  We employ heterodyne interferometry to investigate the effect of a single
organic molecule on the phase of a propagating laser beam. We report on the
first phase-contrast images of individual molecules and demonstrate a
single-molecule electro-optical phase switch by applying a voltage to the
microelectrodes embedded in the sample. Our results may find applications in
single-molecule holography, fast optical coherent signal processing, and
single-emitter quantum operations.
