# Photo-Aligned Nematic Liquid Crystals Enable the Modulation of Thermoplasmonic Heating
## 1 Introduction  
Thermoplasmonics has become one of the most renowned research topics in plasmonics and nano-optics, thanks to the possibility of generating and controlling a great amount of heat at the nanoscale. Thermoplasmonics deals with the heat produced by metallic nanoparticles (NPs) when a radiation with suitable (resonant) wavelength impinges on them [1, 2]. Indeed, when the electrons of the conduction band of a metallic NP oscillate coherently with the electric field of the incident radiation, an increase of the absorption in the corresponding frequency range occurs (\(\omega_{p}\)) that leads to a rapid rise in the temperature of the NP, followed by a heat dissipation into surrounding media [3, 4]. The temperature increase (\(\Delta T\)) of a single spherical NP can be easily expressed as \(\Delta T=Q/(4\pi k_{H}R_{NP})\), where \(R_{NP}\) is the NP radius, \(k_{H}\) is the thermal conductivity of the host medium, and \(Q\) is the heat power density. \(Q\) depends on the absorption cross-section (\(\sigma_{abs}\)) of the NP, and on the intensity (\(I\)) of the incident light through the equation: \(Q=\sigma_{abs}\)\(I\)[1, 2, 3, 4]. For a metallic NP with spherical or rod-like shape, \(\sigma_{abs}\) can be easily calculated using the Mie or Gans theories, respectively, ref. [1, 2, 3, 4] while for different geometries more complex theoretical models are required. Nowadays, the thermoplasmonic heating is used in several research fields such as high precision medicine [5, 6, 7], electronics [8, 9, 10] optics [11, 12, 13], biology [14, 15], and catalysis [16, 17]. All above applications are based on the possibility to finely tune the photo-thermal efficiency of the NPs that can be done by acting on several control parameters such as the intensity of the impinging beam [1, 2, 3, 4], the number (\(N\)) of involved NPs, and the dielectric function of the surrounding medium \(e_{H}\)[18]. The lattercan be achieved by exploiting the properties of smart and thermo-responsive materials such as thermotropic liquid crystals (LCs). These are organic and anisotropic materials responsive to different external perturbations such as electric, optical and magnetic fields, and temperature variation. LCs have been largely used in various research fields ranging from displays [19; 20; 21] to plasmonics [22; 23]. Notably, the temperature sensitivity of the optical properties of LCs have been used as a compelling solution to realize novel methodologies for monitoring and controlling the light-induced heat of NPs [24]. In this work, we report and discuss on the realization of a thermoplasmonic-based optical device made of an array of gold nanoparticles (AuNPs) immobilized on a glass substrate and layered with a photo-aligned NLC. This hybrid system, which represents an all-optical thermoplasmonic device, ingeniously realized through the combination of hard-matter (plasmonic NPs) and soft matter (LCs), is characterized in terms of morphological, optical, and thermo-optical properties. It turns out that the thermoplasmonic heating can be easily controlled by controlling the refractive index of the surrounding LC medium, thanks to the presence of the thin surface command layer. Indeed, starting from a single layer of metal NPs the photothermal response can be controlled by varying the intensity in a specific range, but also with the same power of the resonant radiation through subtle control of the refractive index of the surrounding medium, obtainable using the photo-aligning material and liquid crystal. The all-optical thermoplasmonic device forecasts exciting applications in the field of light-driven thermoplasmonics.