The Effect of Geometry on Thermoplasmonic Phenomena in Metallic Nanoparticles

The work investigates thermoplasmonic phenomena in metallic nanoparticles of the different geometry. The relations for the frequency dependences of nanoparticle overheating and radiation efficiency, as well as the size dependence of Joule number, which characterizes the ability of nanoparticles to generate heat, were obtained. At the same time, the size dependences of the effective electron relaxation rate for cylindrical and disk particles are determined within the frameworks of the equivalent spheroid approach. The frequency dependences of polarizability, absorption and scattering cross-sections, overheating, radiation efficiency and the size dependences of Joule number were calculated for spherical, cylindrical and disk nanoparticles of the different sizes and different metals. It is shown that the number and positions of maxima of absorption and scattering cross-sections and overheating of metallic nanoparticles depend on their geometry and, in the case of 1D-particles, on their sizes (aspect ratio). The splitting of these maxima for cylindrical particles is significantly greater than for disk particles. The calculations demonstrate that the overheating of nanoparticles in the biological transparency windows ranges from fractions to a few degrees, except in the case of nanocylinders with the small aspect ratio, where the overheating maxima of silver nanoparticles fall within the first biological transparency window. It was established that the material of nanoparticles also significantly affects the position of their overheating maximum and is determined by the value of the plasma frequency. It is demonstrated the feasibility of using spherical, disk and short cylindrical nanoparticles in applications where negligible overheating in biological transparency windows is required. In the cases where the significant overheating is required, the use of long nanocylinders is appropriate. Fundamental differences in the behavior of Joule number (ability to generate heat) for particles of different geometries under the variation of their radius / aspect ratio have been found.