# Strategies for Dielectric Contrast Enhancement in 1D Planar Polymeric Photonic Crystals
## 5 Prospects for Polymer Planar Microcavities  
As mentioned above, the research literature does not report any use of the engineered polymers described in the previous paragraph for the fabrication of microcavities. This might be related to the large number of periods needed to maximize light confinement and to achieve lasing or spontaneous emission rate enhancement together with the processability of the new polymer systems, which is often not optimized. On the other hand, lasing and emission rate enhancement represent the main reason why high dielectric contrast is highly desired in planar PhCs. One-dimensional microcavities (MC) are structures similar to DBRs in which a \"defect\" layer is engineered to propagate photons at certain frequencies within the PBG [177, 178, 87]. The defect layer consists of a thin film with different optical thickness (n \(\times\) d) with respect to the others composing the structure. A schematic of a planar microcavity is illustrated in Figure 4a. Its reflectance spectrum is similar to that of DBRs, but a minimum of reflectance within the PBG arises owing to the presence of the defect layer. This minimum corresponds to the cavity mode and allows photon propagation,  
Figure 3: (**a**) Reflectance spectra calculated for DBRs made of 15.5 bilayers of low and high refractive index media with thicknesses of d\({}_{\text{L}}\) = 120 nm and d\({}_{\text{H}}\) = 80 nm employing the refractive index of the following material couples: PS:CA [168], PAA:PVK [91, 98], CA:PVK [106], CA:HBP [134], PAA:IVP [138], Hy:PVK [110], CA:TiO2-PVA [170] and Hy:TiO2-PVA. (**b–d**) Maximum photonic band gap (PBG) reflectance (**b**) and full width at half maximum (FWHM) (**c**) as a function of \(\Delta\)n and (**d**) spectral position \(\lambda_{\text{max}}\) as a function of n\({}_{\text{eff}}\) for the calculated spectra. which is otherwise forbidden for the PBG frequencies. Figure 4 reports the spectra calculated for PS:CA (black line) and Hy:TiO\({}_{2}\)-PVA (red line) microcavities normalized to the spectral position of the cavity mode (\(\lambda_{\text{MC}}\)). Notice that the spectral position of the defect mode within the PBG and the number of cavity modes depend on the symmetry of the photonic structure as well as on the optical thickness of the cavity layer [11; 33; 37; 85; 106]. The spectra were obtained for a defect layer with a thickness of d\({}_{\text{D}}\) = 40 nm and refractive index of n\({}_{\text{MC}}\) = 1.5 sandwiched between two DBRs made of 15.5 periods with d\({}_{\text{L}}\) = 120 nm and d\({}_{\text{H}}\) = 80 nm (i.e., the structures discussed in Figure 3). As discussed in the previous paragraph, the intensity and the spectral width of the PBGs increase with the dielectric contrast of the media composing the two DBRs. Concerning the cavity mode centered at \(\lambda\)/\(\lambda_{\text{MC}}\) = 1, it is sharper for the structure made with the polymer pairing with a larger \(\Delta\)n.  
Sharper and deeper cavity modes are symptomatic of the stronger redistribution of the local density of photonic states (LPDOS) and tighter light confinement [37; 180; 181; 182; 183]. These two parameters play a key role in lasing action and emission rate enhancement [181]. When an emitter is physically positioned in the defect layer and its luminescence spectrum overlaps the microcavity PBG, its emission amplitude is spectrally redistributed by the LPDOS, which is ideally zero at the PBG and infinite at the cavity mode. In real microcavities, the LPDOS is often larger than zero at the PBG and only enhanced at the cavity mode. The LPDOS redistributes the dye emission, which is suppressed at the PBG and funneled when the LPDOS is large at the cavity mode [184; 168]. If the emitter displays amplified spontaneous emission (ASE; i.e., population inversion induced by external optical excitation) spectrally overlapped to the allowed mode, when the gain related to the inversion of population is larger than losses, lasing occurs.