Laboratoire de Physique, Optique Submicronique, Universite´de Bourgogne, Boite Postale 47870, F-21078 Dijon, France~Received 1 August 2000; published 4 April 2001!This article reports on the near-field optical response of a small square grating of gold nanoparticles tailoredby electron-beam lithography. The investigation of the grating is aimed at a deepened understanding ofelectromagnetic interaction among particles due to scattered light fields. Therefore, a photon scanning tunnel-ing microscope is applied to acquire near-field optical images. Two different incident wavelengths are used tocharacterize the intensity and the spatial localization of the electromagnetic near field both in and out ofresonance for the excitation of particle plasmons. Furthermore, the near-field enhancement resulting from theplasmon excitation is evaluated.DOI: 10.1103/PhysRevB.63.165422 PACS number~s!: 78.66.2w, 07.79.Fc, 42.25.FxI. INTRODUCTION