Near- and Far-Field Effects on the Plasmon Coupling in Gold Nanoparticle Arrays

Understanding the plasmon coupling between metal nanoparticles under light irradiation remains a challenging issue for optimizing plasmonic devices for chemical and biological sensing. Here, the optical properties of dense spatially ordered two-dimensional arrays of 50 nm gold nanoparticles are investigated in this aim. Microspectrophotometry experiments are carried out on square arrays and parallel chains, elaborated by electron beam lithography, having different periodicities ranging from 80 to 170 nm. The wavelength, width, and amplitude of the localized surface plasmon resonance (SPR) are quantitatively monitored as a function of both the incident light polarization and the interparticle distance. The experimental findings are then compared with calculations based on the discrete dipole approximation. They match remarkably well these numerical results, not only for the spectral location and the width of the SPR but also for the absolute value of the extinction cross section of the nanoparticles that is linked with the local field enhancement. The variation of the SPR band characteristics as a function of the periodicity of the arrays is investigated in terms of near field and far field coupling effects by discussing the topography of the field amplitude and phase in the arrays Moreover, in order to highlight the influence of phase retardation and radiative effects on the plasmon coupling, the optical properties of equivalent arrays scaled by 1/10, which can be qualitatively described by electrostatic dipolar interactions, are also studied by numerical calculations The discrepancies then observed between the two scales are interpreted. The contributions of the radiative and nonradiative dampings to the width and magnitude of the plasmon resonance are also investigated.