Physical processes in single and multiple photons additive nano-manufacturing of three-dimensional polymeric and metallic structures for advanced optics

Abstract

In the field of nanotechnologies the Two-Photons Direct Laser Writing (TP-DLW) is the most advanced optical technique for creating arbitrarily complex 3D structures in organic resists, featuring details down to 50 nm, well below the diffraction limit. More recently, this technique has been used in “resists” containing a photosensitive metallic precursor, activated by the two-photon absorption (TPA) process, allowing for the creation of metallic nanoparticles clusters inside to the focus figure of a highly focused laser beam, where the TPA threshold intensity is reached. The aim of my PhD work was the elucidation of the physical processes involved in the realization of 3D nanostructures made in different materials for applications in micro-fluidics and advanced optics. In particular, I carried out studies on both isotropic and anisotropic photoresists, and on metallic precursors. Concerning the isotropic photoresists, I have investigated the capabilities and the limits of the TP-DLW technique, on the fabrication of microfluidic systems and elements of millimetric size, with micro- and nano-features printed inside the channels. The best results in printing such millimetric structures in terms of geometrical compliance and fabrication time are achieved, by combining the single (SPA) and the two-photon absorption (TPA) processes. The latter one allowed for the creation of a shell, an internal structural scaffold and eventual microscopic details, whereas the former one to polymerize the bulk of the object. However, the development step of microfluidic systems (i.e. the removal of the un-polymerized resist) is quite challenging in general, due to possible swellings and consequent distortions in the structure geometry. In my PhD, I developed an effective protocol to face this issue. The application of the TP-DLW technique to anisotropic reactive mesogens (RMs) resulted in very interesting achievements, as it allowed for the fabrication of 3D solid structures, maintaining the optical properties of liquid crystals, in combination with the mechanical properties of polymers. Effects of the direct laser writing on the internal molecular order of the reactive mesogens have been thoroughly investigated, to ensure a fine control on the optical properties of 3D objects made in liquid crystalline elastomers. Analyses of the physical processes, which occur during TP-DLW and allow for tuning of the optical response of the printed 3D solid structures are shown. Appropriate doping of the reactive mesogens with dyes and chiral dopant agents were performed to investigate different fields of applications. In particular, a chiral agent confers helical order to the RMs, which show selective Bragg reflection of the impinging light in both wavelength and polarization. Micro-fabrication of 3D chiral structures is a brand new field that is paving the way to the creation of photonic devices, such as micro-laser of defined shape, white light reflective object, anti-counterfeiting and data storage systems. I performed a series of experiments aimed at demonstrating the possibility to manipulate the helical structural order of the liquid crystals during TP-DLW. As a consequence, multi-colour three-dimensional structure can be created. Finally, the possibility to include metallic details in polymeric objects or even to create metallic structures would pave the way for the DLW of metallic/polymeric nano-composites. I performed experiments with polymeric or hydrogel matrices doped with a suitable metallic precursor, in a free surface drop cast, or in cell segregated thin film, onto a glass substrate. In such system, I was able to create 1D gratings made of GNPs stripes with single or multiple laser sweep. I demonstrated that the stripe width increases with the laser power and the exposure time, showing a behaviour similar to the photo-polymerization, as expected. I also analysed the influence of the exposure time over the nano-particles size distribution and density and showed that by suitably adjusting the exposure time it is possible to maximize the occurrence of a given diameter. The experiments were aimed at elucidating the involved physical phenomena, beyond the bare optical absorption. In particular, the key-role of thermal and diffusive processes have been analysed. TPA leads to the photo-reduction of ions of AuCl4 – and the creation of GNPs, but to a local heating of the sample as well. Due to the very fast heating, a thermal shock-wave is generated and is responsible of the local dehydration in the spot area. Due the concentration gradients of the ions of gold precursor and of water, different diffusive processes take place, occurring on different timescales. Therefore, different characteristic times are observed for the ion and the water diffusion, in the polymeric matrix. My experiments demonstrate that the diffusive effects can be exploited for controlling the NPs density and size when a given energy dose is delivered in multiple shots, by tuning the time interval between each shots. Preliminary experiments on the possibility to control the growth of GNPs through the application of specific electric field during TP-DLW were performed as well. Last but not least, the possibility to use TP-DLW of metal precursor to realize smart platform rich in GNPs suitable to different application is shown. In particular, I demonstrate that, controlling the pitch and the size of GNPs stripes, it is possible to create both thermo-platform whose thermal response to external light is tuneable, and detecting substrates for Surface-Enhanced Raman Spectroscopy (SERS). The Raman spectra were recorded from samples immersed in a solution of rhodamine-6G (R6G), as well as, after exposure of the samples in xylene. SERS enhancement factors of up to ~104 were obtained for both rhodamine-6G and xylene.