Journal ArticleWe have investigated the optical emission characteristics of poly(2,5-dioctyloxy-p-phenylenevinylene) (DOO-PPV)  thin films under high intensity ps pulsed laser excitation (Ipump > 1 MW/ cm2). We observed that the emission  spectrum consist of a spectrally narrowed emission (SNE) band (FWHM ~ 10 nm) and superimposed sharp spectral lines. High resolution spectral measurements conducted at room and liquid nitrogen temperatures show that the spectral width of these lines is less than 1 nm. The lines peak positions are independent of film thickness, however, they shifts upon changing the excitation wavelength. Also, the excitation intensity dependence of these narrow lines is found to be  different from that of the SNE band. We argue that the narrow lines are due to Raman scattering gain of the 1st, 2nd and 3d order Raman active modes of DOO-PPV

Shkunov, M. N.

Vardeny, Zeev Valentine

English

The University of Utah: J. Willard Marriott Digital Library

88 Raman scattering gain in DOO-PPV films M.N. Shkunov', W. Gellennann·, A.Fujii··b, K. Yoshinob and Z.V.Vardeny· -nept. of Physics, University of Utah, Salt Lake City, Utah 84112 ~ept. of Electronic Engineering, Faculty of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565, Japan (" " " ABSTRACT We have investigated the optical emission characteristics of poly(2,5-dioctyloxy-p-phenylenevinylene) (DOO-PPV) thin films under high intensity ps pulsed laser excitation (Ipump> 1 MWI cm2). We observed that the emission spectrum consist of a spectrally naIroWed emission (SNE) band (FWHM - 10 nm) and superimposed sharp spectral lines. High resolution spectral measurements conducted at room and liquid nitrogen temperatures show that the spectral width of these lines is less than 1 nm. The lines peak positions are independent of film thickness, however, they shifts upon changing the excitation wavelength. Also, the excitation intensity dependence of these narrow lines is found to be different from that of the SNE band. We argue that the narrow lines are due to Raman scattering gain of the 1'\ 2nd and 3d order Raman active modes ofDOO-PPV. . ' . . ,' Keywords: DOO-PPV, Raman scattering gain, stimulated emission, strongly coupled vibrations. 1. INTRODUCTION The investigation and explanation of the exciton emission behavior in luminescent conducting polymer thin films under high excitation intensities have recently drawn considerable attention 1.7. The high optical gain achieved in these materi-als results in dramatic emission spectral narrowing and radiative lifetime shortening when the excitation intensity ex-ceeds a certain threshold value. For example, for films of the soluble derivative of PPV, namely DOO-PPV, the spectral narrowing process occurs as bright emission with about ten-fold reduced spectral width and subpicosecond time duration, when the exciton density exceeds a threshold level of _5xlO17 cm·3, This nonlinear emission behavior has been attributed to several processes such as amplified spontaneous emission3, cooperative emission or superfluorescence4•5 and mirrorless lasint. We will refer to this emission simply as spectrally narrowed emission (SNE) throughout this paper. In this wolk we report novel spectral features observed in the emission of DOO-PPV films which occur in addi-tion to, or on top of the SNE band. 2. EXPERIMENTAL For our measurements we used thin DOO-PPV films spin-coated on glass substrates. The samples were excited at three different wavelengths: 532, 558 and 563.5 nm, respectively; the laser pulse duration was - 100 ps and the pulse energies varied from 0.1 to 50 J,11. The excitation light was focused onto the polymer film within an area of -1 mm2• The emis-sion spectra were measured with a SPEX 0.6 m triple spectrometer, employing 300, 600 and 1200 grooves/mm gratings and a cooled CCD array. To avoid oxidation, the samples were kept in a cryostat under vacuum of 10.3 torr, where the temperature of the samples could be changed between 80 and 300 K. SPIE Vol. 3145. 0277·786X/97/$10,OO c .Q (f) .~ E (1) "'0 (1) N ro E L-a c c: .Q (f) (f) ·e (1) "'0 (1) .~ ro E L-o c: 1.2 0.6 o 580 a 620 wavelength (nm) 660 Figure 1. Nonnalized emission spectra of DOO-PPV film at room temperature obtained with 532 om excitation, pulse width - 100 PS. repetition rate 100 Hz. The excitation energies per pulse are: (a) 0.11 J.1l. (b) 0.12 J.1l. (c) 0.15 J.1l. (d) 0.20 J.1l. (e) 0.30 J.1l. The DOO-PPV chemical structure is shown in the insert. 2 , o 620 630 640 650 wavelength (nm) Figure 2. Emission spectra for higher excitation intensities, at 80 K. Pump energies are: (a) 0.1 J.1l. (b) 0.65 J.1l. (c) 1.6 J.1l and (d) 5.3 J.1l. 89 90 3. RESULTS AND DISCUSSION -~-'"'' , For 532 run excitation at room temperature the emission spectrum of DOO-PPV follows the typical spectral narrowing process in conducting polymer films: when the excitation energy is increased from 0.1 to 0.3 JJl/pulse the spectral width of the polymer emission collapses from a FWHM of - 80 run to 7 run (Fig. 1 ). However. a further increase of the pump energy leads to the appearance of new. sharp spectral features superimposed on the SNE band. This is illustrated in Fig.2 where we show the emission ofDOO-PPV films measured at 80 K. As the pump energy is further increased from 0.1 to 5.3 JJl/pulse. two sharp peaks emerge on the shoulders of the SNE band at 629 and 640 om. respectively. The FWHM of these sharp lines is less than 1 om. and thus is significantly narrower than the SNE band (Fig.2). Two effects are conceivable for the origin of the observed sharp lines: (1) the presence of a microcavity mode structure or (2) the presence of a nonlinear process associated with Raman scattering, namely Raman gain Considering the first possibility, a cavity length estimate of a possible microcavity, formed by the polymer-air and polymer-glass interfaces. gives d= 10 Jl when using the relationS d = c/2n6.v , (1) where d - cavity length. c - speed of light. n - refraction index of the medium, Av - differen::e in mode frequency. This value for d is at least one order of magnitude larger than the polymer fUm thickness used in our measurements. Another consequence of a microcavity consideration is that we would expect the mode spacing to be different for various fIlm thicknesses. In Fig.3 we show DOO-PPV emission spectra for two films with different thicknesses: 240 om and 900 om thick. respectively. In both cases the spectral separation of the narrow lines is the same. From these two observations we conclude that microcavity effects can be ruled out as giving rise to the narrow emission lines. The excitation intensity needed for observation of the narrow lines was in the order of 2+50 MW Icm2; these are quite high intensity levels. at which nonlinear optical phenomena are expected to play an important role. This fact, to-gether with the sharpness of the narrow emission lines. points at the possibility that stimulated Raman scattering (SRS) is present in our measurements. Under this assumption we expect to observe the following features: (i) the spectral position of the emission lines should change with the shift of the excitation wavelength (ii) the output intensity IsRS should vaty exponentially with the pump intensity Ipump, according to the relation9 IsRs = ISRs (O)*exp(glpump t) (2) where g is the gain coefficient, and l - the length of the active medium. (iii) we expect to observe a threshold effect as well as gain saturation (iv) the Raman lines might be observed not only at first order but also at higher orders. To perform a more detailed study related to the possibility of Raman gain, we measured the 1" order Raman scattering spectrum of DOO-PPV at low excitation intensities. To avoid the strong luminescence background, we used long-wavelength excitation with a CW Ti:sapphire laser operating at 752 om. The Raman spectrum is given in Fig.4 and shows several strong lines located at 1584. 1313, 1285, 1117 and 968 cm· l respectively. According to Sakamoto et al. IO, who measured the Raman spectrum of unsubstituted PPV, the strongest Raman line at 1584 cm·l is assigned to the polymer ring mode corresponding to the VS. of benzene in the Wilson notation We thus expect this phonon line to strongly participate in the Raman gain process at high excitation intensities. Figure 5 shows the emission spectra of a DOO-PPV fUm at high excitation intensity for two additional excita-tion wavelengths, namely 558 and 563.5 om. In both cases the SNE band is centered at 630 om and remains unshifted. However. a very intense, single. sharp emission line appears on the high energy side of the SNE band. From high reso-lution measurements we determined the line width of this sharp line to be less than 3A. The position of this sharp line is shifted at 558 om excitation compared to 563.5 om excitation, and this frequency shift exactly corresponds to the ~ quency difference of the two excitation wavelengths. Therefore, we have a clear indication for the presence of a first order Raman scattering gain in our DOO-PPV films. To identify the Raman active modes participating in the SNE at high excitation intensities we have plotted the first-order Raman spectrum of DOO-PPV together with the polymer emission for the 563.5 om excitation (Figure 6). It • .~ (/) c: cu +J .5 c: .Q (/) (/) ·e cu "'0 cu .~ ro E "-0 c: , 0.5 0 , 0.5 o 14 10 6 2 a b 600 620 640 660 wavelength (nm) Figure 3. DOO-PPVemission spectra for two films of different thickness d, at 80 K, where the excitation wavelength is 532 om. (a) d - 240 run, (b) d - 900 om. 800 1200 1600 Raman Shift (cm-1) Figure 4. DOO-PPV Raman spectrum at low excitation intensity. Excitation source : 10 mW Ti:sappbire laser at 752 om. 91 92 .~ en c: C1> +-' ,5 c: 0 'en en 'E C1> "0 C1> ,t::! "' E ~ 0 c: 1.0 a 0.6 RS 0.2 SNE 1.0 b 0.6 RS SNE I 0.2 600 620 640 660 wavelength (nm) Figure 5. DQO-PPV emission spectra at high excitation intensities for two different excitation wavelengths: (a) Aexc = 563.5 run, (b) Aexc = 558 Dm. The amplified first order Raman scattering lines (RS) are shifted from spectrally narrowed emision (SNE) band. 1.0 '. ;,..:.." ,:i fti ........ 0,6 0.2 600 (b) 620 640 wavelength (nm) (a) 660 ., '. Figu re 6. DOO-PPV emission at high intensity (a) and first order Raman spectrum, obtained at low excitation intensity (b). Excitation wavelength is 563.5 Dm. t , 60 /\ C .~ 40 /' \ / I\¥ CI) c::: Q) +"'" .5 I \ I \ I \ I \ / \ I \ 20 b~ , """-620 630 640 650 wavelength (nm) Figure 7. (a) OOO-PPV emission spectrum at high excitation intensity. Experimental conditions: Aexe = 532 nm, Eexe = 7~, T = 80 K; (b) Second order Raman spectrum of 1285,1313 and 1584 em-l modes is shown as a thin line. (e) Third order Raman spectrum of 968 and 1117 em -1 modes is shown as dashed line. 93 94 is apparent that a very good overlap is obtained for the 1584 cm·1 Raman mode. IT we go back to the case of 532 nm ex-citation, we notice that the frequency shift between the pump and the SNE band is in the order of 3000 cm·I , a shift which is too large for any known Raman mode frequency in DOO-PPV. However, we obtain again good spectral overlap if we consider higher order Raman scattering, as shown in Fig. 7. As we can see, the 640 nm sharp emission peak, on top of the SNE band, overlaps with the second order of the 1584 cm·1 Raman mode (FigA), whereas the 629 nm peak: overlaps with the third order of the 968 cm·1 mode. We conclude therefore, that strong Raman scattering gain is observed only when a spectral coincidence is realized between the fundamental, or higher-oroer Raman mode, and the spectral re-gion of maximum optical gain We thus argue that the observed Raman gain is not due to a simple stimulated Raman processll . Another interesting characteristic of the DOO-PPV emission under high excitation intensity is the excitation intensity dependence for both the SNE band and Raman gain peaks. The threshold excitation intensity for the Raman gain is almost one order of magnitude higher than that for the SNEll. Moreover, the Raman scattering gain seems to saturate at higher excitation intensities than the SNE, and thus dominates the emission spectrum at the highest excita-tion intensitiesl2. 4. CONCLUSIONS We have investigated DOO-PPV emission spectra at high excitation intensities of order 2+50 MW/cm2 and discovered very narrow (0.3 - 1.0 nm) emission lines superimposed on a relatively broad SNE band of - 10 nm width. The appearance of these spectral features can be explained by Raman scattering gain of the first, second and third order Raman active modes of the polymer, the strongest of which is the ring mode at 1584 cm·l. Raman gain appears at rela-tively high excitation intensities whenever the polymer Raman active lines of the frrst, or higher orders coincide with the exciton SNE band. ACKNOWLEDGMENTS We would like to thank S. V. Frolov for the preparation of polymer films. This work was supported in part by the DOE, grant No. FG-03-95 ER 45490, and by the New Energy and Industrial Development Organization (NEDO) 1997 program in Japan I. 2. 3. 4. 5 6 7. 8 . 9. 10. 11. 12. REFERENCES ICSM Proceedings, Synth. Metals 84, pp. 455-476, 1997 N. Tessler, G.J. Denton & R.H. Friend, "Lasing from conjugated-polymer microcavities", Nature, 382, pp. 695-697, 1996. F. Hide, M .A. Dias-Garcia, B.J. Schwartz, M.R. Andersson, Q. Pei and A. Heeger, "Semiconducting polymers: a new class of solid-state laser materials", Science, 273, pp. 1833-1836, 1996. S.V. Frolov, W. Gellermann, M. Ozaki, K. Yoshino and Z.V. Vardeny, "Cooperative emission in 1[-conjugated polymer thin films", Phys. Rev. Lett., 78, pp. 729-732, 1994. H.J. Brouwer, V.V. Krasnikov, A. Hilberer and G. Hadziioannou, "Blue superradiance from neat semicon ducting alternating copolymer films", Adv. Mater., 8, pp. 935-937, 1996. S.V. Frolov, M. Ozaki, W. Gellermann, Z.V. Vardeny and K. Yoshino, , "Mirrorless lasing in conducting polymer poly(2,5-dioctyloxy-p-phenylenevinylene) films", Jpn. J. Appl. Phys., 35, p. L1371, 1996. S.V. Frolov, M. Shkunov, Z.Y. Vardeny, M. Ozaki and K. Yoshino, "Laser action in conducting polymers", Proceedings SPIE Vol. 3145, 1997 A. Yariv, Quantum Electronics, John Wiley and Sons, Inc., New York, 1967. F.T. Arecchi, E.O. Schulz-Dubois, Laser handbook, volume 2, North-Holland PubliShing Company, Amster dam, 1972. A. Sakamoto, Y. Furukawa, M. Tasumi, "Infrared and Raman studies ofpoly(p-phenylenevinylene) and its model compounds", J. Phys. Chem., 96, pp. 1490-1494, 1992. N. Bloembergen, Encounters in Nonlinear Optics, World Scientific, Singapore, p. 339, 1996 M.N. Shkunov, W. Gellermann, Z.v. Vardeny and K. Yoshino (to be published). 

Raman scattering gain in DOO-PPV films

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Raman scattering gain in DOO-PPV films

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