  Laser science has tackled physical limitations to achieve higher power,
faster and smaller light sources. The quest for ultra-compact laser that can
directly generate coherent optical fields at the nano-scale, far beyond the
diffraction limit of light, remains a key fundamental challenge. Microscopic
lasers based on photonic crystals, micro-disks, metal clad cavities and
nanowires can now reach the diffraction limit, which restricts both the optical
mode size and physical device dimension to be larger than half a wavelength.
While surface plasmons are capable of tightly localizing light, ohmic loss at
optical frequencies has inhibited the realization of truly nano-scale lasers.
Recent theory has proposed a way to significantly reduce plasmonic loss while
maintaining ultra-small modes by using a hybrid plasmonic waveguide. Using this
approach, we report an experimental demonstration of nano-scale plasmonic
lasers producing optical modes 100 times smaller than the diffraction limit,
utilizing a high gain Cadmium Sulphide semiconductor nanowire atop a Silver
surface separated by a 5 nm thick insulating gap. Direct measurements of
emission lifetime reveal a broad-band enhancement of the nanowire's spontaneous
emission rate by up to 6 times due to the strong mode confinement and the
signature of apparently threshold-less lasing. Since plasmonic modes have no
cut-off, we show down-scaling of the lateral dimensions of both device and
optical mode. As these optical coherent sources approach molecular and
electronics length scales, plasmonic lasers offer the possibility to explore
extreme interactions between light and matter, opening new avenues in active
photonic circuits, bio-sensing and quantum information technology.
