  Small composite objects, known as Janus particles, drive sustained scientific
interest primarily targeted at biomedical applications, where such objects act
as micro- or nanoscale actuators, carriers, or imaging agents. The major
practical challenge is to develop effective methods for manipulation of Janus
particles. The available long-range methods mostly rely on chemical reactions
or thermal gradients, therefore having mediocre precision and strong dependency
on the content and properties of the carrier fluid. To tackle these
limitations, we propose the manipulation of Janus particles (here, silica
microspheres half-coated with gold) by optical forces in the evanescent field
of an optical nanofiber. We find that Janus particles exhibit stronger
transverse localization and faster propulsion compared to all-dielectric
particles of the same size. The propulsion speed recorded for a 3-$\mu$m
particle with a 20-nm-thick gold cap averages at 2~$\mu$m/s per 1~mW of optical
power, reaching 133 body length/s at 200~mW going through the nanofiber.
