| Abstract |
The main aspects of this PhD thesis are the study, experimental realization and application of a variety of new laser beam shapes of remarkable tunable optical characteristics, which dynamically can be employed for tailored Micro-Structuring of photosensitive materials using the Direct Laser Writing (DLW) technology by Multi-Photon Polymerization (MPP). DLW by Multi-Photon Polymerization (MPP) is a well-established technique for printing μm-scale three-dimensional (3D) structures with tens of nanometers (nm) resolution. When a beam of an ultrafast laser is tightly focused into the volume of a photosensitive material, polymerization is initiated by multiphoton absorption within a volume element called the voxel. By scanning the voxel in space complicated 3D structures can be fabricated. Although the technique provides high resolution structures, at the same time sets serious scaling limitations to the overall size of the fabricated structures leading to time consuming structuring. Since, the last years the technological and industrial advances increase the demands for rapid fabrication, massive production, parallel processing, long scale structuring and 3D printing engineering, in the content of the present PhD thesis, novel fabrication techniques including Shaped Laser beams, Holographic lithography and Focal beam Engineering for advanced, multiscale and tunable laser material’s processing, are being developed, studied, proposed and demonstrated.
The thesis starts with the study of a new type of non-diffractive beams, the ring-Airy beam, which present the exotic property to autofocus with an abrupt fashion while delivering high intensity contrast at the focal point. Tunable-ring Airy beams are experimentally generated and employed for the fabrication of large three-dimensional structures with high resolution using multi-photon polymerization. We demonstrate that these beams can be adjusted to abruptly autofocus over an extended range of working distances while keeping their voxel shape and dimensions almost invariant. This striking property together with the real-time electronically controlled focus tuning makes these beams ideal candidates for long-range multi-photon polymerization. Moreover, the well-controlled remote localized deposition of energy can also impact many other fields of linear and nonlinear optics, like filamentation and remote high-power terahertz generation.
Next, under optical scale down, we demonstrate that the paraxial ring-Airy beams can approach the wavelength limit, while observing a counterintuitive, strong enhancement of their focal peak intensity. Using numerical simulations, we show that this behavior is a result of the coherent constructive action of paraxial and nonparaxial energy flow. A simple theoretical model enables us to predict the parameter range over which this is possible.
An important aspect for laser material processing is the thorough consideration of the focal volume scaling laws which are unveiled during fabrication, especially for high aspect ratio focal volumes. For this reason, we report on the action of exposure time and peak Intensity on the growth of long-range focal volumes in Multiphoton Polymerization. Using modified engineered Bessel beams, we explore the effects that rise during the voxel growth, while we present a counterintuitive action of the expected expansion of the polymerized volumes which is revealed for a specific range of Peak Intensities. We show that there is a regime where the higher exposure time (number of pulses) creates shorter polymerized strings in comparison to lower number of pulses, and we determine the polymerization thresholds.
At the last part of this thesis, a new three-dimensional (3D) holographic focal volume engineering method is demonstrated and employed for advanced Multiphoton Polymerization. In particular a bundle of g18roups of point sources are holographically generated and accurately positioned in space according to a designed geometry and through all-optical multiple sequential micro-displacements of point sources groups in space they lead to the realization of complete 3D arbitrary structures fabricated by Direct Laser Writing without additional optical or mechanical motion support. The microstructures are fabricated without additional optical or mechanical motion support, under parallel processing of multiple spots configuration while the method provides 20-times faster fabrication time in comparison to point-by-point conventional laser polymerization techniques.
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