Prospects for molecular spectroscopy at high spatial resolution

This work discusses the prospects and feasibility of optical spectroscopy and microscopy of single molecules at nanometer resolution via apertureless, antenna-based near-field scanning optical microscopy. First, different near-field optical methods are compared, which detect the weak scattering or fluorescence from a probe-single molecule interaction at high spatial resolution. Specifically, ultimate sensitivities of coherent (bright-field) and non-coherent (dark-field) apertureless near-field microscopes for resonant (e.g., scattering, absorption) and non-resonant (e.g., n-photon fluorescence) imaging applications are discussed and key differences between absorption and scattering/fluorescence methods are elucidated. Secondly, the enhanced near-field interaction between a probe and a luminescent polystyrene nanosphere (doped with fluorescein-like dye) is investigated as a function of the sample size, which reveals a strong probe-induced enhancement of the fluorescence. Finally, the data are used to predict ultimate sensitivities as well as limitations of current apertureless near-field optical techniques.