Design and optimization of a near-field scanning optical microscope for imaging biological samples in liquid.

We describe a near-field scanning optical microscope capable of imaging biological samples in liquid. The microscope uses a straight optical fiber near-field probe and optical shear-force feedback for tip-sample distance regulation. Physical aspects of the design are discussed, and phenomena related to operation in liquid are revealed. Careful calibration of the instrument in air and in liquid is shown, and for the first time to our knowledge, near-field fluorescence images of a biological cell in liquid are presented.