# Stimuli-Responsive Photonic Crystals
## 4 Active Tuning of the Photonic Band Gap
### Electric Stimuli  
Electrically tunable photonic crystals represent an incredible opportunity for technological applications, ranging from colorful displays to sensitive claddings and electrochromic windows. Their electrotunability can be triggered in three ways: (1) reorientation of infiltrated liquid crystals; (2) an electrochemical process; and (3) electrophoretic forces in crystalline colloidal arrays. Liquid crystals (LCs) are a class of materials combining the properties of solid crystals and fluids. According to their alignment axis orientation, LCs assume a different distribution inside the liquid, switching from a randomly distributed phase (nematic) to an oriented phase (smectic or chiral). These materials own different refractive indices in different directions, so by changing their phase it is possible tune their optical properties. Thus, it is possible to tune their dielectric constant by applying an external electric field. LCs are usually infiltrated inside a porous structure [78; 79; 80; 81; 122; 123; 124]. [PERSON] et al. fabricated a porous silicon dioxide/zirconium dioxide 1D photonic crystal infiltrated with a nematic liquid crystal (Figure 3a). The device is tuned by applying an external electric field and so changing the LC alignment, producing a blue shift (Figure 3b) of the peak of 8 nm at 8 V [83]. Electochemically tuned photonic crystals consist in an electrochemical cell immersed in a liquid electrolyte. By applying an electric field, it is possible to activate an electrolytic process, which promotes an oxidation-reduction effect or an acid-base exchange. The stimulus produces an electrostatic repulsion; thus, the original structure undergoes a destabilization due to a localized charge variation. The consequent reorganization of the structure generates a geometric variation of the sample, which, in accordance with Bragg-Snell's law, causes a shift of the peak [46; 84; 85; 86; 125; 126]. In the case of polymers, it is possible to incorporate an electro-responsive material inside the main chain, generating a swelling of the matrix under the application of the field [48; 87; 127; 128; 129]. [PERSON] et al. fabricated a WO\({}_{3}\)-based electrochromic PhC by a facile, reproducible, one-step room temperature glancing-angle electron-beam evaporation (GLAD). By changing the deposition angle, it is possible to obtain layers with different porosities corresponding to different refractive indices. The PhC is then immersed in 1M LiClO\({}_{4}\) in propylene carbonate solution and subjected to an external electric field of \(-1.1\) V (vs. Ag/AgCl). This leads to an electrochromic effect, as reported in Figure 3c. A gradual decrease of the reflectance and a shift of the reflection peak are attributed to colored Li\({}_{3}\)WO\({}_{3}\), which decreases the optical thickness (reduces the refractive index) and increases the light absorption (Figure 3d,e). When an anodic potential is applied (+1.1 V), the process is completely reversed [26]. In recent years, the integration of plasmonic nanoparticles in PhCs has attracted the interest of the scientific community. In these systems, quantized carrier oscillations generate localized surface plasmon resonances (LSPRs) that span over a wide range of wavelengths, depending mostly on the charge carrier density and the surrounding refractive environment. For instance, the charge carrier density in heavily doped metal oxide nanocrystals lies in the infrared (IR) region [130; 131; 132] as it is significantly lower compared with bulk materials (10 \({}^{21}\) cm\({}^{-3}\) and 10 \({}^{23}\) cm\({}^{-3}\), respectively). This allows for easy manipulation of this parameter and, hence, of the dielectric function upon application of external electrochemical bias. In particular, by applying an electric field it is possible to induce a capacitive depletion or accumulation and, consequently, a modulation of opticalproperties [65; 132; 133].