  The generation of coherent radiation in nanostructures has attracted
considerable interest in recent years owing both to the quantum
electrodynamical effects that emerge in small volumes, and to their potential
for future applications. The progress towards exploring this size regime,
however, has been hindered by the lack of a systematic approach to scaling down
the mode as the structures shrink in size and by the high threshold power
required for lasing in small cavities. Here we present a new family of
metamaterial-like nano-cavities consisting of metallic coaxial nanostructures
that solves both of these problems. The proposed nanoscale coaxial cavities can
operate as thresholdless lasers, and their size is not restricted by the mode
cut-off. Using these new nano-cavities we have succeeded to demonstrate for the
first time thresholdless lasing in a broadband gain medium, and the smallest
room temperature, continuous wave, telecom laser to date. The nanoscale coaxial
cavities are the first systematic step towards unveiling the potentials of
ultra small quantum electrodynamical objects in which atom-field interactions
generate new functionalities.
