(a) Schematic diagram of cascade antenna structure with geometry of big and small nanorods being the same as those in Fig. 1, and gap between two adjacent Au nanorods being 15 nm. (b) Resonant electric field intensity distribution of cascaded antenna (top) and individual small antenna (bottom). (c) Spectrum of field intensity enhancement in gap center of individual small antenna (black), optimized two-wire antenna (dashed), and cascaded antenna (red).