# Stimuli-Responsive Photonic Crystals
## 4 Active Tuning of the Photonic Band Gap
### Electric Stimuli
For these reasons, photonic crystals fabricated with plasmonic materials [27; 33; 65; 88; 89; 134] have emerged in the in the last decade. [PERSON] et al. exploited these materials to manufacture 1D photonic crystals composed of alternated layers of WO\({}_{3\times}\) and indium tin oxide (ITO) nanocrystals. In this case, the selected materials show a very similar refractive index in the discharge state (2.19 for ITO and 2.1 for WO\({}_{3\times}\) in the bulk), while charging leads to a strong modification in the WO\({}_{3\times}\) refractive index, thus causing a change of the refractive index contrast (Figure 4a-c). Interestingly, the same procedure can be used to deposit the photonic crystals on an ITO-coated flexible polyethylene terephthalate substrate [90]. Figure 3: (**a**) Schematic representation of a 1D photonic crystal for photonic band gap tuning applying an electric voltage and (**b**) its transmission spectra after the application of an electric field. Adapted from reference [83] under permission from the American Chemical Society. (**c**) Optical effect of an electric field application to 4-bilayer photonic crystals and dense WO\({}_{3}\) film. (**d,e**) Changing of reflectance spectra of PhC with peaks at 450 nm and 550 nm. Adapted from reference [26] under permission from John Wiley & Sons. Another approach relies on the immersion of electrotunable PhCs in liquid electrolytes, with the aim to increase the migration of electrons and ions. Despite the relatively high electrotunability achieved in such devices, the electrolyte can lead to a degradation of the samples and restrict the possibilities of their applications [26; 90; 135]. To address this problem, all solid-state devices have been developed over the past decade, in order to minimize such a detrimental effect [8; 49; 136] For instance, we have recently proposed a 1D electrolyte-free photonic crystal, combining indium tin oxide nanoparticles with TiO\({}_{2}\) nanoparticles on top of a fluorine-doped tin oxide substrate acting as an electrode. The structure was contacted with a top fluorine-doped tin oxide (FTO) substrate and clipped with a paper binder to ensure mechanical stability. By applying a bias to this circuit, charges  
Figure 4: (**a–c**) Cross-sectional SEM image, reflectance spectra and optical image of 1D photonic crystals constituted of indium tin oxide (ITO) NCs and WO\({}_{3\times}\) NC layers (BS1). Adapted from reference [90] under permission from John Wiley & Sons. Effect of an electric tuning and UV curing combination on E-ink. (**d**) Schematic mechanism and (**e**) optical example of lithographical printing by photomask covering. Adapted from reference [91] under permission from John Wiley & Sons. accumulate at the doped semiconductor/TiO\({}_{2}\) interface, leading to an increase of the charge carrier density and an increase of the plasma frequency, according to equation:  
\[\omega_{p}=\sqrt{\frac{Ne^{2}}{m^{*}\epsilon_{0}}} \tag{8}\]  
where \(N\) is the carrier density, \(e\) is the electron charge, \(\epsilon_{0}\) is the dielectric constant under vacuum and \(m^{*}\) is the effective mass [33]. Tunability of photonic crystals can be also achieved by means of electrophoretic forces, which occurs when an external electric field is applied on a high concentration colloidal system. In this way, the particles are in dynamic equilibrium between packing force and electrostatic repulsive force, leading to a specific interparticle distance and, thus, to a specific optical signal. The applied field generates an electrophoretic force between the particles that are forced to reorganize into a more stable structure. Finally, this lattice modification translates into a shift of the photonic band gap [41, 92, 93, 94, 95, 137]. For instance, [PERSON] et al. exploited this mechanism to fabricate an electric-field-assisted multicolor printing (Figure 4d,e) based on electrically tunable and photocurable colloidal photonic crystals [91].