# Strategies for Dielectric Contrast Enhancement in 1D Planar Polymeric Photonic Crystals
## 5 Prospects for Polymer Planar Microcavities
We also notice that the peak corresponding to this enhancement decreases in width with increasing \(\Delta\)n. At left side of this peak mode (\(\lambda\)/\(\lambda_{\text{MC}}\) \(<\) 1, green area), the LPDOS is small until it increases again at \(\lambda\)/\(\lambda_{\text{MC}}\) values increasing with \(\Delta\)n., in agreement with the PBG width observed in Figure 3. Indeed, for \(\Delta\)n = 0.11 (PS:CA), the spectral range in which the LPDOS is low ranges between 0.94 \(<\)\(\lambda\)/\(\lambda_{\text{MC}}\) \(<\) 0.98, and for \(\Delta\)n = 0.58 (Hy:TiO\({}_{2}\)-PVA) it ranges between 0.86 \(<\)\(\lambda\)/\(\lambda_{\text{MC}}\)\(<\) 0.99. The LPDOS is also low at (\(\lambda\)/\(\lambda_{\text{MC}}\)\(>\) 1). On the other hand, this area is dominated by an oscillation of density, which can be assigned to the presence of an intense Fabry-Perot pattern. To estimate the ratio LPDOS\({}_{\text{MC}}\)/LPDOS\({}_{\text{SB}}\), we considered the values at \(\lambda\)/\(\lambda_{\text{MC}}\) = 1 and at \(\lambda\)/\(\lambda_{\text{MC}}\) = 0.96 for all the calculated densities. The ratio is reported in Figure 5b and increases exponentially with dielectric contrast. For PS:CA, this ratio approaches 4 and is larger than 700 for Hy:TiO\({}_{2}\)-PVA. The large effect induced by the tuning of \(\Delta\)n is therefore evident. Concerning the L\({}_{\text{eff}}\), Figure 5c shows that the modal length decreases exponentially with the value of \(\Delta\)n from \(\sim\)10.5 \(\upmu\)m for PS:CA microcavity to \(\sim\)2.2 \(\upmu\)m for Hy:TiO\({}_{2}\)-PVA one. As the entire microcavity  
Figure 5: Relation between the local density of photonic states (LPDOS) and \(\Delta\)n. (**a**) Contour plot of the LPDOS as a function of \(\Delta\)n and normalized wavelength (\(\lambda\)/\(\lambda_{\text{MC}}\)). (**b**) Ratio between the LPDOS at the cavity mode and at the PBG and (**c**) MC effective length versus \(\Delta\)n.  
is 6.2 \(\upmu\)m wide, we can estimate that, for a dielectric contrast smaller than 0.2. the field is not confined into the structures, while for a large dielectric contrast the confinement is optimized and reaches roughly one-third of the full microcavity length for the largest \(\Delta\)n. This value seems to be competitive with respect to inorganic structures. Indeed, emission rate enhancement has been demonstrated for Er\({}^{3+}\) placed into Si/SiO\({}_{2}\) cavities (\(\Delta\)n = 2, L\({}_{\text{eff}}\) = 1 \(\upmu\)m) [220] and for bulk GaAs and GaAs quantum wells placed into AlGaAs/AlAs (max \(\Delta\)n = 0.7, L\({}_{\text{eff}}\) = 1.7 \(\upmu\)m) [221; 222]. The effective length of these microcavities is comparable to those achievable with new polymer-based structures; we can therefore envisage the suitability of new polymer-based microcavities for the Purcell effect and for low threshold lasing. Notwithstanding the fact that the polymers reported in this work have not yet been investigated for applications requiring tight light confinement, this study suggests that the approaches undertaken over the last decade pave the way for these applications. To this end, the processability of the engineered polymers should be investigated in detail and optimized to allow multilayers with extended periodicity, aiming at the fabrication of polymer microcavities. Material processing is indeed a key factor in the fabrication of DBR structures [37]. As previously mentioned, spin-coating deposition is widely used as it provides flat interfaces with a roughness below the nanometer scale and good thickness control [31; 104; 168; 134]. Similarly, dip-coating is increasingly investigated for laboratory scale fabrications [109], although other techniques including layer-by-layer deposition [223] and self-assembling of block copolymers have been investigated in recent years [224; 225]. All these techniques have been widely reported for commercial polymers, but few manuscripts describe the processability of engineered polymers suitable for the fabrication of polymer DBRs.