Discrete Dipole Approximation Calculations of Optical Properties of Silver Nanorod Arrays in Porous Anodic Alumina

The experimental extinction spectrum of 38 nm long silver nanorods grown in the 22 nm pores of an anodic aluminum oxide (AAO) template is theoretically analyzed, using the discrete dipole approximation (DDA) method. A single broadband at 485 nm in the spectrum turns out to be the overlap of the two peaks at 430 and 515 rim, corresponding to the transverse and longitudinal modes of the surface plasmon of the nanorods, respectively, at small angles <= 75 degrees of incidence. Interestingly, the longitudinal mode in the array is blue-shifted by about 50 nm from that of an isolated nanorod while the transverse mode does not shift much. Near the glancing incidence, however, the two modes do not contribute much to the spectrum in the visible range due to significant red-shifting and broadening. We show that the incident angle dependence of the optical properties of the silver array may be utilized in controlling local near-field enhancements for surface-enhanced Raman scattering (SERS) and the maximum enhancements are achievable in the incident angles of 30-45 degrees.