Abstract

The strong near-field enhancement of metallic-tip nanostructures has attracted great interest in scanning microscopy techniques, such as surface-enhanced Raman scattering, near-field scanning optical microscopy and tip-enhanced nonlinear imaging [1,2]. In this paper, we use both experimental and advanced computational methods to examine one important aspect of the tip surface region, namely its tip-substrate current as a function of tip geometry and tip bias. Our experimental approach employs measuring laser photoemission in the presence of a bias applied to a STM tip near a Ag(111) surface. This signal depends sensitively (at the ~10Å level) on the tip height and the position of the tip relative to Ag nanoparticles on otherwise flat Ag(111) terraces, providing a means to spatially resolve the time-resolved signal. In addition, we have combined our experimental approach with three dimensional finite difference domain (3D-FDTD) computation to fully investigate the spatial characteristics of the optical field confinement and localization between a tungsten nanoprobe and an infinite planar silver substrate, with two-color ultra-fast laser excitation scheme [3]. The degree of two-color excited field enhancement, geometry dependence, the exact mechanism of optical tip-substrate coupling and tip-substrate plasmon resonances are significant in realizing a new approach to ultrafast time-resolved measurements of surface electron dynamics [4].

© 2016 Japan Society of Applied Physics, Optical Society of America