ABSTRACT
Some what like radio frequency antennas, resonantly excited nanostructures, especially in the infrared, act as optical antennas that confine energy of electromagnetic radiation to a restricted volume of subwavelength scale. Hence, nanorods with micrometer-sized lengths L, which show so-called antenna resonances in the IR spectral range,16-18 can be termed nanoantennas. However, the simple l/2-dipole behavior associated with radio frequency antennas does not really hold for nanoantennas at optical (including IR) frequencies because the finite penetration depth of the light into the metal as well as the non-negligible diameter D of the antenna lead to a modified relation:16,19
2L = c1 + c2 [ lres / lp ]. (6.1) In Eq. 6.1, lres denotes the photon wavelength of the antenna resonance and lp denotes the plasma frequency of the antenna’s
material. The coefficients c1 and c2 depend on D and the refractive index nmed of the surrounding medium. Basic assumptions in this model are a high aspect ratio of the antenna (D << L) and that the metal can be described by a free-electron gas Drude-type dielectric function with the electronic damping neglected. Since these conditions are sufficiently fulfilled for crystalline gold nanorods with L/D > 10 in the mid and near IR,20 Eq. 6.1 can be used to describe the resonance behavior of isolated or at least noninteracting nanoantennas. For gold in the mid to near IR, the parameter c1 can be negligibly small.
