(color online) (a) PL of bulk gold. Experimental OPL spectrum of gold thin film excited at 405 nm. The inset shows that OPL from bulk gold evolves by a three-step process: a d-band hole is created by 405-nm excitation and scattered within the d-band to a state below the filled sp-band states. The hole subsequently recombines radiatively with an sp-electron (from Ref. [27], Fig. 1). (b) Schematic diagram illustrating surface plasmon polariton (from Ref. [28], Fig. 1(a)). (c) Normalized extinction spectra of as-prepared gold nanorod solutions. Insets show transmission electron microscope images of gold nanorods on a copper grid. The average approximate sizes of nanorods are, from left to right, 37 × 19 nm, 50 × 12 nm, and 50 × 8 nm. Scale bars are 50 nm (from Ref. [15], Fig. 2(b)). (d) Schematic diagram illustrating localized surface plasmon (from Ref. [28], Fig. 1(b)).