UV radiation of powdered ZnO pumped by nanosecond pulses

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1 UV rdition of powdered ZnO pumped y nnosecond pulses V.M.Mrkushev, M.V.Ryzhkov, Ch.M.Briskin *, H.Co, L.A.Zdorozhny c, L.E.Li c, E.I.Gevrgizov c, L.N.Deminets c Institute of Rdio engineering nd Electronics of RAS, 11, Mokhovy st., Moscow, 129, Deprtment on Physics nd Astronomy, Northwestern University, USA c A.V.Shunikov Institute of Crystllogrphy of RAS, Russi ABSTRACT Since the intrinsic lifetime of spontneous recomintion UV rdition in zinc oxide is shorter thn ~2 ps, it is of interest to otin stimulted UV rdition of powdered ZnO with pumping pulses of nnosecond durtion. This will clrify the possiility of qusi-continuous lser rdition in disordered medi. At the sme time, this effect cn open the wy for the developing of cthodoluminescent screen with nrrow spectrum nd short persistence time. Investigtions of UV rdition spectr of powdered zinc oxide nd some disordered films were conducted. The smples were pumped y the 3-rd hrmonics of the two-stge Nd:YAG-lser (λ=355 nm) with the pulse durtion ~1 ns. Mximum energy density of the pumping pulses ws out 16 mj/cm 2. Spectr of spontneous emission were recordered t 3K nd 77K. In some of our powdered smples we chieved lsing t 3K. The threshold vlues of pumping energy density ppered to e pproximtely two orders of mgnitude higher thn those under picosecond pumping. Peculirities of lsing in different smples re demonstrted nd discussed. In the spectr of ZnO films investigted t 3K, the UV nd mximum is situted t ~382 nm, while in powders of ZnO this mximum is locted t ~389 nm. Besides, in the films the long-wvelength prt of the UV nd rodens with increse of pumping power. Keywords: rndom lser, powdered zinc oxide, stimulted UV rdition. 1. INTRODUCTION Stimulted rdition in rndomly inhomogeneous medi ws predicted y Letokhov 1 nd firstly experimentlly demonstrted in the dielectric powders doped with neodymium 2. Then lser effect ws otined nd investigted in the powdered zinc oxide excited y picosecond pulses 3. ZnO is the wide nd-gp semiconductor (~3.3 ev t room temperture) where the inding energy of the exciton mounts pproximtely 6 mev. The recomintion of excitons in zinc oxide gives UV rdition in the rnge nm, with the lifetime of spontneous emission less thn 2 ps. In such sitution it is reltively esy to excite zinc oxide y picosecond pumping ecuse during the pumping pulse the spontneous emission decy prcticlly does not hppen. A quite different sitution tkes plce in the cse of nnosecond excittion, which we use in our reserch. Here the pumping must leve ehind spontneous decy. In other words, initil prt of pumping pulse, shorter thn spontneous lifetime, must provide n excittion energy tht is enough for lsing. Below, we give simplified estimtion of the rise of the threshold t the trnsition from picosecond to nnosecond pumping. Under nnosecond pumping we my expect qusi-continuous lsing, the effect tht ws not investigted in the powders up to now. At the sme time, this effect cn open the wy for the developing of cthodoluminescent screens with nrrow spectrum nd short persistence time. There hs een only pper 4 where pressed powder of ZnO ws pumped y 5 ns pulses. Thus, we conducted the investigtions of UV rdition spectr of powders nd disordered films of zinc oxide mnufctured y different techniques. In some of the powdered smples studied nrrowing of the UV nd ws * chr@mil.cplire.ru; phone ; fx ; Complex Mediums VI: Light nd Complexity, edited y Mrtin W. McCll, Greme Dewr, Mikhil A. Noginov, Proceedings of SPIE Vol (SPIE, Bellinghm, WA, 25) X/5/$15 doi: / Proc. of SPIE 5924Z-1

2 oserved with the increse of the pumping energy density. At the sme time, numer of irregulr nrrow spikes ppered on the top of this nd. These two experimentl effects serve s n evidence of lsing in powdered ZnO excited y nnosecond pumping. 2.EXPERIMENT In our experiments, the smples were pumped with the 3-rd hrmonics (λ=355nm) of the two-stge Q-switched Nd: YAG-lser with the pulse durtion of pproximtely 1 ns nd repetition rte 2 Hz. For vrition of the pumping power with the second stge (mplifier) or without it, we used the set of filters with different trnsmission t 355 nm. The verge pumping power without mplifier ws ~.5mW nd ~2.9mW with it. The re of pumped spot on the smples ws pproximtely equl to cm 2. So, ville densities of energy were up to 16 mj/cm 2. The detection system consisted of the monochromtor MDR-23, the photomultiplier (PET-79), nd oxcr integrtor BCI-28. The dt were cquired y PC. The stepper motor, operted y the sme PC, rotted the diffrction grting of the monochromtor. The system could ccumulte specified numer of dt point t every fixed wvelength. In the experiment, powdered smples were plced in thin qurtz test tues. The mjority of the mesurements were conducted t room temperture (3K) nd few mesurements were mde t the temperture of liquid nitrogen (77K). Spontneous emission ws verged over 4-8 pulses; stimulte emission ws normlly collected without ny verging. 3. SIMPLIFIED ESTIMATION OF THE THRESHOLD INCREASE Let us estimte how mny times the thresholds of lsing under nnosecond pumping will e greter thn under picosecond one. The simplest wy is to suggest tht the pumping pulses re rectngulr. Then the energy of pumping tht flls on the smple during the lifetime of spontneous emission (τ) under long pump must e equl to the energy of short pump E ps. Since τ is less thn 2 ps nd the durtion of nnosecond pulses is ~1 ns the energy of long pump E ns must e more thn 1/.2=5 times greter thn E ps. Somewht more ccurte estimtion of the threshold increse with the using of lnce eqution gives prcticlly the sme result. So, simplified estimtions show tht the threshold density of nnosecond pumping must e pproximtely 5 times lrger thn tht of picosecond one. Actully it turned out tht it is severl times lrger. 4. RESULTS We investigted UV emission of powdered smples mnufctured y different groups nd using different techniques. With the powders, we investigted series of disordered films of zinc oxide mnufctured y therml or mgnetron long sputtering of ZnO on glss, qurtz or spphire sustrte. Beforehnd, the sustrtes were coted with thin (~2-3 nm) lyer of metl (In, Ar or Cu). Under the heting of sustrte, the metllic lyer forms n ssemly of fine droplets. On these droplets columns of zinc oxide grow. The film surfce is presented in the microgrph tht ws mde y scnning electron microscope (SEM) in the Institute of crystllogrphy of RAS (Fig.1). In the Fig.2, where chip of the film is depicted one cn see columnr structure of the film. Proc. of SPIE 5924Z-2

An exmple of such spectrum is depicted in Fig.3(). 1 3 2 2 1 Fig.3. UV-emission spectr of smple #1 t different tempertures: - 3К; 77K Fig.4.

3 Fig.1. The surfce of the film. Fig.2. A chip of the film. 4.1 SPONTANEOUS EMISSION At room temperture, the spectr of spontneous UV emission of powdered zinc oxide hve the mximum t nm nd the ndwidth of the order of nm. An exmple of such spectrum is depicted in Fig.3() Fig.3. UV-emission spectr of smple #1 t different tempertures: - 3К; 77K Fig.4. UV-emission spectr of ZnO film t different levels of excittion: low level (without mplifier), high level (with mplifier) It ws oserved tht intensity in the mximum of UV nd of different smples hs tendency to increse with the incresing of the prticle size (in the rnge etween ~15 nm nd ~2-3µm). When the pumping energy increses, the emission ndwidth decreses. (This will e considered in detil in the next susection). At the temperture of liquid nitrogen, the mximum of UV spectrum is locted t 372 nm nd the ndwidth is pproximtely 5 times nrrower thn tht t room temperture, this is demonstrted in Fig.3 (). The films investigted were mnufctured of the powder (smple #1) ut their spectr hve mximum t 382 nm nd, when the pumping energy increses, their ndwidth does not decrese ut widens towrd long-wve side (Fig.4). Unfortuntely t present we hve no ide out the origin of this effect nd will study it in future. Proc. of SPIE 5924Z-3

The smples investigted significntly differ one from nother oth y the threshold, y the vrition of this intervl nd y the chrcter of the lsing spectr.

4 4.2 LASING When the density of pumping energy increses, the stimulted emission rises. It ppers in the nd nrrowing nd then lsing rises. Different smples switch from spontneous emission to lsing in different intervls of pumping energies. The smples investigted significntly differ one from nother oth y the threshold, y the vrition of this intervl nd y the chrcter of the lsing spectr. Below we present the lsing spectr of smples investigted. It is necessry to point out tht it is impossile to compre the intensities corresponding to different curves in the figures since ech curve hs its own scle. For some of the smples, microgrphs mde y SEM re presented nd pproximte sizes of the powders grins re indicted. The lowest threshold (~2.7 mj/cm 2 ) ws otined for the smples #1 (Fig.5) nd #5 (Fig.7). As one cn see in Fig.6 nd Fig.8 the spectr of these smples re rther nrrow lredy t 2.7 mj/cm 2 nd when the pumping increses further they ecome even nrrower nd the spikes pper. At the sme time, the mximum of the spectr shift to longer wvelengths. As it cn e seen in the figures for other smples, this shift tkes plce for ll of our smples, nd the vlue of the shift mounts up to ~3 nm c Fig.5. Smple # 1 ( µm) Fig.6. UV-emission spectr of smple # 1 t different levels of excittion: 1.7 mj/cm 2, 2.7 mj/cm 2, с 27.7 mj/cm 2 T=3К. It must e noticed tht the spectrl positions nd intensities of spikes re different from one pumping pulse to nother. Chotic chrcter of lsing tht is evidenced itself y irregulr ppernce nd disppernce of intensive rdition from shot to shot ws recorded for ll smples investigted. The sme effect ws descried in Ref. 5. Its origin is yet unknown nd requires more detiled study. Anlogous results were otined in severl other smples. One of the chrcteristic sets of spectr is presented in Fig. 9, where the pumping level increses from Fig. 9 to 9d. Proc. of SPIE 5924Z-4

4 4 3 2 2 1 Fig.7. Smple # 5 (1-3 µm) Fig.8.

15 1 5 15 1 5 15 1 5 15 1 5 d c 37 375 38 385 39

T=3K. The highest threshold (~27 mj/cm 2 ) ws found for the smple #3 (Fig.1 nd Fig.11).

It seems tht only n mplified spontneous emission tkes plce in this cse. pj' 2 2 1 Fig.1. Smple # 3 (.3 -.

5 Fig.7. Smple # 5 (1-3 µm) Fig.8. UV-emission spectr of the smple # 5 t different levels of excittion: 2.7 mj/cm 2, 8 mj/cm 2. T=3К d c Fig.9. UV-emission spectr of the smple # 7: from to d the level of excittion increses. T=3K. The highest threshold (~27 mj/cm 2 ) ws found for the smple #3 (Fig.1 nd Fig.11). The min peculirity of this smple is prcticl lck of spikes even t high level of pumping. It seems tht only n mplified spontneous emission tkes plce in this cse. pj' Fig.1. Smple # 3 ( µm) Fig.11. UV-emission spectr of the smple #3 t different levels of excittion: 27.7 mj/cm 2, mj/cm 2. T=3К. Smples #4 (Fig.12) nd #6 (Fig.13) did not lse t ll up to mximum ville energy density ~16 mj/cm 2. Proc. of SPIE 5924Z-5

Fig.12. Smple # 4 (.2 -.4 µm) Fig.13. Smple #6 (<.

Apprently, these powders consist of smll prticles tht re stuck together forming the lumps. The most interesting results were otined in smple # 2 (Fig.15).

This smple demonstrtes very wek spontneous emission ut when pumping energy increses emission intensity shrply increses nd the width of the nd diminishes.

They hve lmost no pedestl, which is rther lrge in the cse of smples # 1, #5 nd # 7. It is very likely tht this pedestl is conditioned y mplified spontneous emission.

15. UV-emission spectr of the smple #2 t different levels of excittion: 2.7 mj/cm 2, 16 mj/cm 2. T=3К.

The instility of the pumping lser output my led to the distortion of the spectr.

6 Fig.12. Smple # 4 ( µm) Fig.13. Smple #6 (<.2 µm) One should note tht the powder prticles in smples #4 nd #6 hve not visile shrp orders similr to those seen in the SEM imges of the other smples. Apprently, these powders consist of smll prticles tht re stuck together forming the lumps. The most interesting results were otined in smple # 2 (Fig.15). It is prcticlly monodisperse nd its prticles represent spheres with dimeter ~.15 µm (Fig.14). This smple demonstrtes very wek spontneous emission ut when pumping energy increses emission intensity shrply increses nd the width of the nd diminishes. As the result, only few nrrow lines with the width.5-1 nm exist in the spectr. These lines differ from the spikes in the Fig.6, 8 nd 9 significntly. They hve lmost no pedestl, which is rther lrge in the cse of smples # 1, #5 nd # 7. It is very likely tht this pedestl is conditioned y mplified spontneous emission. It seems tht the resonnt feedck in smple #2 is considerly stronger thn in other smples, due to smple s monodispersivity...-w- qi" Fig.14. Smple # 2 (.15 µm) Fig.15. UV-emission spectr of the smple #2 t different levels of excittion: 2.7 mj/cm 2, 16 mj/cm 2. T=3К. In our method of spectrl recording, different spectrl points correspond to different lser pulses. The instility of the pumping lser output my led to the distortion of the spectr. We mde ll possile efforts to stilize the pumping power, however gret vrition of the emission signl from shot to shot ws oserved. This vrition ws significntly greter thn the pumping instility could give. We consider these vritions s the evidence of the chotic chrcter of lsing, similrly to tht oserved in 5. So we cn sy tht the fine structure of the spikes in our spectr sometimes doesn t exctly reflect their rel shpe. But some lines recur mny times, for exmple those in the spectr of smple #2, s it is demonstrted in Fig.16. This cn e considered s n evidence of rel existence of such nrrow lines. They my elong to the resonnt modes. Proc. of SPIE 5924Z-6

7 Fig.16. Smple # 2. Comprison of lsing spectr for different shots. Pumping level 27.7 mj/cm SHORT DISCUSSION Now we will riefly shortly discuss the origin of UV rdition in our experiments. There re t lest two possiilities: exciton recomintion nd recomintion of electron-hole plsm (EHP). The chrcteristic peculirity of the spectr otined y us is the shift of the nd mximum to long-wvelength side when the pumping energy increses. It is known tht the wvelength of exciton recomintion rdition is shifted to the long-wvelength side when the recomintion occurs in the regime of exciton-exciton scttering. (One of two excitons recomines nd the other gets over in the excited stte). The nd corresponding to such recomintion (so-clled P-line), ppering when the pumping level is high enough, does not shift with further pumping rise. But in our experiments continuous shift of the nd mximum ws oserved. The uthors of 6 interpret the UV spectr of ZnO thin films s result of EHP recomintion nd confirm this interprettion y the comprison of the experimentl spectr with the clcultions of nd-gp energy shift s function of crrier density. It is most proly tht in our cse EHP recomintion tkes plce lso since the pumping density is seemingly high enough for the EHP formtion. Actully, energy density 2 mj/cm 2 corresponds to pproximtely photons/cm 2 for pumping wvelength 355 nm. Assuming tht the penetrtion depth is ~.1µm we get ~ photons/cm 3. Relly to estimte the crriers concentrtion it is necessry to tke into ccount the fct tht pumping pulse durtion is much longer thn lifetime of crriers. This mens tht ctul mximum crrier concentrtion (N mx ) will e noticele smller. To estimte this reduce we cn use the pproch suggested in Section 3. Since the lifetime of crriers in EHP is shorter thn the lifetime of exitons, we ll tke for pproximte estimtion τ=.1 ns. Thn N mx is pproximtely 1 times smller thn the density of flling photons. In other words N mx ~ cm -3, tht is evidently enough for EHP formtion. 6. CONCLUSION Spectr of UV spontneous emission nd lsing for ZnO powders were investigted under nnosecond pumping. As it should e wited, the threshold energy density of nnosecond pumping turned out to e considerly higher thn tht of picosecond one. So, under picosecond pumping threshold usully is not lrger thn 2 µj/cm 2 while in our experiments we got mj/cm 2 tht is up to hundred times more. It seems tht not only short intrinsic lifetime of exciton recomintion plys role here ut lso some other fctors tht yet re of unknown nture. One of such fctors my e role of EHP whose lifetime must e shorter thn lifetime of excitons. The smples investigted differ one from the other significntly y the threshold nd y the chrcter of the lsing spectr. It seems to e the result of different morphology of the prticles forming different powders. Proc. of SPIE 5924Z-7

8 Chotic chrcter of lsing tht is evidenced y irregulr ppernce nd disppernce of rdition from shot to shot ws recorded for ll smples investigted. The sme effect ws descried in Ref. 5. Its origin is yet unknown nd requires more detiled study. Acknowledgments: The uthors cknowledge the support from the Russin Foundtion for Bsic Reserch (grnt # ) s well s Prof. V.F.Zolin for vlule help. REFERENCES 1. V. S. Letokhov, Genertion of light y scttering medium with negtive resonnce sorption Zh. Eksp. Teor.Fiz., 53, (1967) [Sov.Phys.JEPT 26, , 1968]. 2. V.M.Mrkushev, V.F.Zolin, Ch.M.Briskin, Powder lser Zh.Prikl. Spectrosk. 45, , H.Co, J.Y.Xu, Y.Ling, A.L.Burin, E.W.Seeling, X.Liu nd R.P.H.Chng. Rndom lsers with coherent feedck. - IEEE J.of Selected Topics in Quntum Electronics, 9, , (23). 4. R.K.Threj, A.Mitr. Rndom lser ction in ZnO.- Appl.Phys. B, 71, , (2). 5. D.Anglos, A.Stssinopoulos, R.N.Ds, G.Zchrikis, M.Psilki, R.Jkuik, R.A.Vi, E.P.Ginnelis nd S.H.Anstsidis Rndom lser ction in orgnic-inorgnic nnocomposites J. Opt. Soc. Am.B, 21, , (24) 6. A.Ymmoto, T. Kido, T. Goto, Y. Chen nd T. Yo Bndgp renormliztion of ZnO epitxil thin films Solid Stte Communictions, 122, 29-32,.22. Proc. of SPIE 5924Z-8

Check your understanding 3

Check your understanding 3 1 Wht is the difference etween pssive trnsport nd ctive trnsport? Pssive trnsport is the movement of prticles not requiring energy. Movement of prticles in ctive trnsport uses energy. 2 A gs tp in the