Calculated extinction cross sections versus wavelength for core–shell nanorods with different cores. Height and radius of cores are fixed at 76 nm and 30 nm, respectively. Thickness of gold shell along axis direction is kept at 7 nm, and along radial direction at 5 nm. Refractive index of the core increases from (a) 1.1 to (b) 1.2 to (c) 1.3, and (d) 1.6 in steps of 0.1.