  It is shown that, in the liquid-filled hollow core of a single-mode photonic
crystal fiber, a micron-sized particle can be held stably against a fluidic
counter-flow using radiation pressure, and moved to and fro (over 10s of cm) by
ramping the laser power up and down. The results represent a major advance over
previous work on particle transport in optically multimode liquid-filled
fibers, in which the fluctuating transverse field pattern renders the radiation
and trapping forces unpredictable. The counter-flowing liquid can be loaded
with sequences of chemicals in precisely controlled concentrations and doses,
making possible studies of single particles, vesicles or cells.
