Localized and Propagating Surface Plasmons in Gold Particles of Near-Micron Size

Abstract

In this letter, we demonstrate that Au triangles between approximately 200 nm and 1.5 μm are transitioning from a predominantly localized surface plasmon resonance (LSPR) character to a regime where both LSPR and a short-range SPR mode are concomitantly present. The Au triangles are active in LSPR between approximately λ = 700 nm for the smaller triangles and λ = 2.5 μm for the largest Au triangles. Triangles with a base length of 200 and 300 nm do not exhibit an active short-range SPR mode, while triangles with 500 and 700 nm and 1.5 μm base lengths show a strong absorption peak in SPR at approximately λ = 800 nm, attributed to the short-range SPR mode. A low sensitivity of <50 nm/RIU is observed with this short-range SPR mode in triangles of near-micron size. Also, Au nanohole arrays exhibit features in both the submicron size, with gold “island” between neighboring nanoholes, and on the order of tens of microns, with gold “strips” along rows of nanoholes. The excitation of nanohole arrays in total internal reflection (TIR) results in the presence of both a short-range SPR mode similar to that of gold triangles and a propagating SPR mode similar to that of a thin Au film of macroscopic surface. For Au nanohole arrays of 820 nm periodicity, TIR experiments result in two overlapping absorption peaks at λ = 650 and 800 nm, corresponding respectively to propagating SPR in thin films (typically around λ = 615 nm) and the short-range SPR mode in Au triangles.