Journal ArticlePhotopumped, pulsed, narrow line laser emission is demonstrated using cylindrical microcavities formed by p-conjugated polymer thin films wrapped around thin glass optical fibers and metal wires with various diameters D. A variety of cavity-dependent resonant laser mode structures were observed, which for D,10 mm contain a single resonant spectral line of less than 1 Å in width. The microring lasers are also characterized by a well-defined, very low threshold excitation intensity, at which beam directionality and polarization degree dramatically increase. These findings open up the  fields of lasers and fiber optics to organic materials

Frolov, S. V.

Vardeny, Zeev Valentine

English

The University of Utah: J. Willard Marriott Digital Library

Plastic microring lasers on fibers and wiresS. V. Frolov and Z. V. VardenyPhysics Department, University of Utah, Salt Lake City, Utah 84112K. YoshinoElectronic Engineering Department, Osaka University, Yamada-Oka 2-1 Suita, Osaka 565, Japan~Received 11 December 1997; accepted for publication 12 February 1998!Photopumped, pulsed, narrow line laser emission is demonstrated using cylindrical microcavitiesformed by p-conjugated polymer thin films wrapped around thin glass optical fibers and metal wireswith various diameters D. A variety of cavity-dependent resonant laser mode structures wereobserved, which for D,10 mm contain a single resonant spectral line of less than 1 Å in width. Themicroring lasers are also characterized by a well-defined, very low threshold excitation intensity, atwhich beam directionality and polarization degree dramatically increase. These findings open up thefields of lasers and fiber optics to organic materials. © 1998 American Institute of Physics.@S0003-6951~98!00115-6#Low-energy excited states in luminescent conductingpolymers ~LCP! are excitonic in nature,1 similar to those ofcommon laser dyes. During the past few years, lasing hasindeed been demonstrated in solutions of several LCP.2 Un-like laser dyes, however, LCP still exhibit strong optical gaineven in the solid state.1 In fact, stimulated emission has beenobserved in thin LCP films at high photoexcitation densities,resulting in spectral narrowing of the polymer emission.3–7This spectral narrowing has been attributed to lasing,3 ampli-fied spontaneous emission,4,5 and superfluorescence.6,7 Thesephenomena require that the optical gain exceeds all com-bined optical losses, such as self-absorption, excited-state ab-sorption, light scattering, and diffraction. Lasing, in addition,requires an optical cavity to produce optical feedback, whichsubsequently forms resonant laser modes. Planarmicrocavities3 have been used to demonstrate photopumpedlaser action in poly~p-phenylene–vinylene! ~PPV! films. Las-ing has also been shown recently in various blends of smallluminescent organic molecules.8,9 Moreover, since doped p-conjugated polymers are able to efficiently conduct electricalcurrent, then ultimately it might be possible to produce anelectrically pumped, all plastic laser diode.Successful fabrication of laser diodes from LCP requiresa sufficiently low laser threshold current. Therefore, in orderto achieve the lowest possible laser threshold, it is first nec-essary to optimize the performance of photopumped polymerlasers. Microcavities ~mcavities! have been shown to furnishan excellent quality factor Q and optimal coupling betweenemission and cavity modes, which results in low laserthresholds.10 Although planar mcavity LCP lasers are com-patible with the present organic light-emitting-diode technol-ogy, and this is desirable for electrical pumping, they sufferfrom unavoidable losses due to imperfect reflections andemission leakage to the sides of the mcavity plane. Hence,the lowest laser threshold excitation intensities cannot beachieved in planar mcavities. Cylindrical or ring mcavities,on the other hand, provide superior two-dimensional opticalconfinement resulting in Q values in excess of 106,11 andthus are much more advantageous for obtaining low laserthresholds.12,13In this letter, we demonstrate that photopumped, pulsed,narrow laser emission lines with very low threshold excita-tion intensities can be obtained using LCP films depositedaround thin optical fibers and metal wires. For the laser ac-tive material we have chosen a derivative of PPV, namely,2,5-dioctyloxy PPV ~DOO-PPV!.14 DOO-PPV has beenshown to be an excellent laser-active medium in the red/yellow spectral range.6,13 The lowest excited states in DOO-PPV are excitons with energy levels similar to those of or-ganic laser dyes, which under optical excitation form a four-level laser system. The polymer laser transition then occursat longer wavelengths compared to the pump wavelength,and thus, population inversion can be achieved at relativelylow excitation densities.4,6To form ring mcavities, optical fibers of various diam-eters D ranging from 5 to 200 mm were dipped into saturatedchloroform solutions of DOO-PPV. The subsequent fastevaporation of the wet polymer film resulted in a self-assembly process in which a thin polymer film was uni-formly coated around the fibers ~Fig. 1, upper inset!. Thinpolymer films of about 1 mm in thickness forming a com-plete cylinder of 100 mm or more in length were obtained inthis way. In order to produce thicker films, this self-assemblyprocedure could be repeated several times. Optical excitationof the ring mcavities was provided by a frequency-doubledoutput ~532 nm! of a Nd:YAG regenerative amplifier pro-ducing 100 ps pulses at a 1 kHz repetition rate. The pumpbeam was focused by a cylindrical lens on one side of thepolymer ring, while the polymer emission was collected onthe other side by a round lens in the plane of the illuminatedring and then analyzed using a 0.6 m spectrometer and acharge-coupled device camera with ;1 Å spectral resolu-tion. In order to minimize photo-oxidation, all samples werecontained in a chamber with flowing N2 gas at room tem-perature.The electrodynamic solution for a polymer ring mcavitycontains a set of resonant modes, of which intensity distribu-tion depends on the thickness d of the film. At d,5 mm thefilm acts as a curved two-dimensional waveguide and thecorresponding resonances in this case are waveguided modesAPPLIED PHYSICS LETTERS VOLUME 72, NUMBER 15 13 APRIL 199818020003-6951/98/72(15)/1802/3/$15.00 © 1998 American Institute of PhysicsDownloaded 24 Nov 2009 to 155.97.11.183. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp~WM! ~Fig. 1, lower inset!. The resonant WM wavelengthslm are given by11–13mlm5pDneff , ~1!where m is an integer and neff is the effective polymer re-fraction index, which in our case is close to that of the bulk,np;1.7–1.8.13 In thicker polymer rings (d/D>0.2), how-ever, the resonant modes are no longer waveguided.15 In fact,these optical modes never reach the film/core interface, andtherefore, are dubbed whispering gallery modes ~WGM!.10,11The light in this case is confined by total internal reflectionsat the film/air interface, and the mode intensity distribution isconcentrated on the outer edge of the film. The WGM wave-lengths are still given by Eq. ~1! with different neff .11Figure 1 shows the laser emission spectra obtained fromDOO-PPV mrings deposited on glass fibers of three differentdiameters. Their characteristic feature is the narrow equidis-tant emission lines, which appear above I0 of order 0.1 nJ/pulse ~Fig. 2!. The laser emission wavelengths are close tothe maximum of the optical gain spectrum, which in DOO-PPV peaks at 630 nm.6,13 The narrow spectral width of theselaser lines, of about 1 Å, is limited by the resolution of ourapparatus and is consistent with the high Q estimated previ-ously for DOO-PPV ring mcavities.13 The spectra in Figs.1~a! and 1~b! are typical of multimode laser emission. Thewavelength of each mode is determined by Eq. ~1!, and themeasured intermodal spacing Dl are in excellent agreementwith those calculated using Dl5lm2lm215l2/pDneffwith neff5np . Since the spectral width G of the optical gainin DOO-PPV is about 10 nm,13 and does not depend on themcavity dimensions, we are able to control the number ofresonant modes by varying D. For Dl.G only a single lasermode appears in the spectrum. Therefore, for very small fi-bers with D of the order of 5 mm, the mring laser containsonly a single mode as shown in Fig. 1~c!.16The characteristic emission intensity dependence on I ofthe mring laser is given in Fig. 2. In addition, the Fig. 2 insetshows that the regular photoluminescence emission from them ring is directional,13 concentrated in the plane of the illu-minated area of the mring within an angle Du of a few hun-dredths of a radian. However, at I0 Du abruptly decreasesfrom 0.06 to 0.015 rad, showing increased directionality inthe lasing regime. We note, however, that it might also bepossible to couple the mring emission directly into the opticalfiber using a proper built-in refraction grating. Such a mlaserwould be fully compatible with the present fiber-optics tech-nology. Moreover, the Fig. 2 inset shows that the polariza-tion degree P ~Ref. 17! of the mring emission also abruptlychanges at I0 . P increases from ;1 for I,I0 to more than100 for I.I0 . We note that the mlaser emission is linearlypolarized along the fiber axis regardless of the excitationbeam polarization. This dramatic increase in P again showsthe resonant character of the laser emission lines in the poly-mer mrings.Unlike optical glass fibers, metal wires, however, are notvery good substrates for a dielectric waveguide. Waveguidedlight on metallic surfaces experiences high absorptionlosses,18 and therefore, lasing is suppressed in thin films de-posited on metal wires. However, it is possible to increase duntil WGM, which are confined to the outer film edge, be-come the dominant modes. The inner surface is no longerimportant in this case, and consequently, may be metallic.Indeed, when we deposited DOO-PPV films around B25 mmmetal fibers, we found that very thin films with d51–2 mm did not lase. However, when we increased d tovalues above 5 mm ~Fig. 3, inset!, we observed lasing WGMas shown in Fig. 3 for aluminum wires, with similar I0 asthat of a glass fiber with the same diameter. As for the glassfibers, Dl in Fig. 3 is determined by D in accordance withEq. ~1!. However, from Fig. 3 we estimate neff for filmsFIG. 1. Normalized emission spectra of lasing mrings formed around thinglass fibers measured at excitation intensities close to the laser thresholdexcitation intensity; the upper spectra are given with vertical offsets. Thefiber diameters D and corresponding intermodal spacing Dl are 20 mm and3.5 nm ~a!, 12 mm and 6.4 nm ~b!, and 5 mm and .10 nm ~c!, respectively.The upper inset shows the structure of the mring laser, and the lower insetillustrates schematically the propagation of waveguided modes inside themring.FIG. 2. Mode peak intensity dependence on the excitation intensity for a 10mm diam ring DOO-PPV mlaser. The upper inset shows the correspondingexcitation intensity dependencies of the polarization degree and the mringemission confinement angle Du ; the lower inset shows a schematic drawingof a photopumped ring mlaser, where Du is defined.1803Appl. Phys. Lett., Vol. 72, No. 15, 13 April 1998 Frolov, Vardeny, and YoshinoDownloaded 24 Nov 2009 to 155.97.11.183. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspdeposited on Al wires to be ;2.3. This indicates that thepolymer refraction index dramatically increases when thepolymer film is deposited on a metallic substrate.19 The dem-onstration of ring mlasers with a metallic core is an importantstep in the pursuit of electrically pumped plastic lasers. Itshows that future mring lasers might be electrically drivenwith the metal core serving as one of the electrodes.In summary, we have achieved efficient photopumpedmultimode and single mode lasing in cylindrical mcavitiesformed by thin films of luminescent conducting polymers.The laser lines spectral width is 1 Å or less. Fabrication ofsuch lasers is possible on thin glass and metal wires. Plasticring mcavities are characterized by high Q and a correspond-ingly low lasing threshold excitation intensity, which ismainly determined by self-absorption and scattering in thepolymer film. Further improvements in the polymers’ qualitywill result in laser thresholds sufficiently low to allow elec-trical pumping. In this case, a ring configuration with a con-ducting core may be the cavity of choice for the plastic laserdiode.The authors thank M. Shkunov, Dr. W. Gellermann, andDr. D. Chinn for assistance with the measurements. Thiswork was done at the Utah Laser Institute and supported inpart by DOE Grant No. FG-03-96-ER4549.1M. Yan, L. J. Rothberg, E. W. Kwock, and T. M. Miller, Phys. Rev. Lett.75, 1992 ~1995!; J. W. Blatchford, S. W. Jessen, L. B. Lin, J. J. Lih, T. L.Gustafson, A. J. Epstein, D. K. Fu, M. J. Marsella, T. M. Swager, A. G.MacDiarmid, S. Yamaguchi, and Hamaguchi, ibid. 76, 1513 ~1996!; S. V.Frolov, M. Liess, P. A. Lane, W. Gellermann, Z. V. Vardeny, M. Ozaki,and K. Yoshino, ibid. 78, 4285 ~1997!.2D. Moses, Appl. Phys. Lett. 60, 3215 ~1992!; H.-J. Brouwer, V. V. Kras-nikov, A. Hilberer, J. Wildeman, and G. Hadziioannou, ibid. 66, 3404~1995!; W. Holzer, A. Penzkofer, S.-H. Gong, A. Bleyer, and D. D. C.Bradley, Adv. Mater. 8, 974 ~1996!.3N. Tessler, G. J. Denton, and R. H. Friend, Nature ~London! 382, 695~1996!.4F. Hide, M. A. Diaz-Garcia, B. J. Schwartz, M. R. Andersson, Q. Pei, andA. J. Heeger, Chem. Phys. Lett. 256, 424 ~1996!; Science 273, 1833~1996!.5H. J. Brouwer, V. V. Krasnikov, A. Hilberer, and G. Hadziioannou, Adv.Mater. 8, 935 ~1996!; G. H. Gelink, J. W. Warman, M. Remmers, and D.Neher, Chem. Phys. Lett. 265, 320 ~1997!.6S. V. Frolov, M. Ozaki, W. Gellermann, K. Yoshino, and Z. V. Vardeny,Jpn. J. Appl. Phys., Part 2 35, L1371 ~1996!; Phys. Rev. Lett. 78, 729~1997!.7X. Long, A. Malinowski, D. D. C. Bradley, M. Inbasekaran, and E. P.Woo, Chem. Phys. Lett. 272, 6 ~1997!.8V. G. Kozlov, V. Bulovic, P. E. Burrows, and S. R. Forrest, Nature ~Lon-don! 389, 362 ~1997!; Appl. Phys. Lett. 71, 2575 ~1997!.9M. Berggren, A. Dodabalapur, R. E. Slusher, and Z. Bao, Nature ~London!389, 466 ~1997!; Appl. Phys. Lett. 71, 2230 ~1997!; Adv. Mater. 9, 968~1997!.10Y. Yamamoto and R. Slusher, Phys. Today 46, 66 ~1993!; J. Rarity and C.Weisbuch, NATO ASI Ser., Ser. E 324, ~1996!.11 J. C. Knight, H. S. T. Driver, and G. N. Robertson, J. Opt. Soc. Am. B 11,2046 ~1994!.12H. P. Weber and R. Ulrich, Appl. Phys. Lett. 19, 38 ~1971!; M. Kuwata-Gonokami, R. H. Jordan, A. Dodabalapur, H. Katz, M. L. Schiling, and R.E. Slusher, Opt. Lett. 20, 2093 ~1995!.13S. V. Frolov, M. Shkunov, Z. V. Vardeny, and K. Yoshino, Phys. Rev. B56, R4363 ~1997!.14We have also tried other LCPs with green emission and obtained similarresults ~see Ref. 13!.15M. Heiblum and J. H. Harris, IEEE J. Quantum Electron. QE-11, 75~1975!.16The single mode regime, however, is observed for I close to I0 .17P5I i /I' , where I i and I' are the intensities of the mring emission withpolarizations parallel and perpendicular to the fiber axis, respectively.18A. Reisinger, Appl. Opt. 12, 1015 ~1973!; I. P. Kaminow, W. L. Mamel,and H. P. Weber, ibid. 13, 396 ~1974!.19DOO-PPV 2 mm films coated on Al or Au substrates exhibited a tenfoldincrease in reflectivity indicating that the index of refraction of such filmscan be considerably higher than that of films coated on dielectric sub-strates.FIG. 3. Normalized emission spectra of mring lasers formed around a thinaluminum wire. The outer diameters D and intermodal spacing Dl aregiven; the inset shows the propagation of the whispering gallery mode insidethe mring.1804 Appl. Phys. Lett., Vol. 72, No. 15, 13 April 1998 Frolov, Vardeny, and YoshinoDownloaded 24 Nov 2009 to 155.97.11.183. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Plastic microring lasers on fibers and wires

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Plastic microring lasers on fibers and wires

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