Scanning near-field fluorescence microscopy of thin organic films at the water/air interface

We present an aperture scanning near-field optical microscope that has the ability of investigating objects at the liquid/air interface, e.g. molecularly thin films floating on a water subphase. For controlling the tip–sample distance, only the transmitted light is used. This signal shows the well-known interference fringes when approaching the tip towards a surface. Using modulation techniques, it is possible to keep the tip at an extremum of the intensity–distance curve. For high-resolution imaging the tip needs to come closer to the sample surface with respect to the regulation set value. We accomplish this by interrupting the distance regulation feedback in the forward scan, and moving the tip across the sample additionally approached by a set value. Thus, we scan the tip at constant height in the near-field of the sample. This excludes artifacts which sometimes arise in the mostly used topography following scan mode. We present transmission and fluorescence contrast images of a thin metal film and of polydiacetylene thin films, both floating on water.