# Strategies for Dielectric Contrast Enhancement in 1D Planar Polymeric Photonic Crystals
## 3 Strategies for the Enhancement of the Dielectric Contrast  
Small differences in the dielectric contrast can provide large effects on the spectral features and thus in the performances of DBRs. Many strategies have been employed to enhance the dielectric contrast in polymer DBRs by lowering or increasing the refractive index of polymer matrices adopting low or high polarizable molecules or fillers. Figure 2 summarizes the refractive index values at a wavelength of 550 nm for components of the polymer DBRs reported in the literature. Figure S2 in the SI reports the entire spectral dispersion of the optical functions together with the reflectance spectra for DBRs built with these materials where available. As mentioned above, the first DBRs were made of PS and CA. This pairing, as evident in Figure 2, allows a dielectric contrast of about \(\Delta\)n = 0.11. Later, several polymers were investigated for DBR fabrication from solution [99]. For instance, PVK substituted PS to increase the dielectric contrast, reaching \(\Delta\)n = 0.21 [100]. For many years, PVK:CA provided the largest dielectric contrast available for DBRs in the visible spectral range [84; 89; 101]. This pairing has indeed been widely used for lasing [102; 103], switchers [104], color purity enhancement in lightening devices [83; 105], and emission control [106]. More recently, the easy filmability of PAA made it promising for DBR fabrications with PVK. DBRs made of PVK:PAA have been reported for lasing [84], switchers [86], and emission control [87]. Recently, perfluorinated polymers (Hy in Figure 2) have been employed as low refractive index media allowing a value of \(\Delta\)n = 0.35 when coupled to PVK [74; 107; 108; 109; 110]. To the best of our knowledge, this is the largest value available for commercial polymers suitable for DBRs. However, the low wettability of perfluorinated polymers film leads to the requirement of the activation of its surface to cast further polymer layers, complicating the fabrication process [108]. Against this background, a great deal of attention has been paid to the engineering of polymer refractive indexes [111]. Besides PhCs, processable high or low refractive index media with optical transparency are highly desirable for a number of applications including, among others, light-emitting devices for displays [112; 113] and antireflective coatings [57; 114]. Very low refractive indexes can be obtained by inserting voids and porosity in the polymer bulk [115; 116; 117; 118; 119; 120]. In contrast, the refractive index can be increased with high filling volumes [109; 118] of inorganic nanoparticles [109; 121; 122; 123] by the addition of a conjugated system with a high density of delocalized electrons or by the introduction of polarizable atoms [116] in the backbone of the polymer chains. A great effort has thus been dedicated to the development of polymers with intrinsic high refractive index values, such as polylimides and thio-derivatives, which report values larger than 1.7 [111; 124; 125; 126]. Highly halogenated polymers [127] and systems containing phosphor [128; 129; 130] display instead values between 1.6 and 1.7. Likewise, hyperbranched polymers with similar refractive index values have often been reported in the literature [131; 132; 133]. In this scenario, several investigations have focused on the implementation of such polymers into DBRs with the aim of enhancing light confinement and thus the performances of optical devices. Hyperbranched polysulphide and inverse-vulcanized polymers [134; 135; 136; 137; 138] have been investigated in depth. Moreover, a great deal of attention has been paid to polymers loaded with high index nano-loads made of metal-oxides [139; 140; 141; 142; 143] and organometallics [144; 145; 146]. One of the first examples regards the use of hyperbranched poly(vinyl sulfide) (HBP, see also Figure S2 of the SI) coupled to CA as a low refractive index material.