  Near-field optical microscopes with two independent tips for simultaneous
excitation and detection can be essential tools for studying localized optical
phenomena on the subwavelength scale. Here, we report on the implementation of
a fully automated and robust dual-tip scanning near-field optical microscope
(SNOM), in which the excitation tip is stationary, while the detection tip
automatically scans the surrounding area. To monitor and control the distance
between the two probes, mechanical interactions due to shear forces are used.
We experimentally investigate suitable scan parameters and find that the
automated dual-tip SNOM can operate stably for a wide range of parameters. To
demonstrate the potential of the automated dual-tip SNOM, we characterize the
propagation of surface plasmon polaritons on a gold film for visible and
near-infrared wavelengths. The good agreement of the measurements with
numerical simulations verifies the capability of the dual-tip SNOM for the
near-field characterization of localized optical phenomena.
