Systematic variation of the internal geometry of a dielectric core-metal shell nanoparticle allows the local electromagnetic field at the nanoparticle surface to be precisely controlled. The strength of the field as a function of core and shell dimension is measured by monitoring the Surface-Enhanced Raman Scattering (SERS) response of nonresonant molecular adsorbates (para -mercaptoaniline) bound to the nanoparticle surface. The SERS enhancement appears to be directly controllable by nanoparticle geometry. A series of silica core-silver shell nanoparticles were constructed using 65 nm and 79 nm cores, upon which silver layers ranging from 5 nm to 20 nm were deposited by an electroless plating method. Raman spectra of nanoshell and adsorbate molecule solutions were obtained at an excitation of 1.0 μm (Nd:YAG). The magnitude of three predominant Stokes modes (390cm-1, 1077cm-1, and 1590cm-1) were theoretically calculated and experimentally measured as a function of nanoshell geometry. The excellent agreement observed between experiment and classical theory indicates that metal nanoshells provide a substrate with a controllable optical near field.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.