LETTERdoi:10.1038/nature10840Thresholdless nanoscale coaxial lasersM. Khajavikhan1, A. Simic1*, M. Katz1*, J. H. Lee1{, B. Slutsky1, A. Mizrahi1, V. Lomakin1& Y. Fainman1The effects of cavity quantum electrodynamics (QED), caused bythe interaction of matter and the electromagnetic field in sub-wavelength resonant structures, have been the subject of intenseresearch in recent years1. The generation ofcoherent radiation bysubwavelength resonant structures has attracted considerableinterest, not only as a means of exploring the QED effects thatemerge at small volume, but also for its potential in applicationsranging from on-chip optical communication to ultrahigh-resolution and high-throughput imaging, sensing and spectro-scopy. One such strand of research is aimed at developing the‘ultimate’ nanolaser: a scalable, low-threshold, efficient source ofradiation that operates at room temperature and occupies a smallvolumeon achip2. Differentresonators havebeenproposedfortherealization of such a nanolaser—microdisk3and photonic band-gap4resonators,and,moredielectric7–10and plasmonic11,12resonators. But progress towardsrealizing the ultimate nanolaser has been hindered by the lack ofasystematic approach to scaling down the size of the laser cavitywithout significantly increasing the threshold power required forlasing. Here wedescribeafamilyofcoaxialnanostructuredcavitiesthat potentially solve the resonator scalability challenge by meansof their geometry and metal composition. Using these coaxialnanocavities, we demonstrate the smallest room-temperature,continuous-wave telecommunications-frequency laser to date. Inaddition, by further modifying the design of these coaxialnanocavities, we achieve thresholdless lasing with a broadbandgain medium. In addition to enabling laser applications, thesenanoscale resonators should provide a powerful platform for thedevelopment of other QED devices and metamaterials in whichatom–field interactions generate new functionalities13,14.The miniaturization oflaser resonators using dielectric or metallicmaterialstructuresfacestwochallenges:(1) the(eigen-)modescalability,implyingthe existence ofaself-sustained electromagnetic fieldregard-less ofthe cavity size, and (2) a relationship between optical gain andcavityloss whichresults inalargeand/orunattainablelasingthresholdas the volume of the resonator is reduced15. Here we propose anddemonstrate a new approach to nano-cavity design that resolves bothchallenges: first, subwavelength-size nano-cavities with modes farsmaller than the operating wavelength are realized by designing aplasmonic coaxial resonator that supports the cut-off-free transverseelectromagnetic (TEM) mode; second, the high lasing threshold forsmall resonators is reduced by utilizing cavity QED effects, causinghigh coupling of spontaneous emission into the lasing mode16,17.When fully exploited, this approach can completely eliminate thethreshold constraint by reaching so-called thresholdless lasing, whichoccurs when every photon emitted by the gain medium is funnelledinto the lasing mode16,17.The coaxiallasercavityis shownin Fig. 1a. Atthe heartofthe cavitylies a coaxial waveguide that supports plasmonic modes and is com-posed of a metallic rod enclosed by a metal-coated semiconductorring18,19. The impedance mismatch between a free-standing coaxialwaveguide and free space creates a resonator. However, our designrecently,metallic5,6,metallo-usesadditionalmetalcoverageontopofthedeviceandthin,low-indexdielectric plugs ofsilicon dioxide (SiO2) at the top end ofthe coaxialwaveguide and air at the bottom end to improve the mode confine-ment. The role of the top SiO2plug is to prevent the formation ofundesirable plasmonic modes at the top interface, between the metalandthe gainmedium. The lowerairplugis usedto allowpump energyinto the cavityand also to couple out the light generated in the coaxialresonator. The metal in the sidewalls ofthe coaxial cavity is placed indirect contact with the semiconductor to ensure the support ofplas-monic modes, providing a large overlap between the modes of theresonator and the emitters distributed in the volume ofthe gain med-ium. In addition, the metallic coating serves as a heat sink that facil-itates room-temperature and continuous-wave operation.Toreducethelasingthreshold,thecoaxialstructuresaredesignedtomaximize thebenefits fromthe modification ofthe spontaneous emis-sion due to the cavity QED effects16,17. Because oftheir small size, themodalcontentofthenanoscalecoaxialcavities is sparse, whichis akeyrequirement to obtain high spontaneous emission coupling into thelasing mode of the resonator. Their modal content can be furthermodified by tailoring the geometry, that is, the radius of the core,the width of the ring, and the height of the gain medium and thelow-index plugs. Note that the number of modes supported by the1Department ofElectrical and ComputerEngineering, UniversityofCalifornia San Diego, 9500 Gilman Drive, La Jolla, California 92093-0407, USA. {Present address: Oracle Labs, 9515 Town Centre Drive,San Diego, California 92121, USA.*These authors contributed equally to this work.RcoreSiO2 plugAir plugh3h2h1Δ1 00 nmStructure ASiO2Rcore = 1 75 nm = 75 nmΔInGaAsPInP1 00 nmStructure BSiO2Rcore = 1 00 nm = 1 00 nmΔInGaAsPInPabcFigure 1 | Nanoscale coaxial lasercavity. a, Diagram ofa coaxial laser cavity;the gain medium is shown in red. See main text for description ofnomenclature. b, c, Scanning electron microscope images ofthe constituentrings in structure A and structure B, respectively. A side view ofthe ringscomprisingthecoaxial structures is seen; therings consistofSiO2on top, andaquantum-well gain region underneath. See main text for details.2 0 4 | N A T U R E |V O L 4 8 2 | 9 F E B R U A R Y 2 0 1 2Macmillan Publishers Limited. 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