In coherent homodyne apertureless scanning near-field optical microscopy (ASNOM) the background field cannot be fully suppressed because of the interference between the different collected fields, making the images difficult to interpret. We show that implementing the heterodyne version of ASNOM allows one to overcome this issue. We present a comparison between homodyne and heterodyne ASNOM through near-field analysis of gold nanowells, integrated waveguides, and a single evanescent wave generated by total internal reflection. The heterodyne approach allows for the control of the interferometric effect with the background light. In particular, the undesirable background is shown to be replaced by a controlled reference field. As a result, near-field information undetectable by a homodyne ASNOM is extracted by use of the heterodyne approach. Additionally, it is shown that field amplitude and field phase can be detected separately.
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