Enhancement of shear-force sensitivity using asymmetric response of tuning forks for near-field scanning optical microscopy.

Resonance characteristics of a tuning fork are investigated to enhance the shear-force detection sensitivity for near-field scanning optical microscopy. In particular, we show that the asymmetric frequency response of a tuning fork can be utilized to increase quality factors and suppress the background feedback signal. The pinning down effect on one side of the main peak can readily elevate vertical sensitivity and stability. A simplified model based on a coupled harmonic oscillator is presented to describe the asymmetric resonance behavior of the tuning fork. We also show improved topographic images of a blue-ray disc and optical images of a chromium pattern on the quartz using the asymmetric resonance.