Journal ArticleWe demonstrate pulsed, photopumped multimode laser emission in the visible spectral range from cylindrical  microcavities formed by conducting polymer thin films deposited around optical fibers. The laser is characterized  by narrow emission lines (~1.5 cm-1), a well-defined excitation threshold, anisotropic emission in both polarization and azimuthal intensity distribution, and high Q(>3000), which leads to a low excitation threshold of order 1 nJ/pulse. We observed two different sets of laser modes; these are wave-guided ring modes in the polymer film and whispering gallery modes close to the optical fiber surface

Frolov, S. V.

Vardeny, Zeev Valentine

English

The University of Utah: J. Willard Marriott Digital Library

Ring microlasers from conducting polymersS. V. Frolov, M. Shkunov, and Z. V. VardenyDepartment of Physics, University of Utah, Salt Lake City, Utah 84112K. YoshinoDepartment of Electronic Engineering, Osaka University, Yamada-Oka 2-1 Suita, Japan~Received 18 June 1997!We demonstrate pulsed, photopumped multimode laser emission in the visible spectral range from cylindri-cal microcavities formed by conducting polymer thin films deposited around optical fibers. The laser is char-acterized by narrow emission lines (;1.5 cm21), a well-defined excitation threshold, anisotropic emission inboth polarization and azimuthal intensity distribution, and high Q(>3000), which leads to a low excitationthreshold of order 1 nJ/pulse. We observed two different sets of laser modes; these are waveguided ring modesin the polymer film and whispering gallery modes close to the optical fiber surface. @S0163-1829~97!53032-3#Recent advances in the synthesis of luminescent conduct-ing polymers ~LCP! followed by a better understanding oftheir optical and electronic properties, have provided evi-dence of optical stimulated emission ~SE! in LCP solutionsand thin films.1–4 SE occurs at a relatively high excitationintensity when the optical gain exceeds optical losses due toself-absorption, light scattering caused by imperfections, andother factors. SE is usually accompanied by a dramatic spec-tral narrowing, as has recently been observed in various con-ducting polymer films.2–4 Even though spectral narrowing isone of the main characteristic properties of lasing,1–2 otherphenomena of high optical gain, such as superfluorescenceand amplified spontaneous emission.2–4 may also produceanalogous spectral narrowing. Laser oscillations, on the otherhand, require a cavity to provide optical feedback.5 The reso-nant nature of the laser cavity establishes a well-definedspectral mode structure and an associated highly anisotropic~i.e., directional! emission pattern.In this paper we investigate photopumped lasing in LCPusing cylindrical microcavity structures of thin films depos-ited on glass fibers. Microcavities provide excellent couplingof spontaneous emission into lasing modes, and a high-cavityquality factor Q which consequently leads to low lasingthresholds.6 Two LCP have been tested: derivatives ofpoly~p-phenylene-vinylene! ~PPV! with lasing in the red,and disubstituted polyacetylene ~DPA! with lasing in thegreen. Multimode, narrow laser lines (;1.5 cm21) with lowthreshold intensities ~1 nJ/pulse! have been demonstrated inthese structures using pulsed photoexcitation. We note thatlow lasing threshold is an important criterion in developingelectrically pumped plastic lasers.In our studies we used soluble derivatives of PPV andDPA as gain media; including DOO-PPV ~Ref. 3! andPDPA-Si(iPr)3 ,7 respectively. Cylindrically shaped thinpolymer films were prepared by dipping commercially avail-able optical fibers into saturated chloroform solutions. Thinpolymer rings were consequently formed around the glasscylindrical core following fast drying in the air. The esti-mated thickness d of the deposited polymer film was about2–3 mm. The self-assembled polymer microstructures in theform of cylindrical microrings are similar to those previouslyreported in Ref. 8, with deposited films of laser dyes blendedin transparent polymers. In the present paper, however, wehave exclusively used pristine conducting polymers as lasingmedia. The excitation source was a Nd: yttrium aluminumgarnet ~YAG! regenerative laser amplifier producing 100 pspulses with a repetition rate of 100 Hz. This laser light waseither frequency doubled ~532 nm!, or tripled ~355 nm! forpumping the DOO-PPV and PDPA-Si(iPr)3 films, respec-tively. The pump beam was focused using a cylindrical lensinto a 100 mm35 mm stripe perpendicular to the polymercoated fiber (B100–200 mm); thus only a small fraction ofthe pump light was absorbed by the polymer film. The pumpbeam polarization could be rotated to be either parallel orperpendicular to the fiber axis. The light emitted from theexcited polymer ring was collected in the plane of the ringwith a round lens and spectrally analyzed using a 0.6 m triplespectrometer and a charge-coupled device array with maxi-mum spectral resolution of about 1.5 cm21.Figure 1 shows the emission spectra of DOO-PPV filmscoated onto two different optical fibers with outside diam-eters D of 200 mm ~a! and 125 mm ~b!, respectively, ob-tained at pumping intensities of order 1 nJ/pulse. In the insetsof Fig. 1~a! we show the DOO-PPV polymer repeat unit and,for comparison, the much broader (;2500 cm21) photolu-minescence ~PL! spectrum. The sharp emission lines seen inFig. 1 appear only above a threshold excitation intensity I0 .Figure 1~b! shows three emission spectra measured below,at, and above I0 . Since the sharp lines are absent at lowintensity, they cannot be due to a quantum electrodynamicenhancement of PL in a mcavity.9 We therefore attributethese lines to laser modes associated with the cylindricalmcavity formed by the thin polymer film. The linewidth ofthe narrow laser lines was limited by the spectral resolutionof the apparatus (1.5 cm21). The inset of Fig. 1~b! alsoshows that the spectrally integrated emission intensity de-pends nonlinearly on the excitation intensity, and this is an-other characteristic feature of lasing. We note that I0 , ex-pressed in terms of absorbed energy/pulse, is of order 1 nJ.This I0 is lower by about three orders of magnitude than thatof the spectral narrowing previously observed in flat thinDOO-PPV films.2RAPID COMMUNICATIONSPHYSICAL REVIEW B 15 AUGUST 1997-IIVOLUME 56, NUMBER 8560163-1829/97/56~8!/4363~4!/$10.00 R4363 © 1997 The American Physical SocietyAnother salient characteristic of lasers is the increaseddirectionality of the emission.5 In the case of microring la-sers, we then expect, for excitation intensities I.I0 , morelight to be emitted into the plane of the ring. We show thisproperty in Fig. 2 by measuring the emission angular depen-dence using the DOO-PPV coated 125-mm-diam fiber. Weintroduced a 1 mm pinhole in front of the collecting lens formonitoring the emission intensity at a given angle u, asshown in Fig. 2, inset. It is seen that the laser emission isconcentrated in a small cone of Du;0.03 rad.10 We notethat the plane of the microring does not coincide with theoptical axis of the apparatus and therefore the peak intensityin the angular dependence of Fig. 2 is slightly shifted awayfrom u50. From Du we can estimate the lateral size z of thelasing ring mode: z'l/Du'20 mm, where l is the emis-sion wavelength ~630 nm!. z is smaller than the size of theexcitation length parallel to the fiber ~100 mm!, indicatingthat it is limited by inhomogeneity in the polymer film. Fur-ther improvements in the film quality may lead to enhanceddirectionality of the microring laser emission.The polymer ring laser emission was found to be linearlypolarized. Its polarization is parallel to the fiber axis, whichis a characteristic property of transverse electric ~TE! lasermodes. The measured polarization ratio P5I i /I' ~I i and I'are the emission intensities with polarizations parallel andperpendicular to the fiber axis, respectively! was found forI.I0 to be P.100. The background PL, however, is char-acterized by P'1 independent of I . We also found that Pdoes not depend on the pump beam polarization.The spectral peak of the polymer ring laser ~Fig. 1! is atthe maximum of the optical gain spectrum, g~l!, which isclose ~but not equal! to one of the PL sideband peaks @Fig.1~a!, inset#. We note that lasing occurs at l where g~l! ex-ceeds the optical losses; this condition is given by9g>2pnplQ , ~1!where np is the polymer index of refraction and Q is thecavity quality factor. Q is usually used to characterize thequality of a resonator mode and is defined as 2p times thenumber of optical cycles required for the cavity mode field todecay to 1/e of its initial value.5 There are three contribu-tions to Q: Qabs is the absorption limited quality factor givenby 2p/al , where a is the absorption coefficient; Qscat isdetermined by scattering losses in the polymer film and itssurface due to imperfections; and Qcav is the cavity finessedetermined by the mcavity geometry. These Q’s are added asfollows:Q215Qabs211Qscat211Qcav21. ~2!For cylindrical mcavities, Qcav is in the range of 104–108.6Qscat strongly depends on the polymer film quality and inprinciple, may be made so large as to eliminate the Qscat21contribution in Eq. ~2!; however, it is the least controllableparameter. Qabs is estimated for our DOO-PPV films froma~630 nm!'30 cm21, to be of order 103–104.11 Since Qabs21@Qscat21 for our films, then from Eq. ~2! Q'Qabs .The value of Q can be directly obtained from the cavityresonance line halfwidth D1/2n below the laser threshold us-ing the relation: Q5n/D1/2n , where n5c/l . However, noneof the resonance lines are observed below the lasing thresh-old @Fig. 1~b!#. In this case the maximum value of D1/2n canbe estimated from the separation Dn between neighboringFIG. 1. Laser spectra of DOO-PPV films coated on glass fibers:~a! D5200 mm, absorbed energy/pulse51.5 nJ; ~b! D5125 mm atvarious absorbed energies/pulse. Insets in ~a! show the polymerrepeat unit and for comparison the broad DOO-PPV PL spectrum;insets in ~b! show the polymer ring structure ~right! and the inte-grated emission intensity dependence on the absorbed excitationenergy/pulse ~left!.FIG. 2. Intensity angular dependence of the DOO-PPV 125 mmring laser emission. The inset schematically shows the experimentalsetup.RAPID COMMUNICATIONSR4364 56FROLOV, SHKUNOV, VARDENY, AND YOSHINOlaser modes, since D1/2n,Dn/2; otherwise no separate laserlines would be observed. We then obtain from Fig. 1~a!D1/2n,5 cm21 and using n516 000 cm21 we estimate Q>33103 for the DOO-PPV ring laser.We can also determine Q from I0 . g is given by sN ,where s is the optical gain cross section and N is the averagenumber density of photogenerated excitons. Using Eq. ~1! atthreshold we getQ5 2pnpsN0l, ~3!where N0 is N at threshold. For DOO-PPV films, np;1.7~Ref. 11! and s;10215 cm2.3,12 N0 can be approximated byN0'Np /V , where Np is the total number of absorbed pho-tons per pulse at I0 and V is the volume of the lasing micro-ring. This assumption is justified in our case, since the exci-ton lifetime in DOO-PPV is 240 ps ~Refs. 3 and 12! and thisis longer than the excitation pulse duration ~100 ps!. For the125-mm-diam fiber we estimate Np533109 and V'1027 cm3. Using Eq. ~3! we then obtain Q'53103, inagreement with the other two estimates. Since all three sepa-rate Q estimates are close to each other, we conclude that Qis in fact limited by self-absorption.The polymer layer acts as a waveguide forming a ringresonator around the glass fiber. The resonant frequencies nmfor the waveguided laser modes ~WM! are given by8nm5mcpDneff, ~4!where m is an integer, c is the speed of light in vacuum, andneff is the effective refraction index in the cylindricalwaveguiding film. neff in turn, is determined13 by the thick-ness d , and the refraction indices of the polymer np and fiberng (51.46). For a relatively thick polymer film, as in ourcase, neff may be approximated14 by neff'np (12d/D)'np . Using this approximation in Eq. ~4!, we calculate theintermodal spectral spacing, Dn5nm2nm215c/pDnp , tobe 9.4 and 15 cm21 for the 200- and 125-mm-diam fibers,respectively. These values are in good agreement with theexperimental Dn in Fig. 1, which are 9.5 and 15.2 cm21,respectively.In addition to the waveguided modes ~WM! in the poly-mer film, the cylindrical glass fiber may also support whis-pering gallery modes ~WGM! propagating close to the fibersurface.5 The resonant frequencies nk of the WGM in theabsence of the polymer film are given by nk5kc/pDneff8 ,where k is the primary mode number and neff8 5n05Ang221/tan21Ang221.5 When the polymer film is depos-ited on top of the glass fiber, neff8 increases and since ng,np , the WGM become lossy. In order to show that WGMmay exist in our geometry, we coated only one side of the200 mm fiber with DOO-PPV. As seen in Fig. 3, lasing couldbe still obtained from this ‘‘one-sided’’ structure. However,due to a lower Q value and limited WGM penetration intothe polymer film, the corresponding I0 in this case is an orderof magnitude higher than that of the complete ring. Asshown in Fig. 3, the WGM of the incomplete ring resonatorare characterized by a larger Dn (510.4 cm21), from whichwe find that neff8 51.53; this value is in agreement with theorysince n0,neff8 ,np .The fiber WGM are not dominant in the complete ringresonator, since WM in the film have higher Q . However,WGM in the fiber are able to modify Qcav of the primaryWM in the polymer ring. Qcav increases whenever the fre-quencies of WM and WGM are in resonance, i.e., nm'nk ,whereas Qcav decreases if the corresponding frequencies aredetuned from each other. As a result, Qcav is periodicallymodulated in n with a period dn determined by the relationdn5DnneffDneff, ~5!where Dneff5neff2neff8 is the difference between neff of theWM and WGM, respectively. It is indeed seen in Fig. 1 thatthe equally spaced laser modes are periodically modulated inintensity. We conjecture that this intensity modulation can beexplained by the variation of Q versus n caused by reso-nances between WGM and WM. From Fig. 1 we find dn594 cm21 for the 200-m-diam fiber and 140 cm21 for theFIG. 3. Laser spectrum of a 200-mm-diam fiber coated withDOO-PPV film on one side, as shown in the inset.FIG. 4. Laser spectrum of PDPA-Si(iPr)3 films coated on a125-mm-diam fiber. The polymer repeat unit is also shown in theinset.RAPID COMMUNICATIONS56 R4365RING MICROLASERS FROM CONDUCTING POLYMERS125-mm-diam fiber. From Dn measured separately for WMand WGM ~Figs. 1 and 3, respectively! we calculate Dneff50.17. Using Eq. ~5! we obtain, in agreement with the ex-periment, dn594 and 150 cm21 for the 200 and 125 mmfibers, respectively.Microring lasers were also made from PDPA-Si(iPr)3 ,which has a broad PL spectrum peaking at 19 000 cm21(;530 nm).7 Figure 4 shows a multimode laser spectrum ofa DPA ring formed around a 125-mm-diam fiber; the poly-mer repeat unit is shown in the inset. We note that I0 here islarger than I0 for DOO-PPV by an order of magnitude. Wemeasured Dn at the red end of the spectrum (18 500 cm21)to be 16.7 cm21, which according to Eq. ~4! corresponds toneff51.53. We also measured Dn516.0 cm21 at the blue endof the spectrum (19 000 cm21) leading to neff51.59. Forthick films neff'np and thus, using the measured laser modestructure, it is possible to accurately determine the absolutevalue and frequency dispersion of np . Similar to DOO-PPV~Fig. 1!, the mode structure in Fig. 4 is periodically modu-lated; this may be also attributed to the resonant couplingbetween WM and WGM in the ring cavity.In summary, we have demonstrated multimode red andgreen lasing from cylindrical microcavities of thin conduct-ing polymer films deposited on optical fibers. The laser emis-sion is characterized by the following properties: ~i! drasticspectral narrowing, where a single mode linewidth is lessthan 1.5 cm21; ~ii! a well-defined threshold excitation inten-sity for lasing; ~iii! a high degree of polarization above thethreshold; and ~iv! increased azimuthal directionality. Theobserved threshold excitation intensities for these microlas-ers are orders of magnitude smaller than those of spectralnarrowing previously reported in flat thin films. This is apromising step towards developing electrically pumped plas-tic lasers.We thank Dr. W. Gellermann for assistance with mea-surements. This work was done at the Utah Laser Instituteand supported in part by DOE Grant No. FG-03-96ER45490.1D. Moses, Appl. Phys. Lett. 60, 3215 ~1992!.2N. Tessler, G. J. Denton, and R. H. Friend, Nature ~London! 382,695 ~1996!; F. Hide et al., Science 273, 1833 ~1996!.3S. V. Frolov et al., Jpn. J. Appl. Phys., Part 2 35, L1371 ~1996!;Phys. Rev. Lett. 78, 729 ~1997!.4H. J. Brouwer et al., Adv. Mater. 8, 935 ~1996!; G. Gelink et al.,Chem. Phys. Lett. 265, 320 ~1997!.5O. Svelto, Principles of Lasers ~Plenum, New York, 1989!.6Y. Yamamoto and R. Slusher, Phys. Today 46, 66 ~1993!; J. C.Knight, H. S. T. Driver, and G. N. Robertson, J. Opt. Soc. Am.B 11, 2046 ~1994!.7K. Tada et al., Jpn. J. Appl. Phys., Part 2 35, L1138 ~1996!.8H. P. Weber and R. Ulrich, Appl. Phys. Lett. 19, 38 ~1971!; M.Kuwata-Gonokami et al., Opt. Lett. 20, 2093 ~1995!.9A. J. Campillo, J. D. Eversole, and H. B. Lin, Phys. Rev. Lett. 67,437 ~1991!.10At excitation intensities below I0 we found that PL is also con-centrated in Du50.09 rad. The emission anisotropy patternabruptly changes at I0 to Du50.03 rad, showing excess direc-tionality for the laser modes.11S. V. Frolov, Z. V. Vardeny, and K. Yoshino ~unpublished!.12S. V. Frolov et al., Phys. Rev. Lett. 78, 4285 ~1997!.13D. L. Lee, Electromagnetic Principles of Integrated Optics~Wiley, New York, 1986!.14M. Heiblum and J. H. Harris, IEEE J. Quantum Electron. QE-11,75 ~1975!.RAPID COMMUNICATIONSR4366 56FROLOV, SHKUNOV, VARDENY, AND YOSHINO

Ring microlasers from conducting polymers

https://collections.lib.utah.edu/dl_files/16/6f/166f2ca91808d3537c76946d5ac6d70b8906cbba.pdf

Ring microlasers from conducting polymers

Abstract

Similar works

Full text

Available Versions

The University of Utah: J. Willard Marriott Digital Library