Size-Dependent Optical Response in Coupled Systems of Plasmons and Electron-Hole Pairs in Metallic Nanostructures

In bulk materials, the collective and individual modes are orthogonal to each other, and no connection occurs in the absence of damping. In the presence of damping, the collective modes, i.e., plasmons decay into hot carriers. In finite systems, the collective and individual modes are coupled by Coulomb interaction. Such couplings by a longitudinal (L) field have been intensively investigated, whereas a coupling via a transverse (T) field has been poorly studied, although plasmons are excited by light irradiated on surfaces and in finite nanostructures. Then, the T field would play a significant role in the coupling between the collective and individual excitations. In this study, we investigate how the T field mediates the coherent coupling. This study is based on the recently developed microscopic nonlocal theory of electronic systems in metals and the results of eigenmode analyses based on this theory. To tune the coupling strength in a single nanorod, we examine three parameters: the rod length Lz, the background refractive index nb, and the Fermi energy epsilon F. We discuss the T-field-induced difference in the spectra of induced current densities to evaluate the coupling by the T field. The T field shifts the collective excitation energy, which causes a finite modulation at both collective excitation and individual excitations. The three parameters can change the energy distance between the collective and individual excitations. Thus, the coherent coupling by the T field is enhanced for the properly tuned parameters. The results of the investigation of the system parameter dependence would give insights into the guiding principle of designing the materials for highly efficient hot-carrier generation.