LETTERS. Generation of single optical plasmons in metallic nanowires coupled to quantum dots

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1 Vol Novemer 7 doi:1.138/nture623 Genertion of single optil plsmons in metlli nnowires oupled to quntum dots A. V. Akimov 1,4 *, A. Mukherjee 1 *,C.L.Yu 2 *, D. E. Chng 1, A. S. Zirov 1,4, P. R. Hemmer 3, H. Prk 1,2 & M. D. Lukin 1 Control over the intertion etween single photons nd individul optil emitters is n outstnding prolem in quntum siene nd engineering. It is of interest for ultimte ontrol over light qunt 1, s well s for potentil pplitions suh s effiient photon olletion 2, single-photon swithing 3 nd trnsistors 4, nd long-rnge optil oupling of quntum its 5,6. Reently, sustntil dvnes hve een mde towrds these gols, sed on modifying photon fields round n emitter using high-finesse optil vities 2,3,5 8. Here we demonstrte vity-free, rodnd pproh for engineering photon emitter intertions 4,9 vi suwvelength onfinement of optil fields ner metlli nnostrutures When single CdSe quntum dot is optilly exited in lose proximity to silver nnowire, emission from the quntum dot ouples diretly to guided surfe plsmons in the nnowire, using the wire s ends to light up. Non-lssil photon orreltions etween the emission from the quntum dot nd the ends of the nnowire demonstrte tht the ltter stems from the genertion of single, quntized plsmons. Results from lrge numer of devies show tht effiient oupling is ompnied y more thn 2.5-fold enhnement of the quntum dot spontneous emission, in good greement with theoretil preditions. Surfe plsmons, or surfe plsmon polritons, re propgting exittions of hrge-density wves nd their ssoited eletromgneti fields on the surfe of ondutor 1. Muh like the optil modes of onventionl dieletri fire, rod ontinuum of surfe plsmon modes n e onfined on ylindril metlli wire nd guided long the wire xis 12,13 (Fig. 1). However, s opposed to dieletri wveguides 14,31, the thin wires n mintin propgtion of Het Γ rd Γ pl Enhnement ftor, P nm wire 1 nm wire.6.3 Effiieny, η 6 8 Distne from wire (nm) Min. Mx. d ρ (µm) z (µm) Figure 1 Rditive oupling of quntum dots to onduting nnowires.,a oupled quntum dot n either spontneously emit into free spe or into the guided surfe plsmons of the nnowire with respetive rtes C rd, C pl., Theoretil dependene of the enhnement ftor P (solid line) nd effiieny of emission into surfe plsmons (dshed line) on distne of the emitter from the nnowire edge. The red (lue) urve orresponds to wire dimeter of 1 nm (5 nm)., Simultions of the eletri field mplitude (ritrry units) emitted y dipole (lue filled irle) positioned 25 nm from 3 θ Sttered intensity (.u.) θ (degrees) one end of onduting nnowire (whose surfe is outlined) 3 mm in length nd 5 nm in dimeter. The vertil sle (r) is enlrged ompred to the horizontl (z) to lerly show the ner field of the surfe plsmons. Upon hitting the fr end ofthe nnowire, some of the surfe plsmonenergy is lerly sttered into the fr-field, while the remining is either lost to dissiption or to k-refletion. h, Emission ngle. d, Amplitude of the Poynting vetor of the light sttered from the fr end of the nnowire, s funtion of h (see ), for wires of dimeter 1 nm (red urve), 5 nm (lue) nd 25 nm (green). 1 Deprtment of Physis, 2 Deprtment of Chemistry nd Chemil Biology, Hrvrd University, Cmridge, Msshusetts 2138, USA. 3 Deprtment of Eletril nd Computer Engineering, Texs A&M University, College Sttion, Texs 77843, USA. 4 P.N. Leedev Physil Institute RAS, Leninskiy prospet 53, Mosow, , Russi. *These uthors ontriuted eqully to this work. 2 7 Nture Pulishing Group

2 NATURE Vol Novemer 7 surfe plsmon modes lolized trnsversely to dimensions omprle to the wire dimeter d, even when it is muh smller thn the optil wvelength l. This suwvelength loliztion is ompnied y drmti onentrtion of optil fields 1,11. In ddition, the surfe plsmon modes propgte with gretly redued veloities euse they involve the motion of hrge-density wves 9,15,16. The emission properties of nnosle optil emitter n e signifintly modified y the proximity of nnowire tht supports surfe plsmons. In priniple, three distint dey hnnels exist. First, diret optil emission into free-spe modes is possile, with rte modified from tht of n isolted quntum dot owing to the proximity of the metlli surfe 17. Seond, the optil emitter n e dmped non-rditively owing to ohmi losses in the ondutor 17. Lst, nd most importntly, the tight field onfinement nd redued veloity of surfe plsmons n use the nnowire to pture the mjority of spontneous rdition into the guided surfe plsmon modes 9, muh like lens with extrordinrily high numeril perture. For n optil emitter pled within the evnesent surfe plsmon mode til, the spontneous emission rte into the surfe plsmons 9 is proportionl to (l/d) 3. In ontrst, the free-spe emission rte n e enhned y t most ftor of four, wheres non-rditive dmping eomes signifint only for very smll wire emitter seprtion 9. Thus, for n optimlly pled emitter, the spontneous emission rte C pl into surfe plsmons n fr exeed the rditive nd non-rditive rtes (C rd nd C nrd, respetively), whih results in highly effiient oupling to surfe plsmons nd enhnement of the totl dey rte (C totl ) ompred to tht of n unoupled emitter (C ). This enhnement n e hrterized y Purell ftor, P 5 C totl /C, whih for thin wires is predited to e lrge 9. We emphsize tht this strong oupling is used y the geometril effet of tight onfinement of the surfe plsmons, nd ours fr wy from the plsmon resonne frequeny of nnowires 18. It does not involve n optil vity 2,3,5 8, nd n e hieved simultneously over rod ontinuum of optil frequenies. Chemilly synthesized CdSe quntum dots 19 pled proximlly to silver nnowires omprise simple experimentl system to investigte the emitter surfe plsmon oupling. As illustrted in Fig. 1, the spontneous emission of quntum dot is split etween photon emission into free spe, whih n e deteted y n optil mirosope, nd the exittion of surfe plsmons (C nrd is negligile for our hosen prmeters, s desried elow). During propgtion long the smooth nnowire, surfe plsmons do not ouple to the oservle fr-field modes of the surrounding dieletri. However, muh like onventionl ntenn, n rupt end of the wire n stter surfe plsmons rditively into fr-field modes, thus filitting their detetion using n optil mirosope. A simultion of this effet is shown in Fig. 1, where quntum dot is pled 25 nm wy from one wire end: wheres the surfe plsmons dey evnesently wy from the nnowire edge, sustntil emission into free spe results from surfe plsmon sttering t the fr end of the wire. Silver nnowires were prepred using solution-phse polyol method with modifitions for surfe pssivtion (Supplementry Informtion). The smples were reted y spinning quntum dots onto glss sustrte, overing them with n,3-nm lyer of poly (methylmethrylte) (PMMA; see Supplementry Informtion for detiled nlysis of the PMMA lyer), nd then depositing dry wires on top. Finlly, the smple ws overoted with thik lyer of PMMA. Snning eletron mirosopy imges reveled tht the dimeters of the silver nnowires were nm (Supplementry Informtion). The losest llowed distne etween the quntum dots nd nnowires is determined y the thikness of the PMMA lyer nd the quntum dot shell rdius (,5 nm), nd is,35 nm (Methods nd Supplementry Informtion). The experimentl setup for studying the quntum dot nnowire system (Fig. 2) is sed on modified onfol mirosope with three snning hnnels. One hnnel (I) ws used for imging nnowires, nd the seond hnnel (II) ws used for imging quntum dots. The third hnnel (III), whih n independently imge ny diffrtion-limited spot within the field of view of the ojetive lens, ws used to detet the sttered surfe plsmons from the nnowire ends. In generl, the oupling etween n optil emitter nd single surfe plsmons should e stronger for thinner wires 9 (Fig. 1). However, for thinner wires, the out-oupling effiieny of surfe plsmons to the fr-field t the wire end dereses owing to lrge wvevetor mismth. In this se, signifint surfe plsmon refletion t the nnowire ends uses stnding surfe plsmon wve 1 Ch II Glss QDs in uffer PMMA Wires PMMA Ch III Ch I Ch I Ch II Ch III Lser Figure 2 Experimentl set-up., Three-hnnel onfol mirosope with 532 nm lser exittion soure., Lyout of smple ontining quntum dots nd nnowires., Left, hnnel I: nnowire imge. Middle, hnnel II: imge of quntum dots. The red irle denotes the position of the oupled quntum dot, nd the sme point is lso denoted in the leftmost imge. 1 µm 7 Nture Pulishing Group Right, hnnel III: the exittion lser ws foused on the quntum dot (red irle). The lrgest right spot orresponds to the quntum dot fluoresene, while two smller spots orrespond to surfe plsmons sttered from the nnowire ends. The lue irle indites the frthest end of the nnowire, used for photon ross-orreltion mesurements. 3

3 NATURE Vol Novemer 7 formtion within the nnowire 12 (Fig. 1) nd eventul energy loss due to heting (ohmi losses). The effet of nnowire dimeter on out-oupling effiieny is illustrted in Fig. 1d, where the intensity of the sttered rdition from the wire end is plotted for different wire dimeters. For 25-nm nnowire, hrdly ny sttering is seen from the end despite the stronger oupling etween the emitter nd surfe plsmons, ut the sttering is signifint for 1-nm wire (this ws verified experimentlly y exiting surfe plsmons diretly with lser foused t one wire end; Supplementry Informtion). Nnowires with d < 1 nm exhiit oth resonle emitter surfe plsmon ouplings nd surfe plsmon to fr-field sttering, nd thus were hosen for the experiments. The lrge ndwidth of the surfe plsmon emitter oupling enles us to perform the experiments t room temperture, where single quntum dot spetrl width exeeds 15 nm (Supplementry Informtion). Figure 2 presents n experimentl demonstrtion of direted emission of quntum dot into surfe plsmons. The leftmost pnel shows onfol refletion imge of silver nnowire reorded with hnnel I. The middle pnel orresponds to fluoresene imge of quntum dots deteted t 655 nm with hnnel II. These two imges were used to determine the positions of the nnowire nd quntum dot reltive to eh other. Owing to the resolution limit of our optil system, the tul distne etween quntum dot nd the nnowire ould not e determined, nd only quntum dots tht pper diretly on top of nnowire were hosen for experiment. The rightmost pnel shows oupled wire dot system imged with hnnel III. When the proximl quntum dot (irled in red) ws exited y the lser, the nnowire ends literlly lit up. The lrge spot round the red irle orresponds to emission from the quntum dot itself, wheres the two other points oinide with the wire ends. Signifintly, high degree of orreltion ws seen etween the time tres of the fluoresene ounts from the quntum dot nd the end of the oupled wire (Fig. 3). These oservtions indite tht the soure of the fluoresene from the wire end is the quntum dot. Photon oinidene mesurements 1 of the quntum dots (Fig. 3) demonstrte tht these quntum dots n only emit single photon t time. In these mesurements, the free-spe fluoresene from the quntum dot ws eqully split into two hnnels using em splitter nd deteted y vlnhe photo-diodes. The oinidenes etween two hnnels were reorded s funtion of time dely t.if the quntum dot emits only one photon t time it n only e reorded t one of the hnnels, nd therefore zero oinidenes re expeted etween the two hnnels t t 5, s seen in Fig. 3. The slight offset from zero n e ttriuted to stry light, drk ounts of the detetors nd the resolution limit of the eletronis (Supplementry Informtion). The light emission t the nnowire end is result of single, quntized surfe plsmons sttering off the ends of the nnowire. This is demonstrted in Fig. 3 y the dip t t 5 in the photon oinidene mesurements etween the free-spe fluoresene of the quntum dot nd emission from the wire end. This ner-zero oinidene is onsequene of the ft tht the single photon emitted from quntum dot n either rdite into free spe or the surfe plsmon modes, ut never oth simultneously. Dt presented in Fig. 3, long with mesured ount rtes, n e used to quntify the oupling strength of the quntum dot to the surfe plsmons. As this oupling retes new dey hnnel for the quntum dot, its dey rte is expeted to inrese. To study this enhnement, oserved oinidene dt were fitted to simple two-level model of quntum dot emission 21 (Fig. 3; see lso Supplementry Informtion). The model inorportes n inoherent pumping rte R from the ground to n exited stte of quntum dot nd dey rte C totl k to the ground stte. In this model, the temporl p width of the nti-unhing dip is given y Dt~ ln ffiffi 2 ð RzCtotl Þ, where the exittion rte R is proportionl to the inident power. Therefore, y extrting Dt from oinidene mesurements s funtion of inident lser power nd extrpolting to R 5, C totl n e otined (Fig. 4). The nturl lifetimes of individul dots ( 3 ns) vry owing to the heterogeneity in their strutures. However, omprison of the lifetime distriutions of 3 oupled nd 1 unoupled quntum dots (Fig. 4) lerly demonstrtes tht sttistilly the lifetime (dey rte) of the exiton in oupled quntum dots is shortened (enhned). The verge lifetime of the oupled (unoupled) quntum dots ws found to e ns ( ns). At the sme time, the distriution for oupled quntum dots hs lrger weight towrds shorter lifetimes. Speifilly, ertin oupled nd unoupled quntum dots exhiited lifetimes s short s 6 ns nd 15 ns, respetively, inditing tht P. 2.5 is hieved for some oupled quntum dot nnowire systems. The pprent effiieny of emission into the surfe plsmons n e estimted y ompring the rtio of photon ounts (n) otined diretly from the dot nd from the wire ends, g m < n ends /(n dot 1 n ends ), nd is found to e,27% for the est oupled quntum dot nnowire system (Fig. 4 nd Supplementry Informtion). This vlue does not ount for the surfe plsmons tht re dissipted efore they reh the wire ends. Correting for the mesured verge sorption lengths in our nnowires llows us to dedue tht the tul effiieny pprohes g < 6 6 1% (Supplementry Informtion), diretly demonstrting Intensity (1 3 ounts s 1 ) Time, t (s) Self-orreltion oinidenes 8 6 Time dely, τ (ns) Figure 3 Demonstrtion of single surfe plsmon genertion., Time tre of fluoresene ounts (red urve) from oupled quntum dot nd sttered light (lue) from the end of the oupled nnowire. Flututions re due to quntum dot linking 19., Seond-order orreltion funtion G (2) (t) of quntum dot fluoresene. The numer of oinidenes t t 5 goes lmost to zero, onfirming tht the quntum dot is single-photon soure. 4 In 2/(R + Γ tot ) 7 Nture Pulishing Group Self-orreltion oinidenes 8 6 Time dely, τ (ns) The width of the dip depends on C totl nd the pumping rte R s shown., Seond-order ross-orreltion funtion etween fluoresene of the quntum dot nd sttering from the nnowire end, otined y oinidenes etween hnnel II (quntum dot) nd hnnel III (wire end). The lk nd red tres in, indite experimentl dt nd est fits, respetively.

4 NATURE Vol Novemer 7 very effiient oupling to surfe plsmons. We note tht this oupling effiieny signifintly exeeds tht reently oserved etween toms nd dieletri nnofires 14,31. The rodnd nture of the strong oupling is demonstrted y ompring the optil spetr ssoited with emission from the quntum dot nd from the wire end. For individul dots rndomly drwn from n inhomogeneous ensemle with l nm, we find tht oth the quntum dot nd wire-end emission exhiit identil,15-nm-wide spetr. This is onsistent with the ility of metlli wires to guide rod rnge of optil frequenies 22 nd with theoretil preditions (Supplementry Informtion) tht strong oupling n e otined for rod ontinuum of frequenies wy from the pek of the oserved plsmon resonnes 18. Further insight into the quntum dot surfe plsmon oupling n e otined y ompring our experimentl oservtions with detiled eletrodynmi lultions 9. Our model of quntum dot emission ner silver nnowire emedded in dieletri medium inludes losses s well s multiple surfe plsmon modes. Figure 1 shows the totl spontneous emission rtes nd the effiieny g 5 C pl /C totl for single surfe plsmon genertion s funtion of quntum dot distne from the wire (d 5 5 nd 1 nm). Here the polriztion of the quntum dot trnsition ws seleted to e rdilly oriented, euse this diretion is expeted to yield the dominnt ontriution to enhnement. For quntum dots positioned 35 nm from the wire nd for 1 nm wire, the lultion yields Purell ftor P < 3.7. The lower enhnement oserved experimentlly n e ttriuted to the ontriutions from other polriztions nd the rndom positioning of the quntum dots wy from the wire. For this distne of seprtion, the non-rditive dey rte (C nrd,.5c ) is predited to e negligile (Supplementry Fig. 1). In ddition to enhned emission into surfe plsmon modes, our theory lso predits moderte inrese in the rditive emission Γ totl + R (ns 1 ) Enhnement, P Exittion power (µw) PMMA thikness (nm) 1/Γ totl normlized distriution d Effiieny ηη, m (%) Lifetime 1/Γ totl (ns) PMMA thikness (nm) Figure 4 Chrteriztion of quntum dot nnowire oupling., The liner dependene of G (2) width on lser power (lk filled irles) is extrpolted to zero power (red filled irle), yielding C totl of quntum dot. The lser power is proportionl to the inoherent pumping rte R of the dot., Normlized histogrms of quntum dot lifetimes. The lk (grey) rs denote the distriution of unoupled (oupled) quntum dots. Overlpping prts of the histogrms re indited y outlined nd vertilly stked rs., Averge Purell enhnement, P, versus PMMA thikness. Red line, verge vlue of P. Height nd width of grey rs indite the stndrd devitions of P nd PMMA thikness, respetively. d, Mesured mximum nd verge effiienies of emission into the surfe plsmons versus PMMA thikness. Blk (red) filled tringles, verge (mximum) pprent oupling effiienies g m, without ompensting for surfe plsmon losses. Red filled dimonds, mximum tul effiieny g, fter ompensting for dissiption. Error rs in,, d indite 61 s.d. 7 Nture Pulishing Group rte, well-known phenomenon for dipoles oriented perpendiulrly to metlli surfe 17. For 1 nm wires nd 35 nm nnowire quntum dot distnes, the surfe plsmon genertion effiieny g is theoretilly estimted to e,5%, whih is onsistent with our oservtions. Further omprison with theoretil preditions is otined y repeting our oservtions with thiker PMMA lyers (Fig. 4, d). These mesurements demonstrte tht oth enhnement nd estimted oupling effiieny rpidly derese s the minimum quntum dot nnowire sping inreses, nd eome very smll for PMMA thiknesses ove 1 nm. These oservtions re lso in good greement with the ove theoretil preditions. The lrge vrines in the Purell ftors otined for different devies re due primrily to vritions in the distne etween quntum dots nd nnowires eyond the minimum llowed distne set y the PMMA lyer thikness. The unique properties of nnosle surfe plsmons hve reently een explored in vriety of fsinting systems, from trnsmission through suwvelength strutures 11 to iomedil devies 1 nd proposls for relizing perfet lenses nd invisiility loks 1. Enhnement of fluoresene 23,24, polriztion-dependent oupling 25,26 nd norml mode splitting 27,28 ner suwvelength strutures hve lso reently een oserved. The present work extends these developments in two prinipl diretions. First, we hve shown experimentlly nd theoretilly tht the present pproh results simultneously in signifint enhnement of surfe plsmon emission nd effiient olletion into guided modes propgting long well-defined diretion. Seond, it estlishes diret oupling etween individul emitters nd individul, quntized surfe plsmons. It thus ridges the fields of nnosle plsmonis nd quntum optis, nd opens up the possiility of using quntum optil tehniques to hieve new levels of ontrol over the intertion of single surfe plsmons nd to relize novel quntum plsmoni devies. In the urrent set-up, the enefits of using smller wires must e lned ginst poor out-oupling to free-spe modes. However, this trde-off n e irumvented y using optimized geometries nd evnesent out-oupling to mode-mthed optil fires 9,16,23. The exellent oupling expeted from these integrted systems n e used, for exmple, for effiient single-photon soures, high resolution mirosopy 29,3 nd sensing, or long-rnge quntum it oupling 5. Furthermore, in suh systems n individul emitter n e mde optilly opque to single inident surfe plsmons, whih n e used to produe lrge optil nonlinerities for reliztion of single-photon trnsistors 4. Beyond these speifi pplitions, the ility to rete nd ontrol individul qunt of urrent osillting t optil frequenies nd ompnied y guided rdition with suwvelength loliztion opens up intriguing new possiilities t the interfe of optis nd eletronis. METHODS SUMMARY Smples were prepred y spin-oting solution of hemilly synthesized CdSe quntum dots (mixed with N 2 B 4 O 7 nd ysteine) onto plsm-lened glss slide t 3, r.p.m. for 6 s under nitrogen tmosphere. Three minutes lter, PMMA (1, 2 nd 3 wt% in toluene for 3, 6 nd 9 nm films) ws spun on top t 6, r.p.m. for 6 s. The quntum dots used do not dissolve in toluene nd re unpertured during the spin-oting proess (experimentlly, we find tht the rrngement of quntum dots on the surfe remins unhnged). A stmp with the modified silver nnowires ws pled on top of the slide nd pressed for few seonds. The stmp ws left there for min nd then gently peeled off, leving nnowires on the PMMA. Finlly, PMMA (2.2 wt%) ws spun on top t 1, r.p.m. for 6 s (Fig. 2). Our onfol mirosope uses.w. 532 nm lser s the exittion soure. It is foused onto the smple using Nikon CFI Pln Fluor 13 oil immersion ojetive NA 1.3, while mirror mounted on glvnometer is used to sn the inoming em. Chnnel II ts s onfol mirosope nd is used to imge single quntum dots, vi fluoresene t 655 nm. Chnnel I is omined with hnnel II using 9:1 em splitter tht direts prt of the refleted lser light towrds detetor nd n e used to imge the silver nnowires. Chnnel III is omined with the min set-up using 5:5 em splitter nd is n independent 5

5 NATURE Vol Novemer 7 imging system. It lso inludes glvnometer whih llows us to imge ny diffrtion limited spot within the field of view to detet fluoresene t 655 nm. Additionl detils of our experimentl set-up re provided in Supplementry Informtion. Reeived 1 April; epted 4 Septemer Hrohe, S. & Rimond, J.-M. Exploring the Quntum: Atoms, Cvities, Photons. (Oxford Univ. Press, New York, 6). 2. Englund, D. et l. Controlling the spontneous emission rte of single quntum in two-dimensionl photoni rystl. Phys. Rev. Lett. 95, 1394 (5). 3. Birnum, K. M. et l. Photon lokde in n optil vity with one trpped tom. Nture 436, 87 9 (5). 4. Chng, D. E., Sørensen, A. S., Demler, E. A. & Lukin, M. D. A single-photon trnsistor using nno-sle surfe plsmons. Nture Phys. dvne online pulition doi:1.138/nphys78 (26 August 7). 5. Cir, J. I., Zoller, P., Kimle, H. J. & Muhi, H. Quntum stte trnsfer nd entnglement distriution mong distnt nodes in quntum network. Phys. Rev. Lett. 78, (1997). 6. Immoğlu, A. et l. Quntum informtion proessing using quntum dot spins nd vity QED. Phys. Rev. Lett. 83, (1999). 7. Hennessy, K. et l. Quntum nture of strongly oupled single quntum dot vity system. Nture 445, (7). 8. Wilk, T., Wester, S. C., Kuhn, A. & Rempe, G. Single-tom single-photon quntum interfe. Siene 317, (7). 9. Chng, D. E., Sørensen, A. S., Hemmer, P. R. & Lukin, M. D. Quntum optis with surfe plsmons. Phys. Rev. Lett. 97, 532 (6). 1. Atwter, H. A. The promise of plsmonis. Si. Am. 296, (7). 11. Genet, C. & Eesen, T. W. Light in tiny holes. Nture 445, (7). 12. Snders, A. W. et l. Oservtion of plsmon propgtion, rediretion, nd fn-out in silver nnowires. Nno Lett. 6, (6). 13. Ditlher, H. et l. Silver nnowires s surfe plsmon resontors. Phys. Rev. Lett. 95, 2573 (5). 14. Nyk, K. P. et l. Optil nnofier s n effiient tool for mnipulting nd proing tomi fluoresene. Opt. Express 15, (7). 15. Tkhr, J., Ymgishi, S., Tki, H., Morimoto, A. & Koyshi, T. Guiding of one-dimensionl optil em with nnometer dimeter. Opt. Lett. 22, (1997). 16. Chng, D. E., Sørensen, A. S., Hemmer, P. R. & Lukin, M. D. Strong oupling of single emitters to surfe plsmons. Phys. Rev. B 76, 354 (7). 17. Chne, R. R., Prok, A. & Siley, R. Moleulr fluoresene nd energy trnsfer ner interfes. Adv. Chem. Phys. 37, 1 65 (1978). 18. Sun, Y., Gtes, B., Myers, B. & Xi, Y. Crystlline silver nnowires y soft solution proessing. Nno Lett. 2, (2). 19. Chung, I., Witkoskie, J. B., Co, J. & Bwendi, M. G. Desription of the fluoresene intensity time tre of olletions of CdSe nnorystl quntum dots sed on single quntum dot fluoresene linking sttistis. Phys. Rev. E 73, 1116 (6).. To, A. et l. Lngmuir-Blodgett silver nnowire monolyers for moleulr sensing using surfe-enhned Rmn spetrosopy. Nno Lett. 3, (3). 21. Lounis, B., Behtel, H. A., Gerion, D., Alivistos, P. & Moerner, W. E. Photon ntiunhing in single CdSe/ZnS quntum dot fluoresene. Chem. Phys. Lett. 329, (). 22. Dikson, R. M. & Lyon, L. A. Unidiretionl plsmon propgtion in metlli nnowires. J. Phys. Chem. B 14, (). 23. Hoherg, M., Behr-Jones, T., Wlker, C. & Sherer, A. Integrted plsmon nd dieletri wveguides. Opt. Express 12, (4). 24. Biteen, J. S., Lewis, N. S. & Atwter, H. A. Spetrl tuning of plsmon-enhned silion quntum dot luminesene. Appl. Phys. Lett. 88, (6). 25. Zhng, J., Ye, Y.-H., Wng, X., Rohon, P. & Xio, M. Coupling etween semiondutor quntum dots nd two-dimensionl surfe plsmons. Phys. Rev. B 72, 136(R) (5). 26. Mertens, H., Biteen, J. S., Atwter, H. A. & Polmn, A. Polriztion-seletive plsmon-enhned silion quntum-dot luminesene. Nno Lett. 6, (6). 27. Belless, J., Bonnnd, C. & Plenet, J. C. Strong oupling etween surfe plsmons nd exitons in n orgni semiondutor. Phys. Rev. Lett. 93, 364 (4). 28. Dintinger, J., Klein, S., Bustos, F., Brnes, W. L. & Eesen, T. W. Strong oupling etween surfe plsmon-polritons nd orgni moleules in suwvelength hole rrys. Phys. Rev. B 71, (5). 29. Klimov, V. V., Duloy, M. & Letokhov, V. S. A model of n pertureless snning mirosope with prolte nnospheroid s tip nd n exited moleule s n ojet. Chem. Phys. Lett. 358, (2). 3. Smolyninov, I. I., Elliott, J., Zyts, A. & Dvis, C. C. Fr-field optil mirosopy with nnometer-sle resolution sed on the in-plne mgnifition y surfe plsmon polritons. Phys. Rev. Lett. 94, 571 (5). 31. Sgué, G., Vetsh, E., Alt, W., Meshede, D. & Rusheneutel, A. Cold tom physis using ultr-thin optil fiers: light-indued dipole fores nd surfe intertions. Preprint t Æ (7). Supplementry Informtion is linked to the online version of the pper t Aknowledgements We knowledge disussions with M. Lonr, J. Doyle, A. Sørensen nd M.-H. Yoon, nd support from the NSF, DARPA, Hrvrd-MIT CUA, Hrvrd CNS, the DTO, the Pkrd Foundtion nd Smsung Eletronis. Author Informtion Reprints nd permissions informtion is ville t Correspondene nd requests for mterils should e ddressed to M.D.L. (lukin@fs.hrvrd.edu) nd H.P. (Hongkun_Prk@hrvrd.edu). 6 7 Nture Pulishing Group