Advanced Photoinitiating Systems for Two-photon Absorption Induced Polymerization

The two-photon polymerization (2PP) is an additive manufacturing technology (AMT) based on cross-linking of photosensitive polymers induced by femtosecond laser pulses. Due to the nonlinear nature of the two-photon absorption process the resulting photochemical reaction is confined to a small volume, inside the focal point of a laser beam. Therefore, it is possible to produce complex 3D structures with resolution below 100 nm. Capability to produce complex 3D structures at such precision level has tremendous potential to advance developments in the fields of nano-optics, microelectronics, and life sciences. Despite the fact that the achievable resolution is superior to other AMTs, advances in the industrial applications of 2PP are compromised by its low processing speed and lack of available specialized materials. In order to advance the 2PP technology to an industrial level it is essential to increase the fabrication speed. An important milestone on the way to this goal is development of effective two- photon photoinitiators (2PIs). Using conventional Z-scan technique it is possible to measure the two- photon absorption (2PA) efficiency of a photoinitiator (PI) at a fixed wavelength. However, matching the absorption spectra of the PI to the wavelength of the laser used for 2PP is equally important. Within this project a unique setup capable of characterizing a complete visible-to-near-infrared 2PA spectra of a PI in a single measurement by means of supercontinuum (SC) generated white light will be developed. The construction, characterization and the measurements, performed with this set-up, represent a substantial goal of the project. While a large 2PA cross-section is an important parameter and requirement for a 2PI, it does not directly translate into an efficient generation of radicals necessary for polymerization. The absorbed energy can lead to fluorescence or can be converted to heat. In both cases the energy for the generation of radicals is lost. Therefore, the practical efficiency of a 2PI needs to be evaluated also directly with 2PP processing. For material research and development a reproducible evaluation system is indispensable for comparison of different materials. Our group at the TU Wien has recently established a comprehensive characterization method for 2PP technique. This method will be utilized and further developed in the framework of the present project. The main goal of this project is the development of advanced PIs for 2PA induced polymerization. It will be achieved by a close interdisciplinary cooperation between the scientific groups having reputable expertise in synthetic chemistry, nonlinear optics and laser physics, and mechanical engineering. The technical realization of the project relies on three major advances: (i) synthesis of novel aromatic ketone-based 2PIs; (ii) development of white light Z-scan for complete characterization of 2PA properties of these PIs; (iii) evaluation of their performance for 2PP at different pulse energies and processing speeds.

The main aims of the present project were to develop advanced photoinitiators for the efficient two-photon absorption induced photopolymerization and a novel method for fast determination of spectral characteristics of nonlinear absorption of such compounds. The challenge in the latter case is that conventional Z-Scan methods are designed for measurements at a single wavelength and are hardly adaptable for spectral characterization. At the same time, matching the spectral absorption characteristics of the photoinitiators with the wavelength used for two- photon polymerization can be detrimental for the efficiency of the process. In the framework of the project a novel Z-Scan setup based on the supercontinuum generated white-light, as an ultra-broadband coherent light source, was developed. Such white-light beam is split into a two-dimensional sheet of light, which has different wavelengths on top to those below. This way it is possible to analyze the absorption characteristics of photoinitiators at different wavelengths in one single measurement. The developed setup was calibrated and used for characterize novel photoinitiators, which were utilized for two-photon polymerization at high scanning speed.