SUPERRESOLUTION IMAGING AND DETECTION OF FLUORESCENCE FROM SINGLE MOLECULES BY SCANNING NEAR-FIELD OPTICAL MICROSCOPY

Scanning near-field microscopy (SNOM or NSOM) is a versatile and attra ctive scanning probe technique for imaging with subdiffraction-limited spatial resolution using visible light. At least three different type s of images can be recorded simultaneously of the selected sample area , such as the topography, the near-field optical transmission, and the fluorescence from excited chromophores. We have built such a microsco pe, especially designed for achieving the high resolution and the sens itivity needed for single molecule detection. We report on optical nea r-field investigations of surface structures and thin polymer films th at are doped with fluorescent dye molecules. The effective aperture di ameters of the fiber tips used in the SNOM experiments were determined by a photon-scanning tunneling microscope (PSTM) giving values betwee n 70 and 160 nm, The transmission imaging of transparent polymer phase gratings reveals the existence of different contrast mechanisms, whic h are either based on the inherent distance dependence of the optical near field or on the periodic change of boundary conditions for the el ectric field component of the light between the aperture and the sampl e. Furthermore, we demonstrate selective irreversible photobleaching o f dye molecules at moderate concentration (10(-5) M) induced locally b y the subwavelength-sized probe tip. Finally, we present fluorescence images showing single molecule detection in a thin solid film. The chr omophores (rhodamine 6G) were embedded at low concentration (10(-7) M) in a 25-nm thin polyvinylbutyral film, A lateral resolution of 160 nm was achieved. We find that the signal strengths of the brightest fluo rescent features vary considerably in a sequence of images (a typical single-molecule behavior), whereas the fluorescence background exhibit s the usual photobleaching behavior of a large ensemble.