# Strategies for Dielectric Contrast Enhancement in 1D Planar Polymeric Photonic Crystals
## 2 Background
Thus, in order to simplify the coupling between DBRs and photoactive materials and reduce costs, a large effort has been dedicated to the development of solution-based fabrication processes. This effort resulted in several new strategies, including the use of polymer solutions and inorganic nanoparticle dispersions for the formation of the DBR building blocks [7; 60], and the use of cholesteric liquid crystals [61; 62; 63] and block copolymers [64; 65; 66]. Currently, amorphous polymer DBRs are the only commercial PhCs available on the square-meter scale [67; 68; 69; 70; 71]. However, the relatively small dielectric contrast available for commercial polymers only allows modest effects when dealing with accurate light control applications [37]. Nevertheless, these systems have been proposed in sensing [72; 73; 74; 75], emission control [76], and lasing [77]. As mentioned above, solution and melt processing requires the mutual processability of the dielectric components of the lattice, thus limiting the amount of materials suitable for the fabrication and in turn reducing the available dielectric contrast achievable in the transparency region of the polymers. Table 1 lists the polymers commonly employed in DBR fabrication. These are divided into two sub-groups: high refractive index polymers, which are usually soluble in low-polarity organic solvents; and low refractive index polymers, which are soluble in orthogonal solvents with respect to the former type. High-index polymers comprise polystyrene (PS), poly(_p_-phenylene oxide) (PPO), and poly(_N_-vinylcarbazole) (PVK). The latter is the commercial polymer with the highest refractive index suitable for DBRs with a PBG in the visible spectral region [37]. Common low-index polymers are poly(acrylic acid) (PAA) and cellulose acetate (CA). The largest dielectric contrast achievable in the visible and near infrared spectral ranges is \(\Delta\)n = 0.21 for the CA:PVK pair [78; 79; 80; 81; 82; 83]. However, PVK is often employed together with PAA [84]. This pairing offers a lower dielectric contrast (i.e., \(\Delta\)n = 0.17) but very high optical quality DBR structures [85; 86; 87; 88; 89]. At times, poly(vinyl alcohol) (PVA) is also reported to be a low refractive index medium [90; 91], while recently bio-derived polymers such as alginate and cellulose are attracting attention for the fabrication of DBRs [75; 92; 93; 94]. However, the need to cast these polymers from water solution limits their application. Indeed, the high surface tension of water and the low wettability of high refractive index polymers often hinder their use. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Polymer** &
\begin{tabular}{c} **Refractive Index** \\ **(at 600 nm)** \\ \end{tabular} & **Solvent** & **Ref.** \\ \hline \multirow{3}{*}{HI} & PVK & 1.68 & Toluene, dichlorobenzene & [95] \\  & PPO & 1.57 & Toluene, carbon tetrachloride & [96] \\  & PS & 1.57 & Toluene & [81; 97] \\ \hline \multirow{2}{*}{LH} & PAA & 1.51 & 2-Methyl-2-pentanol & [98] \\  & CA & 1.46 & Diacetone alcohol & [81; 97] \\ \hline \hline \end{tabular}  
HI = high refractive index, LI = low refractive index, PVK = poly(_N_-vinylcarbazole), PPO = poly(_p_-phenylene oxide), PS = polystyrene, PAA = poly(acrylic acid), CA= cellulose acetate.  
\end{table} 
Table 1: Refractive index values at 600 nm and solvents for common commercial polymers. The spectral dispersions of the refractive indexes of the commercial polymers mentioned above are reported in Figure S1 of the SI. For all of them, we note that the refractive index is quite constant in the near infrared part of the spectrum, while their value increases through the visible range until they reach their maximum value in the ultraviolet frequencies. We then observe that the dielectric contrast achievable for DBRs based on PS:CA, which is an established and widely reported system, is as low as \(\Delta\)n = 0.11, while the minimum value, \(\Delta\)n = 0.07, is obtained by coupling PS with PAA.