An optical tip-sample distance control for a scanning near-field optical microscope

A method of optical distance control for an aperture-SNOM has been developed, which is especially useful for investigations at the liquid/air interface. The method is based on modulations in the transmitted light, which appear when the tip is moved toward the sample. This effect is due to interference. By tracking an extremum of the transmitted light intensity, the distance regulation is not affected by local differences in the transmission of the sample. When taking an image, during the backward scan the distance control keeps the distance between tip and sample constant. During the forward scan the feedback is switched off and the tip is further moved toward the sample, to bring it into the near-field region. Since the feedback is not active at all times, the proposed method is suitable only for flat samples. Images of a solid test specimen, obtained by applying optical distance control are presented in this communication.