Resonant-plasmon field enhancement from asymmetrically illuminated conical metallic-probe tips

By: R. M. Roth, N.-C. Panoiu, M. M. Adams, R. M. Osgood, Jr., C. C. Neacsu, and M. B. Raschke

Optics Express 14, 2921 (2006)

Optical-field enhancement and confinement for an asymmetrically illuminated nanoscopic Au tip suspended over a planar Au substrate is investigated both numerically and experimentally. The spatial field distribution of the tip-sample system was calculated using the full 3D finite-difference time-domain method. The calculation enables investigation of the effects of the substrate-tip placement, angle of incidence, and spectral response. The tip plasmon response leads to a significant (up to ∼70 times) local field enhancement between the tip and substrate. The enhancement is found to be extremely sensitive to the tip-sample separation distance. Tip-enhanced Raman scattering experiments were performed and the numerical results provide a consistent description of the observed field localization and enhancement.