# Strategies for Dielectric Contrast Enhancement in 1D Planar Polymeric Photonic Crystals
## 3 Strategies for the Enhancement of the Dielectric Contrast
In this case, the HBP was obtained by cross-linkingbetween 1,3,5-tripbenzene-triethynebenzene and a di-thiol, reporting a refractive index of 1.72 at 550 nm, corresponding to a DBR dielectric contrast \(\Delta\)n = 0.26 (Figure 2). Successively, [PERSON] and coworkers obtained PVA-titania composites, where amorphous titania (i.e., titanium oxide that contains a certain number of Ti-OH groups) was directly synthetized within a polymer solution. This system was then employed to stabilize commercial titania nanoparticles in order to avoid particle aggregation and light scattering phenomena [109; 147; 148]. Following this approach, the new nanocomposites showed refractive index values as large as 1.9 (PVA\({}_{\text{I}}\) in Figure 2). Such composites were used to fabricate multilayered structures by dip-coating deposition together with a perfluorinated polymer. Notwithstanding the fact that the structure and refractive index of this polymer were not disclosed, we can estimate a value ranging between 1.33 and 1.35, characteristic of perfluorinated compounds, and then suppose a dielectric contrast of \(\Delta\)n = 0.55-0.57 [109]. Later, in a similar approach, a bare hybrid titania-PVA composite (TiO\({}_{2}\)-PVA, without further TiO\({}_{2}\) load) allowed a concentration of the inorganic load as large as 80% v/v, leading to a refractive index value of n = 1.94, which to the best of our knowledge is the highest reported in the literature for optical nanocomposites retaining transparency in the visible spectral range. This medium was then employed together with CA for the fabrication of DBRs, allowing a dielectric contrast of \(\Delta\)n = 0.48 (see also Figure S2 of the SI). In general, nanocomposites have been largely investigated for the fabrication of DBRs [149]. Several research works on loading polymers with TiO\({}_{2}\)[150; 151], SiO\({}_{2}\)[152], ZnO [153; 154; 121], ZrO [155], and even metal nanoparticles [73; 74] have been reported. At times, pristine metal oxide nanoparticle thin films are alternated with polymer layers [156] to achieve higher dielectric contrast or for the modification of the DBR permeability for molecular sensing; the recent literature has indeed reported polymer layers alternated with V\({}_{2}\)O\({}_{5}\)[157; 158], TiO\({}_{2}\)[159; 160; 161; 162; 163; 164], ZnO [165] and CuSCN [166; 167] particles. Lastly, sulfur-rich polymers obtained by inverse vulcanization have been investigated in detail in the field of NIR applications. Polymeric elemental sulfur cross-linked by molecules bearing two or more vinyl-type moieties allows high refractive indexes on one side; however, this implies a noticeable  
Figure 2: Refractive index values at 550 nm for commercial polymers [95; 96; 110; 168] and for engineered polymeric systems employed for the fabrication of DBRs [109; 134; 136; 138; 169; 170]. Notice that the value for CHIIPs is reported in [136; 169] at 600 nm. The value at 550 nm is therefore expected to be larger. Bars represent the dielectric contrast achieved in published DBR structures for commercial polymers (blue bars) and coupling commercial polymers with engineered polymeric media (green bars). The red bar represents the maximum theoretical dielectric contrast achievable within a polymeric system. The dielectric contrast bars are ordered from left to right by publication date. HBP: hyperbranched poly(vinyl sulfide); Hy: perfluorinated polymers. absorption coefficient in the visible part of the spectrum. This approach allows refractive index values larger than 1.9 when divinylbenzene (CHIPs in Figure 2) is employed as a cross-linker, and of 1.83 for a sulfur copolymer with 2,5-disopropenylthiophene (IVP in Figure 2, see also Figure S2 of the SI) [136; 138; 169]. As shown in Figure 2, the largest dielectric contrast reported to date is obtained for DBRs made of titania-based nanocomposite materials.