First Overtone Frequency Stimulated Quartz Tuning Fork Used for Shear-Force Scanning Near-Field Optical Microscopy

The conventional 32.768 kHz tuning fork is stimulated at its first overtone resonant frequency of ～190 kHz for shear-force distance control. The time constant is measured to be 0.54 ms and it decreases about 40 times faster than that of the fundamental frequency (20.76 ms). The cross section of a corn root with height difference of ～3 μm is imaged at a scan speed of 12 μm/s for 256 x 256 pixels.