a faithful modelling of the non-equilibrium melting phenomena occurring during LA at the nanoscale, which is beyond the capabilities of state-of-the-art TCAD continuum models massively used in industrial environments. In this contribution we will present a multiscale atomistic approach to model ultrafast nanoscale phase-changes during LA processes, based on seamlessly coupling a continuum, finite-elements model (FEM) for laser-matter electromagnetic interaction and thermal diffusion over the µm-scale with a superlattice Kinetic Monte Carlo (KMC) scheme, able to model with local atomic resolution the highly crystal-orientation dependent evolution of liquid-solid interfaces [G. Calogero et al, npj Comp. Mater., in press (2022)]. The methodology reproduces the local melt region in the KMC superlattice via mapping of the thermal field and evolving solid-liquid regions back and forth from FEM to KMC solvers, with nanosecond and atomic resolution and for the whole duration of the laser pulse. Implementing the thermal problem via the FEniCS toolkit and the sLKMC model via the MulSKIPS code [https://github.com/MulSKIPS/MulSKIPS], this tool is fully open-source and its formalism can be applied to any system where the atom kinetics is determined by a strongly space- and time-dependent field, such as temperature or strain. In particular we will present benchmarks against phase-field models and experimental data which validate the approach, and show the results of simulated LA processes of a Si(001) surface at various laser fluences and pulse shapes, considering both homogeneous and inhomogeneous nucleation mechanisms, revealing how liquid Si nuclei generate, deform and coalesce during laser irradiation. This work was funded by EU-Horizon 2020 grant No. 871813 (MUNDFAB).

Resume : The structure of 2D materials, characterized by presenting strong bonds with neighbouring atoms in one plane and weak interactions out of that plane, gives rise to attractive functional properties. Most of the advanced applications of these materials require the control of their monolayer/multilayer configurations, the stoichiometry of the compounds or the preferential growth in an out-of-plane orientation. In this work we study the transforming effect, both compositional and morphological, of ultrashort laser pulses on thin films of 2D materials. The objective, besides studying the laser-matter interaction in 2D materials, is to demonstrate the versatility of laser irradiation as a post-synthesis process to improve or locally change the thin-film properties. Specifically, we study the transformation of a WTe2 thin-film under irradiation with 200-fs pulses, exploring the influence of the laser wavelength (1030 nm and 258 nm), the irradiation fluence and the number of pulses by processing single-shot spots, lines, and areas. The WTe2 thin-films were fabricated by a mixed synthesis process of chemical nucleation and vapor phase transformation, forming randomly oriented 2D out-of-plane flakes over a silicon substrate. Both pre- and post-irradiated thin films have been characterized by various techniques, notably by scanning electron microscopy and micro-Raman spectroscopy. Irradiating at 1030-nm wavelength, with peak fluences in the range of 30 to 200 mJ/cm2 (fluence threshold for single-shot estimated at about 50 mJ/cm2), a decomposition of WTe2 is observed. It is evidenced by a greater presence of crystalline Te and the appearance of a higher concentration of WO3 in the resulting films. Morphologically, when scanning a large area, the formation of periodic structures (LIPSS-like) interestingly occurs. This structuring confers an anisotropy in the optical and electrical properties that can be beneficial for the applications. However, a drawback is a fusion-resolidification process that tend to eliminate the originally 2D out-of-plane flakes for the tested conditions. On the other side, by irradiating at 258-nm wavelength, under peak fluences in the range of 20 to 80 mJ/cm2 (fluence threshold for single-shot modification estimated at about 25 mJ/cm2), a very different transformation is produced. From the morphological point of view, the film undergoes nano-pores formation because of a possible selective ablation process, but without forming any periodic structure and without destroying the 2D flakes. Micro-Raman spectroscopy confirms the permanence of WTe2, the partial removal or amorphization processes of Te and the disappearance of WO3. These transformations are shown to be achievable in large area processing (scanning procedures), enhancing the interest of ultraviolet ultrashort laser pulses as a post-synthesis treatment.

Resume : Commercially available Fourier transform infrared (FTIR) spectroscopic devices are well-established in a wide range of application scenarios such as environmental monitoring and (bio)chemical analysis. However, FTIR-based techniques are limited in sensitivity. A hallmark of IR laser spectroscopy is the significantly higher energy density, albeit within a narrower spectral region. Yet, if this emission band overlaps with analyte signatures of interest, a viable alternative for IR spectroscopic analysis with high sensitivity is provided. IR lasers are also of particular interest for analyses in aqueous media, as water itself is a strong absorber in the mid-infrared (MIR) regime. Hence, a higher energy density allows for probing even highly opaque matrices. Quantum cascade lasers (QCL) can be tailored to any specific emission wavelength in the MIR regime, and if several lasers are combined a broad spectral range (> 1,000 cm-1) may be covered [1,2]. For efficiently analyzing opaque samples in attenuated total reflection (i.e., evanescent field absorption) mode, next to the laser light source appropriate MIR waveguides are required matching the emission characteristics of QCLs. If the thickness and geometrical dimensions of the waveguide are on the order of the magnitude of the wavelength of the propagating radiation, a uniform evanescent field is generated maximizing the radiation intensity within that leaking-out radiation leading to maximizing the achievable signal-to-noise (SNR) ratio [3,4]. Diamond is a promising candidate for such thin-film MIR waveguides, as it is transparent across a wide MIR band, chemically inert, and maybe be grown as nanocrystalline diamond (NCD) thin-film waveguides with micrometers thickness suitable for MIR chem/biosensing platforms [5]. In this contribution, we present a novel NCD thin-film waveguide coupled with tunable quantum cascade lasers for the detection and quantification of caffeine in real-world samples. A critical comparison between the caffeine spectra recorded using this sensor technology vs. conventional IR-ATR analysis using FTIR technology will be presented. 1. Teuber, A.; Mizaikoff, B. Cascade Laser Infrared Spectroscopy. In Encyclopedia of Analytical Chemistry; Wiley, 2021; pp. 1–45. 2. Haas, J.; Catalán, E.V.; Piron, P.; Karlsson, M.; Mizaikoff, B. Infrared spectroscopy based on broadly tunable quantum cascade lasers and polycrystalline diamond waveguides. Analyst 2018, 143, 5112–5119, doi:10.1039/c8an00919h. 3. López-Lorente, Á.I.; Karlsson, M.; Österlund, L.; Mizaikoff, B. Diamond Waveguides for Infrared Spectroscopy and Sensing. In Carbon-Based Nanosensor Technology; Kranz, C., Ed.; Springer International Publishing: Cham, 2019; pp. 87–117 ISBN 978-3-030-11864-8. 4. Haas, J.; Catalán, E.V.; Piron, P.; Nikolajeff, F.; Österlund, L.; Karlsson, M.; Mizaikoff, B. Polycrystalline Diamond Thin-Film Waveguides for Mid-Infrared Evanescent Field Sensors. ACS Omega 2018, 3, 6190–6198, doi:10.1021/acsomega.8b00623. 5. López-Lorente, Á.I.; Wang, P.; Sieger, M.; Vargas Catalan, E.; Karlsson, M.; Nikolajeff, F.; Österlund, L.; Mizaikoff, B. Mid-infrared thin-film diamond waveguides combined with tunable quantum cascade lasers for analyzing the secondary structure of proteins. Phys. Status Solidi Appl. Mater. Sci. 2016, 213, 2117–2123, doi:10.1002/pssa.201600134.

Resume : The design of optoelectronic and photonic components based on the InGaN/GaN/AlGaN material system requires a detailed knowledge of the linear and nonlinear optical material properties. In particular for higher light intensities in lasers or high-power LED structures, nonlinear optical effects become increasingly important as the light is guided and amplified in micron- or even sub-micron sized waveguide structures. In this contribution, we will discuss second- and third-order optical effects in GaN and AlxGa1-xN thin films under excitation with femtosecond laser pulses. In particular, we analyze the impact of the Al concentration and the n-type doping level on the two-photon absorption coefficient and determine the two-photon absorption coefficient and the nonlinear index of refraction of GaN for the wavelength range of 550nm – 1550nm. We will also present investigations of the recombination dynamics of photo-excited electron hole-pairs in different GaN-based 3D heterostructures used in nano-LEDs. From the luminescence transients, we extract information about the interplay of radiative and non-radiative recombination processes as well as the screening of internal electric fields. We also demonstrate electroluminescence from hybrid LEDs based on the InGaN/GaN material system where the hole-conductive polymer PEDOT replaces the p-doped GaN layer and part of the sample structuring is performed using a femtosecond laser system.

Resume : Titanium dioxide (TiO2) is actually a reference standard material for photocatalysis. It is potentially able to reduce the problem of environmental pollution in water and in air that are considered among the major humanity issues and it can be also used for the sterilization of surfaces. Indeed irradiation of TiO2 can induce the formation of electron-holes couples with a high oxidation power that oxidize the organic compounds and destroy bacteria and viruses eventually present on its surface.The deposition of good quality crystalline TiO2 on polymers can boost the use of this material in a large plethora of applications. This work reports an innovative method to obtain anatase TiO2 films on polymeric substrates (polyethylene naphthalate, PEN, and polymethyl metacrylate, PMMA). We firstly used a Low Temperature (90 °C) Atomic Layer Deposition (LT-ALD) of amorphous TiO2 on polymeric substrate and afterward, we induced a phase transition from amorphous to crystalline anatase by UV-Pulsed Laser Irradiation. A KrF excimer laser with a low penetration length was used to avoid the heating and damaging of the polymeric substrates. A detailed morphological, structural, and chemical characterization was performed and the diffusion of the heat and the temperature behavior into the TiO2 and polymeric substrates were also simulated and discussed. The effect of the laser fluence and pulse number on the amorphous-crystal transition was deeply investigated. We found that the formation of TiO2 crystals needs: 1) the fluence overcomes a threshold of ~ 45 mJ/cm2; 2) the number of pulses is higher than 10. Moreover, to avoid the formation of TiO2 cracks or degradation of the substrates, both the fluence and the number of pulses must be kept lower than 70 mJ/cm2 and 100 pulses, respectively. The photocatalytic aptitude and self-cleaning properties of the investigated films were demonstrated by the degradation of the methylene blue dye in aqueous solution and by the wettability measurements, respectively. Thus, the matching of the extremely low-temperature growth by ALD with the UV-laser irradiation allows to synthetize anatase TiO2 films on flexible substrates that could open the way to applications in photocatalysis for wastewater treatment and self-cleaning coatings.

Resume : Biopotential signals are used to measure organ function and diagnose diseases. Biopotential electrodes serve as an interface between the biological tissue and the electronic circuit in order to measure and record biopotentials. Wearable health-monitoring systems should be easy to use, free of stigma, and capable of providing high-quality data. Wearable textile electrodes for biopotential sensing are a promising candidate for long-term health monitoring. Smart textiles, which incorporate electronic elements directly into the fabric, offer a seamless way to incorporate sensors into garments for a variety of purposes. This paper describes the direct writing of laser-induced graphene (LIG) on a Kevlar textile for the production of reusable dry electrodes for long-term ECG monitoring. The electrode as-prepared has a high electrical conductivity and skin contact impedance of 100 ± 1 kΩ to 7.9 ± 2.7 kΩ for frequencies ranging from 40 Hz to 1 kHz, which is comparable to conventional Ag/AgCl wet electrodes. The results show comparable performance with significantly lower electrode-skin impedance for clinical-quality. Even after several hours of use, these electrodes cause no skin irritation and work effectively without the need for skin preparation. The proposed dry electrodes are expected to give greater comfort for use over time due to their flexible nature and a better match to the skin's modulus. The fabrication method is simple, cost-effective, and scalable, thus permitting the production of arbitrary-shaped flexible electrodes for long-term biopotential monitoring.

Resume : The precise surface machining of silicon by pulsed laser processing is challenging. Laser ablation enables direct patterning but causes various morphological as well as structural modifications of the surface and sub-surface region due to highly dynamic processes at the surface that occur also at ultrashort pulsed laser ablation. Due to the increasing demand of precisely structured silicon surface in various fields, such as optics and micro-electronic and micro-fluidic, new laser-based ultra-precise surface machining techniques are required. Therefore, the recently developed laser-induced plasma etching process (LIPE) was studied in relation to the remaining chemical as well as structural modifications after the etching of single crystalline silicon. For the studies, a fs-laser (775 nm, 150 fs, 1 kHz) with a pulse energy (EP) of maximum 750 µJ was focused to a CF4/O2 gas mixture at atmospheric pressure igniting a laser-induced plasma in front of the <100>Si sample. The reactive species of the laser-induced plasma (LIP) enable etchings up to a depth of 100 µm. For comparison, a silicon surface was also structured by direct laser ablation. The LIP etched surface that is characterized by SEM, TEM, XPS- and Raman-spectroscopy shows no melting features, no structural surface or subsurface defects and almost no chemical contamination from the etching process besides a silicon oxyfluoride layer with a thickness of approximately 2 nm on top of the atomically ordered silicon. The comparison shows clearly that the severe structural and chemical modifications at laser ablation of silicon can be avoided enabling ultraprecise surfaces machining.

Resume : In this work, we investigate the effect of rare earth ions substitution on the structural, optical, and conduction behaviors of Ba0.85Ca0.12RE0.03Ti0.90Zr0.04Nb0.042O3 (BCRETZN) (RE=Ce, Pr) compound ceramic produced via a solid-state route. The Rietveld analysis of the X-ray pattern at room temperature indicated a tetragonal structure (P4mm) of our compound ceramic. Afterward, the morphology of the ceramics was explored using scanning electronic microscope (SEM) as well as optical response and conduction behavior. The Photoluminescence properties revealed that the BCPrTZN sample gives rise to the green and red photoemissions under laser excitation at 450 nm at RT. Furthermore, for BCCeTZN sample, the photoluminescence spectra showed that strong violets emission bands were obtained, under excitation at 350 nm at RT The presence of Pr and Ce element in such materials may have significant technological promise in novel multifunctional devices.

Resume : Transition metal dichalcogenides (TMDs) are layered materials that have been lately the subject of an intense research effort due to their promise to become a versatile platform for photonics and electronics. Importantly alloying of these materials allows significant enhancement of their optoelectronic characteristics by broadening their spectral response range in photodetection devices. Currently, deposition of 2D materials onto various substrates is performed by transfer of the as-grown films from their substrates where they have been originally synthesized by different methods or isolated by mechanical exfoliation. With the exception of mechanical exfoliation, which is normally used for fundamental studies and prototyping, the methods that are commonly being used for the growth of TMDs involve high temperatures and a special atmosphere to prompt chemical reactions between the precursors, suppress oxidation and Sulphur loss. Recently, it has been reported that it is possible to synthesise MoS2 and WS2 by local laser-induced decomposition of (NH4)2MoS4 and (NH4)2WS4 respectively in ambient conditions. Local laser decomposition and synthesis of TMDs produces tracks of these materials with controllable thickness, which can be closely packed together to cover uniformly large areas. Due to the substrate-agnostic and local nature of this laser-synthesis process, there is no need to transfer the film from substrate to substrate as well as no additional steps for further micro-structuring of the films device fabrication. The reason for that is the ability to remove any unexposed precursor by simply immersing the laser-processed substrate into organic solvents that dissolve the unexposed precursor film preferentially, leaving only the synthesised TMD tracks on the surface of the sample. The simplicity of this method for the synthesis of TMDs implies the possibility of tuning the stoichiometry of the synthesised films by introducing additional components to form a composite source precursor solution. This approach has been used here to synthesise Mo(1-x)WxS2 alloys with variable stoichiometry, which depends upon the relative concentration of the (NH4)2MoS4 and (NH4)2WS4 precursors in the source solution. Tuning of the electrical and optical properties of the resulting film as a function of stoichiometry has been demonstrated. The conductivity of alloys with different compositions has been measured using transmission line measurements. Two-point contact devices with TMD alloy channels of variable lengths were fabricated for transmission line measurements. The results indicate an increase of the conductivity with increasing W content. Furthermore, field effect transistors and a double Schottky barrier photodetector were produced. A detailed analysis of the compositional content and a thorough electrical characterisation of the films will be presented. The authors acknowledge financial support from the Russian Science Foundation (RSF) (grant No. 21-79-20208)

Resume : The properties of different polymorphs of Nb2O5 have recently caught the attention of the scientific community for different emerging applications as energy storage devices, photocatalysis, sensors or resistive switching devices. However, Nb2O5 has some important drawbacks that hamper its practical applicability. First, pure Nb2O5 has a very low electrical conductivity (typically around 10-9 S·cm-1) that hinders the recollection of charge in energy storage devices or decreases the sensitivity in sensors. Second, it has a bandgap in the UV range, preventing its use as photocatalyst in the visible range. The generation of oxygen-deficiency in niobium pentoxide can increase both the electrical conductivity and the visible light absorption. In this work, we have studied the ultrafast-laser processing of Nb2O5 powders to form uniform and compact layers of oxygen-deficient sintered material. Moreover, the layers show an enhancement of several orders of magnitude of the electrical conductivity, which will be very useful for future applications.

Resume : Hybrid silicon/perovskite tandem solar cells with a monolithic architecture (two terminals), require an interconnecting layer between the two sub-cells, which must: (i) efficiently transport each sub-cell carrier types, (ii) have a high vertical conductivity, but low lateral conductivity; and (iii) be optically transparent (low absorption and reflectivity) [1-3]. This interconnection layer can be fabricated as a recombination lawyer using transparent conducting oxides (TCOs) [4], or as a tunnel junction layer achieved either by amorphous silicon (a-Si) [5] or by a crystalline silicon tunnel junction (c-Si TJ) [6]. However, high-temperature perovskite top cell fabrication processes make a-Si and TCOs less appealing due to degradation of their electrical properties at higher temperatures, therefore, c-Si TJ are a promising and viable option. We present our approach to fabricate the interconnecting layer by creating a c-Si TJ on the silicon sub-cell using Gas Immersion Laser Doping (GILD) technique, to achieve an abrupt doping profile required for tunnel junctions. The GILD technique consists in doping a semiconductor by surrounding it with a dopant gas and then using ns laser pulses to melt the surface and hence allow the doping gas to diffuse into the melt before recrystallization [7]. Moreover, we think this approach is scalable, fast (0.5 to 10 cm2/min of laser processed area), cost-effective and compatible with existing silicon cell production lines. The experimental setup consists of two main systems, the reaction chamber and the laser apparatus. The cylindrical reaction chamber is made in aluminium with an IR-transparent window and can hold up to 10×10 cm2 silicon wafers. Connected to the chamber is a gas feed system, capable of delivering both argon and POCl3 through a bubbler containing the liquid phosphorus precursor. A 1064 nm solid-state pumped laser with a 0.04 mJ pulse energy, a 500 kHz pulse rate and a 20 μm beam diameter is coupled to a high speed galvano head, allowing for successive laser rastering sequences that melt the Si wafer surface. Laser rastering parameters, such as distance between consecutive laser spots (d = 1.0, 1.5, 2.0, 2.5 and 3.0 µm) and number of rastering sequences (n = 1, 3, 6, 9 times) were varied and combined to obtain a wide range of 5×10 mm2 laser processed areas with different doping depth profiles [8, 9]. p+ (100)-oriented Cz-Si wafers with a p++ emitter were phosphorus doped. Secondary ion mass spectrometry (SIMS) depth profiling was used to characterize the sample elemental composition. The activity of the dopants is also characterized by the electrochemical capacitance-voltage (ECV). Results show successful surface doping with a flat phosphorus concentration within the 10^19-10^20 cm-3 range, followed by an abrupt “shoulder”. Boron from the emitter accumulates at the maximum melt depth, around 200 to 300 nm, clear evidence of the well-known pile-up phenomenon [10]. Laser rastering parameters, like distance between consecutive laser spots and number of rastering sequences, have a great influence on the shape of the phosphorus doping profile, but also change the underling boron emitter profile due to pile-up. By tuning the laser rastering parameters, the phosphorus and boron depth profiles can be adjusted to further attain the desirable TJ properties. References [1] Y. Ko et al., “Recent Progress in Interconnection Layer for Hybrid Photovoltaic Tandems”, Adv. Mater. 32, 2002196 (2020). [2] M. Jošt et al., “Monolithic Perovskite Tandem Solar Cells: A Review of the Present Status and Advanced Characterization Methods Toward 30% Efficiency”, Adv. Energy Mater. 10, 1904102 (2020). [3] S. Akhil et al., “Review on perovskite silicon tandem solar cells: Status and prospects 2T, 3T and 4T for real world conditions”, Mater. Des. 211 (2021) 110138. [4] S. Albrecht et al., “Monolithic perovskite/silicon-heterojunction tandem solar cells processed at low temperature”, Energy Environ. Sci. 9 (2016) 81. [5] P. Schulze et al., “25.1% High-Efficiency Monolithic Perovskite Silicon Tandem Solar Cell with a High Bandgap Perovskite Absorber”, Sol. RRL 4 (2020) 2000152. [6] F. Sahli et al., “Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency”, Nature Mater. 17 (2018) 820. [7] G. B. Turner et al., “Solar cells made by laser‐induced diffusion directly from phosphine gas”, Appl. Phys. Lett. 39 (1981) 967. [8] G. Gaspar et al., “Sequential silicon surface melting and atmospheric pressure phosphorus doping for crystalline tunnel junction formation in silicon/perovskite tandem solar cells”, Proceedings of the 37th European PV Solar Energy Conference and Exhibition (EU PVSEC), Lisbon, 2020, pp. 765-768. [9] G. Gaspar et al., “Tunnel Junction Formation on Silicon P++ Emitters by Gas Immersion Laser Doping”, Proceedings of the 38th European PV Solar Energy Conference and Exhibition (EU PVSEC), Lisbon, 2021, pp. 521-524. [10] P. Lill, et al., “Boron Partitioning Coefficient above Unity in Laser Crystallized Silicon”, Materials 10 (2017) 189.

Resume : Laser Additive Manufacturing is frequently used for the fabrication of complex materials. Its application to ceramic elements is finding difficulties associated to their characteristic properties. The high temperature gradients reached during the laser treatment generate thermomechanical stresses that lead to the formation of cracks. These can be strongly reduced if the laser treatment is performed at relatively high substrate temperatures, inside a patented Laser Furnace (LF) apparatus. This enables an alternative way to process ceramic and glass surfaces under extreme conditions. In addition, laser irradiation is performed using the Laser Line Scan method, in order to achieve large scale, continuous processes offering the potential for tuning the coating thickness for the applications desired. This method has been applied to process ceramic materials with different properties, demonstrating attractive mechanical and optical properties in 600 x 600 mm clay tiles with decorative patterns. The present work deals with dense dielectric Al2O3 and Al2O3-ZrO2 eutectics, as well as with Bi2Sr2CaCu2O8+ superconductors, with dimensions approaching 60 mm x 60 mm. Laser treatments were performed using CO2 (mid-IR) and fiber (n-IR) lasers. In some cases, sample volume temperatures at which the laser treatment was performed have approached values of 1200 °C, thereby reducing or avoiding the generation of cracks within the ceramic material. Additional ceramic layers were applied by deep coating prior to additional laser furnace cycles, so that the desired coating thickness was achieved. Processing and coating parameters were optimized to assure the adequate integration of the different built-up layers, while maintaining the stability of the resolidified ceramic. In addition, the distribution of the laser energy on the sample surface was adjusted to obtain a stable solidification front along the full sample width and produce an ordered microstructure in both cases, the eutectic and the superconductor ceramics. The results obtained with this new technology will be discussed in connection with the expected properties and applications of both types of products, dielectric and superconducting ceramics, as highlights of the advantages provided by the unique combination of Lasers with continuous furnaces. This paves the way to continuous ceramic manufacturing processes leading to dense microstructures free of microcracks. Acknowledgements: Work funded by the Spanish MCIN/AEI/10.13039/501100011033 (project PID2020-113034RB-I00) and by Gobierno de Aragón (research group T54_20R). B. O. acknowledges to the Scientific and Technological Research Council of Turkey (TUBITAK) for a grant via 2219-Science Fellowships and Grant Programmes Department, with the project number:1059B192000390. Authors also would like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.

Resume : Additive manufacturing using regolith, the main in-situ material resource, is offering a solution for long duration stay on the Moon or Mars. Buildings and tools can be manufactured on demand and save transportation cost and time. To prepare the future exploration and optimize the process beforehand, a Selective Laser Melting (SLM) machine, using a power bed, has been developed by our research team. The experimental process was monitored by a pyrometer. Various lunar soil simulants have been used in this system. This study compares the melted samples made off the various simulants and characterized by XRD. The sample porosity and microhardness were measured to choose the best processing parameters. Furthermore, the process conditions were optimized by a numerical model developed on COMSOL Multiphysics. This model is based on data from the literature to cover material properties such as latent heat of fusion and temperature dependent parameters like the thermal conductivity, the thermal capacity, and the density. The meshing was adapted to the optimize the model efficiency. The laser source was modelled as a gaussian beam. The simulation was done with both a static and a moving beam where the effect of power, beam size and scanning speed was investigated. The evolution of temperature of the melt pool, the deformation of the structures and the evolution of the built-in stress were evaluated. This study shows that the outcome from the numerical model corroborates the experimental results in terms of spot size and temperature of the melt pool. The model predicts the failure of the sample for threshold values of power and scan speed for a given spot size.

Resume : In the field of nuclear fission research, laser-heating constitutes a very powerful technique for submitting various materials (metal, alloy, oxide, etc.) to thermal stresses at a laboratory scale and analysing their response in situ. For several years, CEA in collaboration with CNRS has developed multi-objective laser heating platforms (CHAUCOLASE – Chauffage Contrôlé par Laser - in CNRS/Institut Fresnel and CHARTREUSE in CEA Cadarache) based on high power lasers to address various research topics: study of nuclear fuels at very high temperature, characterization of the thermo-physical properties and samples preparation. These experimental devices rely on high-power Ytterbium fibre lasers, dedicated experimental chambers, specific sample holders, optical systems to generate laser beams of variable shapes and intensities, and lastly the associated instrumentation (pyrometers, high-speed and high-resolution infrared cameras, in-situ microscopes, temperature control systems, etc.). These experimental developments are coupled with theoretical approaches regarding laser/material interactions. These two platforms have been designed to be modular in order to access to many experimental configurations and study different topics that we present in this contribution: 1. Thermal transients. These studies are in line with a general continuous improvement of nuclear power reactors safety. The challenges relate to quantifying the impact of specific thermal transients at extremely high temperatures on the materials used in fuel elements to understand/quantify the mechanisms involved in their behaviours and validate the corresponding fuel performance codes developed at CEA with separate effect approaches. Various types of thermal transients can be reproduced in our facilities: temporal dynamics (from ms to h) and spatial distributions (from few µm to cm) representative of hypothetical accident conditions encountered in reactors. One of the most challenging is the Reactivity-Initiated Accident, which is characterized by a very fast rise and strongly localized temperature. During these experiments, we perform temperature elevations, with the associated diagnostics, of more than 2500K, at the periphery of a UO2 sample, in a few tens of millisecond and we correlate the temperature gradients to the mechanical behaviour of the samples. 2. Thermal properties. The knowledge of material properties at high temperature is critical in the context of the evaluation and understanding of irradiated fuel performances. We have therefore developed two techniques that involve laser material-interactions to evaluate thermal properties of nuclear fuel and their evolution as a function of temperature (up to 3000K). We have implemented both techniques on both facilities. Experiments on graphite, as material model, as well as on UO2, are conducted. We present in this paper results for both techniques which allow thermal conductivity quantification thanks to diffusivity measurements: • The laser-flash method, based on a laser-flash excitation and an infrared thermography, is a non-contact measurement of the thermal response of a studied sample. • The Infrared microscopy technique, which presents a spatial resolution of few tens of microns. It is based on the detection of the surface sample temperature rise distribution induced by the absorption of an intensity-modulated focused laser beam. 3. Laser micro/macro-machining of nuclear fuels. A better-detailed description of the behaviour of the nuclear ceramic during thermal transient requires access to local information within the fuel pellet, from the order of a few hundred of microns to millimetres. Except laser micromachining, few techniques are capable of handling such a range and even fewer are applicable to nuclear fuels. We will present experimental, numerical studies carried out in our platforms, and we will show that such laser cutting technique can produce slices with excellent quality (reduced affected area, no recast layer, no micro-cracks and no debris from ejected material) with high efficiency (cutting depth of 1.5 mm, 60 µm kerf width and 2° taper angle). First experiments show the possibility to apply laser ablation technique to nuclear fuels. This work is also supported by EDF and Framatome in the frame of the Tripartite Institute (CEA/EDF/FRA).

Resume : Laser radiation fluxes accordingly to conventional paradigm propagate in optical waveguides-fibers in frame of the multiple consecutive internal total reflection on the so-called zigzag scheme. It is expected that the laser radiation fluxes propagation takes place in case of the consecutive reflections phasing. But the conventional approach ignores three principle factors connected with appearing of the local interference field of standing radiation wave owing to interference between incident and reflected fluxes, existence of the radiation quasimonochromatism defined by radiation wavelength and it dispersion and the spatial coherence. The total internal reflection phenomenon is accompanied by the laser radiation standing wave interference field appearing with longitudinal and transversal sizes are equal to half of the laser radiation coherence length. Taking into account three pointed factors we showed that the laser radiation fluxes can be transported by optical waveguides on base of the multiple internal total reflection mechanism or in result of the waveguide-resonance fluxes propagation in according with relation between the optical core dimension and half of the radiation coherence length. When the optical core magnitude is higher as the radiation coherence length half laser fluxes will be transported accordingly to the multiple total reflection mechanism. In otherwise case, the waveguide-resonance mechanism of the laser fluxes propagation will be realized. It is known that the light core size in optical fibers is usually smaller as 1 mm, whereas the coherence length of the laser radiation can obtain some hundred meters. This relation allows to affirm that the laser radiation fluxes are transported by optical fiber in frame of the waveguide-resonance propagation mechanism and the conventional paradigm is erroneous and must be revised. The report presents main characteristics of the radiation fluxes waveguide-resonance propagation mechanism and points on its universality.

Resume : Zinc oxide (ZnO) is a very promising material for optoelectronic applications due to its direct wide bandgap (3.37 eV) and large exciton binding energy at room temperature (60 meV). However, the development of blue-UV optoelectronic devices based on ZnO homojunctions is hindered by the difficulty in introducing reproducibly high-quality p-type impurities in ZnO, which exhibits intrinsically n-type conductivity. Therefore, ZnO is usually combined with p-doped semiconductors, most commonly with silicon, which is a narrow-bandgap material and extends the operation of ZnO optoelectronic devices to the visible-near infrared (vis-NIR) spectral range. A wide variety of ZnO/p-Si photodetectors have been developed with high response in the UV-visible spectral range. In some cases, selective detection of UV or visible light has been achieved, which is useful for filterless optoelectronic applications, such as imaging, spectroscopy, and machine vision. In addition to n-ZnO/p-Si heterojunctions, which operate as p-n junction minority-carrier devices, n-ZnO/n-Si isotype heterojunctions form an interesting class of majority-carrier devices. In such devices, a weak built-in potential barrier is formed between the two materials, which enables a two-way current flow. The two-way current flow of the isotype junction is useful for wavelength-selective photodetection, as the net current changes polarity and intensity depending on the illumination wavelength. This is in contrast to the more common p-n junctions, where a strong built-in potential barrier is established between the p- and n-type materials, and which operate based on an entirely different mechanism. In this work, an isotype heterojunction n+-ZnO/n-Si photodetector is developed, demonstrating wavelength-selective or broadband operation, depending on the applied bias voltage. Specifically, it demonstrates a high NIR responsivity at positive bias voltage and a broadband responsivity at negative bias voltage. This feature is complementary to the existing n-ZnO/p-Si selective photodetectors, which detect either UV or visible radiation but do not distinguish between NIR and UV/visible radiation. Additionally, even at self-powered (zero bias) operation, it distinguishes between UV, visible, and near IR (NIR) photons by polarity control of the photocurrent. The photodetector is developed by atomic layer deposition (ALD) of ZnO on n-Si, followed by electric contact deposition and annealing. Photoluminescence measurements reveal high optical quality and improved crystallinity of annealed ZnO on silicon. Photocurrent measurements as a function of illumination wavelength and bias voltage show small negative values in the UV-visible spectral range at zero and positive bias voltage and high positive values in the NIR spectral range. At negative bias voltage, the device shows broadband operation with high photocurrent values across the UV-vis-NIR.

Resume : Breast cancer is the most common type of cancer observed in women. Communication with the tumor microenvironment allows invading breast cancer cells, such as triple negative breast cancer cells, to adapt to specific substrates. To elucidate the mechanisms underlying cancer cell survival, proliferation, and metastatic potential in vitro, a number of parameters should be taken into consideration such as the surface chemistry, surface stiffness, and topographic characteristics. Indeed, the substrate topography modulates the cellular behavior, as it has been shown that topographical features of different length-scales, ranging from nano- to micrometer ones, have a strong influence on cell adhesion, morphology, alignment, and contact guidance. The ideal substrate for tumor cell culturing should be able to combine effectively mechanical properties and macro/nano scale topography in order to achieve the optimal conditions for cell survival and, most importantly, to mimic the microenvironment cells naturally reside in. A number of different materials and micro/nanofabrication techniques have been employed to develop substrates for cell culturing. Silicon-based substrates present many advantages as they are amenable to a wide range of processing techniques and they permit rigorous control over the surface structure. In this work, we investigate and compare the response of MDA-MB-231 cells on laser-patterned silicon substrates with two different topographical scales, i.e., the micro- and the nanoscale, in the absence of any other biochemical modification. We were able to develop silicon surfaces with distinct morphological characteristics by employing two laser systems with different pulse durations (nanosecond and femtosecond) and different processing environments (vacuum, SF6 gas, and water). Laser processing of silicon has been shown to generate a variety of structures at the micro- and nanoscale by tuning the fabrication parameters, such as wavelength, pulse duration, fluence, gas or liquid environment, and the number of incident laser pulses. Laser-patterned silicon substrates have been used for the culture of Schwann cells, fibroblasts, and rat pheochromocytoma (PC12) cells, but have not been applied to TNBC (MDA-MB-231) cell culturing before. Our findings demonstrate that surfaces with micro-topography are repellent, while surfaces with nano-topography are attractive for MDA-MB-231 cell adherence. Furthermore, laser patterning of surfaces allows for localized surface modification and high spatial resolution of the formed structures, thus paving the way for the design of co-culturing substrates and devices based on topographical cues. In this way, invaluable information on the proliferative and migrating behavior of breast cancer cells will be provided, which will enable the formulation of novel treatment approaches.