EXCITONIC TRANSITIONS IN J-AGGREGATES PROBED BY NEAR-FIELD SCANNING OPTICAL MICROSCOPY

Near-field scanning optical microscopy (NSOM) is used to probe the excitonic transitions in J-aggregates of 1,1'-diethyl-2,2'-cyanine iodide grown in poly(vinyl sulfate) thin films. Near-field images of the fluorescence from the excitonic state of the aggregates are obtained with good signal-to-noise and spatial resolution better than 100 nm. Fluorescence spectra recorded as a function of the NSOM tip position along individual aggregates show only slight variations and are very similar to the bulk aggregate spectrum. The absence of spectral broadening due to static inhomogeneities is interpreted as direct evidence for a uniform, well-ordered molecular structure within the aggregates. The excitonic transitions in the aggregates are locally photobleached by the light from the NSOM tip. The spatial extent of photobleaching observed here is limited by the resolution of the instrument; these results are used to place an upper limit of approximate to 50 nm on the physical extent of exciton migration along these aggregates.