# Photo-Aligned Nematic Liquid Crystals Enable the Modulation of Thermoplasmonic Heating
## 3 Results and Discussion  
AuNPs monolayers on glass substrates have been prepared by following the procedure described in the Materials and Methods section. The samples were then characterized by performing a morphological analysis by means of an Atomic Force Microscope (AFM). Figure 1a shows a representative AFM image of uniformly distributed AuNPs with an average diameter of about 20 nm. The inset of Figure 1a shows a photo of the realized sample, which exhibits a distinctive pinkish color associated with the presence of well-dispersed AuNPs. Indeed, the spectral response of the sample (Figure 1b) shows the typical absorption peak of spherical AuNPs centered at the wavelength \(\lambda_{LPR}\) = 522 nm, corresponding to the localized plasmon resonance of the AuNPs [1,2]. Figure 1c shows the absorption spectrum of the photoaligning material PAAD-27 (by BeamCo), dissolved in dimethylformamide, which possesses a broad absorption band centered at 420 nm. PAAD-27 is an azo-based material that is used to promote planar alignment of nematic liquid crystals (NLC); its optical and all-optical properties have been reported elsewhere [28,29]. The AuNPs monolayer has been then covered with the PAAD-27 material (see Materials and Methods section): the absorbance of the obtained sample (AuNPs + PAAD 27) is reported in Figure 1d (green curve). The absorption peak of AuNPs is significantly red shifted, from 522 nm to 544 nm. In general, when metallic nanoparticles are immersed in a large refractive index medium, the resonance is shifted toward longer wavelengths [30]. In fact, the shift observed in Figure 1d is related to a change in the refractive index \(n\) of the surrounding medium: from the air value (\(n_{air}\) = 1) to the PAAD-27 value (\(n_{PAAD}\) = 1.73). This behavior well agrees with the Mie theory [31], which provides an analytical solution of [PERSON]'s equations used to calculate the scattering and the absorption of electromagnetic radiation due to a spherical and isolated metallic NP. Figure 2a sketches the AuNPs monolayer on glass substrate before (top-left) and after the deposition of the PAAD-27 (top-right), whose molecules are randomly aligned. When the sample is illuminated with a polarized UV light (10 min, 2.7 J/cm\({}^{2}\)), a PAAD-27 molecular alignment is induced, which turns out to be perpendicular to the light polarization direction. In fact, when PAAD-27 molecules are optically activated by a light beam whose polarization direction is parallel to their absorption oscillators, they become excited and relax several times with a random alignment; this process results in a new orientation of the PAAD molecules with respect to the initial one, with a consequent excess of molecules in the direction in which the absorbing oscillators are perpendicular to the polarization of the incident radiation. It is a consequence of this process that the PAAD-27 alignment direction can be easily modified by changing the polarization direction of the incident UV light beam. Figure 2b highlights that once the sample is exposed to polarized UV radiation, a further red shift of the plasmonic band is observed with respect to the position registered for the sample in which the AuNPs are covered by amorphous (not aligned) PAAD-27; this further red shift of about 16 nmcan be attributed to the change of the refractive index of the medium surrounding the AuNPs, following the alignment of the PAAD-27 molecules. Finally, we have observed that there is no difference in the shift of the plasmonic band of the sample covered with a PAAD-27 layer when this is acted on by radiations with different (orthogonal) polarizations (blue and magenta curves of Figure 2b). This result points out that the plasmonic response of the AuNPs is not substantially affected by the actual in-plane orientation of the PAAD-27 molecules. A very interesting behavior is observed when monitoring the photothermal response of the sample.