LETTER. Laser cooling of a semiconductor by 40 kelvin

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1 doi:1.138/nture11721 Lser ooling of semiondutor y kelvin Jun Zhng 1 *, Dehui Li 1 *, Renjie Chen 1 & Qihu Xiong 1,2 Optil irrdition ompnied y spontneous nti-stokes emission n led to ooling of mtter, in phenomenon known s lser ooling, or optil refrigertion, whih ws proposed y Pringsheim in In gseous mtter, n extremely low temperture n e otined in diluted tomi gses y Doppler ooling 2, nd lser ooling of ultrdense gs hs een demonstrted y ollisionl redistriution of rdition 3. In solid-stte mterils, lser ooling is hieved y the nnihiltion of phonons, whih re qunt of lttie virtions, during nti-stokes luminesene. Sine the first experimentl demonstrtion in glsses doped with rre-erth metls, onsiderle progress hs een mde, prtiulrly in ytteriumdoped glsses or rystls: reently reord ws set of ooling to out 11 kelvin from the mient temperture, surpssing the thermoeletri Peltier ooler,. It would e interesting to relize lser ooling in semiondutors, in whih exitoni resonnes dominte 7 9, rther thn in systems doped with rre-erth metls, where tomi resonnes dominte. However, so fr no net ooling in semiondutors hs een hieved despite muh experimentl 1 12 nd theoretil 7 9,13,1 work,minlyongroup-iii Vglliumrsenide quntum wells. Here we report net ooling y out kelvin in semiondutor using group-ii VI dmium sulphide nnorions, or nnoelts, strting from 29 kelvin. We use pump lser with wvelength of 1 nnometres, nd otin n estimted ooling effiieny of out 1.3 per ent nd n estimted ooling power of 18 mirowtts. At 1 kelvin, 32-nm pumping leds to net ooling of out 1 kelvin with ooling effiieny of out 2. per ent. We ttriute the net lser ooling in dmium sulphide nnoelts to strong oupling etween exitons nd longitudinl optil phonons (s), whih llows the resonnt nnihiltion of multiple s in luminesene up-onversion proesses, high externl quntum effiieny nd negligile kground sorption. Our findings suggest tht, lterntively, group-ii VI semiondutors with strong exiton oupling ould e hrnessed to hieve lser ooling nd open the wy to optil refrigertion sed on semiondutors. Optil refrigertion hs the dvntges of omptness, freedom from virtion nd the need for ryogen, high reliility 1, nd ould hve pplitions in ll-solid-stte ryooolers 1 nd therml or selfooling lsers 1. Although net ooling hs primrily een demonstrted in vriety of rre-erth metl (REM)-doped glsses nd rystls,,1 17, the lser ooling of diret-ndgp semiondutors, for exmple gllium rsenide (GAs), is more ppeling euse semiondutors llow more effiient pump light sorption, muh lower hievle ooling tempertures nd diret integrility into eletroni nd photoni devies 1 12,18. Nevertheless, the net ooling of GAssed semiondutor hs inherent impediments due to high kground sorption nd low luminesene extrtion effiieny, lthough nti-stokes up-onversion n e redily hieved 9,11,12,18. Aording to Sheik-Bhe/Epstein (SBE) theory 9,19, the ooling effiieny, g (hn, T), of optil refrigertion in semiondutors is given y n f (T) g (hn,t)~g exe g s {1 ð1þ n Here n is the pump lser frequeny, T is the solute temperture of the smple, n f (T) is the men emission frequeny nd h is Plnk s onstnt; g exe is the externl quntum effiieny, written s g exe g e W rd / (g e W rd 1 W nr ), where g e is the luminesene extrtion effiieny nd W rd nd W nr re respetively the rditive nd non-rditive reomintion rtes; nd g s [1 1 /(n, T)] 21 is the sorption effiieny, quntifying the perentge of photons sored tht re engged in ooling, where is the kground sorption oeffiient nd (n, T) is the semiondutor sorption oeffiient. Net lser ooling requires g (hn, T)., whih might e hieved in three relted wys: hving lrge energy differene DE hn f (T) 2 hn; hving g exe pproh unity; nd hving g s pproh unity through the minimiztion of the kground sorption,. In the lssil Pringsheim piture pplied to solids, eh ooling yle removes DE hn f (T) 2 hn < k B T during the thermliztion of old eletrons nd holes, owing to sorption of vrious phonons. In GAs, single -ssisted trnsitions in nd-til sorption (Urh til) hve een proposed to filitte lser ooling 2,21. It hs lso een proposed tht the nd struture n e engineered, for exmple y introduing donor eptor pirs, whih modifies the density of sttes nd ould led to lrger DE, mking the dependene on the other two ftors (g exe nd g s ) less importnt. However, experimentlly tht is diffiult to hieve euse the doping density nd inding energies of oth donors nd eptors hve to e simultneously optimized. We find tht strong exiton oupling in II VI semiondutors suh s dmium sulphide (CdS) n e hrnessed to filitte lser ooling y the nnihiltion of one or more s, leding to the removl of severl k B T units of het in eh ooling yle. Figure 1 shows the nti-stokes photoluminesene spetr of CdS nnoelt exited y 32-nm lser (ottom) nd 1-nm lser (top) t 29 K t three different pumping powers. Strong nti-stokes photoluminesene with pek position of, nm is identified, filitted y resonnt nnihiltion of multiple s (one for the 1-nm lser nd three for the 32-nm lser; hn LO < 37 mev, or 3 m 21 ). We note tht 1-nm pumping led to muh stronger up-onversion, owing to stronger nd-til sorption. Systemti study of the nti-stokes photoluminesene intensity s funtion of lser power (Fig. 1) indites tht two-photon sorption n e exluded t low-tomoderte power (elow,12 mw), t whih our lser ooling experiments re onduted. Further nlysis of the Stokes nd nti-stokes Rmn spetr revels tht oth the nti-stokes omponent nd the 2 nti-stokes omponent re stronger thn their Stokes ounterprts, suggesting tht multiple- nnihiltion domintes over the retion proess. This dominne of the nti-stokes omponent n e redily hieved in CdS under resonnt onditions, tht is, when the lser energy is red-detuned from the exiton energy y roughly n integer multiple of hn LO (Supplementry Fig. 3). Following the theory in ref. 22, we evlute the Hung Rhys ftor, S, whih quntifies the exiton oupling strength (Supplementry Informtion, prt 2). As shown in Supplementry Fig. 3, we find tht S is onsiderly enhned in CdS on the nnosle, in good greement with reent report on ZnTe nnorods 23. Therefore, we speulte tht the resonnt 1 Division of Physis nd Applied Physis, Shool of Physil nd Mthemtil Sienes, Nnyng Tehnologil University, 37371, Singpore. 2 Division of Miroeletronis, Shool of Eletril nd Eletroni Engineering, Nnyng Tehnologil University, 39798, Singpore. *These uthors ontriuted eqully to this work. NATURE VOL 93 2 JANUARY 213 Mmilln Pulishers Limited. All rights reserved 213

2 RESEARCH 2, Intensity (ounts) 2, 1, 1,, 1, 1, 2 ASPL 1 nm K hv LO = 37 mev.7 mw 1. mw 2.7 mw Figure 1 Multiple--ssisted up-onversion spetr of CdS nnoelts., Room temperture nti-stokes photoluminesene (ASPL) spetr of single, 1-nm-thik CdS nnoelt exited y 1-nm (top) nd 32-nm (ottom) lser t three power levels., Power dependene of nti-stokes A ASPL (1.u.) 1 1 nm Liner fitting Liner fitting 1/ K Power (mw) 1 1 photoluminesene pumped t 32 nd 1 nm nd t 29 K. Below,12 mw, the intensity sles linerly with power. Two-photon-sorption-indued photoluminesene strts to tke effet only t high power. A ASPL, integrted pek re of nti-stokes photoluminesene..u., ritrry units. nnihiltion of one or more s filittes the up-onversion in CdS nnoelts owing to the resulting inrese in the exiton oupling, leding to the removl of multiple k B T units of het in eh ooling yle. The mirosopi mehnism y whih multiple LO phonons re tully involved in the up-onversion still remins to e ddressed within the frmework of higher-order exiton oupling. Next we systemtilly evlute the dependene of DE on pump power nd temperture (etween 77 nd 3 K) when pumped y 1- nd 32-nm lsers (Methods Summry). Cooling is possile only if DE is positive. Therefore, the sign nd mgnitude of DE t defined temperture hve importnt implitions for the ooling potentil. Figure 2 nd Fig. 2 show the temperture evolution of nti-stokes Normlized photoluminesene (.u.) Power (mw) K Rmn shift (m 1 ) 1,8 1,2 t 8 mw Temperture (K) 77 K 1 K 13 K 1 K 18 K 2 K 23 K 2 K 293 K 31 K ΔE (mev) Figure 2 Surfe plot of DE s funtion of temperture nd pump power., Anti-Stokes photoluminesene spetr tken from 77 to 3 K using n 8.- mw, 32-nm lser. As the temperture is deresed from 3 to 18 K, the photoluminesene pek grdully lueshifts. Below,18 K, donor eptor pir (DAP) emission eomes pronouned. Both DAP nd exiton emissions lueshift mrginlly., Anti-Stokes photoluminesene spetr of the sme nnoelt exited y.-mw, 1-nm lser. Below 2 K, DAP emission peks pper nd dominte the spetr. The spetr re offset vertilly for lrity, with vertil dshed lines inditing the wvelengths of the pump lsers nd dshed oxes inditing the first-order nti-stokes (22 m 21 ), first-order Stokes (2 m 21 ) nd seond-order (98 m 21 ) Rmn fetures from the silion sustrte in nd Rmn shift (m 1 ) 1,2 1,2 1 1 nm t mw 77 K 1 K 13 K K 18 K 2 K 23 K 2 K K 31 K 3 K 8 2 d 3 1 nm Temperture (K) ΔE (mev)., DE s funtion of temperture nd power for 32-nm pumping. The mximum energy differene, of,173 mev, is otined t round 18 K. Below 18 K, DAP emission peks () pper nd led to pronouned redshifts of the men wvelength of luminesene. d, DE surfe plot for 1-nm pumping. The mximum DE, of,2. mev, is hieved t round 2 K. Below 2 K, the DAP emission peks () pper nd dominte the spetr, leding to the redshift of the men wvelength of luminesene. Beuse the DAP energy levels resonte pproximtely with the pump lser, the DAP emission is muh stronger thn the exiton emission, whih uses DE to e negtive elow 18 K, suggesting tht no net ooling is hievle using 1-nm pumping. In oth nd d, Si Rmn fetures re exluded efore the men emission wvelength is evluted. 2 JANUARY 213 VOL 93 NATURE 213 Mmilln Pulishers Limited. All rights reserved

3 RESEARCH LETTER photoluminesene spetr for lser exittion t (8. mw) nd 1 nm (. mw), respetively. Detiled pek ssignment to the exitons nd donor eptor pirs (DAP) is referred to in Supplementry Informtion, prts 2 3. By extrting the men emission frequeny, we n otin surfe plots of DE hn f (T) 2 hn (Fig. 2, d). For the 32-nm pumping, mximum DE of,173 mev is rehed t round 18 K. With further derese in temperture, DAP emission egins nd leds to redution in DE. Over the whole temperture rnge we investigte (77 3 K), DE remins positive, inditing tht net ooling is possile. For the 1-nm pumping, mximum DE of,3 mev is rehed t round 2 K, elow whih DAP emission egins, leding to trnsition from positive to negtive DE t round 18 K. Therefore, it is impossile to hieve net ooling elow 18 K when pumping t 1 nm, in good greement with our ooling experiments to e disussed (see elow). Over the power rnge we investigte, no strong dependene of DE on power is oserved, s mnifested y the vertilly striped nds in the surfe plots (Fig. 2, d). However, net lser ooling is possile only elow 12 mw, euse the two-photon proess will dominte the luminesene up-onversion when higher power is used (Supplementry Informtion, prt 3.2). We use pump proe luminesene thermometry (PPLT) tehnique to mesure the lser ooling with the smple imge, set-up nd time sequene s shown in Fig. 3. The min ide is to use the temperture dependene of the Stokes photoluminesene pek position for non-ontt lol thermometry, essentilly similr to differentil luminesene thermometry 19 (Supplementry Informtion, prt 3.1). Nevertheless, in our lser ooling experiments the hnge in temperture is suffiient for our high-resolution spetrometer to detet ooling from the shifting of the luminesene pek to shorter 1 μm Pump Proe 73 nm CCD Figure 3 Lser ooling pump proe luminesene thermometry., Flseolour snning eletron mirosope imge of single CdS nnoelt suspended on SiO 2 /Si sustrte. The lser ooling demonstrtion ws onduted on the suspended segment s shown in the inset., Mesurement set-up with two lser ems (pump nd proe) ligned on the sme spot., Time sequene of the PPLT. After every min of pumping (for exmple, using 32-nm lser), the pump lser ws loked momentrily nd the proe Stokes photoluminesene spetrum ws quired immeditely, exited y 2-mW, 73-nm lser. CCD, hrge-oupled devie. wvelengths. We use solid-stte, 32-nm lser nd n rgon ion lser (vriously 1, 2 nd 88 nm) s pump lsers, nd use solid-stte, 73-nm lser s proe em to mesure the ooling or heting. The CdS nnoelts re suspended to derese the therml ondution through sustrtes nd to improve the luminesene extrtion effiieny on hole-ptterned SiO 2 /Si sustrtes (Methods Summry). We first mesure nd evlute the temperture lirtion urves y monitoring the pek shift of the photoluminesene spetrum of the CdS nnoelt exited y the proe em round the strting tempertures, 29 nd 1 K, respetively (Supplementry Fig., d). Then we use these lirtion urves to dedue the temperture hnge used y the lser ooling, DT. Figure nd Fig. d respetively show the Stokes photoluminesene evolution on ontinuous.3-mw, 1-nm lser pumping strting from 29 K nd ontinuous.-mw, 32-nm lser pumping strting from 1 K. After every, min of pumping, the pump lser ws momentrily loked while eh spetrum ws reorded. We oserve pronouned systemti lueshift on lser pumping until stedy stte is rehed in,3 min, suggesting tht mximum net ooling is estlished for the given devie struture nd geometry. After the ooling is stopped (y swithing off the pump lser), the Stokes photoluminesene shifts to longer wvelengths, inditing temperture inrese (dshed urves in Fig., d, shifted vertilly for lrity). the sis of the lirtion urves, DT is plotted in Fig. nd Fig. e, showing respetive net lser oolings of, K for 1-nm pumping nd,2 K for 32-nm pumping, oth t 29 K. At 1 K, only 32-nm pumping n led to net lser ooling of 1 K; 1-nm pumping nnot, owing to redshifting of the men emission wvelength t 1 K, in greement with Fig. 2, d. Pumping t 88 nd 2 nm leds to heting of the smple t ll tempertures in the rnge investigted. Complete dt for ll wvelengths nd tempertures re shown in Supplementry Fig. 7. Knowledge of the sorption t the nd til is ruil to evluting the up-onversion nd lser ooling. At the single-nnoelt level, the diret mesurement of optil sorption remins elusive. We hve devised photoondutivity mesurement to provide diret quntifition of the sorption 2, whih grees well with the sorption oeffiient extrted from the photoluminesene spetr sed on the vn Roosroek/Shokley eqution 2 (Supplementry Fig. 2). Figure shows the Stokes photoluminesene (lk urve) nd the orresponding photoondutivity gin spetr (lue urve nd, on logrithmi sle, red urve) t 29 K. We find tht t, there is out % sorption ompred with the mximum sorption t round 8 nm, wheres t 1 nm, the orresponding sorption is,3%. Figure f shows the normlized temperture hnge s funtion of pump lser wvelength t 29 K (lue urve), lulted on the sis of the SBE model nd n nlysis of ooling power (Supplementry Informtion, prts ). The disrete dt points provide vlues of experimentl temperture hnge, DT, pumped y four different lser lines, tht re in good greement with theoretil nlysis exept for tht orresponding to 88 nm, whih underestimtes the tul heting onsiderly (Supplementry Informtion, prt ). The finl ooling power depends on oth DE nd nd-til sorption. In CdS nnoelts t 29 K, 1 nm seems to e the optiml ooling wvelength with the highest ooling power. The red regions in Fig., f orrespond to the lser ooling til. Two underlying ftors mke net ooling imprtil in GAs-sed semiondutors. These ftors re high prsiti kground sorption nd low luminesene extrtion effiieny, oth of whih led to heting nd so represent the existing hllenges not only in fundmentl mterils siene ut lso in devie engineering in lser ooling 1,1,19. In CdS nnoelts, eh ooling yle n remove more het thn in GAs, s indited y the DE nlysis, euse of the extremely strong exiton oupling (the Fröhlih oupling onstnt in ulk is.1 for CdS nd.7 for GAs 2, nd is even higher on the nnosle, s shown in Supplementry Fig. 3) nd resonnt nti- Stokes up-onversion. When the pump lser (t, for exmple, 1 or NATURE VOL 93 2 JANUARY 213 Mmilln Pulishers Limited. All rights reserved 213

4 RESEARCH Normlized photoluminesene (.u.) K 1 nm,.3 mw Wrming 1 nm off Cooling ΔT (K) K,.3 mw 1 nm,.3 mw 2 nm, 2 mw 88 nm, 3 mw Gin 2 29 K Log(gin) d Normlized photoluminesene (.u.) K,. mw off Wrming Cooling e ΔT (K) K f,. mw 1 nm,.7 mw 2 nm, 3.7 mw 88 nm, 2.2 mw 1 29 K ΔT (K mw 1 ) Heting zone Cooling zone Time (min) Figure Net lser ooling of CdS nnoelts., Evolution of PPLT spetr strting from 29 K, pumped y 1-nm lser with power of,.3 mw. Solid urves represent the ooling yle, nd the dshed urves represent the wrming up fter the pump lser is swithed off. Dshed urves re shifted vertilly for lrity., Temperture hnge, DT, versus time pumped y four lser lines (32, 1, 2 nd 88 nm), using dt extrted from the PPLT spetr (shown in for 1-nm pumping; omplete dt re shown in Supplementry Fig. 7) nd orresponding lirtion urve round 29 K (Supplementry Fig. )., Photourrent gin spetrum (liner sle, lue; logrithmi sle, red) nd Stokes photoluminesene spetrum (lk) of single CdS nnoelt t 29 K. d, Evolution of PPLT spetr for nother nnoelt ) moves to the Urh til of CdS, the kground sorption due to free rriers n no longer e ignored. Nonetheless, our nlysis shows tht the free-rrier sorption in CdS nnoelt is negligile. The luminesene extrtion effiieny pprohes unity s result of the suwvelength thikness of the nnoelt (Supplementry Informtion, prt ). We did not oserve ny emission t the longer-wvelength side of the pump lser wvelength (), suggesting no kground sorption ontriution due to defets or surfe reomintion. Compred with GAs, CdS hs muh smller surfe reomintion veloity 27 nd muh lower Auger non-rditive reomintion oeffiient, owing to the lrger ndgp 28. Our previous dt 29 nd more in-depth nlysis hve indeed reveled tht the rditive reomintion dey lifetime is of the order of tens to hundreds of pioseonds, wheres the non-rditive dey lifetime is three orders of mgnitude lrger thn this t room temperture nd is even greter t low tempertures, justifying the ner-unity externl quntum effiieny (.99%) in CdS nnoelts. Cooling effiienies of,.8% t 29 K nd,2.% t 1 K re estimted from the ove nlysis for 32-nm pumping, nd ooling effiieny of 1.3% t 29 K is estimted for 1-nm pumping (Supplementry Informtion, prt ). The ooling powers re estimted to e,97 nd 18 mw for 32- nd 1-nm pumping t 29 K, respetively, with orresponding therml ondutive lods of out 8 nd 1 mw, in good greement pumped y.-mw, 32-nm lser strting from 1 K (omplete dt re shown in Supplementry Fig. 7). We oserve ooling nd wrming yles similr to those in. e, Temperture hnge, DT, versus time pumped y four lser lines (32, 1, 2 nd 88 nm), using dt extrted from the PPLT spetr (shown in d for 32-nm pumping; omplete dt re shown in Supplementry Fig. 7) nd the orresponding lirtion urve round 1 K (Supplementry Fig. d). f, Summry of mesured mximum DT (pink squres) nd theoretilly lulted temperture hnge (lue urve) normlized to pump power for different pump wvelengths t 29 K. In nd f, the red regions orrespond to the ooling zone. with the ooling power. The rditive lod is negligile ording to lk-ody model for oth wvelengths, suggesting tht the therml ondutive lod domintes the het dissiption (Supplementry Informtion, prt ). Our work demonstrtes sustntil net lser ooling of semiondutor CdS nnoelt y, K from room temperture when pumped y 1-nm lser, nd net ooling y,1 K from 1 K when pumped y 32-nm lser. In priniple, the therml ondutive lod n e further minimized y devie engineering to filitte the lser ooling of CdS nnoelts from mient temperture to tht of liquid nitrogen. Our work opens the wy to the design of mterils for the lser ooling of semiondutors with strong eletron oupling. It would e n immedite dvne to test other II VI mterils in the single-rystl, thin-film or nnomteril morphology for lser ooling. METHODS SUMMARY The CdS nnoelts were synthesized in home-uilt, vpour-trnsport hemilvpour-deposition system. Synthesis nd hrteriztion detils hve een pulished elsewhere 29. The CdS nnoelt ws deposited on indium foil sustrtes or silion sustrtes with 1-nm oxide lyer to evlute the Rmn spetrum (Fig. 1) nd the men emission wvelength (Fig. 2), respetively. The thikness of the nnoelt ws mesured y tomi fore mirosopy (Nnosope III, Veeo Instruments). We onduted experiments using six different lser powers, t every,2-k intervl etween 77 nd 3 K, to generte the DE surfe plot. The lser 2 JANUARY 213 VOL 93 NATURE Mmilln Pulishers Limited. All rights reserved

5 RESEARCH LETTER ooling ws performed on nnoelts suspended ross inverted-pyrmid-shped holes (,3 mm wide) on SiO 2 /Si sustrte ethed using potssium hydroxide solution. The smple ws mounted on the old finger of ontinuous-flow mirosopy ryostt. A PPLT tehnique ws used to ool nd mesure the lol temperture vrition of the nnoelts. The power of proe em of wvelength 73 nm ws kept s low s 2 mw to eliminte the lser heting effet. Both the pump em (Ar ion lser, 88, 2 nd 1 nm; Nd:YAG solid-stte lser, ) nd the proe em (solid-stte lser, 73 nm) were ollimted nd foused thorough 3 ojetive onto the single CdS nnoelt tht rossed n ethed hole. All the spetr were olleted using onfol triple-grting spetrometer (Hori-JY T) in ksttering onfigurtion. With -mm fol length nd 1,8-mm 21 grting, the spetrometer hs spetrl resolution of,. m 21, orresponding to resolution of,.1 nm t wvelength of, nm. Full methods nd more detils out PPLT nd the lirtion n e found in the Supplementry Informtion. Reeived 21 Septemer; epted 2 Otoer Pringsheim, P. Zwei Bemerkungen üer den Untershied von Lumineszenz- und Temperturstrhlung. Z. Phys. A 7, (1929). 2. Phillips, W. D. Noel Leture: Lser ooling nd trpping of neutrl toms. Rev. Mod. Phys. 7, (1998). 3. Vogl, U. & Weitz, M. Lser ooling y ollisionl redistriution of rdition. Nture 1, 7 73 (29).. Epstein, R. I., Buhwld, M. I., Edwrds, B. C., Gosnell, T. R. & Mungn, C. E. Oservtion of lser-indued fluoresent ooling of solid. Nture 377, 3 (199).. Seletskiy, D. V. et l. Lser ooling of solids to ryogeni tempertures. Nture Photon., 11 1 (21).. Seletskiy, D. V. et l. Lol lser ooling of Y:YLF to 11 K. Opt. Express 19, (211). 7. Rivlin, L. A. & Zdernovsky, A. A. Lser ooling of semiondutors. Opt. Commun. 139, (1997). 8. Rupper, G., Kwong, N. H. & Binder, R. Lrge exitoni enhnement of optil refrigertion in semiondutors. Phys. Rev. Lett. 97, 1171 (2). 9. Sheik-Bhe, M. & Epstein, R. I. Cn lser lightoolsemiondutors? Phys. Rev. Lett. 92, 273 (2). 1. Finkeißen, E., Potemski, M., Wyder, P., Vin, L. & Weimnn, G. Cooling of semiondutor y luminesene up-onversion. Appl. Phys. Lett. 7, (1999). 11. Guk, H., Gfroerer, T. H., Renn, M. J., Cornell, E. A. & Bertness, K. A. Externl rditive quntum effiieny of 9% from GAs/GInP heterostruture. Appl. Phys. A, (1997). 12. Imngholi, B., Hsselek, M. P., Sheik-Bhe, M., Epstein, R. I. & Kurtz, S. Effets of epitxil lift-off on interfe reomintion nd lser ooling in GInP/GAs heterostrutures. Appl. Phys. Lett. 8, 811 (2). 13. Rupper, G., Kwong, N. H. & Binder, R. Optil refrigertion of GAs: theoretil study. Phys. Rev. B 7, 223 (27). 1. Khurgin, J. B. Surfe plsmon-ssisted lser ooling of solids. Phys. Rev. Lett. 98, 1771 (27). 1. Nemov, G. & Kshyp, R. Lser ooling of solids. Rep. Prog. Phys. 73, 81 (21). 1. Epstein, R. I. & Sheik-Bhe, M. Optil Refrigertion (Wiley-VCH, 29). 17. Sheik-Bhe, M. & Epstein, R. I. Optil refrigertion. Nture Photon. 1, (27). 18. Eshlghi, S., Worthoff, W., Wiek, A. D. & Suter, D. Luminesene uponversion in GAs quntum wells. Phys. Rev. B 77, 2317 (28). 19. Sheik-Bhe, M. & Epstein, R. I. Lser ooling of solids. Lser Photon. Rev. 3, 7 8 (29). 2. Khurgin, J. B. Bnd gp engineering for lser ooling of semiondutors. J. Appl. Phys. 1, (2). 21. Khurgin, J. B. Role of ndtil sttes in lser ooling of semiondutors. Phys. Rev. B 77, 232 (28). 22. Merlin, R. etl. Multiphonon proesses in YS. Phys. Rev. B 17, (1978). 23. Zhng, Q. et l. Exiton-phonon oupling in individul ZnTe nnorods studied y resonnt Rmn spetrosopy. Phys. Rev. B 8, 818 (212). 2. Li, D. H., Zhng, J., Zhng, Q. & Xiong, Q. H. Eletri field-dependent photoondutivity in CdS nnowires nd nnoelts: exiton ioniztion, Frnz- Keldysh nd Strk effets. Nno Lett. 12, (212). 2. Bsu, P. K. Theory of Optil Proesses in Semiondutors: Bulk nd Mirostrutures (Oxford Univ. Press, 23). 2. Sdo, A., Peter, C., Sf, K. & Arthur, W. Properties of Semiondutor Alloys: Group- IV, III V nd II VI Semiondutors 13 (Wiley, 29). 27. Huppert, D., Evenor, M. & Shpir, Y. Mesurements of surfe reomintion veloity on CdS surfes nd Au interfes. J. V. Si. Tehnol. A 2, (198). 28. Imngholi, B. Investigtion of Lser Cooling in Semiondutors 9. PhD thesis, Univ. New Mexio (2). 29. Xu, X. et l. Dynmis of ound exiton omplexes in CdS nnoelts. ACS Nno, 3 39 (211). Supplementry Informtion is ville in the online version of the pper. Aknowledgements We thnk M. Sheik-Bhe nd R. Merlin for helpful disussions. Q.X. knowledges the support from the Singpore Ntionl Reserh Foundtion through fellowship grnt (NRF-RF29-). This work ws lso supported in prt y the Singpore Ministry of Edution vi Tier 2 grnt (MOE211-T2-2-1) nd strt-up grnt support (M8113) from Nnyng Tehnologil University. Author Contriutions J.Z., D.L. nd Q.X. hd the ide for this work; J.Z., D.L. nd Q.X. designed the experiments; J.Z. nd D.L. performed the experiments; R.C. ptterned the sustrtes; nd J.Z., D.L. nd Q.X. nlysed the dt nd wrote the mnusript. Author Informtion Reprints nd permissions informtion is ville t The uthors delre no ompeting finnil interests. Reders re welome to omment on the online version of the pper. Correspondene nd requests for mterils should e ddressed to Q.X. (qihu@ntu.edu.sg). 8 NATURE VOL 93 2 JANUARY 213 Mmilln Pulishers Limited. All rights reserved 213