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Stimulated emission in poly(p-phenylene–vinylene) films
Synthetic Metals 111–112 Ž2000. 507–508 www.elsevier.comrlocatersynmet
Stimulated emission in poly žp-phenylene–vinylene/ films S.V. Frolov a,) , Z.V. Vardeny b a
Bell Laboratories, Lucent Technologies, 600 Mountain AÕe., Murray Hill, NJ 07974, USA b Physics Department, UniÕersity of Utah, Salt Lake City, UT 84112, USA
Abstract Excitonic emission has been studied in films of polyŽ p-phenylene–vinylene. ŽPPV. derivatives. At high excitation intensities we found an unusual stimulated emission regime characterized by a finely structured spectrum having features as narrow as 0.1 nm. q 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Conducting polymers; Stimulated emission
1. Introduction Various p-conjugated polymers and oligomers produce stimulated emission ŽSE. when excited with short, high-energy laser pulses, w1–13x as evidenced by dramatic spectral narrowing and substantial exciton lifetime shortening. It has been demonstrated that these effects can be described using a simplified ASE model, where spontaneously emitted light is amplified by the same medium as it propagates along the path of the maximum optical gain w2–7,9x. In the case of a thin stripe excitation, we found that SE occurs only in the direction along the stripe w7x. However, upon increasing the spectral resolution of our apparatus, we discovered new characteristics of SE, which do not agree with the ASE picture. 2. Experimental results Thin films of 2,5-dioctyloxy polyŽ p-phenylene–vinylene. wDOO-PPVx were uniformly spin-coated on flat quartz substrates. The polymer films were photoexcited by 100 ps pulses produced by a frequency-doubled Ž532 nm. Nd:YAG regenerative laser amplifier with a repetition rate of 100 Hz. The excitation beam was focused onto a sample with a cylindrical lens, forming a stripe-like excitation area with variable length L and width a. Optical emission from the side of the sample was collected using a round lens )
Corresponding author. E-mail address: [email protected] ŽS.V. Frolov..
positioned in the direction along the excitation stripe. Emission spectra were recorded using a 0.6-m triple spectrometer and a CCD array with spectral resolution of about ˚ 1 A. Fig. 1Ža. shows three SE spectra collected from the same DOO-PPV film at approximately equal excitation intensities I for different values of a. As a decreases, a fine structure progressively develops in the SE spectrum, which initially is smooth at large excitation areas Žlarge a.. We found that the peaks spectral positions and heights changed whenever the excitation area Ž a - 100 mm. was moved across the polymer film; however, the fine spectrum remained unaltered over an extended period of time, if the experimental conditions and sample position were kept the same. It can be argued that the smooth SE band observed at larger a Ž a ) 400 mm. is due to the spectral overlap of numerous narrow peaks originating from different parts of a larger excited area, which, as a result of area averaging, form a band similar to the previously observed SE bands in analogous studies employing a round excitation area w1–9x. Fig. 1Žb. shows the emission spectra measured in DOOPPV films with a s 30 mm at I greater than the SE threshold intensity, IA . Initial spectral narrowing results in a smooth emission band at 630 nm; this is a regular ASE regime described elsewhere w7x. The latter band at higher I Ž I G I B , where I B is a second threshold excitation intensity. transforms into a ‘‘spiky’’ band having features with linewidths as small as 0.1 nm. The emission spectrum at I ) IA is dominated by several narrow peaks. In addition, the emission intensity in this new regime depends linearly
0379-6779r00r$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 4 2 8 - 2
508
S.V. FroloÕ, Z.V. Vardenyr Synthetic Metals 111–112 (2000) 507–508
Fig. 1. Stimulated emission spectra of a DOO-PPV film obtained using a stripe-like excitation area with length Ls1 mm and width a: Ža. SE at different values of a shown in the inset, where Ls1 mm and I ;1.5PIA ;. Žb. SE at different excitation intensities I with as 30 mm: I1 s IA s1 MWrcm2 , I2 s1.25IA , I3 s1.4 IA , I4 s1.6 IA .
on the excitation intesity. These results show a clear separation between the new SE regime and a regular ASE. 3. Discussion We consider few mechanisms, which may conceivably lead to SE with the finely structured spectrum. ŽI. Firstly, the ‘‘spiky’’ SE spectra may be the result of spectral variations in the magnitude of the effective optical gain, geff s g y a , where g and a are the optical gain and loss coefficients, respectively. Such variations can be due to either inhomogeneous broadening ŽIa., e.g. caused by a chain length distribution in p-conjugated polymers, or homogeneous broadening ŽIb. from strong electron–phonon coupling. Narrow bands in the subgap absorption spectrum associated with defects ŽIc. can also lead to similar variations in geff . ŽII. Secondly, structural cavitylike resonances in the gain medium may significantly affect the SE spectrum and produce laser lines. Since there are no obvious cavities in our DOO-PPV films, the laser modes would have to arise from random scattering; this is similar to the scenario originally proposed for photonic paints w14,15x. Even in pristine polymer films there might be a substantial amount of scattering due to impurities and fluctuations in the film’s density and thickness, which may form weak mesoscopic resonances and consequently lead to random lasing.
It has been shown that scenarios Ia–Ic do not provide a satisfactory explanation of this phenomenon w16x. On the other hand, random lasing Žscenario II. is thought to require strong scattering w17x, which is apparently absent in these films. Let us consider, however, a one-dimensional optical amplifier of length L with a gain coefficient g and scattering losses per unit length b , where g ) 1 and b - 1. Then the number of backscattered photons n b is given by n b s ² N :b L, where ² N : is the average number of photons in the amplifier Žin a given mode.. In case of lasing due to random backscattering feedback we can nb g L neglect ASE and find that ² N : s e . Substituting gL this into the previous expression we find the conditions for mirrorless lasing in this amplifier are met when: b s g eyg L Ž 1. Using the variable stripe length method in DOO-PPV films, we measured geff ranging from 10 to 100 cmy1 at different I w7x. Using typical values of L s 2 mm and g s 50 cmy1 , we find from Eq. Ž1. that for exact backscattering b ; 2.3 = 10y3 cmy1 . This in turn requires a total scattering coefficient on the order of 2pbrV ; 0.9 cmy1 . One can see that according to this model, even weak scattering may suffice for random lasing in high gain media. Previously measured optical loss coefficient a of 30 cmy1 in DOO-PPV films w7x may in fact be partially attributed to such scattering. References w1x N. Tessler, G.J. Denton, R.H. Friend, Nature 382 Ž1996. 695. w2x F. Hide, M.A. Diaz-Garcia, B.J. Schwartz, M.R. Andersson, Q. Pei, A.J. Heeger, Science 273 Ž1996. 1833. w3x H.J. Brouwer, V.V. Krasnikov, A. Hilberer, G. Hadziioannou, Adv. Mater. 8 Ž1996. 935. w4x G. Gelink, J.W. Warman, M. Remmers, D. Neher, Chem. Phys. Lett. 265 Ž1997. 320. w5x S.V. Frolov, M. Ozaki, W. Gellermann, K. Yoshino, Z.V. Vardeny, Phys. Rev. Lett. 78 Ž1997. 729. w6x S.V. Frolov, M. Ozaki, W. Gellermann, K. Yoshino, Z.V. Vardeny, Phys. Rev. B 56 Ž1997. R4363. w7x S.V. Frolov, M. Ozaki, W. Gellermann, K. Yoshino, Z.V. Vardeny, Phys. Rev. B 57 Ž1998. 9141. w8x X. Long, A. Malinowski, D.D.C. Bradley, M. Inbasekaran, E.P. Woo, Chem. Phys. Lett. 272 Ž1997. 6. w9x G.J. Denton, N. Tessler, M.A. Stevens, R.H. Friend, Adv. Mater. 9 Ž1997. 547. w10x M. Berggren, A. Dodabalapur, R.E. Slusher, Z. Bao, Nature 389 Ž466. Ž1997. . w11x M. Berggren, A. Dodabalapur, R.E. Slusher, Z. Bao, Appl. Phys. Lett. 71 Ž1997. 2230. w12x M. Berggren, A. Dodabalapur, R.E. Slusher, Z. Bao, Adv. Mater. 9 Ž1997. 968. w13x V.G. Kozlov, V. Bulovic, P.E. Burrows, S.R. Forrest, Nature 389 Ž1997. 362. w14x N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain, Nature 368 Ž1994. 436. w15x F. Hide, B.J. Schwartz, M.A. Diaz-Garcia, A.J. Heeger, Chem. Phys. Lett. 256 Ž1996. 424. w16x S.V. Frolov et al., Phys. Rev. B 59 Ž1999. R5284. w17x S. John, G. Pang, Phys. Rev. A 54 Ž1996. 3642.
Stimulated emission in poly žp-phenylene–vinylene/ films S.V. Frolov a,) , Z.V. Vardeny b a
Bell Laboratories, Lucent Technologies, 600 Mountain AÕe., Murray Hill, NJ 07974, USA b Physics Department, UniÕersity of Utah, Salt Lake City, UT 84112, USA
Abstract Excitonic emission has been studied in films of polyŽ p-phenylene–vinylene. ŽPPV. derivatives. At high excitation intensities we found an unusual stimulated emission regime characterized by a finely structured spectrum having features as narrow as 0.1 nm. q 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Conducting polymers; Stimulated emission
1. Introduction Various p-conjugated polymers and oligomers produce stimulated emission ŽSE. when excited with short, high-energy laser pulses, w1–13x as evidenced by dramatic spectral narrowing and substantial exciton lifetime shortening. It has been demonstrated that these effects can be described using a simplified ASE model, where spontaneously emitted light is amplified by the same medium as it propagates along the path of the maximum optical gain w2–7,9x. In the case of a thin stripe excitation, we found that SE occurs only in the direction along the stripe w7x. However, upon increasing the spectral resolution of our apparatus, we discovered new characteristics of SE, which do not agree with the ASE picture. 2. Experimental results Thin films of 2,5-dioctyloxy polyŽ p-phenylene–vinylene. wDOO-PPVx were uniformly spin-coated on flat quartz substrates. The polymer films were photoexcited by 100 ps pulses produced by a frequency-doubled Ž532 nm. Nd:YAG regenerative laser amplifier with a repetition rate of 100 Hz. The excitation beam was focused onto a sample with a cylindrical lens, forming a stripe-like excitation area with variable length L and width a. Optical emission from the side of the sample was collected using a round lens )
Corresponding author. E-mail address: [email protected] ŽS.V. Frolov..
positioned in the direction along the excitation stripe. Emission spectra were recorded using a 0.6-m triple spectrometer and a CCD array with spectral resolution of about ˚ 1 A. Fig. 1Ža. shows three SE spectra collected from the same DOO-PPV film at approximately equal excitation intensities I for different values of a. As a decreases, a fine structure progressively develops in the SE spectrum, which initially is smooth at large excitation areas Žlarge a.. We found that the peaks spectral positions and heights changed whenever the excitation area Ž a - 100 mm. was moved across the polymer film; however, the fine spectrum remained unaltered over an extended period of time, if the experimental conditions and sample position were kept the same. It can be argued that the smooth SE band observed at larger a Ž a ) 400 mm. is due to the spectral overlap of numerous narrow peaks originating from different parts of a larger excited area, which, as a result of area averaging, form a band similar to the previously observed SE bands in analogous studies employing a round excitation area w1–9x. Fig. 1Žb. shows the emission spectra measured in DOOPPV films with a s 30 mm at I greater than the SE threshold intensity, IA . Initial spectral narrowing results in a smooth emission band at 630 nm; this is a regular ASE regime described elsewhere w7x. The latter band at higher I Ž I G I B , where I B is a second threshold excitation intensity. transforms into a ‘‘spiky’’ band having features with linewidths as small as 0.1 nm. The emission spectrum at I ) IA is dominated by several narrow peaks. In addition, the emission intensity in this new regime depends linearly
0379-6779r00r$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 4 2 8 - 2
508
S.V. FroloÕ, Z.V. Vardenyr Synthetic Metals 111–112 (2000) 507–508
Fig. 1. Stimulated emission spectra of a DOO-PPV film obtained using a stripe-like excitation area with length Ls1 mm and width a: Ža. SE at different values of a shown in the inset, where Ls1 mm and I ;1.5PIA ;. Žb. SE at different excitation intensities I with as 30 mm: I1 s IA s1 MWrcm2 , I2 s1.25IA , I3 s1.4 IA , I4 s1.6 IA .
on the excitation intesity. These results show a clear separation between the new SE regime and a regular ASE. 3. Discussion We consider few mechanisms, which may conceivably lead to SE with the finely structured spectrum. ŽI. Firstly, the ‘‘spiky’’ SE spectra may be the result of spectral variations in the magnitude of the effective optical gain, geff s g y a , where g and a are the optical gain and loss coefficients, respectively. Such variations can be due to either inhomogeneous broadening ŽIa., e.g. caused by a chain length distribution in p-conjugated polymers, or homogeneous broadening ŽIb. from strong electron–phonon coupling. Narrow bands in the subgap absorption spectrum associated with defects ŽIc. can also lead to similar variations in geff . ŽII. Secondly, structural cavitylike resonances in the gain medium may significantly affect the SE spectrum and produce laser lines. Since there are no obvious cavities in our DOO-PPV films, the laser modes would have to arise from random scattering; this is similar to the scenario originally proposed for photonic paints w14,15x. Even in pristine polymer films there might be a substantial amount of scattering due to impurities and fluctuations in the film’s density and thickness, which may form weak mesoscopic resonances and consequently lead to random lasing.
It has been shown that scenarios Ia–Ic do not provide a satisfactory explanation of this phenomenon w16x. On the other hand, random lasing Žscenario II. is thought to require strong scattering w17x, which is apparently absent in these films. Let us consider, however, a one-dimensional optical amplifier of length L with a gain coefficient g and scattering losses per unit length b , where g ) 1 and b - 1. Then the number of backscattered photons n b is given by n b s ² N :b L, where ² N : is the average number of photons in the amplifier Žin a given mode.. In case of lasing due to random backscattering feedback we can nb g L neglect ASE and find that ² N : s e . Substituting gL this into the previous expression we find the conditions for mirrorless lasing in this amplifier are met when: b s g eyg L Ž 1. Using the variable stripe length method in DOO-PPV films, we measured geff ranging from 10 to 100 cmy1 at different I w7x. Using typical values of L s 2 mm and g s 50 cmy1 , we find from Eq. Ž1. that for exact backscattering b ; 2.3 = 10y3 cmy1 . This in turn requires a total scattering coefficient on the order of 2pbrV ; 0.9 cmy1 . One can see that according to this model, even weak scattering may suffice for random lasing in high gain media. Previously measured optical loss coefficient a of 30 cmy1 in DOO-PPV films w7x may in fact be partially attributed to such scattering. References w1x N. Tessler, G.J. Denton, R.H. Friend, Nature 382 Ž1996. 695. w2x F. Hide, M.A. Diaz-Garcia, B.J. Schwartz, M.R. Andersson, Q. Pei, A.J. Heeger, Science 273 Ž1996. 1833. w3x H.J. Brouwer, V.V. Krasnikov, A. Hilberer, G. Hadziioannou, Adv. Mater. 8 Ž1996. 935. w4x G. Gelink, J.W. Warman, M. Remmers, D. Neher, Chem. Phys. Lett. 265 Ž1997. 320. w5x S.V. Frolov, M. Ozaki, W. Gellermann, K. Yoshino, Z.V. Vardeny, Phys. Rev. Lett. 78 Ž1997. 729. w6x S.V. Frolov, M. Ozaki, W. Gellermann, K. Yoshino, Z.V. Vardeny, Phys. Rev. B 56 Ž1997. R4363. w7x S.V. Frolov, M. Ozaki, W. Gellermann, K. Yoshino, Z.V. Vardeny, Phys. Rev. B 57 Ž1998. 9141. w8x X. Long, A. Malinowski, D.D.C. Bradley, M. Inbasekaran, E.P. Woo, Chem. Phys. Lett. 272 Ž1997. 6. w9x G.J. Denton, N. Tessler, M.A. Stevens, R.H. Friend, Adv. Mater. 9 Ž1997. 547. w10x M. Berggren, A. Dodabalapur, R.E. Slusher, Z. Bao, Nature 389 Ž466. Ž1997. . w11x M. Berggren, A. Dodabalapur, R.E. Slusher, Z. Bao, Appl. Phys. Lett. 71 Ž1997. 2230. w12x M. Berggren, A. Dodabalapur, R.E. Slusher, Z. Bao, Adv. Mater. 9 Ž1997. 968. w13x V.G. Kozlov, V. Bulovic, P.E. Burrows, S.R. Forrest, Nature 389 Ž1997. 362. w14x N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain, Nature 368 Ž1994. 436. w15x F. Hide, B.J. Schwartz, M.A. Diaz-Garcia, A.J. Heeger, Chem. Phys. Lett. 256 Ž1996. 424. w16x S.V. Frolov et al., Phys. Rev. B 59 Ž1999. R5284. w17x S. John, G. Pang, Phys. Rev. A 54 Ž1996. 3642.