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Longitudinal pumping of polymer microring lasers
Synthetic Metals 127 (2002) 159–163
Longitudinal pumping of polymer microring lasers T. Ben-Messaoud, S.X. Dou, E. Toussaere*, A. Potter, D. Josse, G. Kranzelbinder, J. Zyss Laboratoire de Photonique Quantique et Mole´culaire (UMR 8537), ENS-Cachan, 61 Av. du Pre´sident Wilson, 94235 Cachan, France
Abstract Pulsed, longitudinal photopumped multi-mode laser emission is demonstrated in the visible spectral range from cylindrical microcavities formed by luminescent polymer thin films coated around optical fibers. Thresholds for laser oscillation for picosecond excitation are 80 pJ/ mm for longitudinal pumping as compared to 4 nJ/mm (5 mJ/cm2 or 0.05 MW/cm2) for transversal pumping. An important advantage of the longitudinal pumping configuration lies in a significantly lower excitation density at threshold in comparison with the transverse pumping configuration. This improvement is ascribed to a larger overlap factor between the optical pump and the gain medium in the longitudinal geometry. The emission spectra of plastic microrings under these different excitation configurations are reported and analyzed. We note that lowering the lasing threshold is an important prerequisite criterion towards the development of electrically pumped plastic lasers. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microcavities; Polymer lasers; Longitudinal pumping; Whispering gallery modes
1. Introduction Organic solid state lasers have been at the origin of the development of laser sources in waveguiding integrated optics format [1] and they experience a revival of interest due to the recent emergence of a wide range of conjugated polymers in the solid state. These polymers have already displayed lasing in various optically pumped configurations including planar microcavities [2,3], distributed feedback (DFB) lasers on plastic substrate [4] or tunable DFB lasers [5,6] as well as in microdisk or microring configurations [7,8]. However, in this last configuration, optical pumping has been reported only in a transverse geometry with respect to the ring. We present here evidence of the interest of longitudinal pumping whereby the optical pump is guided by the optical fiber supporting the microring laser and evanescently coupled to the active medium.
2. Experiment Microring samples are being coated around glass multimode optical fibers that were etched in hydrofluoric acid from their original diameter, 125 mm, to the desired size. * Corresponding author. Tel.: þ33-1-4740-5557; fax: þ33-1-4740-5567. E-mail address: [email protected] (E. Toussaere).
Low concentration acid was used to maintain adequate surface quality. The etched optical fiber was dipped into a solution of DCM–PMMA in anisole, which after quick evaporation, forms a coating around the fiber shaping a complete cylinder of 1.5 mm in thickness, 150 mm in length, and a diameter predetermined by the size of the fiber. The concentration of DCM in PMMA was 2 104 mol/ cm3. The samples can then be used without any further processing. The polymer microrings were photopumped by 100 pslong pulses at 532 nm from a Nd:YAG laser. The two different, longitudinal and transversal, pumping configurations are shown in Fig. 1. In the transverse pumping configuration, the laser beam was first focused through a cylindrical lens, creating a stripe perpendicular to the microring axis, in order to excite modes in a thin cross-section of a single microring sample. The beam was then focused through a spherical lens to further reduce the size of the pumped region. In the longitudinal pumping configuration, the laser beam was coupled directly into the optical fiber. The fiber with the microrings was contained in a cell under nitrogen gas flushing so as to prevent possible degradation of the samples as a result of photo-oxidation. The microring was probed by two microscopic objectives set in the plane of the ring, and the output was carried by a multi-mode optical fiber to a spectrometer and cooled charge coupled device (CCD) camera. The overall spectral resolution of the system is 0.4 nm.
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 6 1 4 - 2
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Fig. 1. Experimental setup for the investigation of microring lasers.
3. Modeling electromagnetic modes in cylindrical microlasers In comparison with a planar microcavity which always experiences losses due to imperfect reflections from two highly reflective mirrors forming the microcavity [2,9], cylindrical microcavities have relatively higher finesse coefficients (i.e. quality factor Q). The light in such microcavities is confined inside the gain medium by total internal reflection, which leads to optical resonances known as whispering gallery modes (WGMs). Optical modes inside a cylinder are given by solutions of the two-dimensional Helmholtz equation [10,11] n2 o 2 r2r;y þ eff2 cðr; yÞ ¼ 0 (1) c where neff is the effective refractive index accounting for mode propagation in the plane of the cylinder, o the optical radial frequency, c the speed of light in vacuum and c the magnetic (resp. electric) field along the cylinder axis of a transverse electric (resp. magnetic) TE (resp. TM) mode. Searching for optical field solutions of the Helmholtz equation in the form of a product cðr; yÞ ¼ RðrÞYðyÞ leads to a decoupling of differential equations for R(r) and Y(y) r2
d2 d RðrÞ þ r RðrÞ ðk2 r 2 þ M 2 ÞRðrÞ ¼ 0 dr dr 2
namely cðr ¼ R0 ; yÞ ¼ 0 where R0 is the outer radius of the ring. In this case the solutions of Eq. (1) are given by XM;N r iMy (4) cðr; yÞ ¼ AM JM e R0 where JM are Bessel functions of the first kind of integer order M, AM is a normalization constant and XM;N ¼ neff oM;N R0 =c is the Nth zero of JM(r). An example of this mode function is shown for R0 ¼ 3 mm and M ¼ 46 in Fig. 2. In our case, only modes with N ¼ 1 are observed since they are less lossy. We abbreviate lM,1 by lM and define neff as the effective index of the lasing modes which is almost independent of the mode number (M, N) in the spectral range of interest. For large (integer) M, the effective cavity length of 2pR0 imposed by the periodic boundary condition results in a WGM condition given by 2pR0 neff ¼ MlM
(5)
(2)
and d2 YðyÞ M 2 YðyÞ ¼ 0 dy2
(3)
where k ¼ neff o=c. One approach to simplify the problem is to assume that guided modes are well confined inside the cavity with strict cancellation of the field at the boundary,
Fig. 2. Plot of electric field cM(r, y) for M ¼ 46, N ¼ 1 and R0 ¼ 3 mm.
T. Ben-Messaoud et al. / Synthetic Metals 127 (2002) 159–163
which leads in turn to the following simple expression for the mode spacing Dl ¼
l2 2pR0 neff
(6)
where l is the wavelength of the mode.
4. Experimental results and data analysis 4.1. Assignment of the photopumped emission lines The emission spectra of microring lasers are analyzed in the light of a Fourier transformation, which allows for the labeling of the photopumped emission lines [12]. The positive harmonics in the transform gives the product of the effective index of refraction, neff, by the laser cavity radius, R0. Feeding the neffR0 value in the expression of XM,N in Eq. (4) allows to assign to the resonant peaks an azimuthal mode number, M, and a radial mode number, N, corresponding to the Bessel function associated to the field distribution in that mode (see Fig. 3). Fig. 3 shows a many peaked lasing spectrum obtained from a microring of approximately 70 mm in diameter. In the two different configurations corresponding to longitudinal (dotted line) and transversal (continuous line) pumping, very similar spectral emission features were observed. The aver-
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age spacing Dl of the emission lines in Fig. 3 was 1.4 nm. Using the strongest emission lines around 616.92 nm, we get from Eq. (6) a product neffR0 ¼ 43:27 mm. The Fourier transform of the emission spectrum provides a more accurate handle towards the determination of neffR0. It may then be used to analyze the emission lines with greater accuracy. Fig. 4 is the Fourier transform of the emission spectrum shown in Fig. 3, plotted on a log scale. When the spectrum is measured in wavevector space (mm1), the Fourier transform will be in pathlength space, measured in micrometer. With such units, an average value of the product neffR0 can be directly obtained from the peak spacing of the transform plot which leads, in the case of Fig. 4, to a value of 43.41 mm. The inset of Fig. 3 shows the difference, dl, between the wavelengths of predicted and experimental peaks, which is almost negligible. Based on the agreement between experimental and model calculations with dl value ranging from 0.25 to 0.24 nm, we conclude that the entire spectrum is well accounted for by a neffR0 value of 43.41 mm and M indices ranging from 419 to 441. 4.2. Threshold characterization Fig. 5(a) and (b) display the lasing threshold determination experiment using, respectively, longitudinal and transverse pumping configurations. For both pumping con-
Fig. 3. Emission spectra of a DCM–PMMA microring laser of approximately 70 mm in diameter; the dotted and the continuous lines corresponding to longitudinal pumping and transversal pumping, respectively. M and N indices are assigned to each laser line (see text). The spacing between modes, Dl, is indicated. Inset shows the difference, dl, between the wavelengths of predicted and experimental peaks.
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Fig. 4. Fourier transform of the experimental laser spectrum given in Fig. 3.
figurations, three spectra from a single DCM–PMMA microring (D ¼ 70 mm) are shown. The lowest line corresponds to a pumping energy just above the lasing threshold; the middle line and the upper line are taken at increasingly higher pump powers. The unit of pump power is mJ/mm, where the length is normalized either to the beam diameter (transversal pumping) or the microring height (longitudinal pumping). We use it for the sake of comparing the thresholds of the two pumping configurations. The lasing thresholds can be inferred from these figures. Whereas the mode structures in both cases are essentially identical, the main difference is the significantly lower excitation intensity in the case of longitudinal pumping. The important advantage in this configuration results from a more isotropic gain distribution in the lateral size of the cylindrical microring. The excitation area in the transversal
pumping configuration (one-sided asymmetrical excitation) being smaller than the lateral size of the cylindrical microring, the ring cavity did not provide any lateral confinement (along the cylinder axis) for the laser modes. A plot of relative emission intensity vs. pump intensity is shown in Fig. 6(a) and (b), plotted on double log scale. The relative intensities in Fig. 6(a) and (b) were obtained by integrating throughout the full resonant peaks. In the case of the transverse pumping configuration, the threshold, 4 nJ/mm (5 mJ/cm2 or 0.05 MW/cm2) is lower than that reported elsewhere with 100 ps pumping [13]. In the case of the longitudinal pumping configuration, the threshold excitation density, 80 pJ/mm for 100 ps pumping, is much lower than in the case of the transversal pumping excitation and has never been reported elsewhere to the best of an knowledge.
Fig. 5. (a) Emission spectra obtained by longitudinal pumping at three different excitation intensities. (b) Emission spectra obtained by transverse pumping at three different excitation intensities.
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Fig. 6. (a) Emission intensity as a function of excitation for longitudinal pumping. (b) Emission intensity as a function of excitation for transversal pumping.
The quality factor Q of the cavity can be inferred from the ratio of one harmonic peak amplitude to the next in the Fourier transform of the spectrum [12]. The cavity Q factor obtained by this method was estimated to be of the order of 103. It should be noted that the spectra are limited by the resolution of our CCD detection system. Thus, the actual value of Q should be higher than this value.
5. Conclusion We have developed cylindrical longitudinally optically pumped microring lasers compatible with both p-conjugated polymers and dye-doped polymers as the active coating material. Such cavities are characterized by high Q, narrow spectral lines and very low laser threshold excitation intensities. This pumping configuration is more homogeneous and efficient than the transverse pumping configuration. The lasing threshold has been lowered by three orders of magnitude compared to the reported values obtained by the transversal pumping. Such low lasing threshold is an important perquisite towards further electrically pumped plastic lasers.
References [1] H. Kogelnik, C.V. Shank, Coupled wave theory of distributed feedback lasers, J. Appl. Phys. 43 (1972) 2328.
[2] N. Tessler, G.J. Denton, R.H. Friend, Lasing of conjugated polymer microcavities, Nature 382 (1996) 695–697. [3] F. Hide, M.A. Diaz-Garcia, B.J. Schwartz, M.R. Andersson, Q.B. Pei, A.J. Heeger, Semiconducting polymers: a new class of solid-state laser materials, Science 273 (1996) 1833–1835. [4] C. Kallinger, M. Hilmer, A. Haugender, M. Perner, W. Spirkel, U. Lemmer, J. Feldmann, U. Scherf, K. Mu¨ llen, A. Gombert, V. Wittwer, A flexible conjugated polymer laser, Adv. Mater. 10 (12) (1998) 920–923. [5] V. Dumarcher, L. Rocha, C. Denis, C. Fiorini, J.M. Nunzi, F. Sobel, B. Sahraoui, D. Gindre, Polymer thin-film distributed feedback tunable lasers, J. Opt. A: Pure Appl. Opt. 2 (2000) 279–283. [6] G. Kranzelbinder, E. Toussaere, D. Josse, J. Zyss, Progress in erasable and permanent polymer based DFB-structures with multicolour tunable laser emission, Synthetic Met. 121 (2001) 1617–1620. [7] S.V. Frolov, A. Fujii, D. Chinn, Z.V. Vardeny, K. Yoshino, R.V. Gregory, Cylindrical microlasers and light emitting from conducting polymers, Appl. Phys. Lett. 72 (22) (1998). [8] S.V. Frolov, M. Shkunov, Z.V. Vardeny, K. Yoshino, Ring microlasers from conducting polymers, Phys. Rev. B 56 (8) (1997) 4363–4366. [9] A. Schulzgen, C. Spiegelberg, M.N. Morrell, S.B. Mendes, B. Kippelen, N. Peyghambarian, M.F. Nabor, E.A. Mash, P.M. Allemand, Near diffraction-limited laser emission from a polymer in a high finesse planar cavity, Appl. Phys. Lett. 72 (3) (1998) 269– 271. [10] R.E. Slusher, A.F.J. Levi, U. Mohideen, S.L. McCall, S.J. Pearton, R.A. Logan, Threshold characteristics of semiconductor microdisk lasers, Appl. Phys. Lett. 63 (10) (1993) 1310–1312. [11] N.C. Frateschi, A.F.J. Levi, Resonant modes and laser spectrum of microdisk lasers, Appl. Phys. Lett. 66 (22) (1995) 2932–2934. [12] R.C. Polson, G. Levina, Z.V. Vardeny, Spectral analysis of polymer microring lasers, Appl. Phys. Lett. 76 (26) (2000) 3858–3860. [13] S.V. Frolov, M. Shkunov, A. Fujii, K. Yoshino, Z.V. Vardeny, Lasing and stimulated emission in p-conjugated polymers, IEEE J. Quant. Electr. 36 (1) (2000) 2–11.
Longitudinal pumping of polymer microring lasers T. Ben-Messaoud, S.X. Dou, E. Toussaere*, A. Potter, D. Josse, G. Kranzelbinder, J. Zyss Laboratoire de Photonique Quantique et Mole´culaire (UMR 8537), ENS-Cachan, 61 Av. du Pre´sident Wilson, 94235 Cachan, France
Abstract Pulsed, longitudinal photopumped multi-mode laser emission is demonstrated in the visible spectral range from cylindrical microcavities formed by luminescent polymer thin films coated around optical fibers. Thresholds for laser oscillation for picosecond excitation are 80 pJ/ mm for longitudinal pumping as compared to 4 nJ/mm (5 mJ/cm2 or 0.05 MW/cm2) for transversal pumping. An important advantage of the longitudinal pumping configuration lies in a significantly lower excitation density at threshold in comparison with the transverse pumping configuration. This improvement is ascribed to a larger overlap factor between the optical pump and the gain medium in the longitudinal geometry. The emission spectra of plastic microrings under these different excitation configurations are reported and analyzed. We note that lowering the lasing threshold is an important prerequisite criterion towards the development of electrically pumped plastic lasers. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microcavities; Polymer lasers; Longitudinal pumping; Whispering gallery modes
1. Introduction Organic solid state lasers have been at the origin of the development of laser sources in waveguiding integrated optics format [1] and they experience a revival of interest due to the recent emergence of a wide range of conjugated polymers in the solid state. These polymers have already displayed lasing in various optically pumped configurations including planar microcavities [2,3], distributed feedback (DFB) lasers on plastic substrate [4] or tunable DFB lasers [5,6] as well as in microdisk or microring configurations [7,8]. However, in this last configuration, optical pumping has been reported only in a transverse geometry with respect to the ring. We present here evidence of the interest of longitudinal pumping whereby the optical pump is guided by the optical fiber supporting the microring laser and evanescently coupled to the active medium.
2. Experiment Microring samples are being coated around glass multimode optical fibers that were etched in hydrofluoric acid from their original diameter, 125 mm, to the desired size. * Corresponding author. Tel.: þ33-1-4740-5557; fax: þ33-1-4740-5567. E-mail address: [email protected] (E. Toussaere).
Low concentration acid was used to maintain adequate surface quality. The etched optical fiber was dipped into a solution of DCM–PMMA in anisole, which after quick evaporation, forms a coating around the fiber shaping a complete cylinder of 1.5 mm in thickness, 150 mm in length, and a diameter predetermined by the size of the fiber. The concentration of DCM in PMMA was 2 104 mol/ cm3. The samples can then be used without any further processing. The polymer microrings were photopumped by 100 pslong pulses at 532 nm from a Nd:YAG laser. The two different, longitudinal and transversal, pumping configurations are shown in Fig. 1. In the transverse pumping configuration, the laser beam was first focused through a cylindrical lens, creating a stripe perpendicular to the microring axis, in order to excite modes in a thin cross-section of a single microring sample. The beam was then focused through a spherical lens to further reduce the size of the pumped region. In the longitudinal pumping configuration, the laser beam was coupled directly into the optical fiber. The fiber with the microrings was contained in a cell under nitrogen gas flushing so as to prevent possible degradation of the samples as a result of photo-oxidation. The microring was probed by two microscopic objectives set in the plane of the ring, and the output was carried by a multi-mode optical fiber to a spectrometer and cooled charge coupled device (CCD) camera. The overall spectral resolution of the system is 0.4 nm.
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 6 1 4 - 2
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Fig. 1. Experimental setup for the investigation of microring lasers.
3. Modeling electromagnetic modes in cylindrical microlasers In comparison with a planar microcavity which always experiences losses due to imperfect reflections from two highly reflective mirrors forming the microcavity [2,9], cylindrical microcavities have relatively higher finesse coefficients (i.e. quality factor Q). The light in such microcavities is confined inside the gain medium by total internal reflection, which leads to optical resonances known as whispering gallery modes (WGMs). Optical modes inside a cylinder are given by solutions of the two-dimensional Helmholtz equation [10,11] n2 o 2 r2r;y þ eff2 cðr; yÞ ¼ 0 (1) c where neff is the effective refractive index accounting for mode propagation in the plane of the cylinder, o the optical radial frequency, c the speed of light in vacuum and c the magnetic (resp. electric) field along the cylinder axis of a transverse electric (resp. magnetic) TE (resp. TM) mode. Searching for optical field solutions of the Helmholtz equation in the form of a product cðr; yÞ ¼ RðrÞYðyÞ leads to a decoupling of differential equations for R(r) and Y(y) r2
d2 d RðrÞ þ r RðrÞ ðk2 r 2 þ M 2 ÞRðrÞ ¼ 0 dr dr 2
namely cðr ¼ R0 ; yÞ ¼ 0 where R0 is the outer radius of the ring. In this case the solutions of Eq. (1) are given by XM;N r iMy (4) cðr; yÞ ¼ AM JM e R0 where JM are Bessel functions of the first kind of integer order M, AM is a normalization constant and XM;N ¼ neff oM;N R0 =c is the Nth zero of JM(r). An example of this mode function is shown for R0 ¼ 3 mm and M ¼ 46 in Fig. 2. In our case, only modes with N ¼ 1 are observed since they are less lossy. We abbreviate lM,1 by lM and define neff as the effective index of the lasing modes which is almost independent of the mode number (M, N) in the spectral range of interest. For large (integer) M, the effective cavity length of 2pR0 imposed by the periodic boundary condition results in a WGM condition given by 2pR0 neff ¼ MlM
(5)
(2)
and d2 YðyÞ M 2 YðyÞ ¼ 0 dy2
(3)
where k ¼ neff o=c. One approach to simplify the problem is to assume that guided modes are well confined inside the cavity with strict cancellation of the field at the boundary,
Fig. 2. Plot of electric field cM(r, y) for M ¼ 46, N ¼ 1 and R0 ¼ 3 mm.
T. Ben-Messaoud et al. / Synthetic Metals 127 (2002) 159–163
which leads in turn to the following simple expression for the mode spacing Dl ¼
l2 2pR0 neff
(6)
where l is the wavelength of the mode.
4. Experimental results and data analysis 4.1. Assignment of the photopumped emission lines The emission spectra of microring lasers are analyzed in the light of a Fourier transformation, which allows for the labeling of the photopumped emission lines [12]. The positive harmonics in the transform gives the product of the effective index of refraction, neff, by the laser cavity radius, R0. Feeding the neffR0 value in the expression of XM,N in Eq. (4) allows to assign to the resonant peaks an azimuthal mode number, M, and a radial mode number, N, corresponding to the Bessel function associated to the field distribution in that mode (see Fig. 3). Fig. 3 shows a many peaked lasing spectrum obtained from a microring of approximately 70 mm in diameter. In the two different configurations corresponding to longitudinal (dotted line) and transversal (continuous line) pumping, very similar spectral emission features were observed. The aver-
161
age spacing Dl of the emission lines in Fig. 3 was 1.4 nm. Using the strongest emission lines around 616.92 nm, we get from Eq. (6) a product neffR0 ¼ 43:27 mm. The Fourier transform of the emission spectrum provides a more accurate handle towards the determination of neffR0. It may then be used to analyze the emission lines with greater accuracy. Fig. 4 is the Fourier transform of the emission spectrum shown in Fig. 3, plotted on a log scale. When the spectrum is measured in wavevector space (mm1), the Fourier transform will be in pathlength space, measured in micrometer. With such units, an average value of the product neffR0 can be directly obtained from the peak spacing of the transform plot which leads, in the case of Fig. 4, to a value of 43.41 mm. The inset of Fig. 3 shows the difference, dl, between the wavelengths of predicted and experimental peaks, which is almost negligible. Based on the agreement between experimental and model calculations with dl value ranging from 0.25 to 0.24 nm, we conclude that the entire spectrum is well accounted for by a neffR0 value of 43.41 mm and M indices ranging from 419 to 441. 4.2. Threshold characterization Fig. 5(a) and (b) display the lasing threshold determination experiment using, respectively, longitudinal and transverse pumping configurations. For both pumping con-
Fig. 3. Emission spectra of a DCM–PMMA microring laser of approximately 70 mm in diameter; the dotted and the continuous lines corresponding to longitudinal pumping and transversal pumping, respectively. M and N indices are assigned to each laser line (see text). The spacing between modes, Dl, is indicated. Inset shows the difference, dl, between the wavelengths of predicted and experimental peaks.
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Fig. 4. Fourier transform of the experimental laser spectrum given in Fig. 3.
figurations, three spectra from a single DCM–PMMA microring (D ¼ 70 mm) are shown. The lowest line corresponds to a pumping energy just above the lasing threshold; the middle line and the upper line are taken at increasingly higher pump powers. The unit of pump power is mJ/mm, where the length is normalized either to the beam diameter (transversal pumping) or the microring height (longitudinal pumping). We use it for the sake of comparing the thresholds of the two pumping configurations. The lasing thresholds can be inferred from these figures. Whereas the mode structures in both cases are essentially identical, the main difference is the significantly lower excitation intensity in the case of longitudinal pumping. The important advantage in this configuration results from a more isotropic gain distribution in the lateral size of the cylindrical microring. The excitation area in the transversal
pumping configuration (one-sided asymmetrical excitation) being smaller than the lateral size of the cylindrical microring, the ring cavity did not provide any lateral confinement (along the cylinder axis) for the laser modes. A plot of relative emission intensity vs. pump intensity is shown in Fig. 6(a) and (b), plotted on double log scale. The relative intensities in Fig. 6(a) and (b) were obtained by integrating throughout the full resonant peaks. In the case of the transverse pumping configuration, the threshold, 4 nJ/mm (5 mJ/cm2 or 0.05 MW/cm2) is lower than that reported elsewhere with 100 ps pumping [13]. In the case of the longitudinal pumping configuration, the threshold excitation density, 80 pJ/mm for 100 ps pumping, is much lower than in the case of the transversal pumping excitation and has never been reported elsewhere to the best of an knowledge.
Fig. 5. (a) Emission spectra obtained by longitudinal pumping at three different excitation intensities. (b) Emission spectra obtained by transverse pumping at three different excitation intensities.
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Fig. 6. (a) Emission intensity as a function of excitation for longitudinal pumping. (b) Emission intensity as a function of excitation for transversal pumping.
The quality factor Q of the cavity can be inferred from the ratio of one harmonic peak amplitude to the next in the Fourier transform of the spectrum [12]. The cavity Q factor obtained by this method was estimated to be of the order of 103. It should be noted that the spectra are limited by the resolution of our CCD detection system. Thus, the actual value of Q should be higher than this value.
5. Conclusion We have developed cylindrical longitudinally optically pumped microring lasers compatible with both p-conjugated polymers and dye-doped polymers as the active coating material. Such cavities are characterized by high Q, narrow spectral lines and very low laser threshold excitation intensities. This pumping configuration is more homogeneous and efficient than the transverse pumping configuration. The lasing threshold has been lowered by three orders of magnitude compared to the reported values obtained by the transversal pumping. Such low lasing threshold is an important perquisite towards further electrically pumped plastic lasers.
References [1] H. Kogelnik, C.V. Shank, Coupled wave theory of distributed feedback lasers, J. Appl. Phys. 43 (1972) 2328.
[2] N. Tessler, G.J. Denton, R.H. Friend, Lasing of conjugated polymer microcavities, Nature 382 (1996) 695–697. [3] F. Hide, M.A. Diaz-Garcia, B.J. Schwartz, M.R. Andersson, Q.B. Pei, A.J. Heeger, Semiconducting polymers: a new class of solid-state laser materials, Science 273 (1996) 1833–1835. [4] C. Kallinger, M. Hilmer, A. Haugender, M. Perner, W. Spirkel, U. Lemmer, J. Feldmann, U. Scherf, K. Mu¨ llen, A. Gombert, V. Wittwer, A flexible conjugated polymer laser, Adv. Mater. 10 (12) (1998) 920–923. [5] V. Dumarcher, L. Rocha, C. Denis, C. Fiorini, J.M. Nunzi, F. Sobel, B. Sahraoui, D. Gindre, Polymer thin-film distributed feedback tunable lasers, J. Opt. A: Pure Appl. Opt. 2 (2000) 279–283. [6] G. Kranzelbinder, E. Toussaere, D. Josse, J. Zyss, Progress in erasable and permanent polymer based DFB-structures with multicolour tunable laser emission, Synthetic Met. 121 (2001) 1617–1620. [7] S.V. Frolov, A. Fujii, D. Chinn, Z.V. Vardeny, K. Yoshino, R.V. Gregory, Cylindrical microlasers and light emitting from conducting polymers, Appl. Phys. Lett. 72 (22) (1998). [8] S.V. Frolov, M. Shkunov, Z.V. Vardeny, K. Yoshino, Ring microlasers from conducting polymers, Phys. Rev. B 56 (8) (1997) 4363–4366. [9] A. Schulzgen, C. Spiegelberg, M.N. Morrell, S.B. Mendes, B. Kippelen, N. Peyghambarian, M.F. Nabor, E.A. Mash, P.M. Allemand, Near diffraction-limited laser emission from a polymer in a high finesse planar cavity, Appl. Phys. Lett. 72 (3) (1998) 269– 271. [10] R.E. Slusher, A.F.J. Levi, U. Mohideen, S.L. McCall, S.J. Pearton, R.A. Logan, Threshold characteristics of semiconductor microdisk lasers, Appl. Phys. Lett. 63 (10) (1993) 1310–1312. [11] N.C. Frateschi, A.F.J. Levi, Resonant modes and laser spectrum of microdisk lasers, Appl. Phys. Lett. 66 (22) (1995) 2932–2934. [12] R.C. Polson, G. Levina, Z.V. Vardeny, Spectral analysis of polymer microring lasers, Appl. Phys. Lett. 76 (26) (2000) 3858–3860. [13] S.V. Frolov, M. Shkunov, A. Fujii, K. Yoshino, Z.V. Vardeny, Lasing and stimulated emission in p-conjugated polymers, IEEE J. Quant. Electr. 36 (1) (2000) 2–11.