Plasmon lasers at deep subwavelength scale

To push the physical limitations of lasers to the nanoscale regime it is necessary to tackle the fundamental challenge of surpassing the diffraction limit. It has been suggested that surface plasmons — light–matter waves trapped on the surface of a conductor — can be used to tightly confine light on very short length scales, but such approaches have been previously hampered by severe losses. Oulton et al. now demonstrate that it is possible to circumvent this problem by utilizing a hybrid between a dielectric waveguide and a conducting surface supporting a plasmon mode, thereby showing the experimental realization of a nanoscale plasmonic laser with an optical mode a hundred times smaller than the diffraction limit. Such hybrid plasmonic coherent light sources offer the possibility to explore extreme interactions between light and matter, and may open important new avenues in optoelectronics. A key challenge is to realize ultracompact lasers that can directly generate coherent optical fields at the nanometre scale, far beyond the diffraction limit. Surface plasmons could be used to tightly confine light on very short lengthscales, but so far this approach has been hampered by ohmic losses at optical frequencies. The experimental demonstration of nanometre-scale plasmonic lasers is now reported, realized using a hybrid plasmonic waveguide — these lasers can generate optical modes a hundred times smaller than the diffraction limit. Laser science has been successful in producing increasingly high-powered, faster and smaller coherent light sources1,2,3,4,5,6,7,8,9. Examples of recent advances are microscopic lasers that can reach the diffraction limit, based on photonic crystals3, metal-clad cavities4 and nanowires5,6,7. However, such lasers are restricted, both in optical mode size and physical device dimension, to being larger than half the wavelength of the optical field, and it remains a key fundamental challenge to realize ultracompact lasers that can directly generate coherent optical fields at the nanometre scale, far beyond the diffraction limit10,11. A way of addressing this issue is to make use of surface plasmons12,13, which are capable of tightly localizing light, but so far ohmic losses at optical frequencies have inhibited the realization of truly nanometre-scale lasers based on such approaches14,15. A recent theoretical work predicted that such losses could be significantly reduced while maintaining ultrasmall modes in a hybrid plasmonic waveguide16. Here we report the experimental demonstration of nanometre-scale plasmonic lasers, generating optical modes a hundred times smaller than the diffraction limit. We realize such lasers using a hybrid plasmonic waveguide consisting of a high-gain cadmium sulphide semiconductor nanowire, separated from a silver surface by a 5-nm-thick insulating gap. Direct measurements of the emission lifetime reveal a broad-band enhancement of the nanowire’s exciton spontaneous emission rate by up to six times owing to the strong mode confinement17 and the signature of apparently threshold-less lasing. Because plasmonic modes have no cutoff, we are able to demonstrate downscaling of the lateral dimensions of both the device and the optical mode. Plasmonic lasers thus offer the possibility of exploring extreme interactions between light and matter, opening up new avenues in the fields of active photonic circuits18, bio-sensing19 and quantum information technology20.