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Significant improvements in the optical gain properties of oriented liquid crystalline conjugated polymer films
Synthetic Metals 155 (2005) 274–278
Significant improvements in the optical gain properties of oriented liquid crystalline conjugated polymer films Ruidong Xia, Mariano Campoy-Quiles, George Heliotis, Paul Stavrinou, Katharine S. Whitehead, Donal D.C. Bradley ∗ Ultrafast Photonics Collaboration, Experimental Solid State Group, Blackett Laboratory, Imperial College London, London SW7 2BZ, UK Available online 2 November 2005
Abstract We report significant improvements in the optical gain properties of highly oriented films of the fluorene-based conjugated polymer poly(9,9dioctylfluorene-co-benzothiadiazole) (F8BT). We demonstrate amplified spontaneous emission (ASE) in optically pumped aligned F8BT planar asymmetric waveguides. The influence of the polymer chain orientation and the excitation polarisation on the optical properties of the materials has been investigated. By selecting specific optimised excitation configurations, high net gain coefficients can be obtained, namely <47 cm−1 compared with <25 cm−1 for standard spin-coated films. The loss coefficients also reduce from α = 7.6 cm−1 (spin-coated film) to 0.9 cm−1 (aligned film). Ellipsometry allows determination of the optical constants and reveals that the refractive indices of the materials are also greatly affected by the orientation of the polymer chains. © 2005 Elsevier B.V. All rights reserved. Keywords: Polyfluorenes; Oriented polymer; Waveguides; Amplified spontaneous emission; Laser; Optical gain properties
1. Introduction Semiconducting polymers have attracted considerable interest as novel gain media for lasers and optical amplifiers due to their high photoluminescence quantum efficiencies (PLQE), large stimulated emission cross-sections and chemically tuneable emission wavelengths [1–3]. Optically pumped lasers have been demonstrated for a wide range of such materials and for several different resonator configurations [4,5]. However, electrically pumped diode lasers fabricated from organic semiconductors have not yet been realised due to the relatively low charge carrier mobilities that act against attainment of the high current densities required to reach threshold conditions [6]. Considerable efforts are consequently being made to improve the intrinsic carrier mobilities in these materials and to increase net optical gain and decrease losses. Poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT) is a promising green-light-emitting polyfluorene copolymer with good electron transport capabilities (the electron mobility is of the order of 10−3 cm2 /V s) [7].
∗
Corresponding author. Tel.: +44 20 75947612; fax: +44 20 75813817. E-mail address: [email protected] (D.D.C. Bradley).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.09.004
Our previous studies have demonstrated highly efficient stimulated emission via amplified spontaneous emission (ASE) in F8BT planar asymmetric polymer waveguides [8]. The good spectral overlap of F8BT emission and Dow Red F absorption allows for efficient energy transfer, and therefore blends of these two materials result in a significant reduction of the pump pulse energy for gain narrowing [9]. In this work, we report a further increase in ASE efficiency through orientation of the F8BT chains. It has been reported that oriented polymer films can exhibit a more than one order of magnitude increase in the charge carrier mobility relative to spin-coated films, thus offering simpler access to high current density devices [10]. However, there is little quantitative information on how the chain alignment affects the gain characteristics and whether the gain properties of a particular material can be enhanced through chain orientation [11–13]. In this paper, we demonstrate stimulated emission in waveguides containing F8BT with a high degree of chain orientation, and investigate the influence that the polymer chain alignment has on the gain properties of the waveguides. Our study shows that high net gain and low loss coefficients can be obtained by employing specific optimised excitation configurations. For ASE in the optimal configuration, the net gain for
R. Xia et al. / Synthetic Metals 155 (2005) 274–278
aligned F8BT increases from the 25 cm−1 value of spin-coated films to 47 cm−1 and the loss reduces from 7.6 to 0.9 cm−1 . Ellipsometrically determined optical constants reveal that the refractive indices of the materials are also greatly affected by the orientation of the polymer chains. 2. Experimental The conjugated polymer F8BT that we have investigated was synthesised and carefully purified at The Dow Chemical Company. The chemical structure of F8BT is shown in Fig. 1a. Oriented polymer planar waveguides were made by spin-coating 150–200 nm thickness films from 20 mg/ml toluene solutions onto polished synthetic quartz (Spectrosil B) substrates that had been precoated with rubbed thin (∼30 nm) polyimide (PI) alignment layers [14]. The absorption and fluorescence intensity of PI were negligibly weak compared to those of F8BT. Alignment was achieved by annealing the samples in a nitrogen atmosphere at 265 ◦ C for 2 min, slowly cooling at 1 ◦ C/min to 235 ◦ C, and then quenching to room temperature. To allow comparison with the aligned samples, “as spin-coated” samples were also fabricated on polished synthetic quartz. The degree of chain orientation was examined using polarised absorption measurements in a Unicam IV UV–vis spectrophotometer. A high degree of polymer chain orientation along the rubbing direction is clearly seen from the absorption spectra in Fig. 1b. The dichroic ratios at the absorption peak wavelengths are D ≈ 2.3 (340 nm) and 9.3 (450 nm). In contrast, the absorption spectra of the spin-coated samples showed no dependence on the polarisation of the incident light, as expected due to the essentially random distribution of polymer chain orientations within the substrate plane. The refractive indices (n) of F8BT films were obtained using a SOPRA rotating polariser ellipsometer. The refractive index of the Spectrosil B substrate varied in the range ns = 1.479–1.457 for wavelengths between 340 and 650 nm. That for polyimide was nPI ≈ 1.66. No optical dichroism was found for the PI film. The aligned F8BT, however, exhibited in-plane birefringence: for light propagating with electric field (E), vector parallel (npar ) and perpendicular (nperp ) to the alignment direction, we found n = 2.1 and 1.6, respectively, at 577 nm (the wavelength at which ASE occurs). For spin-coated F8BT n = 1.69, intermedi-
275
ate between npar and nperp . The refractive index of the polymer layer, np , is always higher than that of the substrate, ns and optical guiding can, thus always be achieved. The higher index value, found in the aligned layers, for polarisation parallel to the alignment direction, has a significant effect on the guiding properties. Waveguide modes, with an electric field vector along the alignment direction, are more strongly confined (over 80% of the modal power) to the active F8BT layer than for spin-coated films (about 60% of the modal power). To induce stimulated emission, the waveguides were optically pumped at 450 nm with a frequency tripled, Q-switched Nd:YAG laser producing 10 ns pulses at a repetition rate of 10 Hz. The pulse energy incident on the samples was adjusted by the insertion of calibrated neutral density filters into the beam path. Glan laser polarisers were used to control the polarisation of the pump beam. The pump beam was focused using a cylindrical lens and spatially filtered through an adjustable slit to create a 400 m × 4 mm excitation stripe on the sample. One end of the stripe was positioned at the substrate edge of the slab waveguide and the edge emission was collected with a fibercoupled grating spectrometer equipped with a CCD detector. At sufficient excitation intensities, the spontaneously emitted photons that are waveguided along the stripe-shaped gain region are amplified via stimulated emission. This process results in most of the light being emitted from the ends of the stripe. Due to the stripe-shaped gain region induced by the excitation beam and the unidirectional orientation of the polymer chains in the waveguides, there are four different experimental configurations with which to study the influence of chain orientation on ASE. These are illustrated together with a schematic of the refractive index ellipse of the aligned polymer in Fig. 2. In case 1, the incident excitation beam is polarised parallel to the chain alignment axis, maximizing the absorption, and thus the effective number of excitations for a given energy density. At the same time, most polymer chains lie perpendicular to the excitation stripe. In this geometry, the transition dipole moments (which lie almost parallel to the chains) emit radiation that propagates along the gain guide, thus maximising the amplification that can occur. In case 2, although the number of excitations is decreased due to the reduced absorption of the pump radiation, the emitted radiation still propagates along the gain guide. In both cases 1 and 2, the emitted light propagates with the E
Fig. 1. (a) The chemical structure of F8BT. (b) Polarised absorption spectra for an aligned F8BT film taken for light polarised parallel (solid line) and perpendicular (dotted line) to the rubbing direction at normal incidence to the substrate plane.
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(iii) The edge emitted spectra for cases 1 and 2 (cf. Fig. 3a inset) are spectrally sharper than for cases 3 and 4. We consider that this is most likely an optical effect associated with waveguiding and are further investigating its origin via modelling.
Fig. 2. Above: schematic of the four different excitation configurations. The solid black lines correspond to the chain alignment, while the gray stripe represents the excitation beam. The polarisation (E) axis of the excitation beam in each of the four different cases is indicated. Below: schematic of the refractive indices for cases 1 and 2 (left) and cases 3 and 4 (right). The dashed line represents the chain axis direction.
vector of its in-plane mode parallel to the alignment direction. A stronger waveguide confinement can be achieved compared with an spin-coated film due to the higher n (2.1) in this direction. Cases 3 and 4 represent unfavourable configurations for ASE since the emitting dipoles are now positioned parallel to the excitation stripe, causing only a small amount of the emitted radiation to be waveguided along the gain region. The E vector of the emitted radiation is perpendicular to the alignment direction. Therefore, the waveguide confinement will be weaker than that of the spin-coated film due to the lower n (1.6) in this direction. 3. Results and discussions Fig. 3a shows the edge-collected emission spectra from an aligned waveguide for the four excitation configurations at very low excitation energy E = 0.02 J. Fig. 3b shows the corresponding spectra together with the spectrum of a spin-coated film at E = 20 J, well above the ASE threshold. Fig. 3c shows the pump-pulse energy dependence of the ASE signal for each of the excitation configurations and for the spin-coated film. The dependence of the emission intensity and spectral profile on the excitation configuration at low pump energy (cf. Fig. 3a) can be summarised as follows: (i) The emission intensity in case 1 (case 3) is stronger than case 2 (case 4) due to a stronger absorption of pump radiation when the excitation light is polarised along the chain axis. (ii) The emission for cases 1 and 2 is stronger than for 3 and 4 respectively due to the better coincidence between the emission axis (defined by the transition dipole direction) and the propagation axis (defined by the pump stripe and location of the optical fibre).
Fig. 3. Emission spectra of the aligned F8BT waveguide as a function of the excitation configuration for pump energies of (a) 0.02 J and (b) 20 J. Insets are the corresponding normalised spectra. (c) Edge emitted output signal at the ASE wavelength as a function of excitation energy for the four-excitation configurations. Case 1: dashed black line, case 2: dotted grey line, case 3: dotted black line, case 4: dashed grey line and the spin-coated film: solid black line (b and c).
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Fig. 4. (a) The net gain coefficients plotted as a function of pump energy for aligned F8BT films in case 1 (open circles), case 2 (filled squares) and for a spin-coated film (filled triangles). (b) Dependence of the emission intensity at λASE on the length of the unpumped region between the end of the excitation stripe and the substrate edge. Data are shown for aligned (case 1 configuration, filled circles) and spin-coated (filled triangles) F8BT films.
As the pump energy increases, the contribution from stimulated emission also increases. This results in the appearance (Fig. 3b) of a spectrally narrow peak centred at the maximum of the net gain spectrum (λ = 577 nm in these samples). The stimulated emission at a given pump energy is stronger (weaker) for both cases 1 and 2 (cases 3 and 4) than for the spin-coated film. In addition, the difference between cases 1 and 2 and the spin-coated film increases with increasing pump energy. At high pump energies, the main ASE peak becomes strongest for case 2. We consider that several factors play a role here: first, there is a large increase in optical confinement for cases 1 and 2 compared with 3 and 4. Second, we note the existence of a second peak at λ = 550 nm, which competes with the mode at λ = 577 nm. Spectral integration of the emission showed cases 1 and 2 to have almost the same total output, implying that in each case the same number of photons is emitted for a given pump energy. However, the weight of the 550 nm peak is higher for case 1 (likewise case 3) than in case 2 (likewise case 4), and hence the emission for case 2 (case 4) at λ = 577 nm becomes stronger than for case 1 (case 3) at higher pump energy. While the influence of the 550 nm peak seems to be reasonably well understood, at present its origin is unclear and is the subject of on-going work. We note, however, that there is a strong experimental correlation between the spectral position of this peak and the thickness of the PI alignment layer (15–50 nm) suggesting that it may involve propagation of a mode at least partially confined within the PI layer. Further investigations are underway. The net gains for oriented F8BT films were investigated using the variable stripe length (VSL) technique [15,16]. Fig. 4a shows the gain coefficients as a function of pump energy (plotted on a logarithmic pump energy scale) for cases 1 and 2 and for a spin-coated film. The gain coefficient for the spincoated film increases approximately linearly before saturating at g = 25 cm−1 (pump energy = 65 J). Conversely, the gain did not saturate for the oriented sample up to pump energy E = 40 J (the maximum polarised pump energy available). The corresponding gains were g = 40 cm−1 (case 1) and 47 cm−1 (case 2).
We have also determined the corresponding loss coefficients for the oriented film. In these experiments, the length of the pump stripe was kept constant (l = 2 mm), but the stripe was gradually translated away from the edge of the sample. Since the emission from the end of the pumped region will remain constant (fixed stripe length), decreases in the detected signal can be used to estimate waveguide losses (mostly absorption and scattering). These are associated with light propagation through an increasing distance of unpumped polymer on the way to the detector. Fig. 4b shows the detected light intensity at λASE from the oriented and the spin-coated waveguide as a function of the stripe displacement from the sample edge. The data were fitted assuming an exponential dependence on length (as appropriate for absorption losses) and the exponential loss coefficient, α, was extracted. In contrast to the strong gain dependence, the α value was almost constant for each of the excitation configurations. We found α = 0.9 cm−1 for the oriented film, much smaller than the α = 7.6 cm−1 value for the spin-coated film. This suggests that annealing induces a significant improvement in film quality leading to a decrease in scattering losses. Again, further investigations will be undertaken to address this issue. 4. Conclusions In summary, we have presented a study of ASE in asymmetric F8BT waveguides for which there is a high degree of chain orientation. We find that the refractive index, the ASE intensity and the gain coefficient are strongly dependent on excitation polarisation, chain alignment direction and pump stripe orientation. Two specific configurations (cases 1 and 2) were identified for efficient ASE. Comparison with spin-coated samples revealed that mono-domain polymer chain alignment increases the gain coefficients by a factor of ≈1.9 due to increased optical confinement and more efficient emission. Alignment also produces a significant decrease in loss coefficient, apparently due to improved film quality. Finally, our methodology also offers some promise for the fabrication of polymer structures with improved charge transport properties that may facilitate construction of an electrically pumped polymer laser.
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Acknowledgements We thank The Dow Chemical Company for providing the F8BT used in these experiments and the UK Engineering and Physical Sciences Research Council (UPC IRC grant GR/R55078) for financial support. References [1] F. Hide, M.A. Diaz-Garcia, B.J. Schwartz, M.R. Andersson, P. Qibing, A.J. Heeger, Science 273 (1996) 1833. [2] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Br´edas, M. L¨ogdlund, W.R. Salaneck, Nature 397 (1999) 121. [3] N. Tessler, Adv. Mater. 11 (1999) 363. [4] G. Heliotis, R. Xia, D.D.C. Bradley, G.A. Turnbull, I.D.W. Samuel, P. Andrew, W.L. Barnes, Appl. Phys. Lett. 83 (2003) 2118. [5] M.D. McGehee, M.A. Diaz-Garcia, F. Hide, R. Gupta, E.K. Miller, D. Moses, A.J. Heeger, Appl. Phys. Lett. 72 (1998) 1536.
[6] M.A. Baldo, R.J. Holmes, S.R. Forrest, Phys. Rev. B 66 (2002) 035321. [7] A.J. Campbell, D.D.C. Bradley, H. Antoniadis, Appl. Phys. Lett. 79 (2001) 2133. [8] R. Xia, G. Heliotis, D.D.C. Bradley, Appl. Phys. Lett. 82 (2003) 3599. [9] R. Xia, G. Heliotis, Y. Hou, D.D.C. Bradley, Org. Electron 4 (2003) 165. [10] M. Redecker, D.D.C. Bradley, M. Inbasekaran, E.P. Woo, Appl. Phys. Lett. 74 (1999) 1400. [11] C. Bauer, G. Urbasch, H. Giessen, A. Meisel, H. Nothofer, D. Neher, U. Scherf, R.F. Mahrt, Chem. Phys. Lett. 3 (2000) 142. [12] G. Heliotis, R. Xia, K.S. Whitehead, G.A. Turnbull, I.D.W. Samuel, D.D.C. Bradley, Synth. Met. 139 (2003) 727. [13] T. Virgili, D.G. Lidzey, M. Grell, D.D.C. Bradley, S. Stagira, M. Zavelani-Rossi, S. De Silvestri, Appl. Phys. Lett. 80 (2002) 4088. [14] M. Grell, D.D.C. Bradley, M. Inbasekaran, E.P. Woo, Adv. Mater. 9 (1997) 798. [15] K.L. Shaklee, R.F. Leheny, Appl. Phys. Lett. 18 (1971) 475. [16] M.D. McGehee, R. Gupta, S. Veenstra, E.K. Miller, M.A. Diaz-Garcia, A.J. Heeger, Phys. Rev. B 58 (1998) 7035.
Significant improvements in the optical gain properties of oriented liquid crystalline conjugated polymer films Ruidong Xia, Mariano Campoy-Quiles, George Heliotis, Paul Stavrinou, Katharine S. Whitehead, Donal D.C. Bradley ∗ Ultrafast Photonics Collaboration, Experimental Solid State Group, Blackett Laboratory, Imperial College London, London SW7 2BZ, UK Available online 2 November 2005
Abstract We report significant improvements in the optical gain properties of highly oriented films of the fluorene-based conjugated polymer poly(9,9dioctylfluorene-co-benzothiadiazole) (F8BT). We demonstrate amplified spontaneous emission (ASE) in optically pumped aligned F8BT planar asymmetric waveguides. The influence of the polymer chain orientation and the excitation polarisation on the optical properties of the materials has been investigated. By selecting specific optimised excitation configurations, high net gain coefficients can be obtained, namely <47 cm−1 compared with <25 cm−1 for standard spin-coated films. The loss coefficients also reduce from α = 7.6 cm−1 (spin-coated film) to 0.9 cm−1 (aligned film). Ellipsometry allows determination of the optical constants and reveals that the refractive indices of the materials are also greatly affected by the orientation of the polymer chains. © 2005 Elsevier B.V. All rights reserved. Keywords: Polyfluorenes; Oriented polymer; Waveguides; Amplified spontaneous emission; Laser; Optical gain properties
1. Introduction Semiconducting polymers have attracted considerable interest as novel gain media for lasers and optical amplifiers due to their high photoluminescence quantum efficiencies (PLQE), large stimulated emission cross-sections and chemically tuneable emission wavelengths [1–3]. Optically pumped lasers have been demonstrated for a wide range of such materials and for several different resonator configurations [4,5]. However, electrically pumped diode lasers fabricated from organic semiconductors have not yet been realised due to the relatively low charge carrier mobilities that act against attainment of the high current densities required to reach threshold conditions [6]. Considerable efforts are consequently being made to improve the intrinsic carrier mobilities in these materials and to increase net optical gain and decrease losses. Poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT) is a promising green-light-emitting polyfluorene copolymer with good electron transport capabilities (the electron mobility is of the order of 10−3 cm2 /V s) [7].
∗
Corresponding author. Tel.: +44 20 75947612; fax: +44 20 75813817. E-mail address: [email protected] (D.D.C. Bradley).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.09.004
Our previous studies have demonstrated highly efficient stimulated emission via amplified spontaneous emission (ASE) in F8BT planar asymmetric polymer waveguides [8]. The good spectral overlap of F8BT emission and Dow Red F absorption allows for efficient energy transfer, and therefore blends of these two materials result in a significant reduction of the pump pulse energy for gain narrowing [9]. In this work, we report a further increase in ASE efficiency through orientation of the F8BT chains. It has been reported that oriented polymer films can exhibit a more than one order of magnitude increase in the charge carrier mobility relative to spin-coated films, thus offering simpler access to high current density devices [10]. However, there is little quantitative information on how the chain alignment affects the gain characteristics and whether the gain properties of a particular material can be enhanced through chain orientation [11–13]. In this paper, we demonstrate stimulated emission in waveguides containing F8BT with a high degree of chain orientation, and investigate the influence that the polymer chain alignment has on the gain properties of the waveguides. Our study shows that high net gain and low loss coefficients can be obtained by employing specific optimised excitation configurations. For ASE in the optimal configuration, the net gain for
R. Xia et al. / Synthetic Metals 155 (2005) 274–278
aligned F8BT increases from the 25 cm−1 value of spin-coated films to 47 cm−1 and the loss reduces from 7.6 to 0.9 cm−1 . Ellipsometrically determined optical constants reveal that the refractive indices of the materials are also greatly affected by the orientation of the polymer chains. 2. Experimental The conjugated polymer F8BT that we have investigated was synthesised and carefully purified at The Dow Chemical Company. The chemical structure of F8BT is shown in Fig. 1a. Oriented polymer planar waveguides were made by spin-coating 150–200 nm thickness films from 20 mg/ml toluene solutions onto polished synthetic quartz (Spectrosil B) substrates that had been precoated with rubbed thin (∼30 nm) polyimide (PI) alignment layers [14]. The absorption and fluorescence intensity of PI were negligibly weak compared to those of F8BT. Alignment was achieved by annealing the samples in a nitrogen atmosphere at 265 ◦ C for 2 min, slowly cooling at 1 ◦ C/min to 235 ◦ C, and then quenching to room temperature. To allow comparison with the aligned samples, “as spin-coated” samples were also fabricated on polished synthetic quartz. The degree of chain orientation was examined using polarised absorption measurements in a Unicam IV UV–vis spectrophotometer. A high degree of polymer chain orientation along the rubbing direction is clearly seen from the absorption spectra in Fig. 1b. The dichroic ratios at the absorption peak wavelengths are D ≈ 2.3 (340 nm) and 9.3 (450 nm). In contrast, the absorption spectra of the spin-coated samples showed no dependence on the polarisation of the incident light, as expected due to the essentially random distribution of polymer chain orientations within the substrate plane. The refractive indices (n) of F8BT films were obtained using a SOPRA rotating polariser ellipsometer. The refractive index of the Spectrosil B substrate varied in the range ns = 1.479–1.457 for wavelengths between 340 and 650 nm. That for polyimide was nPI ≈ 1.66. No optical dichroism was found for the PI film. The aligned F8BT, however, exhibited in-plane birefringence: for light propagating with electric field (E), vector parallel (npar ) and perpendicular (nperp ) to the alignment direction, we found n = 2.1 and 1.6, respectively, at 577 nm (the wavelength at which ASE occurs). For spin-coated F8BT n = 1.69, intermedi-
275
ate between npar and nperp . The refractive index of the polymer layer, np , is always higher than that of the substrate, ns and optical guiding can, thus always be achieved. The higher index value, found in the aligned layers, for polarisation parallel to the alignment direction, has a significant effect on the guiding properties. Waveguide modes, with an electric field vector along the alignment direction, are more strongly confined (over 80% of the modal power) to the active F8BT layer than for spin-coated films (about 60% of the modal power). To induce stimulated emission, the waveguides were optically pumped at 450 nm with a frequency tripled, Q-switched Nd:YAG laser producing 10 ns pulses at a repetition rate of 10 Hz. The pulse energy incident on the samples was adjusted by the insertion of calibrated neutral density filters into the beam path. Glan laser polarisers were used to control the polarisation of the pump beam. The pump beam was focused using a cylindrical lens and spatially filtered through an adjustable slit to create a 400 m × 4 mm excitation stripe on the sample. One end of the stripe was positioned at the substrate edge of the slab waveguide and the edge emission was collected with a fibercoupled grating spectrometer equipped with a CCD detector. At sufficient excitation intensities, the spontaneously emitted photons that are waveguided along the stripe-shaped gain region are amplified via stimulated emission. This process results in most of the light being emitted from the ends of the stripe. Due to the stripe-shaped gain region induced by the excitation beam and the unidirectional orientation of the polymer chains in the waveguides, there are four different experimental configurations with which to study the influence of chain orientation on ASE. These are illustrated together with a schematic of the refractive index ellipse of the aligned polymer in Fig. 2. In case 1, the incident excitation beam is polarised parallel to the chain alignment axis, maximizing the absorption, and thus the effective number of excitations for a given energy density. At the same time, most polymer chains lie perpendicular to the excitation stripe. In this geometry, the transition dipole moments (which lie almost parallel to the chains) emit radiation that propagates along the gain guide, thus maximising the amplification that can occur. In case 2, although the number of excitations is decreased due to the reduced absorption of the pump radiation, the emitted radiation still propagates along the gain guide. In both cases 1 and 2, the emitted light propagates with the E
Fig. 1. (a) The chemical structure of F8BT. (b) Polarised absorption spectra for an aligned F8BT film taken for light polarised parallel (solid line) and perpendicular (dotted line) to the rubbing direction at normal incidence to the substrate plane.
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(iii) The edge emitted spectra for cases 1 and 2 (cf. Fig. 3a inset) are spectrally sharper than for cases 3 and 4. We consider that this is most likely an optical effect associated with waveguiding and are further investigating its origin via modelling.
Fig. 2. Above: schematic of the four different excitation configurations. The solid black lines correspond to the chain alignment, while the gray stripe represents the excitation beam. The polarisation (E) axis of the excitation beam in each of the four different cases is indicated. Below: schematic of the refractive indices for cases 1 and 2 (left) and cases 3 and 4 (right). The dashed line represents the chain axis direction.
vector of its in-plane mode parallel to the alignment direction. A stronger waveguide confinement can be achieved compared with an spin-coated film due to the higher n (2.1) in this direction. Cases 3 and 4 represent unfavourable configurations for ASE since the emitting dipoles are now positioned parallel to the excitation stripe, causing only a small amount of the emitted radiation to be waveguided along the gain region. The E vector of the emitted radiation is perpendicular to the alignment direction. Therefore, the waveguide confinement will be weaker than that of the spin-coated film due to the lower n (1.6) in this direction. 3. Results and discussions Fig. 3a shows the edge-collected emission spectra from an aligned waveguide for the four excitation configurations at very low excitation energy E = 0.02 J. Fig. 3b shows the corresponding spectra together with the spectrum of a spin-coated film at E = 20 J, well above the ASE threshold. Fig. 3c shows the pump-pulse energy dependence of the ASE signal for each of the excitation configurations and for the spin-coated film. The dependence of the emission intensity and spectral profile on the excitation configuration at low pump energy (cf. Fig. 3a) can be summarised as follows: (i) The emission intensity in case 1 (case 3) is stronger than case 2 (case 4) due to a stronger absorption of pump radiation when the excitation light is polarised along the chain axis. (ii) The emission for cases 1 and 2 is stronger than for 3 and 4 respectively due to the better coincidence between the emission axis (defined by the transition dipole direction) and the propagation axis (defined by the pump stripe and location of the optical fibre).
Fig. 3. Emission spectra of the aligned F8BT waveguide as a function of the excitation configuration for pump energies of (a) 0.02 J and (b) 20 J. Insets are the corresponding normalised spectra. (c) Edge emitted output signal at the ASE wavelength as a function of excitation energy for the four-excitation configurations. Case 1: dashed black line, case 2: dotted grey line, case 3: dotted black line, case 4: dashed grey line and the spin-coated film: solid black line (b and c).
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Fig. 4. (a) The net gain coefficients plotted as a function of pump energy for aligned F8BT films in case 1 (open circles), case 2 (filled squares) and for a spin-coated film (filled triangles). (b) Dependence of the emission intensity at λASE on the length of the unpumped region between the end of the excitation stripe and the substrate edge. Data are shown for aligned (case 1 configuration, filled circles) and spin-coated (filled triangles) F8BT films.
As the pump energy increases, the contribution from stimulated emission also increases. This results in the appearance (Fig. 3b) of a spectrally narrow peak centred at the maximum of the net gain spectrum (λ = 577 nm in these samples). The stimulated emission at a given pump energy is stronger (weaker) for both cases 1 and 2 (cases 3 and 4) than for the spin-coated film. In addition, the difference between cases 1 and 2 and the spin-coated film increases with increasing pump energy. At high pump energies, the main ASE peak becomes strongest for case 2. We consider that several factors play a role here: first, there is a large increase in optical confinement for cases 1 and 2 compared with 3 and 4. Second, we note the existence of a second peak at λ = 550 nm, which competes with the mode at λ = 577 nm. Spectral integration of the emission showed cases 1 and 2 to have almost the same total output, implying that in each case the same number of photons is emitted for a given pump energy. However, the weight of the 550 nm peak is higher for case 1 (likewise case 3) than in case 2 (likewise case 4), and hence the emission for case 2 (case 4) at λ = 577 nm becomes stronger than for case 1 (case 3) at higher pump energy. While the influence of the 550 nm peak seems to be reasonably well understood, at present its origin is unclear and is the subject of on-going work. We note, however, that there is a strong experimental correlation between the spectral position of this peak and the thickness of the PI alignment layer (15–50 nm) suggesting that it may involve propagation of a mode at least partially confined within the PI layer. Further investigations are underway. The net gains for oriented F8BT films were investigated using the variable stripe length (VSL) technique [15,16]. Fig. 4a shows the gain coefficients as a function of pump energy (plotted on a logarithmic pump energy scale) for cases 1 and 2 and for a spin-coated film. The gain coefficient for the spincoated film increases approximately linearly before saturating at g = 25 cm−1 (pump energy = 65 J). Conversely, the gain did not saturate for the oriented sample up to pump energy E = 40 J (the maximum polarised pump energy available). The corresponding gains were g = 40 cm−1 (case 1) and 47 cm−1 (case 2).
We have also determined the corresponding loss coefficients for the oriented film. In these experiments, the length of the pump stripe was kept constant (l = 2 mm), but the stripe was gradually translated away from the edge of the sample. Since the emission from the end of the pumped region will remain constant (fixed stripe length), decreases in the detected signal can be used to estimate waveguide losses (mostly absorption and scattering). These are associated with light propagation through an increasing distance of unpumped polymer on the way to the detector. Fig. 4b shows the detected light intensity at λASE from the oriented and the spin-coated waveguide as a function of the stripe displacement from the sample edge. The data were fitted assuming an exponential dependence on length (as appropriate for absorption losses) and the exponential loss coefficient, α, was extracted. In contrast to the strong gain dependence, the α value was almost constant for each of the excitation configurations. We found α = 0.9 cm−1 for the oriented film, much smaller than the α = 7.6 cm−1 value for the spin-coated film. This suggests that annealing induces a significant improvement in film quality leading to a decrease in scattering losses. Again, further investigations will be undertaken to address this issue. 4. Conclusions In summary, we have presented a study of ASE in asymmetric F8BT waveguides for which there is a high degree of chain orientation. We find that the refractive index, the ASE intensity and the gain coefficient are strongly dependent on excitation polarisation, chain alignment direction and pump stripe orientation. Two specific configurations (cases 1 and 2) were identified for efficient ASE. Comparison with spin-coated samples revealed that mono-domain polymer chain alignment increases the gain coefficients by a factor of ≈1.9 due to increased optical confinement and more efficient emission. Alignment also produces a significant decrease in loss coefficient, apparently due to improved film quality. Finally, our methodology also offers some promise for the fabrication of polymer structures with improved charge transport properties that may facilitate construction of an electrically pumped polymer laser.
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