Study of the processing conditions for stainless steel additive manufacturing using femtosecond laser

The use of ultrashort-pulsed (USP) lasers in Additive Manufacturing (AM) enables the processing of different materials and has the potential to reduce the sizes and shapes manufactured with this technology. This work confirms that USP lasers are a viable alternative for Laser Powder Bed Fusion (LPBF) when higher precision is required to manufacture certain critical parts. Promising results were obtained using tailored and own-produced stainless steel powder particles, manufacturing consistent square layers with a series of optimized processing parameters. The critical role of processing parameters is confirmed when using this type of lasers, as a slight deviation of any of them results in an absence of melting. For the first time, melting has been achieved at low pulse repetition (500 kHz) and using low average laser power values (0.5–1 W), by generating heat accumulation at reduced scanning speeds. This opens up the possibility of further reducing the minimum size of parts when using USP lasers for AM

LIPSS manufacturing with regularity control through laser wavefront curvature

Laser-Induced Periodic Surface Structures (LIPSS) manufacturing is a convenient laser direct-writing technique for the fabrication of nanostructures with adaptable characteristics on the surface of virtually any material. In this paper, we study the influence of 1D laser wavefront curvature on nanoripples spatial regularity, by irradiating stainless steel with a line-focused ultrafast laser beam emitting 120 fs pulses at a wavelength of 800 nm and with 1 kHz repetition rate. We find high correlation between the spatial regularity of the fabricated nanostructures and the wavefront characteristics of the laser beam, with higher regularity being found with quasi-plane-wave illumination. Our results provide insight regarding the control of LIPSS regularity, which is essential for industrial applications involving the LIPSS generation technique

Polarization conversion on nanostructured metallic surfaces fabricated by LIPSS

Waveplates modify polarization by generating a phase change. Laser Induced Periodic Surface Structures (LIPSS) have recently started to be studied as waveplates due to the birefringence in-duced by the nanoripples, easily fabricated in a one-step process by laser, where LIPSS morphology is defined by the characteristics of the laser process parameters and the substrate material. The optical properties of these waveplates are defined by LIPSS parameters such as period, depth or width of the ripples. In this work we have deposited thin film coatings on stainless steel samples containing LIPSS for different coating thickness and composition. Results show that thin film coatings are a good candidate for the tunability of LIPSS birefringence since the coating modifies the induced polarization change and reflectivity of the sample depending on coating thickness and composition, as expected from numerical simulations

Direct patterning of periodic semiconductor nanostructures using single-pulse nanosecond laser interference

We demonstrate an effective method for fabricating large area periodic two-dimensional semiconductor nanostructures by means of single-pulse laser interference. Utilizing a pulsed nanosecond laser with a wavelength of 355 nm, precisely ordered square arrays of nanoholes with a periodicity of 300 nm were successfully obtained on UV photoresist and also directly via a resist-free process onto semiconductor wafers. We show improved uniformity using a beam-shaping system consisting of cylindrical lenses with which we can demonstrate highly regular arrays over hundreds of square micrometers. We propose that our novel observation of direct pattern transfer to GaAs is due to local congruent evaporation and subsequent droplet etching of the surface. The results show that single-pulse interference can provide a rapid and highly efficient route for the realization of wide-area periodic nanostructures on semiconductors and potentially on other engineering materials

Femtosecond laser fabrication of monolithic double volume phase-gratings in glass

A diffractive optical element was fabricated by monolithically integrating two volume phase-gratings (VPGs) in the bulk of a single-piece transparent material. A computer model of the diffraction generated by the double volume phase-grating (DVPG) was made with a rigorous coupled wave analysis simulator. Simulations and experiments show that the diffractive behavior of a DVPG can be controlled by arranging the relative displacement and the distance between the VPGs according to Talbot self-imaging planes. In order to diffract the total incident light, the phase accumulation in the VPGs has to be π/2, which was achieved by single-scan femtosecond laser processing of a nanocrystal doped glass as the substrate material. Ex situ microscope images of the cross-sections are presented for laser processed lines in the form of VPGs and DVPGs. The far-field diffraction of DVPGs formed by selectively located VPGs was characterized with a monochromatic 633 nm and a supercontinuum white light. Functional designs of high diffraction efficiency with potential applications in photonics were successfully fabricated in a one-step and free of chemicals process

Study of the processing conditions for stainless steel additive manufacturing using femtosecond laser

The use of ultrashort-pulsed (USP) lasers in Additive Manufacturing (AM) enables the processing of different materials and has the potential to reduce the sizes and shapes manufactured with this technology. This work confirms that USP lasers are a viable alternative for Laser Powder Bed Fusion (LPBF) when higher precision is required to manufacture certain critical parts. Promising results were obtained using tailored and own-produced stainless steel powder particles, manufacturing consistent square layers with a series of optimized processing parameters. The critical role of processing parameters is confirmed when using this type of lasers, as a slight deviation of any of them results in an absence of melting. For the first time, melting has been achieved at low pulse repetition (500 kHz) and using low average laser power values (0.5-1 W), by generating heat accumulation at reduced scanning speeds. This opens up the possibility of further reducing the minimum size of parts when using USP lasers for AM

Tailoring diamond's optical properties via direct femtosecond laser nanostructuring

We demonstrate a rapid, accurate, and convenient method for tailoring the optical properties of diamond surfaces by employing laser induced periodic surface structuring (LIPSSs). The characteristics of the fabricated photonic surfaces were adjusted by tuning the laser wavelength, number of impinging pulses, angle of incidence and polarization state. Using Finite Difference Time Domain (FDTD) modeling, the optical transmissivity and bandwidth was calculated for each fabricated LIPSSs morphology. The highest transmission of ~99.5% was obtained in the near-IR for LIPSSs structures with aspect ratios of the order of ~0.65. The present technique enabled us to identify the main laser parameters involved in the machining process, and to control it with a high degree of accuracy in terms of structure periodicity, morphology and aspect ratio. We also demonstrate and study the conditions for fabricating spatially coherent nanostructures over large areas maintaining a high degree of nanostructure repeatability and optical performance. While our experimental demonstrations have been mainly focused on diamond anti-reflection coatings and gratings, the technique can be easily extended to other materials and applications, such as integrated photonic devices, high power diamond optics, or the construction of photonic surfaces with tailored characteristics in general

Femtosecond laser fabrication of LIPSS-based waveplates on metallic surfaces

A fast and reliable method for the fabrication of polarization modifying devices using femtosecond laser is reported. A setup based on line focusing is used for the generation of LIPSS on stainless steel, processing at different speeds between 0.8 and 2.4 mm/s with constant energy per pulse of 1.4 mJ. SEM and AFM characterizations of the LIPSS show a progressive increase in period as the processing speed increases, while height remains approximately constant in the studied range. Optical characterization of the devices shows an induced change in the polarization of the reflected beam that varies with the processing speed, which allows a controlled fabrication of these devices

A versatile cancer cell trapping and 1D migration assay in a microfluidic device

Highly migratory cancer cells often lead to metastasis and recurrence and are responsible for the high mortality rates in many cancers despite aggressive treatment. Recently, the migratory behavior of patient-derived glioblastoma multiforme cells on microtracks has shown potential in predicting the likelihood of recurrence, while at the same time, antimetastasis drugs have been developed which require simple yet relevant high-throughput screening systems. However, robust in vitro platforms which can reliably seed single cells and measure their migration while mimicking the physiological tumor microenvironment have not been demonstrated. In this study, we demonstrate a microfluidic device which hydrodynamically seeds single cancer cells onto stamped or femtosecond laser ablated polystyrene microtracks, promoting 1D migratory behavior due to the cells' tendency to follow topographical cues. Using time-lapse microscopy, we found that single U87 glioblastoma multiforme cells migrated more slowly on laser ablated microtracks compared to stamped microtracks of equal width and spacing (p < 0.05) and exhibited greater directional persistence on both 1D patterns compared to flat polystyrene (p < 0.05). Single-cell morphologies also differed significantly between flat and 1D patterns, with cells on 1D substrates exhibiting higher aspect ratios and less circularity (p < 0.05). This microfluidic platform could lead to automated quantification of single-cell migratory behavior due to the high predictability of hydrodynamic seeding and guided 1D migration, an important step to realizing the potential of microfluidic migration assays for drug screening and individualized medicine. Published under license by AIP Publishing

Development of a thin film growth system to produce nanostructures through laser interference.

This thesis seeks to initiate an innovative process paradigm for the production of dense arrays of identical nanostructures of precise size, shape, and composition by overcoming all the limitations of conventional nanostructuring. The combination of light-based material structuring, with the advantages of a state-of-the-art thin film growth technique, provides a single step, cost-effective and up-to-date capability for next-generation ordered arrays of nanostructures. New methods to achieve such structures are a vital requirement for the exploitation of devices at the nanometer regime. In our approach, we have developed a system that combines interferometric light patterning with Aerosol Assisted Chemical Vapor Deposition (AACVD). To merge these two techniques, a multidisciplinary setup is required. The system is divided into independent modules that have been designed to complete the hardware, which are; pulsed laser, aerosol generator, beam delivery optics, and reactor. Every single subsystem is defined so it can be easily changed to meet the different needs of each of the processes that have been developed. Therefore, different subsystems must be assembled and validated for the three different processes targeted in this thesis. Firstly, for the AACVD of thin films, the reactor chamber and the gas system are integrated. To validate the deposition technique, Zinc Oxide (ZnO) thin films have been grown. The effects of deposition parameters, such as aerosol flow or substrate temperature are studied, showing a wurtzite crystallization in all cases (from 350 ºC to 400 ºC), of which the one with a higher preferential orientation along the c-axis are the grains grown at 375 ºC. From the growth kinetics study, it is extracted that the activation energy of the aerosol assisted chemical reaction is 1.06 eV. Furthermore, ZnO thin films have been optically and electrically characterized. Secondly, nanosecond lasers have been used to assist the chemical reaction of the AACVD. The laser-matter interaction has been studied through two thermal models, one to study the single pulse thermal effect at the nanosecond scale and the second one to study the thermal accumulation produced by the train of pulses. The thermal accumulation results are corroborated by experimental measurements. When including the AACVD technique in the setup, the laser produces local reaction processes that provide energetically favorable sites for the nucleation or structure of the material. Initial experimental results of the performance of this innovative technique are described in which the temperature stability has distinguished itself as the principal technology limiter. Subsequently, precision laser interference optics and state-of-the-art pulsed lasers have been integrated within materials reactors to produce concentrated light patterns with a pitch of fractions of the laser wavelength. Two lasers with different wavelengths (355 nm and 1064 nm) have been used together with two interference optics approaches. With the 355 nm laser, the light pattern induces local photothermal modifications on the grown surface creating nanostructures. The nanostructures show a concordance between theoretical and experimental periods, 792 nm and 800 nm respectively. The dependence of the height of the nanostructures on the number of pulses follows the Marangoni theory developed for this kind of processes. Gold nanostructured thin films have also been achieved using the LI+AACVD technique, completing one-step processing, which supposes an improvement of the previous similar nanocorrugation techniques. Finally, gas sensor devices to detect NO2 have been developed as an application for the nanostructured ZnO thin films. The nanostructured ZnO-based sensors offer several key advantages compared to the only annealed sensors, such as more responsivity, room temperature gas detection, reduction of the baseline resistance, and improvement of NO2 measurements in wet conditions. Therefore, nanostructured ZnO-based sensors are a step forward for the next generations of NO2 gas detectors.Esta tesis busca iniciar un paradigma de procesos innovadores para la producción de matrices de nanoestructuras con idéntico tamaño, forma y composición, superando todas las limitaciones de la nanoestructuración convencional. La combinación de la estructuración de materiales basada en la luz, con las ventajas de una técnica de crecimiento de película delgada de última generación, proporciona un proceso de un solo paso, rentable y actual para matrices ordenadas de nanoestructuras. Los nuevos métodos para lograr tales estructuras son un requisito vital para la explotación de dispositivos en el régimen nanométrico. Bajo nuestro enfoque, hemos desarrollado un sistema que combina el patrón de luz interferométrica con la deposición de vapor químico asistida por aerosol (AACVD). Para combinar estas dos técnicas, se requiere una configuración multidisciplinaria. El sistema se divide en módulos independientes que han sido diseñados para completar el hardware, los cuales son; láser pulsado, generador de aerosol, óptica de emisión de haz y reactor. Cada uno de los subsistemas está definido de tal forma que pueda modificarse fácilmente para satisfacer las diversas necesidades de cada uno de los procesos que se han desarrollado. Por lo tanto, se deben ensamblar y validar diferentes subsistemas para los tres procesos diferentes que se abordan en esta tesis. En primer lugar, para la AACVD de películas delgadas, el reactor y el sistema de gas están integrados. Para validar la técnica de depósito, se han desarrollado películas delgadas de óxido de zinc (ZnO). Se estudian los efectos de los parámetros de deposición, como el flujo del aerosol o la temperatura del sustrato, mostrando una cristalización en forma de wurtzita para todos los casos (de 350 ºC a 400 ºC), de los cuales los granos crecidos a 375 ºC muestran orientación preferencial alta a lo largo del eje c. Del estudio de la cinética de crecimiento, se ha extraído que la energía de activación de la reacción química asistida por aerosol es de 1,06 eV. Además, las películas delgadas de ZnO han sido caracterizadas óptica y eléctricamente. En segundo lugar, se han utilizado láseres de nanosegundos para ayudar a la reacción química del AACVD. La interacción láser-materia se ha estudiado a través de dos modelos térmicos, uno para estudiar el efecto térmico de único pulso en escala de nanosegundos y el segundo para estudiar la acumulación térmica producida por el tren de pulsos. Los resultados de la acumulación térmica son corroborados por mediciones experimentales. Al incluir la técnica AACVD en la configuración, el láser produce procesos de reacción locales que proporcionan sitios energéticamente favorables para el deposito del material. Se describen los primeros resultados experimentales del funcionamiento de esta innovadora técnica en la que la estabilidad de la temperatura del proceso se ha destacado como el principal limitador de esta novedosa tecnología. Posteriormente, óptica de interferencia láser de precisión y láseres pulsados de última generación se han integrado junto con el reactor para producir patrones de luz concentrados con un paso de fracciones de la longitud de onda del láser. Se han utilizado dos láseres con diferentes longitudes de onda (355 nm y 1064 nm) junto con dos sistemas ópticos. Con el láser de 355 nm, el patrón de luz induce modificaciones fototérmicas locales en la superficie crecida creando nanoestructuras. Las nanoestructuras obtenidas muestran una concordancia entre el periodo teórico y experimental, 792 nm y 800 nm respectivamente. La dependencia de la altura de las nanoestructuras con el número de pulsos sigue la teoría propuesta por Marangoni para este tipo de sucesos. También se ha logrado nanoestructurar capas delgadas de oro mediante la técnica LI+AACVD, consiguiendo un procesado de un solo paso, lo cual supone una mejora de las anteriores técnicas similares de nanocorrugación. Finalmente, se han desarrollado sensores de gas para detectar NO2 como una aplicación para las películas delgadas de ZnO nanoestructurado. Los sensores basados en ZnO nanoestructurado ofrecen varias ventajas frente a los tratados termicamente, como una mayor capacidad de respuesta, detección de gas a temperatura ambiente, reducción de la resistencia de referencia y mejora de las mediciones de NO2 en condiciones de húmedad