LETTERS Plasmon lasers at deep subwavelength scale Rupert F. Oulton 1 *, Volker J. Sorger 1 *, Thomas Zentgraf 1 *, Ren-Min Ma 3 , Christopher Gladden 1 , Lun Dai 3 , Guy Bartal 1 & Xiang Zhang 1,2 Laser science has been successful in producing increasingly high- powered, faster and smaller coherent light sources 1-9 . Examples of recent advances are microscopic lasers that can reach the diffrac- tion limit, based on photonic crystals 3 , metal-clad cavities 4 and nanowires 5–7 . 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 fun- damental challenge to realize ultracompact lasers that can directly generate coherent optical fields at the nanometre scale, far beyond the diffraction limit 10,11 . A way of addressing this issue is to make use of surface plasmons 12,13 , 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 approaches 14,15 . A recent theoretical work predicted that such losses could be significantly reduced while maintaining ultrasmall modes in a hybrid plasmonic waveguide 16 . Here we report the experimental demonstration of nanometre-scale plasmonic lasers, generating optical modes a hundred times smaller than the dif- fraction limit. We realize such lasers using a hybrid plasmonic waveguide consisting of a high-gain cadmium sulphide semi- conductor 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 spon- taneous emission rate by up to six times owing to the strong mode confinement 17 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 circuits 18 , bio-sensing 19 and quantum information technology 20 . Surface plasmon polaritons are the key to breaking down the dif- fraction limit of conventional optics because they allow the compact storage of optical energy in electron oscillations at the interfaces of metals and dielectrics 11–13 . Accessing subwavelength optical length scales introduces the prospect of compact optical devices with new functionalities by enhancing inherently weak physical processes, such as fluorescence and Raman scattering of single molecules 19 and non- linear phenomena 21 . An optical source that couples electronic transi- tions directly to strongly localized optical modes is highly desirable because it would avoid the limitations of delivering light from a macroscopic external source to the nanometre scale, such as low coupling efficiency and difficulties in accessing individual optical modes 22 . Achieving stimulated amplification of surface plasmon polaritons at visible frequencies remains a challenge owing to the intrinsic ohmic losses of metals. This has driven recent research to examine stimulated surface plasmon polariton emission in systems that exhibit low loss, but only minimal confinement, which excludes such schemes from the rich new physics of nanometre-scale optics 14,15 . Recently, we have theoretically proposed a new approach hybridizing dielectric waveguiding with plasmonics, in which a semiconductor nanowire sits on top of a metallic surface, separated by a nanometre- scale insulating gap 16 . The coupling between the plasmonic and *These authors contributed equally to this work. 1 NSF Nanoscale Science and Engineering Centre, 3112 Etcheverry Hall, University of California, Berkeley, California 94720, USA. 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA. 3 State Key Lab for Mesoscopic Physics and School of Physics, Peking University, Beijing 100871, China. –100 0 100 Distance, x (nm) Distance, y (nm) a b CdS Nanowire 489 nm 405 nm z x y d h Ag MgF 2 CdS 100 nm Ag CdS MgF 2 38 nm –100 0 100 |E(x,y)| Figure 1 | The deep subwavelength plasmonic laser. a, The plasmonic laser consists of a CdS semiconductor nanowire on top of a silver substrate, separated by a nanometre-scale MgF 2 layer of thickness h. This structure supports a new type of plasmonic mode 16 the mode size of which can be a hundred times smaller than a diffraction-limited spot. The inset shows a scanning electron microscope image of a typical plasmonic laser, which has been sliced perpendicular to the nanowire’s axis to show the underlying layers. b, The stimulated electric field distribution and direction | E(x, y) | of a hybrid plasmonic mode at a wavelength of l 5 489 nm, corresponding to the CdS I 2 exciton line 25 . The cross-sectional field plots (along the broken lines in the field map) illustrate the strong overall confinement in the gap region between the nanowire and metal surface with sufficient modal overlap in the semiconductor to facilitate gain. Vol 461 | 1 October 2009 | doi:10.1038/nature08364 629 Macmillan Publishers Limited. All rights reserved ©2009