# Fluorinated Polymers for Photonics--From Optical Waveguides to Polymer-Clad Glass Optical Fibers
## 2 Fluorinated Polymers for Passive Optical Waveguides
### Composite Waveguiding Materials Containing SiO\({}_{2}\) and MgF\({}_{2}\) Nanoparticles  
Inorganic materials possess lower coefficients of thermal expansion and consequently lower TOCs compared to organic materials (polymers). Additionally, some inorganic materials (e.g., MgF\({}_{2}\), AlF\({}_{3}\), SiO\({}_{2}\) and CaF\({}_{2}\)) exhibit low or extremely low refractive indices. The combination of inorganic materials in the shape of nanoparticles and organic polymers (nanocomposites) allows tuning of the refractive indices of the resulting waveguide materials. A possible combination of organic and inorganic materials would be a composite in which the inorganic material serves as a filler in a polymeric matrix. Sufficiently small NPs, smaller than 1/20 of the application wavelength (for 1550 nm, smaller than ca. 75-80 nm), produce very weak scattering, and thus this scattering does not make a significant contribution to the optical propagation losses. Therefore, it would be possible, using inorganic NPs, to  
Figure 5: Measurement of propagation optical losses for core ((**a**) \(n\) = 1.5, 0.4 dB-cm\({}^{-1}\)), core with styrene ((b) \(n\) = 1.48, 0.5 dB-cm\({}^{-1}\)) and cladding with only 5%wt of glycidyl acrylate ((**c**) \(n\) = 1.45, 0.4 dB-cm\({}^{-1}\)), (green lines represent exponential fits). Figure 6: Images of different photolithographic structures prepared via core polymer and surface profiling obtained using Dektak 8 profilometer. tune (reduce) the refractive index and TOC. An application of MgF\({}_{2}\) NPs possessing an extremely low refractive index of 1.37 at 1550 nm was rather interesting. The MgF\({}_{2}\) NPs had already been introduced by us into polymeric matrices [29; 30]. We demonstrated a decrease in the refractive indices of the resulting nanocomposites [29]. The combination of inorganic and organic materials (nanocomposites) for application in polymer optical waveguides has received little attention so far. The influence of NPs on the refractive index and TOC is quite understandable theoretically [31], as it is based on their lower \(n\) and TOC values compared to organic polymer materials. However, the effect of the same inorganic NPs dispersed in a polymer matrix on optical propagation losses has not been widely studied so far. Inorganic materials constituting the core of NPs usually do not adsorb at telecommunication wavelengths. Therefore, they should exert a positive influence on optical propagation losses. However, an organic shell, usually required to render inorganic NPs dispersible in a polymer matrix, may have a negative or positive influence on the absorption depending on the ligand used for the modification. In addition, if NPs are not sufficiently small or agglomerate in the matrix during their dispersion or following polymer curing, additional scattering will also increase the optical propagation losses. The introduction of NPs (MgF\({}_{2}\) and SiO\({}_{2}\)) was attempted for both types of waveguide polymer materials [1; 2; 3]. The general idea for both waveguide material systems is shown in Scheme 8. SiO\({}_{2}\) NPs were purchased from Nissan Chemical Industries Ltd. (Tokyo, Japan), Evonik Resource Efficiency GmbH (Wesseling, Germany) and BYK-Chemie GmbH (Wesel, Germany) [1; 2], while MgF\({}_{2}\) NPs were synthesized in-house. For the direct introduction of SiO\({}_{2}\) NPs into the monomer mixtures described in Section 2.1 (Scheme 8, bottom route), we used commercially available SiO\({}_{2}\) NP dispersions (Nissan, Evonik and BYK; diameter: ca. 20 nm). The NP dispersions from Nissan and the Nanopol dispersions from Evonik are based on solvents with different boiling points. The solvents in MEK-ST and Nanopol C784 have lower boiling points and therefore are easier to remove by evaporation or distillation if required. Other commercially available materials (Nanocryl and Nanobyk) contain non-fluorinated acrylate monomers as dispersive media. Figure 8: Schematic representation of nanocomposite fabrication process. It usually starts with NP modification.