# Photo-Aligned Nematic Liquid Crystals Enable the Modulation of Thermoplasmonic Heating
## 3 Results and Discussion
The differences in the measured \(\Delta T\) values acquired for the four different cases is related to the variation of the AuNPs plasmonic band induced by changes in the characteristics of the host medium surrounding the AuNPs. In particular, Figure 2b shows that in the case of not aligned PAAD-27, there is an increase of the sample absorbance, and this is also observed when the PAAD-27 molecules are aligned. However, in this latter case, the difference in the temperature variations are not due to a change in the sample absorbance but to a different value of \(k_{H}\) in the two cases: as a matter of fact, the observation that the plasmonic band does not change for the two different alignment conditions indicates that no variations occur in the refractive index of the PAAD-27, which corresponds to no variation in the Frolich condition. Therefore, a prediction of the sample temperature variation goes through a determination of the \(k_{H}\) value of the medium hosting the AuNPs, which is made of glass and PAAD-27 layer. We can say that the PAAD-27 alignment induces a reversible and repeatable thermal anisotropy due to an induced change in the thermal conductivity value of the PAAD-27, which turns out to be different for the two different polarization directions of the resonant impinging light. As demonstrated in a previous work [18], we are able to calculate the thermal conductivity value by detecting variations of sample temperature as a function of the laser intensity; thus, we can determine the thermal anisotropy of the PAAD-27 layer using Equations (1) and (2). Following the procedure illustrated in [18], the \(k_{H}\) values of the PAAD-27 in the three cases are measured as: 0.77 W(\({}^{\circ}\) m)\({}^{-1}\) for the amorphous PAAD-27, 0.32 W(\({}^{\circ}\) m)\({}^{-1}\) for the \(E_{x}\) aligned PAAD-27 and 0.40 W(\({}^{\circ}\) m)\({}^{-1}\) for the \(E_{y}\) aligned PAAD-27. These results confirm that it is possible to modulate the thermal behavior of PAAD-27 material by means of polarized light. As mentioned above, PAAD-27 can be used to align liquid crystals; for this reason, we have studied the behavior of a sample of AuNPs + PAAD-27 modified by adding a nematic liquid crystal (NLC, see Materials and Methods section) to investigate how a variation of the optical properties of the new medium (represented now by PAAD-27 plus NLC) surrounding the AuNPs, determines variations of the thermal efficiency of the whole system that are not related to a change in the value of the thermal conductivity \(k_{H}\) of some of its components. A sketch of the photo-alignment process of the NLC molecules is reported in Figure 3a. Two limit cases are considered for the configuration of the NLC director as determined by the PAAD-27 photalignement: a planar one, in which the directions of the molecular director on the two sides of the cell are parallel to each other, and a twisted one, in which these directions are perpendicular to each other; details on cell fabrication are reported in the Materials and Methods section. Figure 3b shows polarized optical microscope (POM) images of the photo-exposed area of the sample. The original planar aligned area is outside the red circled zone, while inside, there is a twisted aligned area after it has been exposed to an external light source for 7 min (\(\lambda_{max}\) = 405 nm; \(I\) = 30 mW/cm\({}^{2}\)). The spectral response of the sample shows a decrease of the absorbance in the plasmonic band and a blue shift with respect to the AuNPs+PAAD-27 sample (Figure 3c). This behavior can be ascribed to a reduction of the effective refractive index that the AuNPs monolayer sees in the surrounding medium in both (planar and twisted) alignment configurations due to the presence of the NLC. As a consequence, the corresponding temperature variation results are lower with respect to the sample with PAAD-27 alone (Figure 3d) but still totally tunable in a given range.