PUBLISHED ONLINE: 19 OCTOBER 2015 DOI: 10.1038/NPHOTON.2015.197 Monolyer exitoni lser Yu Ye 1,2,ZiJingWong 1,2,XiufngLu 3,XingjieNi 1,2, Hnyu Zhu 1,XinhuiChen 3,YunWng 1,2 n Xing Zhng 1,2,4 * Two-imensionl vn er Wls mterils hve opene new prigm for funmentl physis explortion n evie pplitions euse of their emerging physil properties. Unlike gpless grphene, monolyer trnsition-metl ihlogenies (TMDCs) re two-imensionl semionutors tht unergo n iniret-to-iret ngp trnsition 1 5, reting new optil funtionlities for next-genertion ultr-ompt photonis n optoeletronis. Although the enhnement of spontneous emission hs een reporte on TMDC monolyers integrte with photoni rystls 6,7 n istriute Brgg refletor mirovities 8,9, oherent light emission from TMDC monolyer hs not een emonstrte. Here, we report the reliztion of two-imensionl exitoni lser y emeing monolyer in miroisk resontor. Using whispering gllery moe with high qulity ftor n optil onfinement, we oserve right exitoni lsing t visile wvelengths. This emonstrtion of two-imensionl exitoni lser mrks mjor step towrs two-imensionl on-hip optoeletronis for high-performne optil ommunition n omputing pplitions. As iret ngp semionutor, trnsition-metl ihlogenie (TMDC) monolyers hve ttrte inresing ttention for eletroni n optoeletroni pplitions ue to their strong light emission ompnie with unique ess to spin n vlley egrees of freeom 10 13. These properties rise from the quntum onfinement n rystl symmetry effet on the eletroni n struture s the mteril is thinne own to monolyer onfigurtion. The emonstrtions of exellent on off rtio trnsistors 14, vlley-hll physis 15, lrge exiton ining energy 16 18, lightemitting ioes 3 5, superonutivity 19, sensors 20 n piezoeletriity 21,22 show the iverse physis n pplitions of this mteril system. However, oherent light emission, or lsing, n essentil step towrs the reliztion of on-hip photoni pplitions, hs not een relize. The esign n frition of mirovities is ruil for two-imensionl lser, whih requires high optil moe onfinement ftor n high qulity ftor Q. Here, we emonstrte two-imensionl exitoni lser using monolyer ouple to miroisk resontor, whih hs high quntum yiel, smll footprint n low power onsumption. TMDCs suh s (Fig. 1) evolve from iniret- to iretngp semionutors s the numer of lyers is reue from ulk to monolyer, with sizle ngps roun 2.0 ev t visile wvelengths. The iret ngps sit t the K n K vlleys (Fig. 1), two non-equivlent momentum vlleys in the reiprol spe of the monolyer protete y the rystl s roken inversion symmetry, proviing rih vlley-ontrsting physis 10 13. The trnsition etween vlene n n onution n-ege is exitoni in nture in suh monolyer system. Strong exitoni fetures, inluing neutrl n reshifte hrge exitons, hve een oserve n stuie 23. The exiton in two-imensionl TMDCs not only governs the emissions properties, ut lso llows for the long-live popultion inversion require to hieve optil gin n possile stimulte emissions. Although Purell enhnement of spontneous emission hs een hieve in photoni rystls n istriute Brgg refletor mirovities 6 9, oherent light emission or lsing from two-imensionl semionuting TMDC hs not een emonstrte ue to the limite mteril gin volume, n the lk of optil onfinement n feek within the tomi monolyer. Here, we report monolyer exitoni lser in miroisk resontor (Fig. 1). By integrting the monolyer into strongfeek photoni vity, the uil-up of stimulte emission n eventully exee the lsing threshol. Miroisks feture lowloss, high-qulity whispering gllery moes (WGMs) tht offer the potentil for ultrlow-threshol lsing 24. Emeing the monolyer etween two ieletri lyers (Si 3 N 4 / /hyrogen silsesquioxne (HSQ)) enles strong optil onfinement n les to lrger mol gin, neessry for n tomilly thin monolyer gin meium. The snning eletron mirosope (SEM) imge in Fig. 1 of the unerut Si 3 N 4 / /HSQ miroisk shows the low siewll roughness tht is essentil to otin high vity Q. The imeter of the HSQ lyer is slightly smller thn tht of the Si 3 N 4 lyer ue to the finite ething seletivity etween the HSQ n Si 3 N 4. The vity resonne is esigne to overlp with the gin spetrum of monolyer (Supplementry Setion I), with the eletri fiel polrize in the plne of the TMDC monolyer to effiiently ouple with the in-plne ipoles of the exitons 25. For Si 3 N 4 /HSQ miroisk struture with imeter of 3.3 µm, we expet strong trnsverse eletri (TE)-polrize WGM t wvelength of 612 nm. The smll imeter of the miroisk ensures other resonne moes re wiely seprte in wvelength to voi moe ompetition. Reuing the numer of moes will lso inrese the spontneous emission ftor n ontriute to the improvement of the lsing threshol. The eletri fiel istriution of the TE 1,24 resonne (ril moe numer l = 1, zimuthl moe numer m = 24) is shown in Fig. 2 (top view) n Fig. 2 (rosssetionl view). The resonnt wvelength mthes the ominnt pek of the mesure lsing spetrum from monolyer emee within 3.3-µm-imeter miroisk t 612.2 nm (Fig. 2), with mesure Q = λ/δλ of 2,604. The snwihe onfigurtion provies two vntges: (1) enhne optil moe overlp, (2) mteril protetion. It is importnt to note tht the optil onfinement ftor of our Si 3 N 4 / /HSQ struture is 30% higher thn in the se where the monolyer is iretly trnsferre onto the top of pre-uilt miroisk (Supplementry Setion I). The enhne onfinement ftor is one orer lrger thn those of low-threshol quntum-ot miroisk lsers 26. It is worth 1 NSF Nnosle Siene n Engineering Center, University of Cliforni, Berkeley, Cliforni 94720, USA. 2 Mterils Sienes Division, Lwrene Berkeley Ntionl Lortory, Berkeley, Cliforni 94720, USA. 3 Hefei Ntionl Lortory for Physil Siene t Mirosle n Deprtment of Physis, University of Siene n Tehnology of Chin, Hefei, Anhui 230026, Chin. 4 Deprtment of Physis, King Aulziz University, Jeh 21589, Sui Ari; These uthors ontriute eqully to this work. *e-mil: xing@erkeley.eu NATURE PHOTONICS ADVANCE ONLINE PUBLICATION www.nture.om/nturephotonis 1
Si W S HSQ Si 3 N 4 mentioning tht the lsing performne of our two-imensionl lser oes not ey even fter one yer, s our snwihe struture protets the monolyer from iret exposure to ir, whih is known to egre its luminesene property 27. In ition, the whispering gllery resonne feture of our struture is further verifie y the three itionl peks in the photoluminesene spetrum, tht is, t 633.7 nm, 657.6 nm n 683.7 nm, orresponing to the TE 1,23, TE 1,22 n TE 1,21 moes in simultion (Supplementry Setion I). Suh moes re only oserve on the low-energy sie of the spetrum, s high-energy photons re more likely to e resore efore oupling to the vity resonne 24. To stuy the emission hrteristis of the monolyer miroisk lser, the evie ws optilly pumpe with n ultrfst lser (190 fs pulse urtion, 80 MHz repetition rte) t 473 nm n 10 K. The pump soure ws swithe on only for 190 fs time intervls, whih re short enough to signifintly minimize heting n ll relte therml effets, llowing opertion t higher pek power ensities ove lsing threshol. The evie hs imeter of 3.3 µm n the emission spetr were ollete ( 50, ojetive of 0.55 NA) s funtion of pump intensity (Fig. 3). At the lowest pump intensity we oserve ro emission orresponing to the photoluminesene of the. For pump intensities etween 3.14 MW m 2 n 22.4 MW m 2, shouler ppers t 612.2 nm, mplifie y the optil feek in the miroisk vity. Aove 22.4 MW m 2, the pek inreses shrply in intensity, showing ler eviene of lsing. In n tomilly thin TMDC monolyer, the optil trnsition spetrum in the non-interting limit exhiits step-like funtion. The strong onfinement enhnes the eletron hole intertion, leing to lrge ining Energy AX K (K ) 1 μm Figure 1 Design of the monolyer exitoni lser., A single lyer of omprising S W S stking, with totl thikness of 0.65 nm. In the top view, eh unit ell n e seen to onsist of two S toms oupying the sme site in the hexgonl lttie, with the W tom resiing t the opposite site., Bn struture t the K (K ) point, showing the iret n A exiton (AX) trnsition, n vlene n splitting ue to spin orit oupling., Shemti imge of monolyer miroisk lser. The snwih struture, Si 3 N 4 / /HSQ, ensures higher onfinement ftor n les to lrger mol gin., SEM of n unerut Si 3 N 4 / /HSQ miroisk, showing the smooth siewll essentil for hieving high vity qulity ftor. energy n shrp exitoni levels 16 18,23, whih further inreses the optil gin n nrrows the gin spetrum ompre with those in the non-interting limit. This moifition of the optil trnsition osilltor strength is preite to result in severl improvements in the lser hrteristis, suh s lower threshol, higher moultion nwith n smller emission linewith 28. The 10 K photoluminesene spetrum t pump intensity of 65.7 MW m 2 (Fig. 3) is well fitte with i-lorentzin urves, where the pek intensity of oth the ro monolyer photoluminesene spetrum kgroun n nrrow vity emission re extrte. In Fig. 3, the pump intensity epenene of the photoluminesene emission (L L urves) is plotte for oth the vity n the kgroun photoluminesene emission. Below the lsing threshol, the vity emission inreses linerly with exittion energy. Aove the threshol, istint kink is oserve in the L L urve, with superliner inrese in the emission output. In ontrst, the monolyer kgroun photoluminesene emission mintins liner epenene on ll pump energies euse the emission is minly from the entre of the miroisk n oes not ouple to the vity moe. In WGMs, eletri fiels re minly lolize t the eges of the miroisk. The eletril fiel istriution for the lsing moe (612.2 nm) n non-lsing moe (633.7 nm) hevily overlp one nother, initing their ompetitive nture in terms of rrier epletion. Compre with the lsing moe (612.2 nm), the 633.7 nm WGM (Fig. 3) oes not show ny lsing ehviour ue to the insuffiient optil gin t wvelength off the resonne of the exitoni trnsition in monolyer. Uner lsing onitions, the 633.7 nm WGM emission lmps fter the lsing threshol, euse the pump power is onverte to the vity lsing photon popultion. The lmping of the ompeting non-lsing moes eyon the lsing threshol is well stuie s n experimentl signture of lsing 29. The lsing ehviour ws lso oserve in nother monolyer lser (Supplementry Setion IV) with high Q, initing the high repetility of this rrngement. NATURE PHOTONICS DOI: 10.1038/NPHOTON.2015.197 612.2 nm TE 1,24 Si HSQ Si 3 N 4 TE 1,23 633.7 nm TE 1,22 657.6 nm TE 1,21 683.7 nm 600 620 640 660 680 Figure 2 Whispering gllery moes of the monolyer exitoni lser., Top view of the simulte eletri fiel istriution of the TE 1,24 resonne., Cross-setionl view of the lulte eletri fiel istriution of the TE 1,24 resonne. The Si 3 N 4 / /HSQ snwih struture enhnes the optil moe overlp with monolyer. Re n lue olours orrespons to the mximum n minimum of the fiel ensity, respetively. The oxe outlines refer to the struture ounry., Experimentl photoluminesene spetrum tken t 10 K when the pump intensity is ove lsing threshol, showing WGMs t 612.2 nm, 633.7 nm, 657.6 nm n 683.7 nm. 2 NATURE PHOTONICS ADVANCE ONLINE PUBLICATION www.nture.om/nturephotonis
NATURE PHOTONICS DOI: 10.1038/NPHOTON.2015.197 LETTERS 48.9 MW m 2 37.8 MW m 2 22.4 MW m 2 3.14 MW m 2 1.68 MW m 2 0.85 MW m 2 1,400 1,200 1,000 800 600 400 200 Dt Bkgroun emission vity 600 605 610 615 620 vity Bkgroun emission 612.2 nm lsing moe 633.7 nm WGM 590 600 610 620 630 0 20 40 60 Pump intensity (MW m 2 ) 0 20 40 60 Pump intensity (MW m 2 ) Figure 3 Oservtion of monolyer exitoni lsing., Stey-stte photoluminesene emission spetr with inresing pump intensity, normlize to pump intensity, illustrting the trnsition from spontneous emission to stimulte emission n lsing., Photoluminesene spetrum for pump intensity of 65.7 MW m 2. Symols show the mesure t, the rown line is fit to the monolyer photoluminesene kgroun emission (from the entre of the miroisk), n the green line is fit tothe vity emission., Monolyer photoluminesene kgroun n vity emissions s funtion of pump intensity. Dshe lines represent liner fits to the experimentl t. The photoluminesene kgroun emission shows liner epenene on the pump intensity, n the green she lines (vity emission) show kink initing the onset of superliner emission n lsing opertion., WGM emission (633.7 nm) lmps fter the lsing threshol. To further verify the lsing ehviour, the emission from our evie ws integrte for 5 s to refully resolve the evolution of the extremely nrrow spetrl linewith s the pump intensity ws grully inrese eyon the lsing threshol. Using rte eqution nlysis, we foun the lsing threshol to e roun 5 8 MWm 2 pump intensity, with n extrte verge threshol mol gin of 78.8 m 1 for the est fitte spontneous emission ftor β of 0.5 (Fig. 4). Due to the high Q of our vity, the Purell effet signifintly enhnes the spontneous emission ftor β, where more spontneous emission is now iretly ouple to the esire lsing moe. Couple with the lrger β ftor, the trnsition kink etween the spontneous emission regime n the stimulte emission smers, leing to soft lsing threshol of 5 8MWm 2 (Supplementry Setions III n V). L L urves for vlues of β of 05, 5 n 1 re lso presente for omprison. High-resolution spetr of vity moe emission t spontneous emission (4.9 MW m 2 ) n the lser osilltion regime (92.1 MW m 2 ) re lso presente. In the spontneous emission regime (Fig. 4), the full-with t hlf-mximum (FWHM) of the moe emission is 0.28 nm n reues to 0.24 nm when the exittion intensity rehes the lser osilltion regime (Fig. 4), lerly showing the linewith nrrowing effet (Fig. 4) oserve in stnr lsers. The smll linewith nrrowing t the threshol is typil for high β ftor lsers 30 33. We foun tht high Q is neessry to hieve lsing osilltion in our monolyer miroisk. A lower vity Q of 1,162, mesure in nother smple, prevents lsing ue to the signifint resontor losses. The internl luminesene quntum yiel, efine y the frtion of sore photons tht emit ritively, is one of the key elements neessry for lser. A high quntum yiel is ruil to hieve lrge optil gin, leing to lower threshol pump power. The quntum yiels of ifferent TMDC monolyers my vry wiely ue to unintentionl oping uring the syntheti/ nturl growth proess of the rystls. We experimentlly mesure the photoluminesene of MoS 2, n WSe 2 monolyers, n quntifie their respetive quntum yiels to etermine the optiml monolyer TMDC mteril for lsing pplitions. The monolyer quntum yiel ws extrte y mesuring the sorption (in ition to photoluminesene), n lirte using stnr rhomine 6G smples s the ontrol 1,34 (Supplementry Setion I). The monolyer t ryogeni tempertures ws foun to hve the lrgest quntum yiel ( 6%), five times n two orers lrger thn the quntum yiels of monolyer WSe 2 n MoS 2, respetively, mking it our optiml TMDC of hoie for optil gin. In summry, we hve emonstrte visile emission from twoimensionl exitoni lser using monolyer s novel gin meium. The ility to generte oherent light rition in n tomilly thin semionuting monolyer pves the wy for twoimensionl oherent ontrol n vn er Wls-se optoeletronis. Seletive exittion of the rrier popultion in one set of two istint vlleys n further le to lsing in the onfine vlley, key element for future vlley optoeletronis. Note e in proof: While this mnusript ws uner review, two relte works were reporte 35,36. NATURE PHOTONICS ADVANCE ONLINE PUBLICATION www.nture.om/nturephotonis 3
NATURE PHOTONICS DOI: 10.1038/NPHOTON.2015.197 10,000 Output intensity (.u.) 1,000 100 10 1 β = 1 β = 0.5 β = 5 β = 05 1 10 Pump intensity (MW m 2 ) 100 0.28 nm 4.9 MW m 2 611.5 612.0 612.5 Dt Fitting sum vity 613.0 0.30 0.28 FWHM (nm) 0.26 30,000 20,000 10,000 0.24 nm Dt Fitting sum vity 0.24 0 92.1 MW m 2 1 10 Pump intensity (MW m 2 ) 100 611.5 612.0 612.5 613.0 Figure 4 Chrteriztions of two-imensionl exitoni lsing., Experimentl t n rte eqution nlytil fits. The est fit to the experimentl t gives threshol pump intensity of 5 8MWm 2 with spontneous emission ftor, β, of0.5.thefits to β of 05, 5 n 1 re lso presente for omprison., FWHM versus input pump intensity. Linewith nrrowing of the lsing moe is oserve s the exittion intensity exees the lsing threshol. The re she line is guie to the eye. The linewith error r is too lrge t low pump intensity to etermine the rel linewith., High-resolution spetrum in the spontneous emission regime (4.9 MW m 2 ). The vity emission (green urve) gives FWHM of 0.28 nm., High-resolution spetrum in the lsing osilltor regime (92.1 MW m 2 ). The vity emission (green urve) gives FWHM of 0.24 nm. Reeive 6 Ferury 2015; epte 17 Septemer 2015; pulishe online 19 Otoer 2015 Referenes 1. Mk, K. F., Lee, C., Hone, J., Shn, J. & Heinz, T. F. Atomilly thin MoS 2 : new iret-gp semionutor. Phys. Rev. Lett. 105, 136805 (2010). 2. Splenini, A. et l. Emerging photoluminesene in monolyer MoS 2. Nno Lett. 10, 1271 1275 (2010). 3. Ross, J. S. et l. Eletrilly tunle exitoni light-emitting ioes se on monolyer WSe 2 p n juntions. Nture Nnoteh. 9, 268 272 (2014). 4. Pospishil, A., Furhi, M. M. & Mueller, T. Solr-energy onversion n light emission in n tomi monolyer p n ioe. Nture Nnoteh. 9, 257 261 (2014). 5. Bugher, B. W. H., Churhill, H. O. H., Yng, Y. & Jrillo-Herrero, P. Optoeletroni evies se on eletrilly tunle p n ioes in monolyer ihlogenie. Nture Nnoteh. 9, 262 267 (2014). 6. Wu, S. et l. 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NATURE PHOTONICS DOI: 10.1038/NPHOTON.2015.197 32. Gong, Y. et l. Nnoem photoni rystl vity quntum ot lser. Opt. Express 18, 8781 8789 (2010). 33. Mohieen, U. & Slusher, R. E. Semionutor mirolser linewiths. Phys. Rev. Lett. 73, 1785 1788 (1994). 34. Ammer, F., Penzkofer, A. & Weiner, P. Conentrtion-epenent fluoresene ehvior of oxzine 750 n rhomine 6G in porous silite xerogel monoliths. Chem. Phys. 192, 325 331 (1995). 35. Wu, S. et l. Monolyer semionutor nnovity with ultrlow threshols. Nture 520, 69 72 (2015). 36. Slehzeh, O., Djvi, M., Trn, N. H., Shih, I. & Mi, Z. Optilly pumpe twoimensionl MoS 2 lsers operting t room-temperture. Nno Lett. 15, 5302 5306 (2015). Aknowlegements The uthors knowlege finnil support from the US Air Fore Offie of Sientifi Reserh uner wr no. FA9550-12-1-0197 (Optil Design n Chrteriztion), n the Light Mteril Intertion in Energy Conversion Energy Frontier Reserh Center fune y the US Deprtment of Energy, Offie of Siene, Offie of Bsi Energy Sienes uner wr no. DE-AC02-05CH11231 (Mterils Synthesis n Lithogrphy). The uthors lso thnk A. Grine for his help in mesuring the pssive Q ftor of the vity. Author ontriutions X.Z., Y.W., Y.Y. n Z.J.W. oneive the projet. X.L. n X.C. grew ulk rystls. Y.Y., Z.J.W. n H.Z. evelope the smple esign n frite the smples. Y.Y., Z.J.W. n X.N. performe the mesurements. Z.J.W. n Y.Y. performe the numeril simultion. Y.Y. n Z.J.W. rrie out the t nlysis. Y.Y., Z.J.W., X.Z. n Y.W. wrote the mnusript. X.Z. n Y.W. guie the reserh. All uthors ontriute to isussions. Aitionl informtion Supplementry informtion is ville in the online version of the pper. Reprints n permissions informtion is ville online t www.nture.om/reprints. Corresponene n requests for mterils shoul e resse to X.Z. Competing finnil interests The uthors elre no ompeting finnil interests. LETTERS NATURE PHOTONICS ADVANCE ONLINE PUBLICATION www.nture.om/nturephotonis 5