  Cavity-free efficient coupling between emitters and guided modes is of great
current interest for nonlinear quantum optics as well as efficient and scalable
quantum information processing. In this work, we extend these activities to the
coupling of organic dye molecules to a highly confined mode of a nanofiber,
allowing mirrorless and low-threshold laser action in an effective mode volume
of less than 100 femtoliters. We model this laser system based on
semi-classical rate equations and present an analytic compact form of the laser
output intensity. Despite the lack of a cavity structure, we achieve a coupling
efficiency of the spontaneous emission to the waveguide mode of 0.07(0.01), in
agreement with our calculations. In a further experiment, we also demonstrate
the use of a plasmonic nanoparticle as a dispersive output coupler. Our laser
architecture is promising for a number of applications in optofluidics and
provides a fundamental model system for studying nonresonant feedback
stimulated emission.
