Synthetic Metals 111–112 Ž2000. 567–570 www.elsevier.comrlocatersynmet
Optical gain spectra from doped polymeric waveguides Kevin P. Kretsch a,) , Colin Belton a , Stephen Lipson a , Werner J. Blau a , Fryad Z. Henari b, c Steffen Pfeiffer c , Hartwig Tillmann c , Hans-Heinrich Horhold ¨ a
Department of Physics, Trinity College Dublin, Dublin 2, Ireland Department of Physics, UniÕersity of Bahrain, Isa Town, Bahrain Friedrich-Schiller UniÕersity, Institute for Organic Chemistry and Macromolecular Chemistry, Jena, Germany b
c
Abstract We report on single-pass optical gain measurements in polyŽstyrene. waveguides doped with polymeric model compounds under picosecond excitation. Using a wavelength-dependent model of amplified spontaneous emission, we produce optical gain spectra for these materials. Large net optical gains are produced for small input pulse energies. A figure of merit describing the potential of a material for use as a laser medium is presented. Comparisons with other recent publications indicate that organic materials can compete with inorganic semiconductors for optically pumped applications. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Simulated emission; Optical gain; Waveguides; Polymer; Laser
1. Introduction Much work is in progress to understand the processes of emission and amplification of light in conjugated polymers and to define structure–property relationships as an aid to molecular engineering. Low molecular weight model compounds are finding use in this regard, allowing studies of the effect of chemical functional groups on optical processes in a controlled and convenient fashion. Thorough optical characterisation of these materials is essential to determine their potential for use in active optical devices, whether optically or electrically pumped. We report optical gain studies involving two model compounds: 1,4-bis-Ž4X-diphenylamino-styryl.-benzene and 1,4-bis-Ž a-cyanostyryl.-2,5-dimethoxybenzene. We will refer to the compounds as SP35 and G33, respectively, in keeping with recent publications involving the same materials. Table 1 lists the optical properties and of these two compounds in polyŽstyrene. host. Fig. 1 shows the chemical structure of the compounds.
)
Corresponding author. E-mail address:
[email protected] ŽK.P. Kretsch..
Synthesis of the compounds and their analogues is described by Rost et al. w1x Pfeiffer et al. w2x and Birckner et al. w3x. Lasing characteristics of SP35 in toluene solutions w4x and stimulated emission and triplet absorption cross-sections for G33 are reported by Henari et al. w5x. The high fluorescence quantum yields, f f , in polyŽstyrene. matrices bode well for use of these compounds in ‘active’ optical media. The measurement of optical gain presented is similar to the method used by Shaklee and Leheny w6x in semiconductor crystals, and by Sorek et al. w7x in organic doped glass waveguides. This technique involves the measurement of spontaneous Žfluorescent. emission, amplified via stimulated emission as it passes through the excited volume of a sample. We present a model of amplified spontaneous emission that explicitly includes wavelength-dependent optical gain. We use this model to produce optical gain spectra for the above model compounds. 2. Theory The ASE theory usually begins with a one-dimensional approximation w6,7x describing the rate of change of fluorescence intensity with length of the pumped region: dl s AP0 q gI, Ž 1. dx
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 3 0 8 - 2
K.P. Kretsch et al.r Synthetic Metals 111–112 (2000) 567–570
568
where I is the fluorescence intensity propagating along the x-axis, g s g X y a is the net optical gain, g X is the ‘actual’ gain, a accounts for all loss mechanisms, and AP0 describes spontaneous emission proportional to pump intensity. The solution of this equation is: Is
AP0 g
exp Ž gl . y 1 4 ,
Ž 2.
where l is the length of the pumped stripe. To include wavelength dependence of the fluorescence amplification, we express the fluorescence quantum yield, f f , as w8x:
ff s
lb
Hl
Ef Ž l . d l ,
Ž 3.
a
where Ef Ž l. is the fluorescence quantum distribution, la and l b are the limits of the fluorescence spectrum. Considering the AP0 term in Eqs. 1 and 2, it is apparent that A must be proportional to the f f , if AP0 is to describe spontaneous emission proportional to pump intensity: A s Cf f ,
Ž 4a .
where C is a constant. We can therefore also write: A Ž l . s c Ž l . Ef Ž l . ,
Ž 4b .
where cŽ l. is the wavelength-dependent expression of C. By substitution of Eqs. 4a, 4b into Eq. 2, we can now write an equation describing ASE, which includes wavelength dependence: I Ž l. s
c Ž l . Ef Ž l . P0 g Ž l.
exp Ž g Ž l . l . y 1 4 ,
Ž 5.
where g Ž l. is the net gain per unit length at wavelength l. The exponential term indicates that small changes in g Ž l. can produce appreciable changes in output intensity over sufficient path lengths. This narrows the fluorescence output as some wavelengths are amplified to a greater extent than others.
Fig. 1. Chemical structures of the model compounds under study. 1,4-BisŽ4X-diphenylaminostyryl.-benzene, referred to as SP35, 1,4-bis-Ž acyanostyryl.-2,5-dimethoxybenzene, referred to as G33.
As the total fluorescence intensity, I, in Eq. 2 is equal to the integrated intensity of the fluorescence spectrum, i.e., Is
lb
Hl
™ 0 Ýl I Ž l. ,
I Ž l . d l s lim
a
dl
we can write: I A Ý exp Ž g Ž l . l . .
Ž 6. Ž 7.
l
For g Ž l.l < 1, then the sum in Eq. 7 is approximately equal to expŽ g max Ž l. l .. Therefore, the net gain, g, for the integrated intensity as expressed in Eq. 2 is very close to the maximum gain value of g Ž l. in Eq. 7. We can now compare any single-pass fluorescence amplification experiment, whether measured for discrete wavelengths, or for the total fluorescence intensity. Neither Ef Ž l. nor cŽ l. is constant under conditions where stimulated emission is important. As fluorescence amplification becomes observable, this increases the apparent emission efficiency along the pumped stripe for the amplified wavelengths, but decreases the efficiency at all wavelengths in all other emission directions. For our purposes, we assume that for large values of g Ž l., the exponential term in Eq. 5 dominates over changes in Ef Ž l. or cŽ l. and general features of the optical gain spectra will not be affected. To obtain an optical gain spectrum, we measure emission spectra from the sample as a function of pump length and apply a nonlinear least squares routine to each wavelength, adjusting g Ž l. and AP0 to obtain the best fit to Eq. 5. 3. Experiment
Table 1 Optical properties of the model compounds under study in solid polyŽstyrene. host. F is the figure of merit described in this report. Absorption, lma x Žnm. Emission, lma x Žnm. Quantum yield, f f Gain, l ma x Žnm. g ma x Žcmy1 . g Ž l. FWHM Žnm. F Žnm cm Jy1 .
SP35
G33
390 470, 490 0.85 487 19.8 12 4.2=10 8
410 505 0.65 510 15.5 15 4.1=10 8
The samples are produced by spin coating from a polyŽstyrene.rortho-xylene solution, doped with 2 wt.% of the model compounds, onto silicon wafers with a 1.5 mm thermal oxide overlayer. Refractive index and film thickness were measured with a prism film coupler w9x. Waveguide propagation losses at l s 632.8 nm are below 3.0 dB cmy1 , measured during prism coupling experiments. We note that the fluorescent intensity continues to increase for pump lengths in excess of 9 mm, despite the drop in both pump intensity Ždue to beam profile. and available excited states Ždepleted via stimulated emission.. This
K.P. Kretsch et al.r Synthetic Metals 111–112 (2000) 567–570
suggests that ground state re-absorption and scattering losses across the fluorescence band can be neglected for short pump lengths. The thickness of the polymer films were chosen to support two guided modes, TE 0 and TM 0 , at the fluorescence maximum of the samples, resulting in thickness of 0.45 mm for G33-doped films and 0.4 mm for SP35-doped films. All sample preparation and optical measurements were performed in air at room temperature. The samples were pumped using a frequency-tripled Nd:YAG laser, producing 0.9 mJ pulses of 35 ps duration at 355 nm. The beam profile at the sample is 30 mm long by 0.1 mm wide, with the center of the beam and the edge of the sample coincident. The maximum pulse intensity incident on the sample is 8.7 = 10 5 W cmy2 . The pump length is adjusted by means of a moveable shutter. Output fluorescence is detected using a cooled CCD spectrometer. The maximum pump length used for the calculations was 3 mm. The change in intensity at the center of the pump beam is small over this length and the pump intensity is assumed constant across this region.
molecules are excited by the pump pulse and that stimulated emission dominates. We estimate Nchr ; 10 18 cmy3 for both model compounds implying a minimum value of s SE,eff s 2 = 10y1 7 cm2 . This agrees with the value reported by Henari et al. w5x of s SE,eff s 5.5 = 10y1 7 cm2 Ž l s 514.5 nm. for G33 in toluene solution. On either side of the gain peak, there are regions exhibiting negative values of g Ž l.. We attribute excited singlet absorption to be principally responsible for the long wavelength negative values of g Ž l.. The short duration pump pulse allows us to neglect triplet–triplet absorption w8x. Similar spectral features are present in studies on other materials in a wide range of different experimental configurations w10–13x. The short wavelength negative g Ž l. values are attributed primarily to fluorescence output saturation via stimulated emission. To enable a comparison between different materials for use in laser devices, a figure of merit is useful. As tunability of potential devices will be important, we favour not only large net gains, but also large gain bandwidths, characterised by the full width at half maximum of the gain spectrum, f. We therefore suggest a figure of merit, F, of the form:
4. Results and discussion Fs Fig. 2 shows the optical gain spectra obtained for the doped polyŽstyrene. waveguides at the maximum incident pump intensity of 8.7 = 10 5 W cmy2 . Both SP35-and G33-doped samples exhibit a single well-defined peak in g Ž l., at l s 487 nm and 510 nm respectively, with G33 also exhibiting a weak shoulder near 520 nm. The maximum net gains obtained are g s 19.8 cmy1 for SP35 and g s 15.5 cmy1 for G33. The full width at half maximum for the gain spectra are 12 and 15 nm, respectively. By estimating the density of dopant molecules in the polyŽstyrene. matrix, we can set a lower limit on the effective stimulated emission cross-section, s S.E.,eff s s SE y sabs by using the relation: g s Nchr s SE ,eff ,
Ž 8.
where g is the net optical gain and Nchr is the chromophore number density. This assumes that all dopant
Fig. 2. Optical gain spectra produced using a nonlinear least squares fit to Eq. 5 at incident pump power of 8.7=10 5 W cmy2 . Solid circles are for SP35-doped waveguides, open circles are for G33-doped waveguides.
569
g max f Pabs
,
Ž 9.
where Pabs is the absorbed pump pulse energy density to produce g max , the maximum value of g Ž l.. We choose pulse energy density ŽJ cmy2 . over pump intensity ŽW cmy2 . to enable comparisons of measurements using different pump pulse widths. We estimate that 20% of the incident pump pulse is absorbed by the samples, including reflection from the Si substrate. This gives values of F s 4.2 = 10 8 nm cm Jy1 and F s 4.1 = 10 8 nm cm Jy1 for SP35- and G33-doped films, respectively. It is instructive to compare the figure of merit of our polymer waveguides with those obtained from previous optical gain studies. We estimate a figure of merit for ladder type polyŽ para-phenylene., LPPP 1 w14x, to be F s 8.3 = 10 7 nm cm Jy1. For BuEH-PPV w15x, we estimate F s 3.3 = 10 8 nm cm Jy1 , despite large optical losses 2 . For GaInNrGaN double heterostructures w13x, we estimate F s 4.7 = 10 7 nm cm Jy1 , and for AlGaNrGaN DHs w16x, F s 2.7 = 10 7 nm cm Jy1 . This suggests that organic materials may be suitable candidates for optically pumped
1 For LPPP, we use g s 50 cmy1 at 22 kW cmy2 at 10 Hz, 6 ns pulse width from Ref. w17x, f f 20 nm from Fig. 2 of Ref. w13x. We estimate that 90% of the pump pulse is absorbed from Ref. w17x. The gain bandwidth may be an overestimate. 2 Ref. w15x reports linear optical losses of 44 cmy1 for Bu-EH-PPV waveguides, suggesting 99% loss over a 1 mm path length. If these losses are primarily scattering-induced, the gain values obtained by McGehee et al. w15x may be inflated by regenerative feedback via the scattering process.
570
K.P. Kretsch et al.r Synthetic Metals 111–112 (2000) 567–570
laser media and can surpass semiconductors for optically pumped laser applications. 5. Conclusion We have presented a wavelength-dependent model of amplified spontaneous emission and applied the model to measurements of polyŽstyrene. waveguides doped with polymeric model compounds. Both dopant molecules exhibit large optical gains for small input pulse energies. We suggest a figure of merit to enable a comparison of materials for use in optically pumped laser. This may extend the use of single-pass fluorescence amplification experiments in determining the potential of a material as a laser medium. Acknowledgements The authors wish to thank Dr. David Gray ŽTCD. for assistance with the Nd:YAG laser during the course of this work. This work is funded by European Union ESPRIT project a28580,‘‘LUPO’’. References w1x H. Rost, A. Teuschel, S. Pfeiffer, H.H. Horhold, Synth. Met. 84 ¨ Ž1997. 269.
w2x S. Pfeiffer, H.-H. Horhold, H. Boerner, N. Nikol, W. Busselt, Proc. ¨ SPIE 3476 Ž1998. 258. w3x E. Birckner, U.-W. Grummt, H. Rost, S. Pfeiffer, H. Tillmann, H.-H. Horhold, J. Fluoresc. 8 Ž1998. 73. ¨ w4x F.Z. Henari, K.P. Kretsch, W.J. Blau, H. Rost, S. Pfeiffer, A. Teuschel, H. Tillman, H.H. Horhold, Photon Sci. News 4 Ž1. Ž1998. 5. w5x F.Z. Henari, H. Manaa, K.P. Kretsch, W.J. Blau, H. Rost, S. Pfeiffer, A. Teuschel, H. Tillman, H.H. Horhold, Chem. Phys. Lett. ¨ Ž1999. accepted for publication. w6x K.L. Shaklee, R.F. Leheny, Appl. Phys. Lett. 18 Ž1971. 475. w7x Y. Sorek, R. Reisfeld, I. Finkelstein, S. Ruschin, Appl. Phys. Lett. 66 Ž10. Ž1995. 1169. w8x W. Holzer, A. Penzkofer, S.-H. Gong, W. Blau, A. Davey, Opt. Quantum Electron. 29 Ž1997. 713. w9x R. Ulrich, R. Torge, Appl. Opt. 12 Ž12. Ž1973. 2901. w10x A. Haugeneder, M. Neges, C. Kallinger, W. Spirkl, U. Lemmer, J. Feldmann, M.-C. Amann, U. Scherf, J. Appl. Phys. 85 Ž2. Ž1999. 1124. w11x B. Schweitzer, G. Wegmann, H. Giessen, D. Hertel, H. Bassler, R.F. ¨ Mahrt, U. Scherf, K. Muller, Appl. Phys. Lett. 72 Ž23. Ž1998. 2933. ¨ w12x E. Bourdin, A. Davey, W. Blau, S. Delysse, J.M. Nunzi, Chem. Phys. Lett. 275 Ž1997. 103. w13x G. Frankowsky, F. Steuber, V. Harle, F. Scholz, A. Hangleiter, ¨ Appl. Phys. Lett. 68 Ž26. Ž1996. 3746. w14x C. Zenz, W. Graupner, S. Tasch, G. Leising, K. Iskra, J. Flieser, T. Neger, J. Appl. Phys. 84 Ž10. Ž1998. 5445. w15x M. McGehee, R. Gupta, S. Veenstra, E.K. Miller, M.A. Diaz-Garcia, A.J. Heeger, Phys. Rev. B 58 Ž11. Ž1998. 7035. w16x S.T. Kim, H. Amano, I. Akasaki, N. Koide, Appl. Phys. Lett. 64 Ž12. Ž1994. 1535.
