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 dev eloped, which is especially useful for investigations at the liquid/ai r interface. The method is based on modulations in the transmitted lig ht, which appear when the tip is moved toward the sample. This effect is due to interference. By tracking an extremum of the transmitted lig ht intensity, the distance regulation is not affected by local differe nces in the transmission of the sample. When taking an image, during t he backward scan the distance control keeps the distance between tip a nd sample constant. During the forward scan the feedback is switched o ff and the tip is further moved toward the sample, to bring it into th e near-field region. Since the feedback is not active at all times, th e proposed method is suitable only for flat samples. Images of a solid test specimen, obtained by applying optical distance control are pres ented in this communication.