Pushing the Limits of Near-field Microscopy

摘要

Recent advances in apertureless near-field scanning optical microscopy (ANSOM) allow unprecedented optical and spectroscopic (fluorescence and Raman) resolution on the scale of 20-30 nm~(1,2). The key behind this technique is the probe geometry, incident light polarization and type of illumination (two - photon excitation (TPE)). The key behind the high optical spatial resolution is the creation of an electric field enhancement (f) between the specimen and the probe apex. Utilizing a 3D Finite Difference Time Domain (FDTD) technique for the modeling of tip shapes coupled with the use of a dual-beam Focused Ion Beam (FIB), it has been possible to design tips with high electric field enhancement factors in order to achieve ultra-high resolution spectroscopic images. This geometrical optimization has seen little investigation. Up to now, ion beams and chemical etching techniques have been employed to create different probe geometries. These two methods provide good reproducibility and sharp apexes, but are limited in creation of the overall end probe diameters to nominally 10-30 nm. This work investigates the possibility of utilizing a method for creating more favorable geometries for achieving a higher spatial resolution. This involves the fabrication of probes through the extraction of Gilbert-Taylor (G-T) cone geometries by high field extraction, which can achieve ＜ 10 nm end diameters. Utilizing FDTD, the potential field enhancement factors of the G-T geometry are determined.