A near-field scanning optical microscope with a high Q-factor piezoelectric sensing element

A tapping-mode near-field scanning optical microscope utilizing a piezoelectric microtuning fork as its height-sensing element is described. We have developed a method for modifying and attaching an optical fiber to the tuning fork that allows the assembly to retain a Q factor of up to 9000, substantially higher than the Q factors described so far in the literature for tuning-fork-based instruments. The method involves reducing the diameter of the cladding of the optical fiber down to 17-25 mu m using several chemical etching steps, before the fiber is attached to the tuning fork. A sharp upturn in the Q factor is observed when the fiber diameter d drops below similar to 25 mu m. An analysis showing that the stretching force constant of a bent fiber is proportional to d(4) accounts for the great sensitivity of the Q factor to the fiber diameter. The high Q factors result in improved force sensitivity and allow us to construct a tapping-mode instrument without the use of additional dithering piezoelements. (C) 1999 American Institute of Physics. [S0003-6951(99)01738-6].