Title: Continuous-wave virtual-state lasing from cold ytterbium atoms,
Abstract: While conventional lasers are based on gain media with three or four real
levels, unconventional lasers including virtual levels and two-photon processes
offer new opportunities. We study lasing that involves a two-photon process
through a virtual lower level, which we realize in a cloud of cold ytterbium
atoms that are magneto-optically trapped inside a cavity. We pump the atoms on
the narrow $^1$S$_0$ $\to$ $^3$P$_1$ line and generate laser emission on the
same transition. Lasing is verified by a threshold behavior of output power
vs.\ pump power and atom number, a flat $g^{(2)}$ correlation function above
threshold, and the polarization properties of the output. In the proposed
lasing mechanism the MOT beams create the virtual lower level of the lasing
transition. The laser process runs continuously, needs no further repumping,
and might be adapted to other atoms or transitions such as the ultra narrow
$^1$S$_0$ $\to$ $^3$P$_0$ clock transition in ytterbium.