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Optical gain measurements in polymethyl methacrylate plastic doped with perylimide dyes
Journal of Luminescence 94–95 (2001) 55–58
Optical gain measurements in polymethyl methacrylate plastic doped with perylimide dyes H. Manaa*, S.M. Al-Alawi Physics department, University of Bahrain, P.O. Box 32038, Isa Town, Bahrain
Abstract The effective optical gain is measured in a polymethyl methacrylate plastic slab doped with perylimide dyes and found to be positive. The experimental set-up is based on a single exciting laser beam, and the detection of a one pass amplified light emitted along the length of an optically excited sample. The photoluminescence output intensity varies exponentially with the length of the excited region. Under moderate pumping intensities, net laser gains of 53 and 36 cm1 are measured for yellow KF 241 and red BASF 339, respectively. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Lasers; Perylimide dyes; Optical gain
In the last several years, there has been a great interest in studying organic dyes embedded in various solid-state matrices [1–3]. The main goal is to replace liquid dye lasers by solid-state dyes that can combine wavelength tunability in the visible domain, high fluorescence quantum efficiency, compactness, non-volatility, non-toxicity and mechanical stability [4]. In spite of the inconvenience of heat dissipation, solid-state dye lasers also have the advantage of a low laser oscillation threshold due to their high-stimulated emission cross-sections. The diversity of applications of solid materials doped with organic dyes is due to the wide range of types of matrices that the dyes can be embedded in. For example, optical wave guiding is possible in the organically modified silicon (ORMSIL) doped with perylimide dyes. Alternatively, fluorescent solar collectors made of transparent plastic doped with appropriate organic dyes *Corresponding author. Fax: +973-682-582. E-mail address: [email protected] (H. Manaa).
can be used to collect and convert sunlight into electricity [5,6]. In this work, we have performed direct measurements of the effective optical gain coefficient in two samples of polymethyl methacrylate (PMMA), one doped with perylimide BASF 339, and the other with KF241. The samples have the form of parallelepiped slabs of dimensions (85 50 2.9) mm3, with well polished surfaces. The concentration of the perylimide dye in PMMA is 10–4 M for both KF241 and BASF 339. The chemical structure of these peryline derivative dyes has already been published in Refs. [7,8], and their absorption spectra are shown in Fig. 1. The spectral shapes are very similar, and exhibit strong absorption bands corresponding to the allowed optical transition So -S1 : The absorption crosssection peaks are estimated from the absorption spectra and found to be 2.4 10–16 cm2/molecule at a wavelength of 577 nm for KF and 3.5 10–16 cm2/molecule at a wavelength of 526 nm for BASF.
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 2 7 8 - 2
56
H. Manaa, S.M. Al-Alawi / Journal of Luminescence 94–95 (2001) 55–58
Fig. 1. Absorption spectra of PMMA doped with KF241 and BASF 339 perylimide dyes.
It is known that the optical gain coefficient is the main parameter to determine the laser potentiality of a medium. We can write geff ¼ gse a;
ð1Þ
where geff is the effective gain coefficient, and gse is the stimulated emission gain which is related only to the strength of the stimulated emission crosssection sse in the material, and a is the total optical losses per unit length of the sample. The losses are mainly due to excited-state absorption from the emitting energy level, and to self-absorption. In terms of cross-section, this equation can be rewritten as seff ¼ sse sloss ;
ð2Þ
where seff is the effective stimulated emission cross-section and it is related to the effective gain coefficient by the equation seff ¼ geff =N; where N is the density of excited molecules and it depends on the conditions of the experiment. Several experimental methods are usually employed to carry out optical gain and/or excited state absorption measurements [9,10]. The pump/ probe technique has been widely used to perform such measurements. It measures the amplification of an external probe beam passing through the studied medium subject to strong pulsed excitation. The disadvantage of this method lies in the difficulty of obtaining both a homogeneously excited medium and perfectly overlapped beams,
and these affect the precision of the measurements. Moreover, in order to work with a good signalto-noise ratio, the exciting laser pulses must be energetic enough to create a large enough density of excited molecules. This condition may not be fulfilled for all kinds of materials because of damage limitations. A direct measurement method using only one exciting beam is possible, when the studied material can have a high gain coefficient. This is the case for certain semiconductors or organic molecules. Here, one external pulsed beam is used to excite the system, and the gain coefficient is directly deduced from the measurement of the output fluorescence [11]. This work uses the latter method, but with a continuous laser as the excitation source instead of a pulsed laser. A 100-mW Argon laser beam is focused with the help of a cylindrical lens and an appropriate slit, to a horizontal line on the surface of the sample. Both of the main laser spectral lines at 488 and 514 nm are well absorbed by the sample. (Around 40% and 80% of the incident beam are absorbed in BASF and KF respectively). The sample is mounted on a translation stage, allowing a controlled variation of the length of the excited region. The spontaneous fluorescence emitted in parallel to the horizontal line is amplified by stimulated emission through the excited volume. The emitted light is then collected, and focused with a lens at the entrance of a 1200 grooves/mm monochromator, and detected with a sensitive silicon photodiode. The signal is amplified and averaged with a Lock-in amplifier. Care is taken to ensure that only the light passing through the excited volume before reaching the edge of sample is detected. The dependence of the fluorescence spectra in BASF and KF on the excitation length are shown in Figs. 2 and 3. As the length of the excited region passes from 0.10 to 1.00 cm, the fluorescence intensity increases by a factor >10, and the spectra becomes broader. The amplification factor also increases with increasing wavelengths. In the domain of shorter wavelengths, 525–540 nm in KF and 550–590 nm in BASF, the amplification factor is p10. However, beyond these wavelengths, in both systems, the amplification factor
H. Manaa, S.M. Al-Alawi / Journal of Luminescence 94–95 (2001) 55–58
Fig. 2. Emission spectra of PMMA doped with KF241 perylimide, measured with two different excited region lengths (up: x ¼ 1 cm; down: x ¼ 0:1 cm).
emission spectra. This mechanism competes with the stimulated emission and decreases the effective gain coefficient. Furthermore, it is well known that the stimulated emission cross-section increases dramatically with increasing wavelengths since seff is proportional to l4 [12]. Consequently, the optical gain increases with larger wavelengths, and the stimulated emission spectrum becomes broader. The emission spectra obtained with small excitation lengths contain only two bands. These are situated at l ¼ 535 and 575 nm in KF, and l ¼ 605 and 650 nm in BASF. When the excitation length is increased, a third band appears in the emission spectrum at l ¼ 620 nm in the KF sample. The presence of this band is expected because in this kind of molecules, the fluorescence spectrum associated with the optical transitions So -S1 usually exhibits a mirror image relative to the So -S1 absorption transition [13,14]. In the second sample, the investigation could probably be done beyond 750 nm to observe such a behavior. The effective optical gain in the two samples is measured using the experimental method explained above. The effective optical gain geff is related to the output fluorescence intensity I; and the length of the excited strip x; by the equation [11,15]: IðxÞ ¼ ðIs A=geff Þ½expðgeff xÞ 1;
Fig. 3. Emission spectra of PMMA doped with BASF 339 perylimide, measured with two different excited region lengths (up: x ¼ 1 cm; down: x ¼ 0:1 cm).
becomes >10. This behavior indicates that the measured fluorescence spectra are the result of the superposition of spontaneous fluorescence and stimulated emission spectra. Optical gain is achieved when the emitted photons travel through the excited region, and induce the emission of similar photons via the stimulated emission mechanism. The small optical gain coefficient at shorter wavelengths may result from self-absorption due to the overlapping between the absorption and the
57
ð3Þ
where Is is the spontaneous emission rate per unit volume and A is the cross-section area of the excited region. The output fluorescence is measured at the peak of the second emission band in each sample, i.e. at l ¼ 575 nm in KF, and at l ¼ 650 nm in BASF. The output emission is measured for excited strip length varying from 0 to 5 mm. However, in order to avoid inhomogeneity problems in the excited region, only the first millimeter is considered in the calculations. The dependence of the fluorescence intensity I on the excited strip length x is presented in Fig. 4. The exponential increase in the intensity I as a function of the strip length x is a direct indication of the presence of positive gain. In order to evaluate the effective gain geff ; we have modeled the measured intensity using Eq. (3). geff and Is A are taken as adjustable parameters in the fitting
58
H. Manaa, S.M. Al-Alawi / Journal of Luminescence 94–95 (2001) 55–58
good quality samples that were used in these measurements.
References
Fig. 4. Emission intensity as function of the length of the excited region. Squares: KF241, Triangles: BASF 339. Solid line is the fitting of experimental data using gain equation.
calculation. The agreement between the experimental data and the theory is reasonable. The maximum effective optical gain is found to be 53 cm1 in KF 241 and 36 cm1 in BASF 339. In conclusion, we found that the optical gain coefficient in perylimide dyes doped PMMA plastic is large enough to make this material a potential candidate as a gain medium for wavelength tunable laser operation.
Acknowledgements The authors would like to acknowledge D.J.S. Birch and R.E. Imhof for providing the
[1] R. Reisfeld, E. Yariv, H. Minti, Opt. Mater. 8 (1997) 31. [2] R. Reisfeld, H. Minti, E. Yariv, SPIE 3176 (1997) 240. [3] Y.L. Xue, G.B. Smith, A.T. Baker, IEEE J. Quantum Electron. 34 (1998) 1380. [4] R. Reisfeld, R. Gvishi, Z. Burshtein, J. Sol–Gel Sci. Tech. 4 (1995) 49. [5] Y.L. Xue, G.B. Smith, A.T. Baker, SPIE 2893 (1996) 296. [6] G. Seybold, G. Wagenblast, Dyes Pigments 11 (1989) 303. [7] R. Gvishi, R. Reisfeld, J. de Phys. IV 1 (1991) 199. [8] S.M. Al-Alawi, D.J.S. Birch, R.E. Imhof, in: Al-Alawi, H. (Ed.), Proceedings of the Solar Energy Prospect in the Arab world conference, Vol. 108, Pergamon Press, Oxford, 1986. [9] F.Z. Henari, H. Manaa, K.P. Kretsch, W.J. Blau, H. Rost, . S. Pfeiffer, A. Teuschel, H. Tillmann, H.H. Horhold, Chem. Phys. Lett. 307 (1999) 163. [10] H. Manaa, R. Moncorge, F. Deghoul, Y. Guyot, L. Merkle, Chem. Phys. Lett. 238 (1995) 333. [11] K.L. Shaklee, R.F. Leheny, Appl. Phys. Lett. 18 (1971) 471. [12] H. Manaa, R. Moncorge, Y. Guyot, Phys. Rev. B 48 (1993) 3633. [13] R. Gvishi, R. Reisfeld, Z. Burshtein, Chem. Phys. Lett. 213 (1993) 338. [14] R. Gvishi, R. Reisfeld, Z. Burshtein, Chem. Phys. Lett. 212 (1993) 463. [15] Y. Sorek, R. Reisfeld, I. Finkelstein, S. Ruschin, Appl. Phys. Lett. 66 (1995) 1169.
Optical gain measurements in polymethyl methacrylate plastic doped with perylimide dyes H. Manaa*, S.M. Al-Alawi Physics department, University of Bahrain, P.O. Box 32038, Isa Town, Bahrain
Abstract The effective optical gain is measured in a polymethyl methacrylate plastic slab doped with perylimide dyes and found to be positive. The experimental set-up is based on a single exciting laser beam, and the detection of a one pass amplified light emitted along the length of an optically excited sample. The photoluminescence output intensity varies exponentially with the length of the excited region. Under moderate pumping intensities, net laser gains of 53 and 36 cm1 are measured for yellow KF 241 and red BASF 339, respectively. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Lasers; Perylimide dyes; Optical gain
In the last several years, there has been a great interest in studying organic dyes embedded in various solid-state matrices [1–3]. The main goal is to replace liquid dye lasers by solid-state dyes that can combine wavelength tunability in the visible domain, high fluorescence quantum efficiency, compactness, non-volatility, non-toxicity and mechanical stability [4]. In spite of the inconvenience of heat dissipation, solid-state dye lasers also have the advantage of a low laser oscillation threshold due to their high-stimulated emission cross-sections. The diversity of applications of solid materials doped with organic dyes is due to the wide range of types of matrices that the dyes can be embedded in. For example, optical wave guiding is possible in the organically modified silicon (ORMSIL) doped with perylimide dyes. Alternatively, fluorescent solar collectors made of transparent plastic doped with appropriate organic dyes *Corresponding author. Fax: +973-682-582. E-mail address: [email protected] (H. Manaa).
can be used to collect and convert sunlight into electricity [5,6]. In this work, we have performed direct measurements of the effective optical gain coefficient in two samples of polymethyl methacrylate (PMMA), one doped with perylimide BASF 339, and the other with KF241. The samples have the form of parallelepiped slabs of dimensions (85 50 2.9) mm3, with well polished surfaces. The concentration of the perylimide dye in PMMA is 10–4 M for both KF241 and BASF 339. The chemical structure of these peryline derivative dyes has already been published in Refs. [7,8], and their absorption spectra are shown in Fig. 1. The spectral shapes are very similar, and exhibit strong absorption bands corresponding to the allowed optical transition So -S1 : The absorption crosssection peaks are estimated from the absorption spectra and found to be 2.4 10–16 cm2/molecule at a wavelength of 577 nm for KF and 3.5 10–16 cm2/molecule at a wavelength of 526 nm for BASF.
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 2 7 8 - 2
56
H. Manaa, S.M. Al-Alawi / Journal of Luminescence 94–95 (2001) 55–58
Fig. 1. Absorption spectra of PMMA doped with KF241 and BASF 339 perylimide dyes.
It is known that the optical gain coefficient is the main parameter to determine the laser potentiality of a medium. We can write geff ¼ gse a;
ð1Þ
where geff is the effective gain coefficient, and gse is the stimulated emission gain which is related only to the strength of the stimulated emission crosssection sse in the material, and a is the total optical losses per unit length of the sample. The losses are mainly due to excited-state absorption from the emitting energy level, and to self-absorption. In terms of cross-section, this equation can be rewritten as seff ¼ sse sloss ;
ð2Þ
where seff is the effective stimulated emission cross-section and it is related to the effective gain coefficient by the equation seff ¼ geff =N; where N is the density of excited molecules and it depends on the conditions of the experiment. Several experimental methods are usually employed to carry out optical gain and/or excited state absorption measurements [9,10]. The pump/ probe technique has been widely used to perform such measurements. It measures the amplification of an external probe beam passing through the studied medium subject to strong pulsed excitation. The disadvantage of this method lies in the difficulty of obtaining both a homogeneously excited medium and perfectly overlapped beams,
and these affect the precision of the measurements. Moreover, in order to work with a good signalto-noise ratio, the exciting laser pulses must be energetic enough to create a large enough density of excited molecules. This condition may not be fulfilled for all kinds of materials because of damage limitations. A direct measurement method using only one exciting beam is possible, when the studied material can have a high gain coefficient. This is the case for certain semiconductors or organic molecules. Here, one external pulsed beam is used to excite the system, and the gain coefficient is directly deduced from the measurement of the output fluorescence [11]. This work uses the latter method, but with a continuous laser as the excitation source instead of a pulsed laser. A 100-mW Argon laser beam is focused with the help of a cylindrical lens and an appropriate slit, to a horizontal line on the surface of the sample. Both of the main laser spectral lines at 488 and 514 nm are well absorbed by the sample. (Around 40% and 80% of the incident beam are absorbed in BASF and KF respectively). The sample is mounted on a translation stage, allowing a controlled variation of the length of the excited region. The spontaneous fluorescence emitted in parallel to the horizontal line is amplified by stimulated emission through the excited volume. The emitted light is then collected, and focused with a lens at the entrance of a 1200 grooves/mm monochromator, and detected with a sensitive silicon photodiode. The signal is amplified and averaged with a Lock-in amplifier. Care is taken to ensure that only the light passing through the excited volume before reaching the edge of sample is detected. The dependence of the fluorescence spectra in BASF and KF on the excitation length are shown in Figs. 2 and 3. As the length of the excited region passes from 0.10 to 1.00 cm, the fluorescence intensity increases by a factor >10, and the spectra becomes broader. The amplification factor also increases with increasing wavelengths. In the domain of shorter wavelengths, 525–540 nm in KF and 550–590 nm in BASF, the amplification factor is p10. However, beyond these wavelengths, in both systems, the amplification factor
H. Manaa, S.M. Al-Alawi / Journal of Luminescence 94–95 (2001) 55–58
Fig. 2. Emission spectra of PMMA doped with KF241 perylimide, measured with two different excited region lengths (up: x ¼ 1 cm; down: x ¼ 0:1 cm).
emission spectra. This mechanism competes with the stimulated emission and decreases the effective gain coefficient. Furthermore, it is well known that the stimulated emission cross-section increases dramatically with increasing wavelengths since seff is proportional to l4 [12]. Consequently, the optical gain increases with larger wavelengths, and the stimulated emission spectrum becomes broader. The emission spectra obtained with small excitation lengths contain only two bands. These are situated at l ¼ 535 and 575 nm in KF, and l ¼ 605 and 650 nm in BASF. When the excitation length is increased, a third band appears in the emission spectrum at l ¼ 620 nm in the KF sample. The presence of this band is expected because in this kind of molecules, the fluorescence spectrum associated with the optical transitions So -S1 usually exhibits a mirror image relative to the So -S1 absorption transition [13,14]. In the second sample, the investigation could probably be done beyond 750 nm to observe such a behavior. The effective optical gain in the two samples is measured using the experimental method explained above. The effective optical gain geff is related to the output fluorescence intensity I; and the length of the excited strip x; by the equation [11,15]: IðxÞ ¼ ðIs A=geff Þ½expðgeff xÞ 1;
Fig. 3. Emission spectra of PMMA doped with BASF 339 perylimide, measured with two different excited region lengths (up: x ¼ 1 cm; down: x ¼ 0:1 cm).
becomes >10. This behavior indicates that the measured fluorescence spectra are the result of the superposition of spontaneous fluorescence and stimulated emission spectra. Optical gain is achieved when the emitted photons travel through the excited region, and induce the emission of similar photons via the stimulated emission mechanism. The small optical gain coefficient at shorter wavelengths may result from self-absorption due to the overlapping between the absorption and the
57
ð3Þ
where Is is the spontaneous emission rate per unit volume and A is the cross-section area of the excited region. The output fluorescence is measured at the peak of the second emission band in each sample, i.e. at l ¼ 575 nm in KF, and at l ¼ 650 nm in BASF. The output emission is measured for excited strip length varying from 0 to 5 mm. However, in order to avoid inhomogeneity problems in the excited region, only the first millimeter is considered in the calculations. The dependence of the fluorescence intensity I on the excited strip length x is presented in Fig. 4. The exponential increase in the intensity I as a function of the strip length x is a direct indication of the presence of positive gain. In order to evaluate the effective gain geff ; we have modeled the measured intensity using Eq. (3). geff and Is A are taken as adjustable parameters in the fitting
58
H. Manaa, S.M. Al-Alawi / Journal of Luminescence 94–95 (2001) 55–58
good quality samples that were used in these measurements.
References
Fig. 4. Emission intensity as function of the length of the excited region. Squares: KF241, Triangles: BASF 339. Solid line is the fitting of experimental data using gain equation.
calculation. The agreement between the experimental data and the theory is reasonable. The maximum effective optical gain is found to be 53 cm1 in KF 241 and 36 cm1 in BASF 339. In conclusion, we found that the optical gain coefficient in perylimide dyes doped PMMA plastic is large enough to make this material a potential candidate as a gain medium for wavelength tunable laser operation.
Acknowledgements The authors would like to acknowledge D.J.S. Birch and R.E. Imhof for providing the
[1] R. Reisfeld, E. Yariv, H. Minti, Opt. Mater. 8 (1997) 31. [2] R. Reisfeld, H. Minti, E. Yariv, SPIE 3176 (1997) 240. [3] Y.L. Xue, G.B. Smith, A.T. Baker, IEEE J. Quantum Electron. 34 (1998) 1380. [4] R. Reisfeld, R. Gvishi, Z. Burshtein, J. Sol–Gel Sci. Tech. 4 (1995) 49. [5] Y.L. Xue, G.B. Smith, A.T. Baker, SPIE 2893 (1996) 296. [6] G. Seybold, G. Wagenblast, Dyes Pigments 11 (1989) 303. [7] R. Gvishi, R. Reisfeld, J. de Phys. IV 1 (1991) 199. [8] S.M. Al-Alawi, D.J.S. Birch, R.E. Imhof, in: Al-Alawi, H. (Ed.), Proceedings of the Solar Energy Prospect in the Arab world conference, Vol. 108, Pergamon Press, Oxford, 1986. [9] F.Z. Henari, H. Manaa, K.P. Kretsch, W.J. Blau, H. Rost, . S. Pfeiffer, A. Teuschel, H. Tillmann, H.H. Horhold, Chem. Phys. Lett. 307 (1999) 163. [10] H. Manaa, R. Moncorge, F. Deghoul, Y. Guyot, L. Merkle, Chem. Phys. Lett. 238 (1995) 333. [11] K.L. Shaklee, R.F. Leheny, Appl. Phys. Lett. 18 (1971) 471. [12] H. Manaa, R. Moncorge, Y. Guyot, Phys. Rev. B 48 (1993) 3633. [13] R. Gvishi, R. Reisfeld, Z. Burshtein, Chem. Phys. Lett. 213 (1993) 338. [14] R. Gvishi, R. Reisfeld, Z. Burshtein, Chem. Phys. Lett. 212 (1993) 463. [15] Y. Sorek, R. Reisfeld, I. Finkelstein, S. Ruschin, Appl. Phys. Lett. 66 (1995) 1169.