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Semiconducting polyfluorenes as materials for solid-state polymer lasers across the visible spectrum
Synthetic Metals 140 (2004) 117–120
Semiconducting polyfluorenes as materials for solid-state polymer lasers across the visible spectrum夽 Ruidong Xia, George Heliotis, Donal D.C. Bradley∗ Ultrafast Photonics Collaboration, Blackett Laboratory, Experimental Solid State Group & Centre for Electronic Materials and Devices, Imperial College London, London SW7 2BZ, UK Received 8 February 2003; received in revised form 14 February 2003
Abstract We report a detailed study of the gain properties of three polyfluorenes: poly(9,9-dioctylfluorene-co-9,9-di(4-methoxyphenyl)fluorene (F8DP), poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and a Dow proprietary copolymer, Dow Red F. The emission spectra of these conjugated polymers span the full visible spectrum from 400 to 800 nm. We observe amplified spontaneous emission (ASE) with peak wavelengths at 452, 576 and 685 nm for F8DP, F8BT and Red F, respectively. Low stimulated emission thresholds in the region of 0.1–0.45 J per pulse are demonstrated for planar asymmetric waveguides. Variable stripe length gain measurements at the peak ASE wavelengths show large net gains up to 66 cm−1 . Very low loss coefficients, in the range of 3.2–14.8 cm−1 , were also found. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Polyfluorenes; Asymmetric waveguides; Amplified spontaneous emission; Optical gain properties
1. Introduction Semiconducting (conjugated) polymers are now attracting considerable attention as a new materials class for use in electronics and optoelectronics. Following encouraging successes in the commercialization of electroluminescent displays, there is strong interest in developing these materials for use as gain media in lasers and optical amplifiers [1,2]. Fluorene-based polymers (polyfluorenes) are very attractive conjugated polymers for such applications, due to their high photoluminescence (PL) and electroluminescence efficiencies, low stimulated emission thresholds and high stability in air [3–7]. These features may eventually allow such organic laser sources to compete with inorganic semiconductor lasers in a number of applications such as optical communications and optical sensing. Here, we present a detailed study of the properties of a variety of semiconducting polyfluorenes as gain media. We demonstrate amplified spontaneous emission (ASE) in the blue emitting poly(9,9-dioctylfluorene-co-9,9-di(4-metho夽 The Publisher regrets that this paper was not published as part of the special issue ‘Proceedings of the Fifth International Topical Conference on Optical Probes of Conjugated Polymers and Organic & Inorganic Nanostructures, Venice, February 9–14th, 2003 [SYNMET 139, No. 3 (2003)]. ∗ Corresponding author. Tel.: +44-20-75947612; fax: +44-20-75813817. E-mail address: [email protected] (D.D.C. Bradley).
xyphenyl)fluorene (F8DP), the green-yellow emitting poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and a Dow proprietary red emission copolymer, known as Dow Red F. The characteristics of the gain narrowing of planar asymmetric waveguides are measured as a function of excitation wavelength and intensity and film thickness. Low ASE thresholds (∼600 W/cm2 ) are demonstrated. Gain and loss measurements at the peak ASE wavelengths show that the waveguides can exhibit large net gain and have very low loss coefficients, making these materials attractive for solid-state lasers with wavelengths that span the entire visible spectrum.
2. Experimental The conjugated polymers were synthesized at The Dow Chemical Company. Planar waveguides were made by spin casting 100–350 nm thick films from 20 mg/ml toluene solutions onto polished synthetic quartz substrates. The absorption (dashed lines) and PL (dottted lines) of the three polymers are shown in Fig. 1. It can be seen that the absorption and emission bands are substantially separated in wavelength, which can lead to reduced self-absorption at the emission wavelengths—a potentially important factor for achieving high gains and low thresholds. The PL emission spectra of these three polymers span the complete visible
0379-6779/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00235-2
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range from 400 to 800 nm. To induce stimulated emission, the samples were photopumped using a Q-switched Nd:YAG laser pumped type-II BBO optical parametric oscillator, that delivered 10 ns pulses at a repetition rate of 10 Hz. The pump beam was focused into a stripe of approximately 400 m × 4 mm. The pump wavelengths were chosen to be close to the absorption maxima of individual peaks in the absorption spectra of each polymer, i.e. λex = 355 nm for F8DP; 337 and 440 nm for F8BT; 337, 440 and 532 nm for Dow Red F. The energy of the pulses was controlled using a set of calibrated neutral density filters. The edge emission spectra were collected using a fiber-coupled grating spectrometer equipped with a charge-coupled device (CCD) detector. A full description of the measurement system can be found elsewhere [8,9].
3. Results and discussions We have investigated the spectral emission characteristics of the polymer waveguides as a function of the excitation energies at various excitation wavelengths. Only broad spontaneous emission spectra were observed at low excitation energies. However, at sufficiently high excitation intensities, the spontaneously emitted photons that are waveguided along the stripe-shaped gain region are amplified via stimulated emission (or ASE). In this process, the emission spectrum exhibits a sudden narrowing in linewidth and increase in intensity. Fig. 1 (solid lines) presents the ASE spectra of F8DP, F8BT and Dow Red F above threshold. Excitation of the different peaks (or shoulders) that appear in the absorption spectra of F8BT and Dow Red F gave no detectable wavelength shift of the ASE peaks for these two polymer waveguides. The ASE thresholds, however, exhibit a strong excitation wavelength dependence. Fig. 2 shows the pump–pulse energy dependence of the ASE from F8BT at the ASE wavelength for pump wavelengths of 337 (䊉) and 440 nm (䉱), respectively. The inset shows the corresponding variation of the emission linewidth. If the ASE thresh-
Fig. 1. Normalized absorption (dashed lines), photoluminescence (dotted lines) and ASE (solid lines) spectra from (i) F8DP, (ii) F8BT and (iii) Dow Red F slab waveguides.
Fig. 2. Edge emitted output intensity from F8BT at the ASE wavelength as a function of excitation energy. Data are reported following excitation at λex = 337 (䊉) and 440 nm (䉱). The emission FWHM linewidth dependence is inset.
old is defined as the pump intensity at which the emission FWHM is reduced to half its value at low pump power (PL), then, when pumping the sample at 337 nm, the ASE threshold is Eth = 1.1 J. The threshold reduced to 0.45 J when the pump wavelength was shifted to 440 nm. Note that the absorption strength of F8BT is almost the same at these wavelengths as shown in Fig. 1 (ii). This may imply that energy transfer between the states responsible for the absorption peaks at 330 and 455 nm (dashed line in Fig. 1 (ii)) is incomplete. According to Eth = nex hc/λex (where c is the speed of light in vacuum and h is Planck’s constant) and accounting for the difference in wavelength, the threshold difference suggests a 45% loss in the effective number of photons, nex , during the non-radiative transfer from the peak at 330 nm to the peak at 455 nm. In a very similar way, the lowest ASE threshold for Dow Red F waveguides was obtained by pumping the absorption shoulder at 550 nm. Another interesting phenomenon deserving special comment is the large reduction in the residual spontaneous emission at λ = 650 nm when the pump wavelength for Red F is shifted from 337 to 532 nm. It is also worth noting that the ASE threshold measured in air exhibits exactly the same value as that measured in a 10−2 Torr vacuum chamber. This can be interpreted as a result of the high thermal and oxidative stability of the polyfluorenes studied here. The results of the ASE investigations for the three fluorene-based polymer are summarized in Table 1, which includes data for the ASE wavelength, FWHM linewidth and lowest threshold pump energy together with the corresponding pump wavelength. We have also studied the ASE behavior of the polymer waveguides as a function of film thickness. For thicknesses over 800 nm, the films showed no spectral narrowing at any pump energy up to the damage threshold. It is possible that scattering at grain boundaries results in high optical losses, that prevent sufficient net gain from being achieved.
R. Xia et al. / Synthetic Metals 140 (2004) 117–120
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Table 1 A summary of the ASE properties of the semiconducting fluorene polymers F8DP, F8BT and Red F Materials
λpump (nm)
λASE (nm)
Ith (kW/cm2 )
ASE FWHM (nm)
gnet,max (cm−1 )
σ (×10−16 cm2 )
α (cm−1 )
F8DP F8BT Red F
355 440 532
452 576 685
0.6 2.8 2.8
5 8 10
66 22 24
6.57 7.09 7.73
14.8 7.6 3.2
λASE and Ith are the ASE wavelength and pump threshold intensity at a pump wavelength of λpump ; ASE FWHM is the ASE linewidth well above threshold; gnet,max is the maximum measured net gain coefficient; σ is the calculated gain cross section; and α is the waveguide loss coefficient.
However, the waveguides need to be of a minimum thickness in order that there is a propagating mode at the peak of the gain spectrum [10]. The net gains of the materials were investigated using the variable stripe length (VSL) technique [11,12]. Fig. 3 shows the emitted light intensity from a F8BT waveguide as a function of excitation length for different pump energies at a pump wavelength of 440 nm. To calculate the gain coefficients, we have fitted the data to an exponential function. The inset in Fig. 3 shows the net gain coefficients as a function of pump energy for F8BT waveguides at a pump wavelength of 337 and 440 nm. The net gain increases more or less linearly up to a saturation value when plotted on a logarithmic pump energy scale. Higher gain is obtained at a pump wavelength of 440 nm for each pump energy. The maximum gain of the F8BT waveguides we have studied is about 22 cm−1 at a pump energy of 7.78 J. The net gain coefficients for the other polymer waveguides were measured in the same way. Their maximum net gain coefficients are listed in Table 1. We also estimated the gain cross section, σ, from the measured saturation energy density ES /A, using the expression (ES /A) = hc/λσ, where A is the pump stripe area, and λ is
the ASE wavelength. The order of magnitude value for the gain cross section is found to be σ ≈ 7 × 10–16 cm2 for all of the polymers studied here (Table 1). The accuracy of this value is largely limited by the measurement error in the saturation excitation density. One of the keys to reducing the pump intensity needed for net gain is to reduce the loss, that arises from scattering and self-absorption. The large spectral shift between absorption and emission wavelength lowers the self-absorption losses and results in low ASE thresholds for these polymers. Although our films appear to be quite homogeneous, it is possible that there are inhomogeneities at the submicrometer length scale and that these cause scattering. Therefore, to more fully characterize the conjugated polymers as potential laser materials, we also measured the waveguide losses. In these experiments, we kept the length of the pump stripe constant and moved the pump stripe away from the edge of the sample. Assuming that the emission from the end of the pump stripe I0 is constant, the emission from the edge of the sample should decrease as a result of waveguide losses. The output intensity can be expressed as I = I0 exp(−αx), where x is the length of the unpumped region between the end of the pump stripe and the edge of the sample and α is the loss coefficient. The intensity of the emission can be measured as a function of x. By curve fitting these data to an exponential, we obtained the loss coefficient of the individual polymer waveguides. These are also listed in Table 1. The very small values observed are considered to be indicative of the excellent film forming properties of the polymers that we have studied.
4. Conclusions
Fig. 3. Excitation stripe length dependence of the ASE intensity in a F8BT waveguide at pump wavelength λex = 440 nm for pump energies of 1.0 (䊉), 2.0 (䊐), 4.0 (䉱), 8.0 (䊊) and 20.0 J (䉬). Inset are the net gain coefficients g plotted as a function of pump energy E and for pump wavelengths of 337 (䊉) and 440 nm (䉱).
In summary, we have presented a detailed study of the gain properties of three polyfluorenes. Low threshold light amplification was demonstrated in the blue, green and red spectral ranges via ASE in optically pumped planar asymmetric waveguides. Gain and loss measurements at the peak ASE wavelengths show large net gains, 22 cm−1 ≤ g ≤ 66 cm−1 with corresponding gain cross sections, 6.6 × 10–16 cm2 ≤ σ ≤ 7.8 × 10–16 cm2 and low losses, 15 cm−1 ≥ α ≥ 3 cm−1 . The low thresholds for stimulated emission in these polymers, coupled with their broad PL spectra, offer great promise for the fabrication of low cost, highly tuneable, solid-state lasers.
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R. Xia et al. / Synthetic Metals 140 (2004) 117–120
Acknowledgements The authors thank The Dow Chemical Company for providing the three polymers used in these experiments. We are grateful to the United Kingdom Engineering and Physical Sciences Research Council (Ultrafast Photonics Collaboration) for financial support and we thank Dr. Yanbing Hou, Dr. Graham Turnbull and Prof. Ifor Samuel for useful discussions and Dr. Mattijs Koeberg for experimental assistance.
References [1] N. Tessler, Adv. Mater. 11 (1999) 363. [2] M.D. McGehee, A.J. Heeger, Adv. Mater. 12 (2000) 1655.
[3] R.B. Fletcher, D.G. Lidzey, D.D.C. Bradley, M. Bernius, S. Walker, Appl. Phys. Lett. 77 (2000) 1262. [4] M. Redecker, D.D.C. Bradley, M. Inbasekaran, E.P. Woo, Appl. Phys. Lett. 73 (1998) 1565. [5] M. Redecker, D.D.C. Bradley, M. Inbasekaran, W.W. Wu, E.P. Woo, Adv. Mater. 11 (1999) 241. [6] A.J. Campbell, D.D.C. Bradley, H. Antoniadis, Appl. Phys. Lett. 79 (2001) 2133. [7] T. Virgili, D.G. Lidzey, D.D.C. Bradley, Adv. Mater. 12 (2000) 58. [8] G. Heliotis, D.D.C. Bradley, G.A. Turnbull, I.D.W. Samuel, Appl. Phys. Lett. 81 (2002) 415. [9] R. Xia, G. Heliotis, D.D.C. Bradley, Appl. Phys. Lett. 82 (2003) 3599. [10] M. Marcuse, Theory of Dielectric Waveguides, Academic Press, New York, 1974 (Chapter 1). [11] K.L. Shaklee, R.F. Leheny, Appl. Phys. Lett. 18 (1971) 475. [12] M.D. McGehee, R. Gupta, S. Veenstra, E.K. Miller, M.A. Diaz-Garcia, A.J. Heeger, Phys. Rev. B 58 (1998) 7035.
Semiconducting polyfluorenes as materials for solid-state polymer lasers across the visible spectrum夽 Ruidong Xia, George Heliotis, Donal D.C. Bradley∗ Ultrafast Photonics Collaboration, Blackett Laboratory, Experimental Solid State Group & Centre for Electronic Materials and Devices, Imperial College London, London SW7 2BZ, UK Received 8 February 2003; received in revised form 14 February 2003
Abstract We report a detailed study of the gain properties of three polyfluorenes: poly(9,9-dioctylfluorene-co-9,9-di(4-methoxyphenyl)fluorene (F8DP), poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and a Dow proprietary copolymer, Dow Red F. The emission spectra of these conjugated polymers span the full visible spectrum from 400 to 800 nm. We observe amplified spontaneous emission (ASE) with peak wavelengths at 452, 576 and 685 nm for F8DP, F8BT and Red F, respectively. Low stimulated emission thresholds in the region of 0.1–0.45 J per pulse are demonstrated for planar asymmetric waveguides. Variable stripe length gain measurements at the peak ASE wavelengths show large net gains up to 66 cm−1 . Very low loss coefficients, in the range of 3.2–14.8 cm−1 , were also found. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Polyfluorenes; Asymmetric waveguides; Amplified spontaneous emission; Optical gain properties
1. Introduction Semiconducting (conjugated) polymers are now attracting considerable attention as a new materials class for use in electronics and optoelectronics. Following encouraging successes in the commercialization of electroluminescent displays, there is strong interest in developing these materials for use as gain media in lasers and optical amplifiers [1,2]. Fluorene-based polymers (polyfluorenes) are very attractive conjugated polymers for such applications, due to their high photoluminescence (PL) and electroluminescence efficiencies, low stimulated emission thresholds and high stability in air [3–7]. These features may eventually allow such organic laser sources to compete with inorganic semiconductor lasers in a number of applications such as optical communications and optical sensing. Here, we present a detailed study of the properties of a variety of semiconducting polyfluorenes as gain media. We demonstrate amplified spontaneous emission (ASE) in the blue emitting poly(9,9-dioctylfluorene-co-9,9-di(4-metho夽 The Publisher regrets that this paper was not published as part of the special issue ‘Proceedings of the Fifth International Topical Conference on Optical Probes of Conjugated Polymers and Organic & Inorganic Nanostructures, Venice, February 9–14th, 2003 [SYNMET 139, No. 3 (2003)]. ∗ Corresponding author. Tel.: +44-20-75947612; fax: +44-20-75813817. E-mail address: [email protected] (D.D.C. Bradley).
xyphenyl)fluorene (F8DP), the green-yellow emitting poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and a Dow proprietary red emission copolymer, known as Dow Red F. The characteristics of the gain narrowing of planar asymmetric waveguides are measured as a function of excitation wavelength and intensity and film thickness. Low ASE thresholds (∼600 W/cm2 ) are demonstrated. Gain and loss measurements at the peak ASE wavelengths show that the waveguides can exhibit large net gain and have very low loss coefficients, making these materials attractive for solid-state lasers with wavelengths that span the entire visible spectrum.
2. Experimental The conjugated polymers were synthesized at The Dow Chemical Company. Planar waveguides were made by spin casting 100–350 nm thick films from 20 mg/ml toluene solutions onto polished synthetic quartz substrates. The absorption (dashed lines) and PL (dottted lines) of the three polymers are shown in Fig. 1. It can be seen that the absorption and emission bands are substantially separated in wavelength, which can lead to reduced self-absorption at the emission wavelengths—a potentially important factor for achieving high gains and low thresholds. The PL emission spectra of these three polymers span the complete visible
0379-6779/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00235-2
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R. Xia et al. / Synthetic Metals 140 (2004) 117–120
range from 400 to 800 nm. To induce stimulated emission, the samples were photopumped using a Q-switched Nd:YAG laser pumped type-II BBO optical parametric oscillator, that delivered 10 ns pulses at a repetition rate of 10 Hz. The pump beam was focused into a stripe of approximately 400 m × 4 mm. The pump wavelengths were chosen to be close to the absorption maxima of individual peaks in the absorption spectra of each polymer, i.e. λex = 355 nm for F8DP; 337 and 440 nm for F8BT; 337, 440 and 532 nm for Dow Red F. The energy of the pulses was controlled using a set of calibrated neutral density filters. The edge emission spectra were collected using a fiber-coupled grating spectrometer equipped with a charge-coupled device (CCD) detector. A full description of the measurement system can be found elsewhere [8,9].
3. Results and discussions We have investigated the spectral emission characteristics of the polymer waveguides as a function of the excitation energies at various excitation wavelengths. Only broad spontaneous emission spectra were observed at low excitation energies. However, at sufficiently high excitation intensities, the spontaneously emitted photons that are waveguided along the stripe-shaped gain region are amplified via stimulated emission (or ASE). In this process, the emission spectrum exhibits a sudden narrowing in linewidth and increase in intensity. Fig. 1 (solid lines) presents the ASE spectra of F8DP, F8BT and Dow Red F above threshold. Excitation of the different peaks (or shoulders) that appear in the absorption spectra of F8BT and Dow Red F gave no detectable wavelength shift of the ASE peaks for these two polymer waveguides. The ASE thresholds, however, exhibit a strong excitation wavelength dependence. Fig. 2 shows the pump–pulse energy dependence of the ASE from F8BT at the ASE wavelength for pump wavelengths of 337 (䊉) and 440 nm (䉱), respectively. The inset shows the corresponding variation of the emission linewidth. If the ASE thresh-
Fig. 1. Normalized absorption (dashed lines), photoluminescence (dotted lines) and ASE (solid lines) spectra from (i) F8DP, (ii) F8BT and (iii) Dow Red F slab waveguides.
Fig. 2. Edge emitted output intensity from F8BT at the ASE wavelength as a function of excitation energy. Data are reported following excitation at λex = 337 (䊉) and 440 nm (䉱). The emission FWHM linewidth dependence is inset.
old is defined as the pump intensity at which the emission FWHM is reduced to half its value at low pump power (PL), then, when pumping the sample at 337 nm, the ASE threshold is Eth = 1.1 J. The threshold reduced to 0.45 J when the pump wavelength was shifted to 440 nm. Note that the absorption strength of F8BT is almost the same at these wavelengths as shown in Fig. 1 (ii). This may imply that energy transfer between the states responsible for the absorption peaks at 330 and 455 nm (dashed line in Fig. 1 (ii)) is incomplete. According to Eth = nex hc/λex (where c is the speed of light in vacuum and h is Planck’s constant) and accounting for the difference in wavelength, the threshold difference suggests a 45% loss in the effective number of photons, nex , during the non-radiative transfer from the peak at 330 nm to the peak at 455 nm. In a very similar way, the lowest ASE threshold for Dow Red F waveguides was obtained by pumping the absorption shoulder at 550 nm. Another interesting phenomenon deserving special comment is the large reduction in the residual spontaneous emission at λ = 650 nm when the pump wavelength for Red F is shifted from 337 to 532 nm. It is also worth noting that the ASE threshold measured in air exhibits exactly the same value as that measured in a 10−2 Torr vacuum chamber. This can be interpreted as a result of the high thermal and oxidative stability of the polyfluorenes studied here. The results of the ASE investigations for the three fluorene-based polymer are summarized in Table 1, which includes data for the ASE wavelength, FWHM linewidth and lowest threshold pump energy together with the corresponding pump wavelength. We have also studied the ASE behavior of the polymer waveguides as a function of film thickness. For thicknesses over 800 nm, the films showed no spectral narrowing at any pump energy up to the damage threshold. It is possible that scattering at grain boundaries results in high optical losses, that prevent sufficient net gain from being achieved.
R. Xia et al. / Synthetic Metals 140 (2004) 117–120
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Table 1 A summary of the ASE properties of the semiconducting fluorene polymers F8DP, F8BT and Red F Materials
λpump (nm)
λASE (nm)
Ith (kW/cm2 )
ASE FWHM (nm)
gnet,max (cm−1 )
σ (×10−16 cm2 )
α (cm−1 )
F8DP F8BT Red F
355 440 532
452 576 685
0.6 2.8 2.8
5 8 10
66 22 24
6.57 7.09 7.73
14.8 7.6 3.2
λASE and Ith are the ASE wavelength and pump threshold intensity at a pump wavelength of λpump ; ASE FWHM is the ASE linewidth well above threshold; gnet,max is the maximum measured net gain coefficient; σ is the calculated gain cross section; and α is the waveguide loss coefficient.
However, the waveguides need to be of a minimum thickness in order that there is a propagating mode at the peak of the gain spectrum [10]. The net gains of the materials were investigated using the variable stripe length (VSL) technique [11,12]. Fig. 3 shows the emitted light intensity from a F8BT waveguide as a function of excitation length for different pump energies at a pump wavelength of 440 nm. To calculate the gain coefficients, we have fitted the data to an exponential function. The inset in Fig. 3 shows the net gain coefficients as a function of pump energy for F8BT waveguides at a pump wavelength of 337 and 440 nm. The net gain increases more or less linearly up to a saturation value when plotted on a logarithmic pump energy scale. Higher gain is obtained at a pump wavelength of 440 nm for each pump energy. The maximum gain of the F8BT waveguides we have studied is about 22 cm−1 at a pump energy of 7.78 J. The net gain coefficients for the other polymer waveguides were measured in the same way. Their maximum net gain coefficients are listed in Table 1. We also estimated the gain cross section, σ, from the measured saturation energy density ES /A, using the expression (ES /A) = hc/λσ, where A is the pump stripe area, and λ is
the ASE wavelength. The order of magnitude value for the gain cross section is found to be σ ≈ 7 × 10–16 cm2 for all of the polymers studied here (Table 1). The accuracy of this value is largely limited by the measurement error in the saturation excitation density. One of the keys to reducing the pump intensity needed for net gain is to reduce the loss, that arises from scattering and self-absorption. The large spectral shift between absorption and emission wavelength lowers the self-absorption losses and results in low ASE thresholds for these polymers. Although our films appear to be quite homogeneous, it is possible that there are inhomogeneities at the submicrometer length scale and that these cause scattering. Therefore, to more fully characterize the conjugated polymers as potential laser materials, we also measured the waveguide losses. In these experiments, we kept the length of the pump stripe constant and moved the pump stripe away from the edge of the sample. Assuming that the emission from the end of the pump stripe I0 is constant, the emission from the edge of the sample should decrease as a result of waveguide losses. The output intensity can be expressed as I = I0 exp(−αx), where x is the length of the unpumped region between the end of the pump stripe and the edge of the sample and α is the loss coefficient. The intensity of the emission can be measured as a function of x. By curve fitting these data to an exponential, we obtained the loss coefficient of the individual polymer waveguides. These are also listed in Table 1. The very small values observed are considered to be indicative of the excellent film forming properties of the polymers that we have studied.
4. Conclusions
Fig. 3. Excitation stripe length dependence of the ASE intensity in a F8BT waveguide at pump wavelength λex = 440 nm for pump energies of 1.0 (䊉), 2.0 (䊐), 4.0 (䉱), 8.0 (䊊) and 20.0 J (䉬). Inset are the net gain coefficients g plotted as a function of pump energy E and for pump wavelengths of 337 (䊉) and 440 nm (䉱).
In summary, we have presented a detailed study of the gain properties of three polyfluorenes. Low threshold light amplification was demonstrated in the blue, green and red spectral ranges via ASE in optically pumped planar asymmetric waveguides. Gain and loss measurements at the peak ASE wavelengths show large net gains, 22 cm−1 ≤ g ≤ 66 cm−1 with corresponding gain cross sections, 6.6 × 10–16 cm2 ≤ σ ≤ 7.8 × 10–16 cm2 and low losses, 15 cm−1 ≥ α ≥ 3 cm−1 . The low thresholds for stimulated emission in these polymers, coupled with their broad PL spectra, offer great promise for the fabrication of low cost, highly tuneable, solid-state lasers.
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R. Xia et al. / Synthetic Metals 140 (2004) 117–120
Acknowledgements The authors thank The Dow Chemical Company for providing the three polymers used in these experiments. We are grateful to the United Kingdom Engineering and Physical Sciences Research Council (Ultrafast Photonics Collaboration) for financial support and we thank Dr. Yanbing Hou, Dr. Graham Turnbull and Prof. Ifor Samuel for useful discussions and Dr. Mattijs Koeberg for experimental assistance.
References [1] N. Tessler, Adv. Mater. 11 (1999) 363. [2] M.D. McGehee, A.J. Heeger, Adv. Mater. 12 (2000) 1655.
[3] R.B. Fletcher, D.G. Lidzey, D.D.C. Bradley, M. Bernius, S. Walker, Appl. Phys. Lett. 77 (2000) 1262. [4] M. Redecker, D.D.C. Bradley, M. Inbasekaran, E.P. Woo, Appl. Phys. Lett. 73 (1998) 1565. [5] M. Redecker, D.D.C. Bradley, M. Inbasekaran, W.W. Wu, E.P. Woo, Adv. Mater. 11 (1999) 241. [6] A.J. Campbell, D.D.C. Bradley, H. Antoniadis, Appl. Phys. Lett. 79 (2001) 2133. [7] T. Virgili, D.G. Lidzey, D.D.C. Bradley, Adv. Mater. 12 (2000) 58. [8] G. Heliotis, D.D.C. Bradley, G.A. Turnbull, I.D.W. Samuel, Appl. Phys. Lett. 81 (2002) 415. [9] R. Xia, G. Heliotis, D.D.C. Bradley, Appl. Phys. Lett. 82 (2003) 3599. [10] M. Marcuse, Theory of Dielectric Waveguides, Academic Press, New York, 1974 (Chapter 1). [11] K.L. Shaklee, R.F. Leheny, Appl. Phys. Lett. 18 (1971) 475. [12] M.D. McGehee, R. Gupta, S. Veenstra, E.K. Miller, M.A. Diaz-Garcia, A.J. Heeger, Phys. Rev. B 58 (1998) 7035.