  We present single-mode nanowire (NW) lasers with ultralow threshold in the
near-infrared spectral range. To ensure the single-mode operation, the NW
diameter and length are reduced specifically to minimize the longitudinal and
transverse modes of the NW cavity. Increased optical losses and reduced gain
volume by the dimension reduction are compensated by excellent NW morphology
and InGaAs/GaAs multi-quantum disks. At 5 K, a threshold low as 1.6 {\mu}J/cm2
per pulse is achieved with a resulting quality factor exceeding 6400. By
further passivating the NW with an AlGaAs shell to suppress surface
non-radiative recombination, single-mode lasing operation is obtained with a
threshold of only 48 {\mu}J/cm2 per pulse at room temperature with a high
characteristic temperature of 223 K and power output of ~ 0.9 {\mu}W. These
single-mode, ultralow threshold, high power output NW lasers are promising for
the development of near-infrared nanoscale coherent light sources for
integrated photonic circuits, sensing, and spectroscopy.
