Dual wavelength SNOM imaging of monolayers of J-aggregated cyanine dye molecules

Bortchagovsky EG, Heimel J, Fuchs H, Fischer UC

Abstract

Scanning Near-Field optical Microscopy (SNOM) using a metal covered tetrahedral tip as a probe was used for near field optical imaging of monolayers consisting of domains of J-aggregated dye molecules on a thin film of gold as a substrate. The SNOM was operated using the tip at the same time for electronic tunneling and a constant current STM mode was used to control the distance and acquire a topographic image simultaneously with the SNOM image. The SNOM was operated by exciting the tip with two diode lasers at 635 nm and 670 nm simultaneously. Two preparations of dye layers were investigated using dye molecules with characteristic spectral features at the chosen wavelengths. The transmitted signal was separately detected at the two wavelengths such that transmission SNOM images were recorded at the two wavelengths simultaneously. Characteristic features of the dye preparations could be observed which are different for the two wavelengths, but a clear correlation of the spectral features of the SNOM images with the adsorbed dye molecules could not be obtained. The difficulties in an assignment of spectral features of the images to the ones of the adsorbed dye molecules seem to be related to the rather complex mechanism of image formation in the SNOM mode where the coherent SNOM signal is due to an interference between the light emitted from the tip acting as a source and and the emission, of the sample induced by this source.