Fluorescence lifetime imaging with near-field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) is a high-resolution scanning probe technique capable of obtaining simultaneous optical and topographic images with spatial resolution of tens of nanometers. We have integrated ti me-correlated single-photon counting and NSOM to obtain images of fluoresce nce lifetimes with high spatial resolution. The technique can be used to me asure either full fluorescence lifetime decays at individual spots with a s patial resolution of < 100 nm or NSOM fluorescence images using fluorescenc e lifetime as a contrast mechanism. For imaging, a pulsed Ti:sapphire laser was used for sample excitation and fluorescent photons were time correlate d and sorted into two time delay bins. The intensity in these bins can be u sed to estimate the fluorescence lifetime at each pixel in the image. The t echnique is demonstrated on thin films of poly(9,9'-dioctylfluorene) (PDOF) . The fluorescence of PDOF is the results of both inter- and intrapolymer e mitting species that can be easily distinguished in the time domain. Fluore scence lifetime imaging with near-field scanning optical microscopy demonst rates how photochemical degradation of the polymer leads to a quenching of short-delay intrachain emission and an increase in the long-delay photons a ssociated with interpolymer emitting species. The images also show how intr a- and interpolymer species are uniformly distributed in the films.