INFLUENCE OF DETECTION CONDITIONS ON NEAR-FIELD OPTICAL IMAGING

The process of image formation in transmission mode scanning near-fiel d optical microscopy is analyzed both theoretically and experimentally . Changes in the dielectric and topographic properties of the sample i nfluence not only the total transmitted intensity, but also its angula r distribution in the far field. This opens up an additional source of optical information about the sample. Some of this additional informa tion is retrieved by separate but simultaneous detection of the radiat ion emitted at angles smaller (allowed light) and larger (forbidden li ght) than the critical angle of total internal reflection, respectivel y. Different experimental setups and their respective advantages are d iscussed. High resolution, constant height mode optical images of test structures are compared with theoretical predictions. Forbidden-light optical images frequently provide enhanced resolution and/or contrast as compared to allowed light images. For small phase objects, in cont rast to amplitude objects, a contrast reversal between forbidden and a llowed light images is observed. (C) 1998 American Institute of Physic s. [S0021-8979(98)02923-5]