Observation of nanometric metallic particles with an apertureless scanning near-®eld optical microscope Sakina Benrezzak, Pierre Michel Adam, Jean Louis Bijeon * , Pascal Royer Laboratoire de Nanotechnologie et d'Instrumentation Optique LNIO), Universit e de technologie de Troyes, 12, rue Marie Curie ± BP 2060, 10010 Troyes Cedex, France Received 23 November 2000; accepted for publication 29 June 2001 Abstract The near ®eld of an isolated gold metallic nanoparticle laid on a quartz substrate has been investigated using an apertureless scanning near-®eld optical microscope SNOM). We performed a near-®eld absorption spectra of a gold nanoparticle, emphasizing an electromagnetic resonance. In this study, we were interested in the process of image formations in SNOM. In this context, we prove that the optical image contain near-®eld information by varying dif- ferent parameters such as wavelength, amplitude oscillation of the tip, polarization of the light, size of the particles. The atomic force microscopy images recorded simultaneously with the optical images were an useful tool to characterize the particle shape and therefore to show the sphere deformation when deposited on the substrate. We used several hy- pothesis to explain the nanoparticle resonance position with the help of established calculations and complementary experiments. The optical response of a metallic nanoparticle is also suited in order to understand the physical phe- nomena involved in the surface enhanced raman scattering and more generally to improve the resolution in local spectroscopy. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Non-linear optical methods; Atomic force microscopy; Plasmons; Gold 1. Introduction Scanning near-®eld optical microscopy SNOM) is considered as a new type of microscope which overcomes the limit in lateral resolution of the classical optical microscope. Indeed, the electro- magnetic ®eld scattered by an object illuminated by a light source contains not only the propagating waves far ®eld) but also the nonpropagating waves evanescent waves). These evanescent waves contain the subwavelength information of the sample surface related to the high spatial fre- quencies and lie closely to the surface of the object at a distance d < k. The SNOM is thus hoped to determine the optical properties of the nanostruc- tures of the sample by localization and analysis of the evanescent waves. SNOM is based on a sharp nanoscopic probe which scans the surface at a subwavelength dis- tance from it. Many SNOM con®gurations use a small aperture as a probe usually realized by coating with a metal the extremity of a tapered optical ®ber. In this case, the resolution of the Surface Science 491 2001) 195±207 www.elsevier.com/locate/susc * Corresponding author. Tel.: +33-325-715830; fax: +33-333- 25715675. E-mail address: bijeon@utt.fr J.L. Bijeon). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602801)01404-2