Real-Time TDDFT Investigation of Optical Absorption in Gold Nanowires

Using a real-time TDDFT method, a set of linear gold nanowires Au_m (m = 4, 6, 8, 10, 12) are investigated to understand the plasmon-like behavior that results from resonant excitation of a superposition of single-electron transitions. These characteristic excitations of gold nanowires have been previously investigated via linear-response TDDFT calculations, and the results from these two approaches are compared. Real-time TDDFT provides dynamical information about how the electron populations change during excitations in these systems. This study also investigates the relationship between the d-band transitions and the plasmon-like states in gold nanowires. In this work, the longitudinal and transverse absorption peaks are studied after dipolar excitation, and the effects of changing the length of the nanowire are examined. The time evolution of the single-particle transitions and the interplay between different transitions involved in the plasmon-like excitations of model gold nanowires are also investigated. The lowest-energy longitudinal excitation occurs around 1-2 eV in the optical absorption spectra; this peak redshifts with increasing nanowire length. A splitting in the longitudinal peak is present due to the involvement of interband transitions. The frequency of the transverse mode, which lies around 6-7 eV in the absorption spectra, tends to stay constant as the nanowire length increases. The time-dependent occupation numbers and their Fourier transformed spectra reveal that a dominant single-particle transition (ς_n → ς_n+1) can be identified in the longitudinal peaks, which is coupled with less probable d-band transitions (d → ς). In contrast, the transverse modes are constructed from a coupling of two or more single-particle transitions with a ς_n → Î _n character.