NSOM(Near field scanning optical microscopy) is performed by scanning a subwavelength optical probe in the vicinity of a sample surface. The optical probes have an aperture radius of the order of 50 to 200nm. The probe-sample separation should be less than the aperture radius to obtain optical information with spatial resolution of the order of the aperture radius. For this reason, probe-sample distance regulation plays a central role in NSOMs. The most popular technique to regulate the probe-sample distance is to vibrate the probe parallel to the sample surface and sense a decrease in the vibration amplitude of the probe.
In this paper, the probe is modeled as a 2`nd order mass-spring-damper system driven by a harmonic force. The primary cause of the decrease in vibration amplitude is due to an increase in damping force - shear force - between the surface and the probe. Using the model, damping constant and resonance frequency of the probe can be calculated as a function of probe-sample distance. The characteristic of shear force will be shown more detail.
Using the shear force characteristic and mechanical resonance of piezoelectric tuning fork, we can control the position of the optical probe about 0 to 50nm above the sample. The feedback signal to regulate the probe-sample distance can be used independently for surface topography imaging. The result of the shear force image will be shown.