# Light-Matter Complex Interactions in Stereolithographies
## 4 Production of Free Radicals by Light
### One-Photon Process
#### 4.3.5 Two-Photon Stereolithography  
Three-dimensional printing by one-photon photopolymerization transforms a liquid resin into a solid material [2]. There are two phenomena to consider: first, the addition (already at the scanning stage) of support pillars of the object under construction is necessary to ensure the smooth printing process of parts with complex shapes. Second, The removal of the supports, usually by hand, is a time-consuming process. Moreover, the realization of very thin layers is generally delicate (cf. \(\mu\)-stereolithography) and consequently, developing 3D printing by photopolymerization without support is a research direction which can lead to relevant advantages. Proof of Concept  
With a long working distance microscope objective (focal length 4 mm, numerical aperture 0.42, working distance 20.5 mm), an average power of 15 mW at 515 nm, pulse  
Figure 17: Principle of the two-photon stereolithography setup. Figure 18: Multi-photon micro-stereolithography (**left**: sculpture according to [PERSON] et al. [79]; **right**: Eiffel Tower element according to [PERSON] et al. [82]). duration of 500 fs, and a 1 kHz repetition rate, a proof of concept was performed by polymerizing an acrylic resin, HDDA (1,6-Hexanediol diacrylate), in a spectroscopic cell (see Figure 11). In a first step, the beam focal point is placed at the exit surface of the small reservoir so as to be able to hang a horizontal beam in a second step. Taking into account the characteristics of the beam and the objective, the diameter of the beam at the focal plane is estimated at about ten micrometers (the diffraction limit with this objective is about 1 \(\upmu\)m in air). In a second step, the tank is moved so that the focal point approaches the entrance face (cf. Figure 19 highlighting the 6 mm long object thus created); finally, in a third step, the beam is moved along a vertical line, allowing the fabrication of a bayonet-shaped physical object (1.3 cm high). This demonstrates the possibility of reaching dimensions corresponding to the main 3D market. This simple proof of concept could be used to go further and make applicable parts. Removal of Layers  
Since there can be a volume change related to the polymerisation [2], the part being built should preferentially be supported to prevent it from being deformed by its gravitational fall (Stokes' law). This effect can be notably limited by increasing the viscosity of the resin (which is not a problem since we are working without a layer by layer process), or by adding various fillers to the resin (but which keep the medium transparent to the laser wavelength) that will make the medium practically solid or with solid behavior. Figure 20 gives an example of a realization of a 3D part in a solid reactive medium [86; 87; 88; 89]. The rather classical setup is shown on the right-hand side of the figure (see [90]). Figure 19: Proof of concept (the lens of on the left part of the figure). Figure 20: Part (realized without layer and without support) and two-photon 3D device [91]. In these experiments, we used conventional photochemical initiators (those used for one-photon polymerizations) and standard acrylic resins with known polymerization kinetics after free radical production [2]. Voxels Size vs. Time  
By focusing the laser beam inside the reactive fluid, the initiator is excited by biphotonic absorption and leads to polymerization. Depending on the local flux density, the curve of Figure 2 is followed: no polymerization reaction as long as the inhibitors are not consumed (zone I); then, beginning of polymerization with a fluid material at the beginning which becomes pasty (zone II), before a hardening phase (zone III). To study the relationship between the concentration of the initiator, the luminous flux, and the size of a voxel, a device where we produce a very elongated ellipsoid was chosen to have the best accuracy of measurement of the long axis of the voxel, by taking out the voxel with a small clamp, the short axis can be partially degraded.