Study of laser material interactions and applications to surface property augmentation

Recently, laser material interaction has been widely used in many industries as well as research areas because it can achieve low-cost, one-step, and chemical-free manufacturing processes to modify surface properties of target materials. Accordingly, tailored alterations induced by incident laser processing parameters and its characteristics have been applied to particular applications.First, blue diode laser with continuous-wave property can process large area due to high optical output power to augment the performances of thin films via their phase transformation, such as crystallization and graphitization. As a first application, I systematically analyzed the crystallization mechanism of amorphous silicon thin film and the associated thermal deformation of the glass substrate during blue diode laser annealing (BLA) process. BLA was able to grow poly-crystalline silicon laterally (4 µm ×10 µm) with high quality on glass substrates. Furthermore, the thermal deformation induced by BLA was numerically modeled to demonstrate how to control the heat conduction from the thin film that affects the substrate. It is shown that optimizing the scanning speed and incident laser power, the depth of thermal deformation could be comparable to the roughness of the silicon film. Moreover, laser-annealed polydopamine (LAPDA) thin film was demonstrated via BLA to overcome challenges of its inherently weak wear resistance and high surface roughness. LAPDA film was shown to be >100-fold more scratch resistant than pristine PDA and even better than hard inorganic substrates. Furthermore, PDA nanoparticles weakly adsorbed on the surface were selectively removed during BLA, resulting in a smoothened surface. More importantly, BLA achieves such augmentations while preserving the attractive functionality inherent to PDA as demonstrated by grafting of an antifouling polyethylene glycol polymer onto LAPDA film, offering superior mechanical stability and biofouling resistance compared to conventional PDA modified with the same polymer. In addition, femtosecond laser (fs) has been employed to precisely tailor surface properties, such as surface morphology, and the associated optical properties. More specifically, I fabricated laser-induced blackbody emitters (LIBEs) on various metal substrates and graphite to augment the thermal radiative energy transport for solar energy harvesting and storage applications. Consecutive fs laser pulse irradiation results in double-length scale structures consisting of microstructures decorated with nanoparticles (NPs). Such meta-surfaces confine incident light in the cavities between recessed areas, thereby increasing the spectral absorptivity close to 1. The performances of LIBEs were experimentally evaluated for concentrating solar power and thermophotovoltaic applications. Furthermore, high throughput data generation of optical meta-surfaces fabricated by fs laser was pursued for Machine-Learning to find laser processing parameters which can confer optimal optical properties to maximize the efficiency of solar energy harvesting applications. A total of 35,280 unique surfaces were fabricated on stainless steel substrates via high throughput fs laser micromachining setups. Moreover, high throughput measurement of optical properties of all the laser-fabricated surfaces was accomplished via a custom Fourier Transform Infrared microscope spectrometer. The unique combination of three-dimensional laser processing parameter space (laser power, scanning speed, and line spacing) can result in a diverse pool of spectral emissivity data, which are employed to train the ML model. In-situ probing diagnostics for the ablation dynamics induced by lasers are essential to achieve the better understanding of how materials interact with the incoming laser and are removed from the target surface. Specifically, to analyze ablation mechanisms of silver thin film induced by fs laser, I established time-resolved scattering, emission imaging, and associated emission spectroscopy techniques under different laser fluences and background gas pressures. It is found that nanoparticles were released in the vertical direction from the target sample at fluences near the ablation threshold. At higher fluences, plasma plume was formed at the center of the laser beam and NPs were released in oblique trajectories from the peripheral area of the laser-irradiated spot. The average ejection speed of these NPs increases with the laser fluence and as the background gas pressure drops. Lastly, high power rare-earth doped fiber lasers have a capability to remove a large amount of materials from the target surface due to deep melting process. Nevertheless, they entail complex ablation dynamics due to prolonged interactions between material ejecta and incoming laser. Accordingly, ablation dynamics of aluminum induced by single pulse of ytterbium fiber laser were investigated using aforementioned probing techniques, and they allowed us to study a sequence of material ejections across three different phases: (1) ejection of particles and liquid Al columns, (2) secondary detonation of accumulated materials over the surface, and (3) molten liquid Al pool oscillating on the surface and large droplet ejection. Atomic Al and AlO plasma emission were observed during the entire lifetime of the event, verifying the formation of oxidized Al vapor during its interaction with air.

Advisor: Grigoropoulos, Costas P
Zorba, Vassilia