Surface plasmon resonance linear and nonlinear response in a single nanorod H. Baida, D. Christofilos, P. Maioli, A. Crut, N. Del Fatti, and F. Vallée FemtoNanoOptics Group LASIM, Universite de Lyon – CNRS, 43 Bd du 11 Novembre, 69622 Villeurbanne - France ABSTRACT The optical extinction spectra of single gold nanorods are investigated using a spatial modulation spectroscopy technique. The experimental results are compared to the computed spectra of nanoellipsoids using the dipolar approximation or a generalization of the Mie theory, focussing on the width of the longitudinal surface plasmon resonance. This is shown to be consistent with that deduced from the theoretical models using different available sets of dielectric constant for bulk gold, without introducing surface broadening effect. Extension of this approach to investigation of the ultrafast nonlinear optical response of a single gold nanorod is also discussed. Keywords: metal nanoparticles, surface plasmon resonance, single particle spectroscopy 1. INTRODUCTION The appearance of new resonances in nanometric particles and the possibility they offer to design the linear and nonlinear optical properties of nano-structured materials have led to considerable interest in the academic and industrial domains. In metal nanoparticles, the main feature is the surface plasmon resonance (SPR) whose characteristics, wavelength, lineshape and amplitude, depend on the size, shape, structure and environment of the particle, yielding the unique possibility of manipulating the optical field at a subwavelength scale. The SPR is a consequence of the resonant interaction of the metal electrons with the electromagnetic field that leads to a large local field enhancement in and in the close vicinity of the particles [1-3]. This local field effect modifies all the responses depending locally on the field, such as the nonlinear optical response of the particle or the luminescence or Raman scattering of surrounding molecules. Optimization of the SPR characteristics and their adaptation to specific applications have thus led to considerable amount of works during the last decades, with, in particular, the goal of increasing the amplitude of the local field enhancement in nanoobjects with sharper tips and model their optical response [2,4]. With the advance of synthesis techniques, metal nanoparticles with different shapes and composition and, consequently, various optical responses can now be produced [5,6]. Among the different synthesized nanoobjects, nanorods are of special interest because of the large amplitude and the tunability of their SPR whose wavelength can be adjusted from the visible to the near infrared by adapting their aspect ratio [7,8]. Though a good degree of shape and size control of nanorods has now been reached, their geometrical parameters are still too polydispersed to avoid strong inhomogeneous effects in their optical response. This is due to the large sensitivity of their surface plasmon resonance characteristics on the nanorod aspect ratio, which shows-up in a SPR width much larger than the computed homogeneous width in ensemble measurements [9-11]. This dispersion is preventing not only precise comparison of the experimental and computed spectra, but also quantitative analysis of nanorod efficiency in applications such as those based on field enhancement effects. The width of the resonance, i.e., its quality factor, is here a key parameter that can only be properly defined at the single nanoparticle level. Single nanorods were first investigated using dark-field microscopy, yielding spectral information on their main SPR, i.e., corresponding to light polarized along their long axis [1,12]. However, for the sizes of usually synthesized nanorods (15-20 nm in their smaller dimension), the optical response is dominated by absorption, precluding full investigation of their spectral features using scattering-based methods (i.e., longitudinal and transverse SPR and interband absorption). An extinction based method, namely the spatial modulation spectroscopy (SMS), for optical detection and spectroscopy of single metal nanoparticles have been recently demonstrated enabling quantitative determination of the extinction cross-section of a nanoobject down to a few nanometers [13-15]. Using this approach we have investigated the extinction Invited Paper Plasmonics: Nanoimaging, Nanofabrication, and Their Applications IV, edited by Satoshi Kawata, Vladimir M. Shalaev, Din Ping Tsai, Proc. of SPIE Vol. 7033, 703319, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.795124 Proc. of SPIE Vol. 7033 703319-1 2008 SPIE Digital Library -- Subscriber Archive Copy