Direct observation of localized surface plasmon coupling J. R. Krenn, J. C. Weeber, A. Dereux, E. Bourillot, and J. P. Goudonnet Optique Submicronique, Universite ´ de Bourgogne, Boı ˆte Postale 47870, F-21078 Dijon Cedex, France B. Schider, A. Leitner, and F. R. Aussenegg Institut fu ¨r Experimentalphysik, Universita ¨t Graz, Universita ¨tsplatz 5, A-8010 Graz, Austria C. Girard CEMES, 29 rue Jeanne Marvig, BP 4347, F-31055 Toulouse Cedex 4, France Received 16 March 1999 We report on the direct observation of localized surface plasmon coupling using a photon scanning tunneling microscope. The surface plasmons are excited in gold nanostructures tailored by electron beam lithography. Electromagnetic energy transfer from a resonantly excited nanoparticle to a nanowire, which is not directly excited by the incident light is observed. Our experimental results appear to be in good agreement with theoretical computations based on Green’s dyadic technique. S0163-18299908931-6 I. INTRODUCTION Nanoscale noble metal structures can exhibit anomalous optical extinction in the visible spectral range due to reso- nantly driven electron plasma oscillations localized surface plasmons, LSP’s. 1 The spectral position of the LSP reso- nance depends mainly on the structure geometry and the po- larization state of the exciting light. 2 Due to its resonant character LSP excitation is accompanied with a strongly lo- calized field enhancement around the nanostructures. 2 These properties make such structures interesting in the context of future nanooptical applications, i.e., guiding and controlling light on the submicrometer scale. Although intensively investigated with far-field methods, little is still known about the optical near-fields around nanoscale metal structures. Such knowledge is highly desir- able in order to tailor well-defined nanooptical devices, a task that is feasible today using advanced lithography tech- niques. Indeed, direct mapping of LSP near fields is acces- sible due to the development of near-field optical microscopy. 3 This technique has already been employed to study various metal geometries supporting LSP’s. Experi- mental work focused on the local field enhancement and the spatial field distribution around nanoparticles 4,5 and nanos- cale defects on metal thin films 6–10 were recently reported. Additionally, coupling effects of LSP fields were investi- gated. These effects are expected to occur for nanostructures in immediate vicinity of each other and can be described as dipole-dipole coupling 2,11 or in terms of near-field scattering. 12 A laterally squeezed optical near field due to LSP coupling was recently observed on an ensemble of lin- early aligned gold nanoparticles 13 . Electromagnetic energy transport along a closely packed linear chain of nanoscale gold spheres following a local excitation was proposed in Ref. 14. Another example of LSP coupling was discussed for a local-field enhancement between a metal-probe tip and a metal sample in apertureless scanning near-field optical microscopy. 15 Despite these rather attractive properties there are to date, to the best of our knowledge, no spatially re- solved experimental data demonstrating electromagnetic coupling between two well-defined isolated metal nanostruc- tures. In this paper, we use a photon scanning tunneling microscope 16 PSTM to demonstrate an example of electro- magnetic coupling between individual gold nanostructures. To evidence such a coupling, we use a resonant single par- ticle as a local source to excite a nanowire placed in the close vicinity of the nanosource. The long axis of the nanowire is oriented such that it is not directly exicted by the incident field. Since the resonance frequency of the surface mode sustained by a metal particle depends strongly on its geom- etry, a proper choice of the incoming field’s wavelength and polarization state allows such a selective excitation of the single particle. II. EXPERIMENTAL AND THEORETICAL BACKGROUND In our PSTM setup the sample is illuminated by an eva- nescent surface wave generated by total internal reflection through a glass prism of a TM-polarized laser beam angle of incidence 60°). Two kinds of lasers were used in the follow- ing experiments, a red He-Ne with a wavelength of  =633 nm and a Titane:Sapphire tunable from  =700 to  =820 nm. The resulting optical near fields are probed by a tapered dielectric optical fiber produced by a standard heat-and-pull technique. The signal picked up by the fiber is converted with a photomultiplier. The fiber tip is approached to the sample with a piezo actuator until an exponential increase of the detected signal reveals the evanescent field and thus the immediate vicinity of the sample surface within the decay length of the evanescent field in the order of 150 nm. The PSTM images provide maps of the spatial distribution of the optical near-field intensity, which are acquired by scanning the fiber tip in a constant height over the sample surface. Between two successive images the tip is approached a few nanometers closer to the sample surface until the tip touches the sample structure, thereby indicating the position of the structures. Before this final approach, no change in the tip morphology could occur. The PSTM images shown here PHYSICAL REVIEW B 15 AUGUST 1999-I VOLUME 60, NUMBER 7 PRB 60 0163-1829/99/607/50295/$15.00 5029 ©1999 The American Physical Society