Abstract
Commonly used shear force scanning near-field optical microscopy (SNOM) of soft biological samples is more critical to implement in aqueous environment than in air. A tapping-mode distance control based on a rectangular piezoelectric bimorph cantilever attached vertically by a straight fiber tip as force sensor for SNOM is introduced. The bimorph lever serving as both the probe dither and the force responder operates in flexural mode with a spring constant k=3.7×103N∕m. The sensitivity of the sensor is enhanced through the increase in lever’s quality factor (Q) and the usage of a higher eigenmode. Experimental results reveal that the describe sensor can operate in liquid with an effective Q up to 103 at its second eigenfrequecy f2=18.8kHz. High sensitivity of the sensor is demonstrated by imaging soft biological samples. Near-field optical resolution of better than 100nm on red blood cells in water is obtained. Compared to the existing tapping mode SNOM setups, our approach is compact, sensitive, lacking in parasitic optical background, and easy to practice in liquid.
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