# Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience
## 3 Photonic Applications
Together with the optical properties of the cholesteric phase, known as selective reflection, the structure reflects left- or right-handed circular polarized light, depending on the handedness of the CLC helix, while the wavelength of the reflected circular polarized light depends on the pitch and average refractive index of the helix. By removing the NIR irradiation, the reverse process would spontaneously occur [287]. In other recent work, the authors designed an adaptive smart window whose optical transmittance could be switched between transparent and opaque states due to the phase transition between the SmA* and the N* phase induced by the photothermal effect of dispersed graphene (Figure 10c) [288]. Light-driven phase transitions in LCs are a fascinating topic from both the scientific and the technological point of view. Apart from the photothermal effect [289], such phase transition can also be obtained by the photoisomerization ability of azo benzene dyes [290; 291; 292; 293; 294], which, at the same time, is also usually used for fabricating LC lasers [295; 296; 297; 298]. However, because these dyes are not nanoparticles, they will not be discussed within the framework of this review. However, since we have mentioned LC lasers on several occasions, it is sensible to go into somewhat more detail by discussing quantum dots (QDs) in this respect. As we mentioned above, QDs are semiconductor nanoparticles with unique physical characteristics such as high quantum yields, large absorption cross-section, tunability of fluorescence emission,  
Figure 10: **(a)** Schematic figure of the photothermal process of reversible handedness inversion. **(b)** Dynamic photonic reflection colours of self-organized helical superstructure upon NIR irradiation. Reproduced with permission from [287]. Copyright (2016) John Wiley and Sons. **(c)** Polymer-stabilized graphene-containing CLC thin film as a photothermal material. Reproduced with permission from [288]. Copyright (2017) ELSEVIER. excellent colour purity, broad excitation spectra, good stability and long lifetimes. As a result, QDs have already been widely used as a nanodopant in LC materials for introducing additional photonic functionalities to LCs including LC lasers. LC lasers are usually made by doping laser dyes (e.g., DCM) into cholesteric LCs (CLCs), which, due to the selective reflection, are photonic band gap materials. A CLC has a 1D modulation of the refractive index due to its unique self-organized helical superstructures, in which rod-like LC molecules self-assemble into monomolecular layers, with their long axes continuously rotating along the helical axis, perpendicular to the director. The helical superstructure is described by its pitch length \(P\) and the helical handedness, where \(P\) is the distance along the helix axis over which the director rotates 360\({}^{\circ}\), while the helix handedness is the direction of rotation. According to [PERSON]'s law, such helical superstructures can selectively reflect circularly polarized light with the same handedness as the helix and the reflection wavelength is determined by its pitch \(\lambda=<\)_t\(n\)\(>\)P_, where \(<\)_t\(n\)\(>\)_ is the average refractive index. Thus, CLCs are usually considered a 1D photonic crystal with an adjustable photonic bandgap (PBGs) via the temperature dependence of the pitch \(P\). After doping the gain medium (e.g., dyes and QDs), the CLC helix acts as a resonator necessary for laser emission, which can be tuned by various external stimuli. Compared to conventional LC lasers, which are fabricated by dispersing laser dyes into CLCs, LC lasers based on QDs can lase emissions with improved features such as narrower line width, lower lasing threshold and low intensity to noise ratios, as well as high stability due to the 3D quantum confinement effect of QDs [131; 299; 300; 301] (Figure 11). On the other hand, amplified spontaneous emission (ASE) from colloids of QDs dispersed in LCs has also been investigated widely [301]. [PERSON] et al.