We use the discrete dipole approximation (DDA) method to investigate the optical extinction spectra and the E-field
enhancement distribution of Ag nanorods with different topologic shapes, such as cylindrical-, needle-, period-, L-,
parallel-, U-shaped, and helical nanorod structures. Compared to Ag nanorods with a simple shape such as cylinder and
needle, irregular nanorod structures show many distinct plasmon resonance modes over a large spectral range. More hot
spots are observed for nanorods with more kinks and bendings, such as period-shaped and L-shaped nanorods, and the
E-field distribution depends on the polarization and propagation direction of the excitation light. For the parallel-nanorod
structure, when the incident E-field is perpendicular to the nanorod axis, only transverse plasmon modes are excited.
However, for a U-shaped nanorod, longitudinal plasmon modes can also be stimulated along the vertical nanorod
direction. Helical nanorod interacts differently with left- and right-handed circular polarized lights, which provides
another way to tune the plasmon peak and arrange the E-field distribution.