(a) Field intensity enhancements varying with wavelength for different lengths of big Au nanorod. (b) Field intensity enhancements varying with wavelength for different sizes of the small Au nanorod, i.e., Width W and height H both are 8 nm, 10 nm, 12 nm, 15 nm and the corresponding lengths are L = 47 nm, 54 nm, 60 nm, 68 nm, respectively. (c) Field intensity enhancements varying with wavelength for different gap sizes of small antenna.