# Strategies for Dielectric Contrast Enhancement in 1D Planar Polymeric Photonic Crystals
## 2 Background  
Figure 1a reports the structure of a DBR. The simple PhC alternates thin films of high and low refractive index materials assembled into a planar 1D lattice, with a periodicity comparable with visible and near infrared wavelengths. When a light beam impinges on this structure, the beam is reflected and refracted (transmitted) by any interface between the two materials. All the beams generated by this process interfere with each other, creating a diffraction pattern in the visible and near-infrared spectral regions that is detectable by means of reflectance or transmittance spectroscopy. When the interference between all the reflected (transmitted) beams is constructive (disruptive), diffraction peaks are detectable in reflectance (transmittance) spectra as maxima (minima). These peaks correspond to the PBG of the structure [14]. Intuitively, the larger the dielectric contrast in the lattice, the more efficient the effect. Figure 1b reports the modelled reflectance response typical for a polymer and an inorganic DBR. Both the structures are made of the same number of layers and have the same layer thicknesses. In detail, the DBRs are made of 15.5 bilayers where the high refractive index (HI) material has a geometric thickness of d\({}_{\text{H}}\) = 80 nm, while the low-index material (LI) has a thickness of d\({}_{\text{L}}\) = 110 nm. The data were modeled using a transfer matrix method (TMM) formalism which has been previously described and is reported in the Supporting Information (SI) [37]. Both the structures show an intense reflectance peak which corresponds to the first-order PBG of the DBRs. The background of both spectra is dominated by a Fabry-Perot interference pattern caused by the partial light reflection at the external surfaces of the crystals. The spectra of the two systems ths appear similar in terms of their general features. On the other hand, the PBG of the polymer DBR is blue-shifted, more than 15% less intense and almost four times sharper than the inorganic counterpart. In agreement with the Bragg-Snell law, at normal incidence, the spectral position of the stop-band (\(\lambda_{\text{max}}\)) is a function of the periodicity of the lattice (d\({}_{\text{H}}\) + d\({}_{\text{L}}\)) and the effective refractive index of the structure (\(\lambda_{max}=2\) (\(d_{H}+d_{L}\)) ) \(\sqrt{n_{\mathit{eff}}}\), see SI for details). Figure 1: (**a**) Schematic of a distributed Bragg reflector (DBR) structure. (**b**) Typical reflectance spectra for a polymer (\(\Delta\)n = 0.2, red line) and an inorganic (\(\Delta\)n = 1, black line) DBR made of 31 alternated layers of low (110 nm) and high (80 nm) refractive index materials. Figure 1b shows how the larger dielectric contrast typical of inorganics, which easily approaches \(\Delta\)n = 1, increases the spectral width and the intensity of the PBG with respect to polymers showing lower contrast (\(\Delta\)n = 0.2 in Figure 1). Indeed, as for other photonic structures, DBRs have been traditionally made of inorganic media with high dielectric contrast by vacuum fabrication methods [38; 39; 40; 41] and employed for several applications related to light control, including lasing [42; 43], mirrors and filters [44; 45; 46], and LEDs [47; 48]. Most of these systems, which show a refractive index difference with components larger than \(\Delta\)n = 1, [49; 50; 51], are available commercially [52], but time and cost-demanding fabrications such as sputtering, e-beam evaporations [53; 54], or epitaxial vacuum methods [55; 56] are required for their production. Furthermore, inorganic multilayers lack flexibility and are usually characterized by poor mechanical properties. Finally, the high temperatures and low pressures used for their fabrication are incompatible with the processing of thin film electronics, which are based on organic materials such as sensitive dyes and emitters [57; 58; 59].