Optical properties of gold-silica-gold multilayer nanoshells
Optical properties of concentric and offset gold-silica-gold multilayer nanoshells are computationally investigated using a Mie-based code and a finite-element simulation package. A plasmon hybridization theory is used to interpret the optical tunability. For concentric multilayer nanoshells, the interaction between the plasmon modes on the inner core and the outer shell results in dual plasmon resonances. The low-energy dipole mode is red-shifted by reducing the spacing (i.e., the intermediate silica layer) between the core and the shell. Compared to silica-gold core-shell nanoshells, the extra tunability brought by the inner gold core allows multilayer nanoshells to achieve sub-100 nm dimensions with relatively thick gold shells while maintaining the near-infrared optical properties for absorption-based biomedical applications. For multilayer nanoshells with reduced geometrical symmetry (i.e., the inner core is offset from the center), modes of different orders interact. The mixed interaction introduces the dipolar (bright) characteristic into the higher-order (dark) modes and improves their coupling efficiency to the excitation light. The excitation of the dark modes attenuates and red-shifts the dipole mode and gives it higher-order characteristics.