Novel scanning near-field optical microscope (SNOM)/scanning confocal optical microscope based on normal force distance regulation and bent etched fiber tips

Abstract

We report on a unique combination of a novel scanning near-field optical microscope/scanning confocal optical microscope based on normal force distance regulation with bent etched fiber tips. The use of normal force detection makes all classic atomic force microscope operation modes such as contact mode and tapping mode accessible to scanning near-field optical microscopy (SNOM), thus allowing near-field optical imaging at sample distances unavailable to shear force regulated distance control. The SNOM laser light is pulsed or detected synchronously to the fiber tapping amplitude to illuminate the sample at a predetermined height above the sample. Bent etched fiber tips are used, whose fiber diameters are additionally etched down to yield resonant frequencies between 60 and 300 kHz, Q factors around 180 and spring constants in the range of 80–400 N/m. The typical roughness of the tip cone surfaces is shown to be due to etching in a meniscus. For the first time, simultaneously acquired constant gap width mode topography and constant height mode near-field optical images are presented, allowing topography induced contrast to be separated from true optical contrast such as absorption and fluorescence.