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Amplified spontaneous emission from a new 4-triarylamine substituted 1,8-naphthalimide semiconductor oligomer
Chemical Physics Letters 409 (2005) 105–109 www.elsevier.com/locate/cplett
Amplified spontaneous emission from a new 4-triarylamine substituted 1,8-naphthalimide semiconductor oligomer Wu Lu, Guoli Tu, Bo Zhong, Dongge Ma *, Lixiang Wang, Xiabin Jing, Fosong Wang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Graduate School of the Chinese Academy of Sciences, PR China Received 28 March 2005; in final form 20 April 2005 Available online 23 May 2005
Abstract Amplified spontaneous emission has been observed in a new semiconductor oligomer of 2-decyl-6-{[4 0 -(naphthalene-1-yl-phenylamino)-biphenyl-4-yl]-[4-(naphthalene-1-yl-phenyl-amino)-phenyl]-amino}-benzo[de]isoquinoline-1,3-dione (4-triarylamine substituted 1,8-naphthalimide TAANPI) doped polymer films pumped by the second harmonic of a Nd:YAG laser. The dependence of the threshold and gain on the oligomer concentration in polymer was studied in detail. It was found that the semiconductor oligomer shows low threshold, high gain and low loss even though the doped oligomer concentration is up to 60%, indicating a low concentration quenching effect. This demonstrates that the oligomer could be a promising candidate as gain medium for organic diode lasers. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of the semiconductor properties in organic materials, rapid progress has been made in the synthesis and characterization of organic semiconductors and in their development as active materials for use in electronic and optoelectronic devices, such as in the field of the organic semiconductor lasers (OSLs) [1]. High photoluminescence efficiencies and large stimulated emission cross section coupled with spectra covering from the blue to far IR, which are still not easily accessible by most commonly used inorganic semiconductor materials, make the organic semiconductors interesting for scientists and researchers as gain medium in lasers. At present, amplified spontaneous emission (ASE) and lasing action have been achieved from various organic materials, including polymers [2], organic small molecules [3], dye-doped dendrimers [4], *
Corresponding author. Fax: +86 431 5262873. E-mail address: [email protected] (D. Ma).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.04.101
and dye-doped liquid crystals (LCs) [5,6]. Many types of laser structures suitable for organic lasers, for example, wave-guiding traveling arrangements [7], permanent one-dimensional distributed feedback (1D DFB) [8], permanent two-dimensional distributed feedback lasers (2D DFB) [9], micro-ring, micro-disc, micro-droplet, hollow-fiber, and distributed Bragg reflectors (DBRs) [10] have been demonstrated. In the case of molecularly doped systems, usually the doping concentration must be maintained low due to the existence of concentration quenching [11]. Furthermore, in some organic molecules it is also difficult to obtain lasing action because of strong intermolecular interaction. Therefore, it is important to develop new organic laser materials with weak intermolecular interaction. 1,8-Naphthalimide derivatives have been used as fluorescence probes in biological cells [12], and become attractive candidates for use as n-type materials in organic light-emitting diodes (OLEDs) due to their high electron affinities and low reduction potentials [13–15]. However, many 1,8-naphthalimide derivatives have
106
W. Lu et al. / Chemical Physics Letters 409 (2005) 105–109
N
0.8 N
0.6 O
N O C10H21
0.4 0.2
Output Intensity (a.u.)
1.0 N
0.0
550
600 650 700 750 800
PL spectrum 0.012mJ/Pulse 0.015mJ/Pulse 0.030mJ/Pulse 0.043mJ/Pulse 0.048mJ/Pulse 0.060mJ/Pulse
Wavelength (nm) Fig. 1. Normalized emission spectra collected from the edge of a 5-mm-long strip at different pump intensities (TAANPI concentration is 30%). Inset shows the chemical struture of TAANPI.
low luminescent efficiency at room temperature due to strong intersystem crossing to their triplet states [16,17]. It is well known that 1,8-naphthalimide substituted at the 4-position with electron-donating groups has high fluorescent quantum yields and tunes easily the emission wavelength [18]. Here, we studied the lasing action of 4-triarylamine substituted 1,8-naphthalimide (TAANPI) (the chemical structure was shown in inset of Fig. 1). The TAANPI emits red light and shows high efficiency. The studies of amplified spontaneous emission (ASE) found that the TAANPI shows high net gain, low threshold and loss even though the concentration of doping TAANPI in PS is up to 60%, indicating that the TAANPI is a promising lasing medium material. We investigated the amplified spontaneous emission (ASE) and lasing action of the new oligomer TAANPI doped polystyrene (PS) films at different doping concentrations in detail.
2. Experimental The oligomer TAANPI was synthesized via Ullmann reaction in our laboratory. The films were made by spincoating onto quartz substrate from chloroform solution containing polystyrene (PS) with TAANPI at different doping concentrations. The film thickness is about 800 nm. The refractive index of the doped polymer films, which depends on the dopant concentration, was measured to be in the range of 1.55–1.62 at the central wavelength of 640 nm. The refractive index and the thickness of each film were measured by variable angle spectroscopic ellipsometry (JY Horba). The photoluminescence quantum efficiency of the films was measured by the integrating sphere method. The experimental set-up to investigate the amplified spontaneous emission properties of the TAANPI poly-
mer films followed [19]. The frequency doubled 532 nm line of 10 Hz, 10 ns pulse duration, Q-switched Nd:YAG laser (Spectra-Physics) was used as the excitation source for the lasing emission. The output pulse energy of the pump laser was controlled using neutral density filters. An adjustable slit and a cylindrical lens were used to shape the beam into a strip with a width of 1 mm and a length that could be continuously varied. The films were pumped at normal incidence with the long axis of the pump beam perpendicular to the edge of the sample. The output signals were detected by fibercoupled CCD spectrometer (JY SPEX CCD3000). The pumped energies from the laser were measured using a calibrated laser power and energy meter (Gentec).
3. Results and discussion The emissive spectra of the films as a function of the excitation intensity are showed in Fig. 1. At low pumped energy (<0.043 mJ pulse1), the emissive spectra exhibit broad peaks of spontaneous emission similar to the normal PL spectra of the TAANPI film. If the pumping energy is increased above the threshold, there is a substantial reduction in the full-width at half-maximum (FWHM) of the emitting light. The peak wavelength of the narrowed emissive spectra occurs red-shift with respect to the PL spectrum due to self-absorption effects. This collapse of the emission spectrum is one of the signatures of the presence of amplified spontaneous emission (ASE). In order to elucidate that the spectrum narrowing phenomena are a result of ASE, the FWHM of the emission and the output emission intensity integrated over all wavelengths as a function of the pumped intensity are plotted in Fig. 2. The threshold pumped energy for ASE can be easily discerned at 0.043 mJ pulse1,
W. Lu et al. / Chemical Physics Letters 409 (2005) 105–109
6 5 FWHM (nm)
60 4
50
3
40 30
2
20
1
10
0
0.00
0.02 0.04 0.06 Pumped Intensity (mJ/Pulse)
Output Intensity (a.u.)
70
0.08
Fig. 2. The output emission intensity integrated over all wavelengths as a function of pump intensity (closed squares) and the dependence of the full-width at half-maximum (FWHM) on the pumped intensity (TAANPI concentration is 30%) (open squares).
corresponding to an areal energy density of 0.86 mJ pulse1 cm2 when the dopant concentration is 30%. For pumped intensity below the threshold value, the FWHM of the emission is broad and the emission intensity increases in proportion to the pump intensity slowly. Above the threshold, the emission intensity increases sharply and the FWHM of the emission spectrum decreases sharply, and then keeps less steep, even stable due to the onset of gain saturation. The threshold values of the TAANPI doped PS films at different doped concentrations are summarized in Table 1. The case of 30% TAANPI concentration shows the lowest ASE threshold of 0.86 mJ pulse1 cm2. The threshold is yet much lower than that of DCM:PS film [20] even though the TAANPI concentration in PS is up to 60%. This value is even lower than that of some other organic molecules [22]. This indicates that the introduction of the triarylamine groups within the 1,8-naphthalimide reduces significantly the concentration quenching effects between 1,8-naphthalimide molecules due to triarylamine group larger steric hindrance. The low concentration quenching existing in TAANPI offers great promising organic gain medium material for the fabrication of low cost solid-state lasers with low threshold.
In order to fully demonstrate the ASE and lasing characteristics of the TAANPI:PS films, the gain and loss characteristics of the waveguides were also studied in detail. The net gain was measured by using the usually variable-stripe-length method [21], which has been widely used to characterize gain in both bulk inorganic and organic semiconductor slab geometries. For the case of ASE, the output emission intensity I(k) should obey the following equation [21]: AðkÞ I p GðkÞ L I ðkÞ ¼ e 1 ; ð1Þ GðkÞ where A(k) is a constant related to the cross-section for spontaneous emission, Ip is the pumped energy intensity, G(k) is the net gain coefficient and L is the pumped stripe length. Fig. 3 shows the output intensity at peak of emission spectrum as a function of excitation stripe length for different pump energies. By fitting the experimental data to Eq. (1), the net gain coefficient can be obtained. For example, for the case of 30% TAANPI, at pump energy of 0.075 mJ pulse1, the net gain was 13.59 cm1 at 641.6 nm, whereas for pump energy of 0.12 mJ pulse1, the gain was 19.76 cm1 and for the energy of 0.15 mJ pulse1, the net gain reached 26.59 cm1. Fig. 4 shows the output intensity at peak 8
Output Intensity (a.u.)
80
107
0.075mJ/Pulse 0.120mJ/Pulse 0.150mJ/Pulse Fitting Line
6
4
2
0.0
0.1 0.2 0.3 Pumped Region Length (cm)
0.4
Fig. 3. The dependence of the emission intensity at peak wavelength on the excitation length at indicated pump intensities. (TAANPI concentration is 30% and kpeak = 641.6 nm.)
Table 1 The dependence of threshold and gain on the doped concentrations, the optical gain values of TAANPI:PS films were measured at 0.15 mJ pulse1 System
Dye concentration (%) by weight
kASE (nm)
Ith (mJ pulse1 cm2)
PL efficiency (%)
Gain (cm1)
TAANPI:PS film
15 30 40 50 60
640.58 641.62 644.24 645.81 647.37
0.94 0.86 1.46 1.50 1.62
32.4 27.5 20.1 17.5 15.8
17.26 26.59 18.61 14.56 9.26
108
W. Lu et al. / Chemical Physics Letters 409 (2005) 105–109
6 Output Intensity (a.u.)
ple, thus the loss coefficient can be estimated. Fig. 5 shows the output intensity at kASE = 641.6 nm as a function of X. By fitting, a loss coefficient of 2.47 cm1 was achieved in the 30% TAANPI concentration in PS film. This value is much more smaller than 44 cm1 loss coefficient of poly[2-butyl-5-(2 0 -ethyl-hexyl)-1,4-phenylene vinylene] [19] and 30 cm1 loss coefficient of poly[(2,5dioctyloxy)-1,4-phenylene vinylene] [23], and also lower than that of DCM1 doped PS film [20]. The low loss coefficient should be attributed to the high PL efficiency and low self-absorption or scattering of TAANPI.
15% 30% 40% 50%
4
2
0
4. Conclusions 0.0
0.1 0.2 0.3 0.4 Pumped Region Length (cm)
0.5
Fig. 4. The dependence of the emission intensity at peak wavelength on the excitation length at indicated doped concentration (pumped intensity is 0.15 mJ pulse1).
of emission spectrum as a function of excitation stripe length at different TAANPI doping concentrations. The corresponding results of gain values for different TAANPI concentrations were given in Table 1. There exists an optimal concentration for a higher net gain value. The loss coefficient of the waveguide was measured by using the method, where we kept the pumped length constant (l = 0.5 cm) and moved the pumped region away from the edge of the sample. Since the emission from the end of the pump stripe remains constant, the detected signal from the edge of the sample should decrease as I = I0 e(aX), where a is the waveguide loss coefficient, and X is the length of the unpumped region from the end of the pump region to the edge of the sam10
We have investigated amplified spontaneous emission (ASE) of a new semiconductor oligomer 1,8-naphthalimide derivative (TAANPI) doped polymer films. A spectral narrowing red emission was achieved, and the narrowing phenomenon was attributed to the amplified spontaneous emission (ASE) mechanism, which is seldom reported on the 1,8-naphthalimide derivatives as gain medium. Due to the existence of less concentration quenching effects, TAANPI doped polymer films show low threshold, large net gain, and low loss even though the doping concentration is up to 60%. Our results indicate that TAANPI is an attractive candidate as gain medium to realize red organic semiconductor diode lasers.
Acknowledgements The author (Dr. D. Ma) is grateful to Hundreds Talents Program of Chinese Academy of Sciences, National Education Committee Foundation, and National Personnel Ministry Foundation for financial support.
Output Intensity (a.u.)
Experimental Data Fitting Line
References 1
0.1
0.01 0.00
0.04 0.08 0.12 0.16 Unpumped Region Length (cm)
Fig. 5. The intensity of light emitted at kpeak = 641.6 nm from the edge of a waveguide as a function of the distance between the pump stripe and the edge of the TAANPI:PS film.
[1] M.D. McGehee, A.J. Heeger, Adv. Mater. 12 (2000) 1655. [2] N. Tessler, Adv. Mater. 11 (1999) 363. [3] V.G. Kozlov, V. Bulovic, P.E. Burrows, M. Baldo, V.B. Khalfin, G. Parthasarathy, S.R. Forrest, J. Appl. Phys. 84 (1998) 4096. [4] S. Yokoyama, A. Otomo, S. Mashiko, Appl. Phys. Lett. 80 (2002) 7. [5] O. Masanori, K. Mashiro, G. Dirk, H. Wolfgang, Y. Katsumi, Adv. Mater. 14 (2002) 306. [6] F. Araoka, K.-C. Shin, Y. Takanishi, K. Ishikawa, H. Takezoe, Z.G. Zhu, T.M. Swager, J. Appl. Phys. 94 (2003) 279. [7] Y. Yoshida, Y. Nishihara, R. Ootake, A. Fuji, M. Ozaki, K. Yoshino, H.K. Kim, N.S. Baek, S.K. Choi, J. Appl. phys. 90 (2001) 6061. [8] 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. [9] G.A. Turnbull, P. Andrew, W.L. Barnes, I.D.W. Samuel, Phys. Rev. B 67 (2003) 165107.
W. Lu et al. / Chemical Physics Letters 409 (2005) 105–109 [10] U. Scherf, S. Riechel, U. Lemmer, R.F. Mahrt, Curr. Opin. Solid State Mater. Sci. 5 (2001) 143. [11] W.-C. Wu, H.-C. Yeh, L.-H. Chan, C.-T. Chen, Adv. Mater. 14 (2002) 1072. [12] Y.Q. Gao, R.A. Marcus, J. Phys. Chem. A 106 (2002) 1956. [13] F. Cacialli, R.H. Friend, J. Appl. Phys. 83 (1998) 2343. [14] J. Gan, Q.L. Song, X.Y. Hou, K. Chen, H. Tian, J. Photochem. Photobiol. A 162 (2004) 399. [15] D. Kolosov, V. Adamovich, P. Djurovich, M.E. Thompson, C. Adachi, J. Am. Chem. Soc. 124 (2002) 9945. [16] V. Wintges, P. Valat, J. Kossanyi, L. Biczok, A. Demeter, T. Berces, J. Chem. Soc., Faraday Trans. 90 (1994) 411.
109
[17] A. Demeter, T. Berces, L. Bicsok, V. Wintges, P. Valat, J. Kossanyi, J. Phys. Chem. 100 (1996) 2001. [18] V. Wintges, P. Valat, J. Kossanyi, A. Demeter, L. Biczok, T. Berces, New J. Chem. 20 (1996) 1149. [19] M.D. McGehee, R. Gupta, S. Veenstra, E.K. Miller, M.A. DiazGarcia, A.J. Heeger, Phys. Rev. B 58 (1998) 7035. [20] W. Lu, B. Zhong, D.G. Ma, Appl. Opt. 43 (2004) 5074. [21] K.L. Shaklee, R.F. Leheny, Appl. Phys. Lett. 18 (1971) 475. [22] M. Ichikawa, T. Tachi, M. Satsuki, S. Suga, T. Koyama, T. Yoshio, J. Photochem. Photobiol. A 158 (2003) 219. [23] S.V. Frolov, Z.V. Vardeny, Phys. Rev. B 57 (1998) 9141.
Amplified spontaneous emission from a new 4-triarylamine substituted 1,8-naphthalimide semiconductor oligomer Wu Lu, Guoli Tu, Bo Zhong, Dongge Ma *, Lixiang Wang, Xiabin Jing, Fosong Wang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Graduate School of the Chinese Academy of Sciences, PR China Received 28 March 2005; in final form 20 April 2005 Available online 23 May 2005
Abstract Amplified spontaneous emission has been observed in a new semiconductor oligomer of 2-decyl-6-{[4 0 -(naphthalene-1-yl-phenylamino)-biphenyl-4-yl]-[4-(naphthalene-1-yl-phenyl-amino)-phenyl]-amino}-benzo[de]isoquinoline-1,3-dione (4-triarylamine substituted 1,8-naphthalimide TAANPI) doped polymer films pumped by the second harmonic of a Nd:YAG laser. The dependence of the threshold and gain on the oligomer concentration in polymer was studied in detail. It was found that the semiconductor oligomer shows low threshold, high gain and low loss even though the doped oligomer concentration is up to 60%, indicating a low concentration quenching effect. This demonstrates that the oligomer could be a promising candidate as gain medium for organic diode lasers. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of the semiconductor properties in organic materials, rapid progress has been made in the synthesis and characterization of organic semiconductors and in their development as active materials for use in electronic and optoelectronic devices, such as in the field of the organic semiconductor lasers (OSLs) [1]. High photoluminescence efficiencies and large stimulated emission cross section coupled with spectra covering from the blue to far IR, which are still not easily accessible by most commonly used inorganic semiconductor materials, make the organic semiconductors interesting for scientists and researchers as gain medium in lasers. At present, amplified spontaneous emission (ASE) and lasing action have been achieved from various organic materials, including polymers [2], organic small molecules [3], dye-doped dendrimers [4], *
Corresponding author. Fax: +86 431 5262873. E-mail address: [email protected] (D. Ma).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.04.101
and dye-doped liquid crystals (LCs) [5,6]. Many types of laser structures suitable for organic lasers, for example, wave-guiding traveling arrangements [7], permanent one-dimensional distributed feedback (1D DFB) [8], permanent two-dimensional distributed feedback lasers (2D DFB) [9], micro-ring, micro-disc, micro-droplet, hollow-fiber, and distributed Bragg reflectors (DBRs) [10] have been demonstrated. In the case of molecularly doped systems, usually the doping concentration must be maintained low due to the existence of concentration quenching [11]. Furthermore, in some organic molecules it is also difficult to obtain lasing action because of strong intermolecular interaction. Therefore, it is important to develop new organic laser materials with weak intermolecular interaction. 1,8-Naphthalimide derivatives have been used as fluorescence probes in biological cells [12], and become attractive candidates for use as n-type materials in organic light-emitting diodes (OLEDs) due to their high electron affinities and low reduction potentials [13–15]. However, many 1,8-naphthalimide derivatives have
106
W. Lu et al. / Chemical Physics Letters 409 (2005) 105–109
N
0.8 N
0.6 O
N O C10H21
0.4 0.2
Output Intensity (a.u.)
1.0 N
0.0
550
600 650 700 750 800
PL spectrum 0.012mJ/Pulse 0.015mJ/Pulse 0.030mJ/Pulse 0.043mJ/Pulse 0.048mJ/Pulse 0.060mJ/Pulse
Wavelength (nm) Fig. 1. Normalized emission spectra collected from the edge of a 5-mm-long strip at different pump intensities (TAANPI concentration is 30%). Inset shows the chemical struture of TAANPI.
low luminescent efficiency at room temperature due to strong intersystem crossing to their triplet states [16,17]. It is well known that 1,8-naphthalimide substituted at the 4-position with electron-donating groups has high fluorescent quantum yields and tunes easily the emission wavelength [18]. Here, we studied the lasing action of 4-triarylamine substituted 1,8-naphthalimide (TAANPI) (the chemical structure was shown in inset of Fig. 1). The TAANPI emits red light and shows high efficiency. The studies of amplified spontaneous emission (ASE) found that the TAANPI shows high net gain, low threshold and loss even though the concentration of doping TAANPI in PS is up to 60%, indicating that the TAANPI is a promising lasing medium material. We investigated the amplified spontaneous emission (ASE) and lasing action of the new oligomer TAANPI doped polystyrene (PS) films at different doping concentrations in detail.
2. Experimental The oligomer TAANPI was synthesized via Ullmann reaction in our laboratory. The films were made by spincoating onto quartz substrate from chloroform solution containing polystyrene (PS) with TAANPI at different doping concentrations. The film thickness is about 800 nm. The refractive index of the doped polymer films, which depends on the dopant concentration, was measured to be in the range of 1.55–1.62 at the central wavelength of 640 nm. The refractive index and the thickness of each film were measured by variable angle spectroscopic ellipsometry (JY Horba). The photoluminescence quantum efficiency of the films was measured by the integrating sphere method. The experimental set-up to investigate the amplified spontaneous emission properties of the TAANPI poly-
mer films followed [19]. The frequency doubled 532 nm line of 10 Hz, 10 ns pulse duration, Q-switched Nd:YAG laser (Spectra-Physics) was used as the excitation source for the lasing emission. The output pulse energy of the pump laser was controlled using neutral density filters. An adjustable slit and a cylindrical lens were used to shape the beam into a strip with a width of 1 mm and a length that could be continuously varied. The films were pumped at normal incidence with the long axis of the pump beam perpendicular to the edge of the sample. The output signals were detected by fibercoupled CCD spectrometer (JY SPEX CCD3000). The pumped energies from the laser were measured using a calibrated laser power and energy meter (Gentec).
3. Results and discussion The emissive spectra of the films as a function of the excitation intensity are showed in Fig. 1. At low pumped energy (<0.043 mJ pulse1), the emissive spectra exhibit broad peaks of spontaneous emission similar to the normal PL spectra of the TAANPI film. If the pumping energy is increased above the threshold, there is a substantial reduction in the full-width at half-maximum (FWHM) of the emitting light. The peak wavelength of the narrowed emissive spectra occurs red-shift with respect to the PL spectrum due to self-absorption effects. This collapse of the emission spectrum is one of the signatures of the presence of amplified spontaneous emission (ASE). In order to elucidate that the spectrum narrowing phenomena are a result of ASE, the FWHM of the emission and the output emission intensity integrated over all wavelengths as a function of the pumped intensity are plotted in Fig. 2. The threshold pumped energy for ASE can be easily discerned at 0.043 mJ pulse1,
W. Lu et al. / Chemical Physics Letters 409 (2005) 105–109
6 5 FWHM (nm)
60 4
50
3
40 30
2
20
1
10
0
0.00
0.02 0.04 0.06 Pumped Intensity (mJ/Pulse)
Output Intensity (a.u.)
70
0.08
Fig. 2. The output emission intensity integrated over all wavelengths as a function of pump intensity (closed squares) and the dependence of the full-width at half-maximum (FWHM) on the pumped intensity (TAANPI concentration is 30%) (open squares).
corresponding to an areal energy density of 0.86 mJ pulse1 cm2 when the dopant concentration is 30%. For pumped intensity below the threshold value, the FWHM of the emission is broad and the emission intensity increases in proportion to the pump intensity slowly. Above the threshold, the emission intensity increases sharply and the FWHM of the emission spectrum decreases sharply, and then keeps less steep, even stable due to the onset of gain saturation. The threshold values of the TAANPI doped PS films at different doped concentrations are summarized in Table 1. The case of 30% TAANPI concentration shows the lowest ASE threshold of 0.86 mJ pulse1 cm2. The threshold is yet much lower than that of DCM:PS film [20] even though the TAANPI concentration in PS is up to 60%. This value is even lower than that of some other organic molecules [22]. This indicates that the introduction of the triarylamine groups within the 1,8-naphthalimide reduces significantly the concentration quenching effects between 1,8-naphthalimide molecules due to triarylamine group larger steric hindrance. The low concentration quenching existing in TAANPI offers great promising organic gain medium material for the fabrication of low cost solid-state lasers with low threshold.
In order to fully demonstrate the ASE and lasing characteristics of the TAANPI:PS films, the gain and loss characteristics of the waveguides were also studied in detail. The net gain was measured by using the usually variable-stripe-length method [21], which has been widely used to characterize gain in both bulk inorganic and organic semiconductor slab geometries. For the case of ASE, the output emission intensity I(k) should obey the following equation [21]: AðkÞ I p GðkÞ L I ðkÞ ¼ e 1 ; ð1Þ GðkÞ where A(k) is a constant related to the cross-section for spontaneous emission, Ip is the pumped energy intensity, G(k) is the net gain coefficient and L is the pumped stripe length. Fig. 3 shows the output intensity at peak of emission spectrum as a function of excitation stripe length for different pump energies. By fitting the experimental data to Eq. (1), the net gain coefficient can be obtained. For example, for the case of 30% TAANPI, at pump energy of 0.075 mJ pulse1, the net gain was 13.59 cm1 at 641.6 nm, whereas for pump energy of 0.12 mJ pulse1, the gain was 19.76 cm1 and for the energy of 0.15 mJ pulse1, the net gain reached 26.59 cm1. Fig. 4 shows the output intensity at peak 8
Output Intensity (a.u.)
80
107
0.075mJ/Pulse 0.120mJ/Pulse 0.150mJ/Pulse Fitting Line
6
4
2
0.0
0.1 0.2 0.3 Pumped Region Length (cm)
0.4
Fig. 3. The dependence of the emission intensity at peak wavelength on the excitation length at indicated pump intensities. (TAANPI concentration is 30% and kpeak = 641.6 nm.)
Table 1 The dependence of threshold and gain on the doped concentrations, the optical gain values of TAANPI:PS films were measured at 0.15 mJ pulse1 System
Dye concentration (%) by weight
kASE (nm)
Ith (mJ pulse1 cm2)
PL efficiency (%)
Gain (cm1)
TAANPI:PS film
15 30 40 50 60
640.58 641.62 644.24 645.81 647.37
0.94 0.86 1.46 1.50 1.62
32.4 27.5 20.1 17.5 15.8
17.26 26.59 18.61 14.56 9.26
108
W. Lu et al. / Chemical Physics Letters 409 (2005) 105–109
6 Output Intensity (a.u.)
ple, thus the loss coefficient can be estimated. Fig. 5 shows the output intensity at kASE = 641.6 nm as a function of X. By fitting, a loss coefficient of 2.47 cm1 was achieved in the 30% TAANPI concentration in PS film. This value is much more smaller than 44 cm1 loss coefficient of poly[2-butyl-5-(2 0 -ethyl-hexyl)-1,4-phenylene vinylene] [19] and 30 cm1 loss coefficient of poly[(2,5dioctyloxy)-1,4-phenylene vinylene] [23], and also lower than that of DCM1 doped PS film [20]. The low loss coefficient should be attributed to the high PL efficiency and low self-absorption or scattering of TAANPI.
15% 30% 40% 50%
4
2
0
4. Conclusions 0.0
0.1 0.2 0.3 0.4 Pumped Region Length (cm)
0.5
Fig. 4. The dependence of the emission intensity at peak wavelength on the excitation length at indicated doped concentration (pumped intensity is 0.15 mJ pulse1).
of emission spectrum as a function of excitation stripe length at different TAANPI doping concentrations. The corresponding results of gain values for different TAANPI concentrations were given in Table 1. There exists an optimal concentration for a higher net gain value. The loss coefficient of the waveguide was measured by using the method, where we kept the pumped length constant (l = 0.5 cm) and moved the pumped region away from the edge of the sample. Since the emission from the end of the pump stripe remains constant, the detected signal from the edge of the sample should decrease as I = I0 e(aX), where a is the waveguide loss coefficient, and X is the length of the unpumped region from the end of the pump region to the edge of the sam10
We have investigated amplified spontaneous emission (ASE) of a new semiconductor oligomer 1,8-naphthalimide derivative (TAANPI) doped polymer films. A spectral narrowing red emission was achieved, and the narrowing phenomenon was attributed to the amplified spontaneous emission (ASE) mechanism, which is seldom reported on the 1,8-naphthalimide derivatives as gain medium. Due to the existence of less concentration quenching effects, TAANPI doped polymer films show low threshold, large net gain, and low loss even though the doping concentration is up to 60%. Our results indicate that TAANPI is an attractive candidate as gain medium to realize red organic semiconductor diode lasers.
Acknowledgements The author (Dr. D. Ma) is grateful to Hundreds Talents Program of Chinese Academy of Sciences, National Education Committee Foundation, and National Personnel Ministry Foundation for financial support.
Output Intensity (a.u.)
Experimental Data Fitting Line
References 1
0.1
0.01 0.00
0.04 0.08 0.12 0.16 Unpumped Region Length (cm)
Fig. 5. The intensity of light emitted at kpeak = 641.6 nm from the edge of a waveguide as a function of the distance between the pump stripe and the edge of the TAANPI:PS film.
[1] M.D. McGehee, A.J. Heeger, Adv. Mater. 12 (2000) 1655. [2] N. Tessler, Adv. Mater. 11 (1999) 363. [3] V.G. Kozlov, V. Bulovic, P.E. Burrows, M. Baldo, V.B. Khalfin, G. Parthasarathy, S.R. Forrest, J. Appl. Phys. 84 (1998) 4096. [4] S. Yokoyama, A. Otomo, S. Mashiko, Appl. Phys. Lett. 80 (2002) 7. [5] O. Masanori, K. Mashiro, G. Dirk, H. Wolfgang, Y. Katsumi, Adv. Mater. 14 (2002) 306. [6] F. Araoka, K.-C. Shin, Y. Takanishi, K. Ishikawa, H. Takezoe, Z.G. Zhu, T.M. Swager, J. Appl. Phys. 94 (2003) 279. [7] Y. Yoshida, Y. Nishihara, R. Ootake, A. Fuji, M. Ozaki, K. Yoshino, H.K. Kim, N.S. Baek, S.K. Choi, J. Appl. phys. 90 (2001) 6061. [8] 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. [9] G.A. Turnbull, P. Andrew, W.L. Barnes, I.D.W. Samuel, Phys. Rev. B 67 (2003) 165107.
W. Lu et al. / Chemical Physics Letters 409 (2005) 105–109 [10] U. Scherf, S. Riechel, U. Lemmer, R.F. Mahrt, Curr. Opin. Solid State Mater. Sci. 5 (2001) 143. [11] W.-C. Wu, H.-C. Yeh, L.-H. Chan, C.-T. Chen, Adv. Mater. 14 (2002) 1072. [12] Y.Q. Gao, R.A. Marcus, J. Phys. Chem. A 106 (2002) 1956. [13] F. Cacialli, R.H. Friend, J. Appl. Phys. 83 (1998) 2343. [14] J. Gan, Q.L. Song, X.Y. Hou, K. Chen, H. Tian, J. Photochem. Photobiol. A 162 (2004) 399. [15] D. Kolosov, V. Adamovich, P. Djurovich, M.E. Thompson, C. Adachi, J. Am. Chem. Soc. 124 (2002) 9945. [16] V. Wintges, P. Valat, J. Kossanyi, L. Biczok, A. Demeter, T. Berces, J. Chem. Soc., Faraday Trans. 90 (1994) 411.
109
[17] A. Demeter, T. Berces, L. Bicsok, V. Wintges, P. Valat, J. Kossanyi, J. Phys. Chem. 100 (1996) 2001. [18] V. Wintges, P. Valat, J. Kossanyi, A. Demeter, L. Biczok, T. Berces, New J. Chem. 20 (1996) 1149. [19] M.D. McGehee, R. Gupta, S. Veenstra, E.K. Miller, M.A. DiazGarcia, A.J. Heeger, Phys. Rev. B 58 (1998) 7035. [20] W. Lu, B. Zhong, D.G. Ma, Appl. Opt. 43 (2004) 5074. [21] K.L. Shaklee, R.F. Leheny, Appl. Phys. Lett. 18 (1971) 475. [22] M. Ichikawa, T. Tachi, M. Satsuki, S. Suga, T. Koyama, T. Yoshio, J. Photochem. Photobiol. A 158 (2003) 219. [23] S.V. Frolov, Z.V. Vardeny, Phys. Rev. B 57 (1998) 9141.