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

Near-field scanning optical microscopy (NSOM) is used to probe the exc itonic transitions in J-aggregates of 1,1'-diethyl-2,2'-cyanine iodide grown in poly(vinyl sulfate) thin films. Near-field images of the flu orescence from the excitonic state of the aggregates are obtained with good signal-to-noise and spatial resolution better than 100 nm. Fluor escence 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 du e to static inhomogeneities is interpreted as direct evidence for a un iform, well-ordered molecular structure within the aggregates. The exc itonic 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 ar e used to place an upper limit of approximate to 50 nm on the physical extent of exciton migration along these aggregates.