We studied optical properties of hexagonally arranged gold nanoparticle trimer assembly using optical microscopy. We found that the scattering spectrum showed multiple resonances in the visible region. We simulated the optical properties by assuming a pair of trimers as a model structure. The simulated spectrum reproduced the resonance features observed, and were assigned to plasmon-modes excited. Surface-enhanced Raman Scattering (SERS) activity of the assembly was found to be dependent on the excitation wavelength. The highest SERS enhancement was achieved with 633 nm excitation, reaching 108. The wavelength-dependency of the activity indicates that plasmonic fields are responsible for the enhancement. (C) 2015 Elsevier B.V. All rights reserved.