WEDNESDAY MORNING 585 parameterscan be computed from the Theresults of thecalculations show nelsare then available for the excited measured cross-relaxation ratesand good good qualitativeagreement with the ex- atoms;one is togive the energy to agreementwith experiment is obtained. perimental work2B3 andearlier perturba- xenon atoms by means of collisions of the Fig. 2 shows acomparison of the com- tiontheoretical considerations.4 second kind, the other is to decay through puted and experimental linewidth param- cascade to a binding level where excited leters for theexperiments of Dietel.2 molecules areformed. Theprobability for the first channel totake placeincreases withthe partial * W. Dietel, Phys. Lett., vol. 29A, p. 268, 1969. pressure of the xenon in the mixture and, therefore, as indicatedby the experi- mental results, it seemsthat the 3.467-p xenon lasertransition is originated by atom-atom collisions withexcited argon. The other known laser transitions of the N.8 GasLaser With a Saturable Ab- xenon areeither originated by direct sorber, R. Salomaaand S. Stenholm, N.9 EnergyTransfer Excitationas electronexcitation or by molecule-atom Research Institute for Theoretical Phys- Mechanism in Noble-Gas Mixtures, collision. ics,University of Helsinki,Finland. Y. Binur, R. Shuker, A. Szoke, and E. Theexact dependence of thevarious Zamir, Tel-AvivUniversity, TeLAviv, mechanisms on thepartial pressure and ‘Thispaper calculates thenonlinear po- Israel. electron excitationproperties are cur- larization of a cell containing absorber rentlyunder investigation. gas bythe method used forthe single- Energy transfer via collisions o’f the sec- modeamplifier by Stenholm and Lamb.1 ondkind is a known mechanismfor A laserconsisting of bothan amplifier achieving population inversion in gas and an attenuator cell within the optical lasers,lhowever, most of thenoble gas resonance cavity is discussed. The solu- lasers withthe exception of the He-Ne tionin the single-mode case is given in are excited by direct collisions with elec- terms of continued fractions, but for most trons of an electric discharge. Evenin N.10 OpticallyPumped Molecular Io- properties the lowest order approximation gas mixtures, suchas He-Xe, it isbe- dineVapor Phase Laser, R. L. Byer, to these suffices (REA).The case where lieved that the Xe is directly excited by R. L. Herbst,and H. Kildal, Micro- theDoppler width considerably exceeds the electrons.2 waveLaboratory, Stanford UniversiLy, thenatural linewidth leads to analytic Spectroscopic measurements done lately Stanford,Calif., and M. D.Levenson, expressions forsome special cases, but inthis laboratory3 pointed out the pos- Gordon McKay Laboratories,Harvard other cases are easily treated numerically. sibility of very efficient excitationby University,Cambridge, Mass. Theabsorber cell istaken to be more atom-atom or molecule-atom collisions in We have observed stimulated emission in strongly saturated than the amplifier at a noblegas mixtures such as argon-xenon, optically pumped molecular iodine vapor. given field intensity by choosing different krypton-xenon, and argon-krypton. This A &-switched Nd:YAG laser operating at atomic linewidths. Thiscan be achieved measurement shows that at a relatively varioussecond-harmonic wavelength by keeping the cells at different pressure. high range of pressures of the donor atom pumps selected vibrational-rotational lev- A brief discussion of the influence of pres- (~200torr) and low concentration of the els of the B3IIOu+t XI&,’- electronic sureon the properties of the laser cell acceptor, the cross section for the energy transition. This leads to population inver- is given. transfer can be as high as 10-13 cm2. sion between the excited B state and the It is found that the laser with a satur- In this paper we would like to present higher vibrational levels of the X state ableabsorber in one-mode operationhas results of measurementsthat were done yielding a series of molecular laser transi- a bistable steady state for some parameter in high-pressure argon-xenon system. A tions from 544 to 1.335 nm. ranges. If the intensity is small it decays new xenonlaser transition at 3.467 -C Pumpingan 8-cm Brewster angle cell to zero but if it exceeds a certainthres- 0.003 JA,which mas identified as either the at roomtemperature we haveobtained hold valuethe intensity finds a new 7j[3/21, + 7d[5/2I2 orthe 7j[3/2l1 + peakoutput powers of 1 W in asingle nonvanishingstable operating point. The 7d[5/212 atomic xenon transition was line (618 nm) at 0.5 percent conversion stability of thesystem is considered in foundtogether with the well-known4 efficiency. We used a 5.5 percenttrans- a two-dimensional planehaving the transitions at 2026, 2627,and 2651 p. The mittingoutput mirror and a Brewster pumpingrates of the amplifier and ab- new transitiondoes not appear in pure angle quartz prism toselect individual sorber cells as its axes. Byvarying the xenon discharge or in krypton-xenon mix- lines. Forthe present 190-ns pump pulse absorbingstrength one can get a situa- ture,despite the similarity in - energy each iodine molecule is usedonce and tion where the field intensitydrops dis- transfer mechanisms in both systems. The the maximum output power is limited by continuously to zero from a finite value, upperlevel of thenew transition is lo- the number of iodine molecules. For the giving a situationanalogous to afirst- catedonly 160 cm-1 apartfrom the 8-cm cell and a 19.5-cm cavitythe laser orderphase transition in contradistinc- metastable 4s’[lJ211 argon level. threshold is as low as a few microjoules of tion to the analog of a second-order t,ran- The pressuredependence of the new pump energy. sition provided by normal laser operat.ion. lines intensity also differs from. that of the We have pumped the iodine laser with Underthe assumption that only one previously known lines whereas these lines the second-harmonic output of three mode is oscillating (thestrong mode) a do appearin xenon partial pressures as Nd:YAG laserlines at 530.6, 532.0,and perturbationtreatment is developed for low as 10 p torr and their peak intensity 536.7 nm.We observelaser actionfrom the other weakmodes in the presence of is between 0.5 and 1 torr. The new 3467 ,U the excited B state to every second vibra- the strong one. From this the st.ability of line appears only in xenon partial pressure tional level of the X state from v” = 2 to thesolution with onlythe strong mode of 1 torr and above. Due to the high con- v” = 74. Each transition is a doublet with oscillating can be investigatedfor each centration of argonin the argon-xenon the R and P lines separatedby a few modeseparately as the interaction be- mixture it is clear that most of the elec- angstroms.Since theiodine absorption tween the weak modes is negligible. The trons energy is utilized to raise the argon spectrum is very dense near 530 nm, the analysis is carried out for standing-wave atomsto their excitedlevels. Twochan- pump laser with a linewidth of 0.2 A modes within the Fourier expansion tech- pumpsmore than one transition. We nique of footnote 1. Results of numerical thereforehave inserted a tiltedetalon work will displaythe parameter regions = A. Javan, Phys. Rev. Lett., vol. 3, p. 87, 1959. = s. E. Schwarz, T. A. DeTemple, and R. Tacy, within the Nd:YAG cavity to narrow the where one-mode opera,tionis stable. Atti. Phys.Lett vol 17 p. 305 1970. line widthand to allowselective excita- Gedankeg J. ’Joriner, B.’ Raz, and A. Szoke to be published. tion single vibrational-rotational levels. 4w‘. L. Faust R. A. McFarlane C. R. N This allows unambiguousspectral assign- 1 S. Stenholm and W. E. Lamb, Phys. Rev., Patel and C. G.’ B. Garrett, Appl. ’Phys. Lett.; vol. 181, p. 618, 1969. vol. 4: p. 85, 1962. ments of theiodine laserfrequencies. 586 WIIDKESDAY MORNISG

The iodine laser gainline width is 1.5 thermopile9 calibrnied at 337 @m usinga 0.2 Theory of Self-Focusing (Inviled), GHz, which is larger thanthe Doppler HCN laser of known power. Peak power J. Marburger, University of Xoulhern width. This is due to the expected hyper- is determined usinga repetitionrate of California,Los Angeles, Calif. fine splitting.1 We have measured the 3 s-1 andan estimated pulsew-idth of saturatedgain by measuring the iodine 1 ps. laser builduptime. The single passgain Laseroscillation occurs upto a pres- for an 8-em cell at room temperature is sure of 10 torr, a factor of ten higher 0.3 Self-Focusingand Self-Phase Mod- 23 percent, which is in good agreement than the upper pressure limit reported by ulationin Picosecond Pulse Oscilla- with acalculated gain of 19 percent. Changand Bridges.3 Output pulses are tors,1 R. L. Carman, J. Fleck, and L. Thereare several possible applications about 1 ps long and vary in amplitude by James, PhysicsDepartmenL, Lawrence forthe molecular iodine laser. They in- asmuch as 50 percent, possibly because Livermore Laboratory, Livermore, Calif. clude improved spectroscopicmeasure- of varyingpump mode st,ructure.Near 04550. ments,determination of relaxation rates theoptimum pressure,signals can be andstudy of the hyperfine structureby measured usinga low-resolution helium- Great difficulty hasbeen encountered in Lamb-dip spectroscopy. In addition,the filled monochromator.At this pressure obtaining reproduciblesources of pulses shorterthan 100 ps. This becomes in- iodine laser may be a useful frequency only the line at 496 -I- 1 pmis observed. marker across the visible andnear in- We have also observed TEA laser pumped creasingly truefor durations less than fraredspectrum. oscillation in CHZOH at 570 pnl with peak 100 ps, and especially inthe case of ouput power about 20 mW. Nd:glass.2 Usinga high-speed framing A notablefeature of thepresent laser camera: we have examined thetemporal HInsch, M. D. Levenson, and A. L. is itssomewhat low efficiency. Although development of thenear field of mode Phys. Rev. Lett., 1971. vol. 26, p. 946, the pumping power has been increased by locked ruby, Nd:YAG and Nd:glass oscil- the use of a TEA laser from 1.5 kW to lators. We havefound that spatial hot -1 MW, the output only rises by a factor spots develop frominitially uniform of 102. This could be caused by an oscil- beams (using a linear flashlamp configura- lating bandwidth for the TEA laser that tion which provides uniform pumping). In N.11 10-W MethylFluoride Laser at is broader than the pumping transition in both ruby and Nd:YAG, the intensity re- 496 pm: F.Brown,2 Francis Bitter Na- methyl fluoride. Recent measurements1oJl quired for this breakup is about one order tionalMagnet Laboratory, Massachu- indicate a TEA laseroscillating band- of magnitudehigher than for Nd:glass. settsInstitute ofTechnology, Cam- widt,h of -200 MHzin the absence of Thehot spots reduce in diameter until bridge,Mass. 09139, E. Silver, Depart- forcedmode locking and a mode-locked cavity diffraction partially compensates ment of Physics, Williams College, Wil- bandrvidth -400 MHz. Since the collision- forthe shrinking or damage in the laser liamstown, Mass, C. E. Chase and K. J. broadenedlinewidth of theabsorbing media ormirrors occur. Since thesame Button, Francis Bitter National Magnet transition in CH3F is -50 MHz at 1 torr,3 saturable absorber solvent, dichloroethane, Laboratory,Massachusetts Institute of onlypart of the available TEA laser mas used in allsystems, its effect was Technology,Cambridge, Mass, B. Lax, pumping power is absorbed resultingin similar.The effects can be completely explained by self-focusing due to the las- Francis BitterNational Magnet Labo- an apparent efficiency of 10-20 percent. inghost, where thenonlinearity in Nd: ratory, Massachusetts Institute of Tech- is anticipated that TEA-laser pumped It glass is about one order of magnitude nology, Cambridge, Mass.,and the De- lasers will find a number of im- FIR larger2 than in ruby4 or Nd:YAG. partment of Physics,Massachusetts portant applications. Thepresent laser, Institute of Technology,Cambridge, withits relatively high power, may also Stable reproducibleoscillators of <2.5- Mass. be used in plasmadiagnostics to replace mm diameter can be made for ruby and the HCN laser. In the case of inter- Nd:YAG for cavity length of 1.5 m. Such Optically pumped far-infrared (FIR) laser oscillators will be described. Bandwidth- actionin gases hasbeen described by ferometricplasma density measurements the 496-pm laser would beadvantageous limited pulse durations of >20 ps have Changand eo-workers.3-7 The strong- been reproduciblyobtained in diffraction becauseplasmas of 1014 to 1015 ions per estreported line is at 496-pm linein limitedbeams from Nd:YAG. In ruby methyl fluoridewith peak power 0.1 W, cubic cent,imeter have higher optical dis- persion at this wavelength thanat 337 pulse durations down to 5 ps have been achieved by pumping with a conventional obtained in diffraction limitedbeams. 1.5 kW &-switched CO, laser. Thispaper pm. In the field of infrared cyclotron resonance, thepresent lasercould extend Below 5 ps, the pulses arefound to be reports excitation of thisline with a noticably no longer bandwidthlimitcd. 1-MWtransversely excited atmospheric measurements of effective masses in semi- conductors to higher values. Magnetic re- rlttemptsto produce difhaction limitcd pressure (TEA) GOz laser6 to produce 10 operation at >2.5-mm beamdiameter W of peak FIR power. sonance of a wide variety of materids is a,lso now readily, possible at 496 pm at have failed dueto self-focusing inboth Theexperimental arrangement origi- ruby and Nd:YAG for simple linear cav- magnetic field strengths generallyavail- nally used is similar to that. of Chang and ity geometries of 1.5-m length. More com- able. Bridges3 with the important exception of plicated geometries have shown promise. the TErl laser.Oscillation is confined In the case of Kd:glass we havebeen predominantly to a single roktional line 9 Scientech,Inc., model3600 with 3602 en- ableto operate anywherebetween band- [P(20) at 9.55 pml of CO, byuse of a closure. width limited durations of -1 ps, as mea- planealuminum master grating in the 10 0. R. Wood R. L. Abramsand T. J. Bridges, Appl. Pkys. Lett., vol. 17,'~.376, 1970. sured by two-photon fluorescence (TPF), cavity. An improved FIR cavity involving u R. H. Abramsand 0. R. Wood, Appl. Phys. to 200 em-1 fullwidth at half maximum folded optical delay-linetechniques has Lett., vol. 19, p. 518, 1971. (FWHM) at 5 ps. Simplelinear cavities alsobeen designed andtested. Average at 1.5-m length work well atabout 100 power isdetermined using a radiation MSV/cmz inbeams of <3 mmfor Nd: glass. Bandwidthand diffraction limited pulses of about 10 ps are readily obtained. 1 This work was supported by the USAF Office of ScientificResearch Grant AF-AFOSR-1284-67 As thepeak intensity is increased, self- andthe National Scignce Foundation. SESSION 0 focusing results in damage of the mirrors, 2 On leavefrom Williams College,W'illiams- town, Mass. 02167. as can be seen fromnear field photog- 3T. Y. Changand T. J. Bridges, Opt. Com- Wednesday,May 1972 A.M.- mum., vol. 1, p. 4231970. 10, 8:30 4T. Y. Chang, 'J?. J. Bridges,and E. G. Burk- 12:oo 1 This work was supported by the U. S. Atomic hardt, ApApl. Phys. Lett. vol. 17, p. 249, 1970. Energy Commission. 2 6 ~ Appl. Phys. Lltt., vol.17, p. 357,1970. N. G. Basov, et ol., Son. Pl%ys.-JETP, vol. 6 T. k. Chang, Opt. Comnzzm., vol. 2, p. 77, 0.1 CountingQuanta: The Physics of 33, p. 289, 1971. 1o?n_,. ". 3M A.Duguay J. W. Hansen,and S. L. 7T. Y. Changand J. D. "Gee, Appl. Phys. Photon Statistics (Invited), R. J. Glau- ShapiGo, IEEE I. brelztztm Electrom., vol. QE-6, Lett vol. 19 p 103 1971. her? HarvardUniversity, Cambridge, p. 7251970. SA. J. Bgauiieu, 'AAppZ. Phys. Lett., vol. 16, p. 4 C.' R.Giuliano and J. H. Marburger, Pkys. 504, 1970. ivlass. Rev. Lett., vol.27, p. 905, 1971.