Prasad, G R

Venkateshan, S P

Yoganarasimha, A

IACS Institutional Repository

InrUan J. Phya. MB, 24-31 (1980) Photobleaching of laser ~ye rhodamine 6G OR 1'1'"sa<1, A Yog~ll~fa.~imha, Ilond S P Vonl~lloteBha.n* L .... r L~bor"tory, Dnpartmont of M •• h .... i.,,1 Englllooring, Iudl&n Institute of SOlonce, B .... Saloro 600012 Ablltr ••• , The ... -eversibl. photobloaohing of Uhodamino 60 occuring duo to Jlu.sb lamp pumping baA bE'l'n st,ndlod WhldO tba dye is subject to lasel' action. Tho ~b.orption spectrum of th~ dy" bhowe tho groou .. ' build ~p of tho b1.~,ed. molecule. with t,he number uf nash lamp pulse.. The rau m the mtensity of tho lRsl'r output thu.t oeelllb 'Wlth thE' IlumbAl' of lassl' pulliE'B W88 found td., be correlated to the rOUl1ctlOD lfl thA abborptolotl of 1.he dye. A strong dOI)Ondehct· of t,ho rBtu of photoblon.chlD~ OJ) '\.h(\ sizf't of thl~ dye coll hD8 a.lso boon found. ThiB hM hmm ~xpla.i1l.cd a!!! boing ItUD tIl thl' dyn moloc111(l1~ cont.aint"d withm the le.:-:lM' modo vo]uffi(' pIloIt,o.ldllA" m flhmulu.tod eomiKsions. whl10 thA rema.im.i(lr of t,ho eXClto(J dye in thl' dyo ("(11118 und~r~mnp; mC:ll'(~ly joI,p()IltBrnOOUB fluoroSCNlCP 8lld uiJIOl' non ra.diative tleexoitu.t,JODR. 1. Introduction Organic dyoK \1110(1 in dyo lasers huffor IlflJgJ'e,shivc Cl,,~ofJOI'atlOn elito to iJ'mvorHibitl bl .. anhing (Seh,,!'ol' 1973, Sn"vcly 1969) that O"curB eluring the onul'~C ul' tho in·a,· diatioTL of j,he dye mo\oc:ulog hy pump r ... <1I(11ioCl. It l'!lo~ huon cHt.imllotllCi (lppon et al 1971) thllot tIle hfo timo oi (I ely" mol()(lu\c like RhodlllnhlC1 110 is a.bullt 40 macuM·· whon ~ubjce't to IL OW pump l)oam of POWl1l' WS watti"m' (roughly tho tJlre~hold pump powur fur RhudIloUlinll1iG) from all Algnn iOIl Ja~uf !lot.ol45 A Tho rato of blea.ching howl\vor, ne,plmds in gCll1orllo1 Ull f,e,vera.! pal'''mCltl'f~, liJle tJ\Cl nature of the suJvcut, it,s pH, t,empuratul'O, prOflOnce 01' aUMtI\lCO of triplllt quc\lclwfK OJ' prom{)tof~, 1JJ.o ~pcctral a.nd tempeJJ'al nat.uro of tho llUmp and WhUtJWf the crv radiatIOn that may be prCRont m t.hc pump l·a.dia.tion iH inhibited 01' noi, the l'tISono.tuf geometl'yand pump coupling configuration in cDoBO of lial!h lamp pumped dye l!L:!crs, otc. The mlll~llque'llcltoH nf this photnhlolloching a)'o : 110 rcrluotion in the number of fluoroKccllt monomet's alUi a. duoroaso in tilt> fiUOI'OSOellt quantum efficiency, an increll>1od IIobxurption in the UV This papor pcrtaina to 1111 investigation of photob\eDoching of Ute ol'ganic dye Rhomaninc 6G used in " IIIIsh lamp pumped IlLllOf. Because of tho oomplox °P •• santlyat Dopartment of Engineering "nd Applied Boie""", YaJe Ul>ivereity, Now Haven, CT, USA, •• Thle computation ,s b .... d on ~ quantum emciency of abont 5 X 10-7 mol<!. cuI •• bl.""hod/ab80rbod photon for Rhodamine 6G in othanol eolution measured by Ippen ot al (1971). Photob!eaching o/laser dye rhodamine 25 dependenoe of the blellohing ra,ta on ma,ny pa,ra,meters listed above, t,he experi-monts reported III Iitertlotul'o have not considered all the possible effects. For example, the re~ulta of .ubjecting a dyo to nlm'ow ha,nd (e.g., la~er) or broad ba.nd irradilltion (fillsh la,mp) should he expeeted to he different as also when tho dye is moroly irra.diatcct or whil(l it iii mado to undorgo lasor BttlOll. 'l'his has indeed been found to be the Cll,so Ul our mVeatigatioll. In OUr oxperiments the differenceA hetween tho two COoJ:!eB (1) wherein tho dyo is merely u'l'lIdiatod with a flash IlImp [lnd (2) whol e the dyo is ma,do to laso with the Slime pump, hllvo boen eXElmiuAd 111 dctall. 2. Experimental details A Imolll' Xonon flaHh lamp (1l0 mm <11'0 lellg(,)\) waR Il""d as pump, and it was ooupled to the dye coll by means of a.ll olliptic cavity. Tho dyo cell (125 mm long) cOllsisted of a. fused sIlica 01 borosll tllbv of 8 mm i.d. and Wa.B provided with quartz window/!. A second 1asor Jloa.d was used m .... ddltion to the ono just described. The only diffvronces were in tho ItJllgth of 1,ho Xonon fla~h lamp (50 mm ElrQ) Ilnd the length (65 llim) and diamoter of th" dyo ceIl Throe different borosil tubes of diameter 8·2 mm, 6·5 mm a.ud 5·5 min wore uHed 1'110 la.sor output power lionel duration Wtl1'0 monitorod by an EO lI.nd 0 Lite mike, calibrated Silicon photo. diode. The signa.l from the photodiodo wa.s recorded on a Tektronix: 7633 storllgo oscillosoope. The light fl'om Hash lamp wa.s also monitored by a Bepllratc photodiodo a.nd I'ccordoll simultllneou.ly to obtain the ftllsh Illmp riBe time a.nd its duratioll. A low indueta.nce stora.go clIpll.citer (40 ,iF, 5 kV) wa.s diroctly disohargod through tho flash lllmp without a.ny extemal inductanco, USing a. parallel triggsl·. (PrllBa.d, Venkll~eshall and Yogana.rllBimha 1978). Reagent grade Rhodamine 60 (BDH chemicals) was used Ile such without any further purifica.tion. Metha.nol (AR grade) was the solv~nt used in all th(l studies plesented hom. Tbe dYtl solution of 1 x 10-4 mola.r concentration was introduoed into the dye cdl or extracted by meElnl> of a glllPB sytinge. The dye in tl'6 cell WIIS romoved after a predeteI1l~ined number of shotq a,l1d hs absorption spectrum \Vos obtll.in~d. 3, Results and di.cu •• ion The. mllximum il1Pllt energy to the fla~h lamp wos a.bout 160 joules per pulse. When the quartz dy~ coli wa.a usod. it was obeel'vod that t,ho laser IIctioll ceased after 5 or 0 shots in the lOllgor oavity. This indioates thllt photobleaohing had reduced the llumbol' of availablo finol'cscent monomer dye molecules to auoll lID extent thllt threshold could nut bo roaohed. The i1bsorptioll _pcctlum of the 4 26 Prasad, Yoganarasimha and Venkateshan b h d i . h wn l·n ]Ngur(l 1. It is to he noted here tha.t the relativo absorp-leao e (yo l~ sOd tion of" the bloached dye with TOspoet. to the unblnach(ld dyo has been rocorde 0.3 02 01 \, BLEACHED o~---------i---=~:;~=--------'~(J~u:s;t~a~';t:.r~c;'.~s;s~at;ti~~;1 o 0.05 of Lasing) \ 11 Shots· 9 Shots 5 Shots 0.05 O~----------~~~==~~------~~-----:JISSh~o~t~S;-~ .0·05 _0.1 200 300 400 500 600 WaveLength (nm) F1rure 1. Hel .. tive absorptIOn speotr .. of partloJly and fully bleached dyo. l~e"ult. "'·0 for tho long CUNity dye coli of dillmeter 7·8 mm. by havmg tho lllllJlu!\uhec1 dye Ul th" rufci"cnco (Joll IIml tho bloached dye in tIle ~a.mple ooll of" tho dual boa.m ~poctrophot,()J1l(Jt,or. Since it, i,,; woll u~ta.blished (Loth a.l1d Meyor 1973) that tho ultravlOlot, phOt,Oll~ havo a groa.tol' pl'opOllJ!ity to oaUHO photocltllllliclIj rell.ctiou~, tho fUBed ~ilioa tUbe wa.~ ropla.ood by 80 bureail Photobleaching of laser dye rhodamine 27 glass tuhe (also 8 mm i d.). It. waR found tllELi the la.~(lr action could cont.inue for mOl'e than 30 shots. However the laSH1' output powcr undergoes a monotonic decrease after remaining nearly constant for tho first 2 or 3 shots. The number of pubes during which lasing continues, depends on the diametel' of dye cell, input pump energy, conoentration of dYft, etc:. Figure 1 also shows the absorption spectra of thc dye subjected to 3, 5, 9 and II shots. It is Been from an examina~ion of this figure that at 550 nm (tho wave length of the peak absorption of Rhodaminc 6G) tho reduction in absorp-tion is monotonica,lly increasing with the number of shots the dye was subjected ~o. When the speotrum of this partially blcach(,d dyo was compru'od with that of dye merely sUbjocted to the samo llUmhor of fla.sh la,mp irradiations (withont lasing), it was notioed that tho absorption docrease waR very much largcr in tho lattor case This decreasc in the relative ahsorption coefficient between tho fresh dye and irra,diated dye is a, mQasure of tho numhel' of dye molecules bleaohed. For e~ample, if wo o~amine the eMil of dye subjected to 4 identica.l flash la,mp pulses while lasing or while not lasing, the rat.io of the relativo ahsorp-tion for the lased to merely irradiated dy(, is 0·44 (and it bocomes 0·47 for 6 shots) After ahout 7 or 8 pulses of mcre irradiat.lOn, the dyc doea not lase if an attempt is made to lasc it with tho same pump pOWer, and iR t.hen consi!1ered to be "fully hleaohed". From tho moasured absorption coefficients the number of molccules bleaohed has been found. The number oJ bloached moleoules'" is about 0·02 X 1018 when "fully bloached", (1.c., for the given maximum pump energy of 160 joules, this solution dOM not lase), and is 0·22xlOlO/ml and 0·5 X 1016 for dye subject to 4 shots while lasing and wl1ilo 1I0t lasing respectivcly This implies a much la,rger bleaching rate in the absence of lasing action, which was al~() noticed in the experiment.R of Oet.tillgor and Dew\,y (1976), wh'O, howev~r URad laser e~citabon WCbOl' (1973) and Bem' and Weber (1972) found that tho addition of a triplet qucncher like COT (CaHa) reduc.os the blea.ohiug rate. This observation in conjunction with the faci that the life time of the first cxcited triplet Rtate T 1 is much longer (:::: 10-6 ROCS) than that of the excited singlet state 8, and that the t/l'iplet state ill more chemically reactivo than t.he singlet Rtatc, implies that it is the dye in tho triplet state that iR primarily being photohleached (Britt a,nd Moniz 1972, Marling et al 1970. YMnashita and Kashiwagi 1976). The oxperiments described above whorein we have found that the propensity for the dyc to be destroyed faster whon the dye is morely excitod a,nd allowed to de-excite sponta.nonusly, than whcn stimulated emissions are also taking plaoe lendR further support to the o.bovo hypothesis. In the experiments in whioh • The number of dye molecules in the fresh (unbleached) dye solution is 6·02 X 10'·/ml (oorresponding to (\ ooncentre,tion of 10-< mole/liter), 28 Prasad, Y oganarasimha a,nd Venkats8han jllosor IIoction IS 1Io1~o tllking pls.ce, the dea.ctlvation of the 81 state occurs primarily by: (a) sponto.ncou~ omissIon, (b) i·I,-T1 CfOSS oVer, and (0) stimulated emirlBion, Tho stiOlula.ted oOlissionH tho.t ocollr whilo tlto dy~ is lasing aa.uso a!lIrger number of 81 molecules to return dlreotly to tJlO ground state G, than in the cue when no !s.sing ocours In the latter situation a la.rger s.ceumulation of moleculos in the triplet state oecurR thu~ giving a high~r photobJoaohing rate. For the long ca.vit,y laser :!<'iguro 2 shows tIle gra.dnal decreaso in t,ho peak output power of t,he JaIlOr with tho lluml)oT of pulses As pointed out earlier, tho dip in speotrum a~ 550 urn in li'igm(l 1 i8 proportione.l to the reduction in numbor of o.otivo dyo molecules whioh is growmg with tile numbor of pll~B . • • O.50~ ___ -= _____ -L_--l , 5 10 No. of Shots N Ffpre 2. tI~duotion of normwi.od 1a ••• output (PH/P,) with number of ~boto N. P, ,. the laser poak power output. So a. l'rOHS correlation can now be made between ilia extent of this dip to tho pea.k power output. This is ai-own in Figure 3, and indiCa.toB tha.t the lasor output pOWer ~allB as more and more dye moleculaR a.to bl~&rhed ont. The fa.irly wide sce.tter In the data. points is to bo oxpected, beea.USe, eVen in 1;1le most llophietioatod tlash lamp pumped lal!erR, $hot to shot ve,riationa in intensity a.re in \IKe-ellS of 10%. Photobleaching of laser dye rhodamine 29 Furthormore in tho IIxperiment~ reported hero, oaclL experimental points repro· sents a IIOpara~e sequence of I!hot~. In ordor to roduce tho oon~equent errors a. modular spectrophotometer ha.s been oOl1l.ltructod in our la.boratory to diroctly soan tho speotrum of the dye while it ill within the dye cuvotte in between SUOCIlS· sive 1a.sor shots. Theso experiments are now in progress a.nd their rosults will be prosented elsewhere. 01'"------e o o a .C 0 U'I U'I .... - 0.1 a o 0 II -tt '''' l' -0.2 ~ -0.25'!"----_~~ ___ ---J o 0.5 1.0 PH/P, Plpr. 3. Correlation of the ratIO of 8b.orptI~n coefficients of \aBed ("N) to fl'esh dyo solution ("0) WIth (PH/PI)' Absorption mell8llr~d over 10m. path length. Tho intensity of ra.dlllotion within thc dye coll will bo nonuniform and depend· ing on the TEM mode struoture the inicnllity distribution 1\as olle or more muima in the centro of the 0011 and dooays to negligible values outside tfIo modo volume. If the diameter of the dye 0011 iR large!." than the radjalox~llDt of the mode volume, then, sinco the dyo conta.ined in tho region outside tho mode volume shOUld have 110 higher bloa.ching rato (because no stimula.ted emissions are ooouring in this region) the ov(\ra.lllife of the dye in a. larger cell must bc lower than tha.t of 110 smaller dye cell. Tha.t this ill indeed the olloSll has boen domonstraoted (see Figaro 4) by using dye cella of different dia.meters 5·5, 6·5 and 8·2 mm. 30 Prasad, Y oganarasimha and Venkatl!Bhan In theso oxpel'l1n~nt" the la.ser was, mado to oscillate in the TEMoD mo~e (Prasad et a! 1979). The increase in ~iamet~r of dye oanalso chang~ the radl~! dist.ribution of pump power dellsLty m tho dyc: FOI·. 110 conoentr~~lOn of 10 molcsjhter, the absorption eoeffioicllt of t.he dYI'. LS as high !loB 12 em at 550 nm N PlJure 4. Effect of dye coli diameter on the "laaehing rate, fOT the ahort cavity. althouglL it is smaller at other wavelengths. ThuB o.n increase in diameter of the coli would cause a. reduction in the pump power distribution at the center of tJIB cell. Since the Ia.ser iSoOpera.ting in tho TEMoo mode, such that the stimulated light intensity is a ma.ximum at the center of the coU and the pump powor dis-tribution is suoh a~ to have Ia.rger popula.tions of 81 (a.nd honce T t ) away from the oenter of' the dye cell, tho effect of inoreo.sing the diamet.er is t,o further enhance tho ra.te of photoble!\ching A drllomatic incr-eo.so in the number of shots boforo the irreversible bloElching leo.ds to o~Bsation of laser action is Bllell as the mode volume fillo a. IElrgor portion of the dye cell. A detailed a.nalysis of this phenomenon (involving a. solution of the transient ra.te equa.tions of the la.ser) is necessary for quantito.tiv6 descriptions of tbe!'o effeots. Suell an analysil! is under way and the ]'esults will be presented elsewhere. Aclmowledgment We gratefully acknowledge the help provincd by Dr. M. Nallasamy in the initial ph a.ea of thiR work. Photobleaching of lUBer dye rhodamine RelereDC. Beer D and Weber J 1972 Opt;"'8 Oomm. 5 307 Britt A D and Momz W B 19721111111111. J. Quant. Eloo. JQ.E 8 913 Ippen E P, Sha.nk a v a.nd Dienes A 1971 IEEE J. Quallt Eleo. JQ.E7 17S Marling J B, Gregg D W o.nd Wood L 1970 Appl. Phy,. Lett8. 1'7 527 Oettmger P E a.nd Dowey C F J1' 19761111111111 J. Quant. Eke. JQ.E12 96 31 Prasad C R, Venkatesho.n S P aud Yogo.narllsimha A 197B FlaB" Lamp Pumped Dye Laoer, Laa.r Lab. Repllrl ME LLR 2, Dept. of Mwk. Engg., IISc., Bri1lgalo,.c Pl'asad C R, Yoga.narasimha A, Vonkateflhan S P and Nagarllju l' a 1979 Rev. Sci. In8tr1tm. 50 1161 Sohafer F P 1973 ill Dye Laaer8, Ed F. P. SChafer, Springer Verlag Snavely B B 1969 Pro •. 1ElIIE. 57 1374 Webor J 1973 Optio8 O~mm. 7420 Ya.mllShit. M and Kaeluwagi H 1976 IElIIE, J. QuuIII. lille •. JQ.E12 90 

Photobleaching of Laser Dye Rhodamine 6G

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Photobleaching of Laser Dye Rhodamine 6G

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