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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 10 — May. 9, 2011
  • pp: 10009–10016
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Strongly confined, low-threshold laser modes in organic semiconductor microgoblets

Tobias Grossmann, Sönke Klinkhammer, Mario Hauser, Dominik Floess, Torsten Beck, Christoph Vannahme, Timo Mappes, Uli Lemmer, and Heinz Kalt  »View Author Affiliations


Optics Express, Vol. 19, Issue 10, pp. 10009-10016 (2011)
http://dx.doi.org/10.1364/OE.19.010009


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Abstract

We investigate lasing from high-Q, polymeric goblet-type microcavities covered by an organic semiconductor gain layer. We analyze the optical modes in the high-Q cavities using finite element simulations and present a numerical method to determine the cutoff thickness of the gain layer above which the whispering gallery modes are strongly confined in this layer. Fabricated devices show reduced lasing thresholds for increasing gain layer thicknesses, which can be explained by a higher filling factor of the optical modes in the gain layer. Furthermore, reduced lasing threshold is accompanied by a red-shift of the laser emission.

© 2011 OSA

1. Introduction

Besides a low-loss microresonator, a large oscillator strength gain medium has to be integrated in the device in order to achieve low lasing thresholds. The latter is desirable for utilization of compact pumping sources [9

9. S. Klinkhammer, T. Grossmann, K. Lull, M. Hauser, C. Vannahme, T. Mappes, H. Kalt, and U. Lemmer, “Diode-pumped organic semiconductor microcone laser,” IEEE Photon. Technol. Lett. 23(8), 489–491 (2011). [CrossRef]

], ideally even incoherent light sources, such as light-emitting diodes. For this, gain materials can be directly integrated to the microcavity’s material prior to the lithographic structuring. This is realized, e.g., in sol-gels doped with rare-earth ions [1

1. H.-S. Hsu, C. Cai, and A. M. Armani, “Ultra-low-threshold Er:Yb sol-gel microlaser on silicon,” Opt. Express 17(25), 23265–23271 (2009). [CrossRef]

,10

10. L. Yang, T. Carmon, B. Min, S. M. Spillane, and K. J. Vahala, “Erbium-doped and Raman microlasers on a silicon chip fabricated by the sol-gel process,” Appl. Phys. Lett. 86(9), 091114 (2005). [CrossRef]

] or in polymers doped with dye molecules [11

11. T. Grossmann, S. Schleede, M. Hauser, M. B. Christiansen, C. Vannahme, C. Eschenbaum, S. Klinkhammer, T. Beck, J. Fuchs, G. U. Nienhaus, U. Lemmer, A. Kristensen, T. Mappes, and H. Kalt, “Low-threshold conical microcavity dye lasers,” Appl. Phys. Lett. 97(6), 063304 (2010). [CrossRef]

]. Alternatively, a gain medium can be deposited onto the cavity subsequent to the lithographic structuring, e.g., by spin-coating [12

12. B. Min, S. Kim, K. Okamoto, L. Yang, A. Scherer, H. Atwater, and K. Vahala, “Ultralow threshold on-chip microcavity nanocrystal quantum dot lasers,” Appl. Phys. Lett. 89(19), 191124 (2006). [CrossRef]

14

14. H. S. Choi, X. Zhang, and A. M. Armani, “Hybrid silica-polymer ultra-high-Q microresonators,” Opt. Lett. 35(4), 459–461 (2010). [CrossRef] [PubMed]

] or sputtering [15

15. G.-D. Kim, G.-S. Son, H.-S. Lee, K.-D. Kim, and S.-S. Lee, “Refractometric sensor utilizing a vertically coupled polymeric microdisk resonator incorporating a high refractive index overlay,” Opt. Lett. 34(7), 1048–1050 (2009). [CrossRef] [PubMed]

].

2. High-Q microgoblet cavities

In the following, the manufacturing and the optical properties of the passive cavities are described before we turn to the analysis of the lasing properties.

In order to achieve low laser thresholds after deposition of the active layer, the Q factor of the WGM-microcavity has to be high. The cavities onto which the gain layer is deposited are goblet-shaped polymeric microcavities, which are made of the low-loss, thermoplastic polymer poly(methyl methacrylate) (PMMA) and are directly processed on a silicon substrate, see [7

7. T. Grossmann, M. Hauser, T. Beck, C. Gohn-Kreuz, M. Karl, H. Kalt, C. Vannahme, and T. Mappes, “High-q conical polymeric microcavities,” Appl. Phys. Lett. 96(1), 013303 (2010). [CrossRef]

] for a detailed description. After lithographic structuring of PMMA-microdisks, the silicon is isotropically etched using XeF2. A subsequent thermal reflow step results in goblet-shaped microcavities depicted in the scanning electron micrograph Fig. 1(a)
Fig. 1 (a) Scanning electron micrograph of an array of passive PMMA-microgoblets standing on silicon pedestals. (b) Transmission spectrum showing WGM-resonances with loaded Q factors of 3.2·106 and 2.0·106 inferred from the resonance linewidth, which is determined by a Lorenzian fit.
. These microcavities have a significantly reduced surface roughness compared to the microdisks before thermal treatment, resulting in a Q-factor enhancement by two orders of magnitude.

For measuring the quality factors of the WGMs in the microgoblets, a single-mode, tunable, external-cavity laser (linewidth 200 kHz) with wavelengths around 1300 nm is used. Tapered optical fibers (SMF-28) with minimum waist diameters of approximately 1 µm are utilized to evanescently excite WGMs of the cavity. For resonator-waveguide positioning, the tapered fiber is mounted on a five axis positioning stage with a resolution of 20 nm. The transmitted intensity is recorded by a photodiode. The Q factors are determined by measuring the linewidth (full width at half maximum) of the Lorentzian-shaped dips in the transmission spectrum, recorded by sweeping the laser wavelength.

Figure 1(b) shows a resonance spectrum around 1301 nm of a goblet microcavity with a maximum diameter of 40 µm. The highest measured Q factor here is 3.2·106, indicating a smooth cavity surface with low surface-scattering losses of the WGMs. The Q factors in this wavelength region are mainly limited by the absorption of PMMA and are expected to be even higher in the visible [7

7. T. Grossmann, M. Hauser, T. Beck, C. Gohn-Kreuz, M. Karl, H. Kalt, C. Vannahme, and T. Mappes, “High-q conical polymeric microcavities,” Appl. Phys. Lett. 96(1), 013303 (2010). [CrossRef]

,16

16. Y. Takezawa, N. Taketani, S. Tanno, and S. Ohara, “Empirical estimation method of intrinsic loss spectra in transparent amorphous polymers for plastic optical fibers,” J. Appl. Polym. Sci. 46(10), 1835–1841 (1992). [CrossRef]

].

3. Finite element simulations

In order to analyze the influence of an additional organic semiconductor coating with higher refractive index (nAlq3:DCM = 1.72) than the PMMA-cavity (nPMMA = 1.49) on the optical modes, we perform finite element simulations with JCMwave’s simulation software package JCMsuite. We calculate the eigenvalues (frequencies) and the respective electric field distributions of the WGMs by solving Maxwell’s equations with the eigensolver JCMresonance, using third order finite elements with adaptive mesh refinement [17

17. J. Pomplun, S. Burger, L. Zschiedrich, and F. Schmidt, “Adaptive finite element method for simulation of optical nano structures,” Phys. Status Solidi B 244(10), 3419–3434 (2007). [CrossRef]

]. In order to realize transparent boundary conditions, the simulation uses the adaptive perfectly matched layers method (PML).

Figure 2(a)
Fig. 2 (a) Computational domain for the finite element simulations showing the modeled cavity geometry and materials. The inset shows an enlarged view of the cavity rim with the gain material Alq3:DCM on top of the PMMA-resonator and the initial triangulation for the computation. (b) Intensity distribution and resonance wavelengths of the TM0,0(274)-mode for three different thicknesses of the Alq3:DCM-layer. For a thickness of d = 200 nm of the Alq3:DCM-layer, the mode is guided within the gain layer.
shows the computational domain for modeling of microgoblet-lasers, where the rotational symmetry of the resonator is used to reduce computational costs. The inset of Fig. 2(a) shows an enlarged view of the resonator rim including the Alq3:DCM-layer and the triangulation of the structure. The Alq3:DCM-layer covers the upper half of the microresonator due to deposition of gain material from above by thermal evaporation. Each mode is characterized by the polarization of its electromagnetic field (tranverse electric (TE) or transverse magnetic (TM)), the azimuthal mode number m (integer number of wavelengths in the plane of the cavity), the axial mode number l and the radial mode number n. The modes in the following will be denoted as TE/TMn,l(m).

In order to derive a quantitative method for determining the cutoff-thickness, above which the mode is guided in the gain-layer, the effect of an increasing gain-layer thickness on the resonance wavelengths, the mode volumes and the filling factors of the WGMs were investigated in finite element simulations, in which the gain-layer thickness is varied between 5 and 300 nm in steps of 5 nm. As already noted in Fig. 2(b), the resonance wavelength of the same WGM increases with increasing thickness of the Alq3:DCM-layer, as the mode is shifted to the gain layer and thus propagates along a slightly larger radius with a higher effective refractive index. This property is depicted in more detail in Fig. 3(a)
Fig. 3 Results of finite element calculations showing (a) the resonance wavelengths, (b) the mode volumes and the filling factors of the (c) TE0,0(274)- and (d) TM0,0(274)-modes as function of the Alq3:DCM-layer thickness.
. Above a thickness of approximately 75 nm the splitting between TE and TM polarization increases, which also indicates a movement of the WGM intensity distribution towards the cavity surface, as shown in Fig. 2(b). Besides the effect of an increasing resonance wavelength, a change in the mode volume with increasing thickness of the gain layer can be observed in Fig. 2(b). Therefore the mode volume was calculated according to the following equation, where the dielectric constant is denoted as ε [18

18. M. Oxborrow, “Traceable 2-D finite-element simulation of the whispering-gallery modes of axisymmetric electromagnetic resonators,” IEEE Trans. Microw. Theory Tech. 55(6), 1209–1218 (2007). [CrossRef]

]:
V=ε|E(r)|2dVmax{ε|E(r)|2}.
(1)
Figure 3(b) shows the mode volume as function of the Alq3:DCM-layer thickness for the TM/TE0,0(274)-mode. For both modes, the mode volume drops above a certain gain-layer thickness, due to the localization in the Alq3:DCM. This enables identification of a weak and a strong confinement regime, indicated in Fig. 3(b). For the TM0,0(274)-mode, the mode volume drops by a factor of three through the transition from weak to strong confinement.

Although a change in resonance wavelength and mode volume of the WGMs for larger thicknesses indicates guidance of the modes in the gain layer, a quantitative cut-off criterion cannot be inferred from these quantities. For this, we investigate the filling factor of the mode in the gain layer, defined as fraction of the electric energy density in the gain layer and the total electric energy density [18

18. M. Oxborrow, “Traceable 2-D finite-element simulation of the whispering-gallery modes of axisymmetric electromagnetic resonators,” IEEE Trans. Microw. Theory Tech. 55(6), 1209–1218 (2007). [CrossRef]

]:

Fg=gε|E(r)|2dVε|E(r)|2dV.
(2)

4. Lasing in organic semiconductor coated microgoblets

In addition to the optical losses, the microcavity determines the laser emission spectrum. Figure 4(b) shows a spectrum above threshold obtained from a microgoblet laser with a 200 nm thick Alq3:DCM-layer. The multi-mode emission spectrum shows laser modes with linewidths of 80 pm, limited by the resolution of the spectrometer (1800 lines/mm grating). The wavelength spacing between observed lasing modes is smaller than the calculated free spectral range of about 2 nm due to the presence of higher-order transverse modes in addition to the fundamental cavity modes. Furthermore, the envelope of the laser emission is shifted to larger wavelengths for increasing Alq3:DCM-layer thicknesses, shown in Fig. 4(c). This behavior is attributed to increased absorption of dye molecules for thicker gain layers due to an increased filling factor of the modes in the gain layer and results in red-shifted net gain spectra of the laser dye. This is accompanied by a decrease of the lasing thresholds due to an increased concentration of dye molecules within the WGMs. These effects can be described by a modified standard dye laser model [11

11. T. Grossmann, S. Schleede, M. Hauser, M. B. Christiansen, C. Vannahme, C. Eschenbaum, S. Klinkhammer, T. Beck, J. Fuchs, G. U. Nienhaus, U. Lemmer, A. Kristensen, T. Mappes, and H. Kalt, “Low-threshold conical microcavity dye lasers,” Appl. Phys. Lett. 97(6), 063304 (2010). [CrossRef]

,20

20. M. M. Mazumder, G. Chen, R. K. Chang, and J. B. Gillespie, “Wavelength shifts of dye lasing in microdroplets: effect of absorption change,” Opt. Lett. 20(8), 878–880 (1995). [CrossRef] [PubMed]

], where the number of dye molecules per mode volume depends on the thickness of the gain layer.

5. Conclusion

Acknowledgments

This work has been supported by the DFG Research Center for Functional Nanostructures (CFN) Karlsruhe, by a grant from the Ministry of Science, Research, and the Arts of Baden-Württemberg (Grant No. Az:7713.14-300) and by the German Federal Ministry for Education and Research BMBF (Grant No. FKZ 13N8168A) T.M.’s Young Investigator Group (YIG 08) received financial support from the Concept for the Future of the Karlsruhe Institute of Technology (KIT) within the framework of the German Excellence Initiative. T.G. gratefully acknowledges financial support of the Deutsche Telekom Stiftung. T.G., M.H., T.B., C.V., and S.K. are pursuing their Ph.D. within the Karlsruhe School of Optics and Photonics (KSOP). We acknowledge support by Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of Karlsruhe Institute of Technology. Furthermore, we acknowledge JCMwave GmbH for academic use of their JCMsuite.

References and links

1.

H.-S. Hsu, C. Cai, and A. M. Armani, “Ultra-low-threshold Er:Yb sol-gel microlaser on silicon,” Opt. Express 17(25), 23265–23271 (2009). [CrossRef]

2.

J. Yang and L. J. Guo, “Optical sensors based on active microcavities,” IEEE J. Quantum Electron. 12(1), 143–147 (2006). [CrossRef]

3.

E. P. Ostby and K. J. Vahala, “Yb-doped glass microcavity laser operation in water,” Opt. Lett. 34(8), 1153–1155 (2009). [CrossRef] [PubMed]

4.

W. Fang, D. B. Buchholz, R. C. Bailey, J. T. Hupp, R. P. H. Chang, and H. Cao, “Detection of chemical species using ultraviolet microdisk lasers,” Appl. Phys. Lett. 85(17), 3666–3668 (2004). [CrossRef]

5.

H. Rong, S. Xu, Y.-H. Kuo, V. Sih, O. Cohen, O. Raday, and M. Paniccia, “Low-threshold continuous-wave Raman silicon laser,” Nat. Photonics 1(4), 232–237 (2007). [CrossRef]

6.

P. Thilakan, G. Sasikala, and I. Suemune, “Fabrication and characterization of a high Q microdisc laser using InAs quantum dot active regions,” Nanotechnology 18(5), 055401 (2007). [CrossRef]

7.

T. Grossmann, M. Hauser, T. Beck, C. Gohn-Kreuz, M. Karl, H. Kalt, C. Vannahme, and T. Mappes, “High-q conical polymeric microcavities,” Appl. Phys. Lett. 96(1), 013303 (2010). [CrossRef]

8.

A. M. Armani, A. Srinivasan, and K. J. Vahala, “Soft lithographic fabrication of high Q polymer microcavity arrays,” Nano Lett. 7(6), 1823–1826 (2007). [CrossRef] [PubMed]

9.

S. Klinkhammer, T. Grossmann, K. Lull, M. Hauser, C. Vannahme, T. Mappes, H. Kalt, and U. Lemmer, “Diode-pumped organic semiconductor microcone laser,” IEEE Photon. Technol. Lett. 23(8), 489–491 (2011). [CrossRef]

10.

L. Yang, T. Carmon, B. Min, S. M. Spillane, and K. J. Vahala, “Erbium-doped and Raman microlasers on a silicon chip fabricated by the sol-gel process,” Appl. Phys. Lett. 86(9), 091114 (2005). [CrossRef]

11.

T. Grossmann, S. Schleede, M. Hauser, M. B. Christiansen, C. Vannahme, C. Eschenbaum, S. Klinkhammer, T. Beck, J. Fuchs, G. U. Nienhaus, U. Lemmer, A. Kristensen, T. Mappes, and H. Kalt, “Low-threshold conical microcavity dye lasers,” Appl. Phys. Lett. 97(6), 063304 (2010). [CrossRef]

12.

B. Min, S. Kim, K. Okamoto, L. Yang, A. Scherer, H. Atwater, and K. Vahala, “Ultralow threshold on-chip microcavity nanocrystal quantum dot lasers,” Appl. Phys. Lett. 89(19), 191124 (2006). [CrossRef]

13.

A. Tulek, D. Akbulut, and M. Bayindir, “Ultralow threshold laser action from toroidal polymer microcavity,” Appl. Phys. Lett. 94(20), 203302 (2009). [CrossRef]

14.

H. S. Choi, X. Zhang, and A. M. Armani, “Hybrid silica-polymer ultra-high-Q microresonators,” Opt. Lett. 35(4), 459–461 (2010). [CrossRef] [PubMed]

15.

G.-D. Kim, G.-S. Son, H.-S. Lee, K.-D. Kim, and S.-S. Lee, “Refractometric sensor utilizing a vertically coupled polymeric microdisk resonator incorporating a high refractive index overlay,” Opt. Lett. 34(7), 1048–1050 (2009). [CrossRef] [PubMed]

16.

Y. Takezawa, N. Taketani, S. Tanno, and S. Ohara, “Empirical estimation method of intrinsic loss spectra in transparent amorphous polymers for plastic optical fibers,” J. Appl. Polym. Sci. 46(10), 1835–1841 (1992). [CrossRef]

17.

J. Pomplun, S. Burger, L. Zschiedrich, and F. Schmidt, “Adaptive finite element method for simulation of optical nano structures,” Phys. Status Solidi B 244(10), 3419–3434 (2007). [CrossRef]

18.

M. Oxborrow, “Traceable 2-D finite-element simulation of the whispering-gallery modes of axisymmetric electromagnetic resonators,” IEEE Trans. Microw. Theory Tech. 55(6), 1209–1218 (2007). [CrossRef]

19.

T. A. Beierlein, B. Ruhstaller, D. J. Gundlach, H. Riel, S. Karg, C. Rost, and W. Rieß, “Investigation of internal processes in organic light-emitting devices using thin sensing layers,” Synth. Met. 138(1-2), 213–221 (2003). [CrossRef]

20.

M. M. Mazumder, G. Chen, R. K. Chang, and J. B. Gillespie, “Wavelength shifts of dye lasing in microdroplets: effect of absorption change,” Opt. Lett. 20(8), 878–880 (1995). [CrossRef] [PubMed]

OCIS Codes
(140.7300) Lasers and laser optics : Visible lasers
(160.4890) Materials : Organic materials
(160.5470) Materials : Polymers
(140.3948) Lasers and laser optics : Microcavity devices

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: April 8, 2011
Revised Manuscript: May 4, 2011
Manuscript Accepted: May 4, 2011
Published: May 6, 2011

Citation
Tobias Grossmann, Sönke Klinkhammer, Mario Hauser, Dominik Floess, Torsten Beck, Christoph Vannahme, Timo Mappes, Uli Lemmer, and Heinz Kalt, "Strongly confined, low-threshold laser modes in organic semiconductor microgoblets," Opt. Express 19, 10009-10016 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-10-10009


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References

  1. H.-S. Hsu, C. Cai, and A. M. Armani, “Ultra-low-threshold Er:Yb sol-gel microlaser on silicon,” Opt. Express 17(25), 23265–23271 (2009). [CrossRef]
  2. J. Yang and L. J. Guo, “Optical sensors based on active microcavities,” IEEE J. Quantum Electron. 12(1), 143–147 (2006). [CrossRef]
  3. E. P. Ostby and K. J. Vahala, “Yb-doped glass microcavity laser operation in water,” Opt. Lett. 34(8), 1153–1155 (2009). [CrossRef] [PubMed]
  4. W. Fang, D. B. Buchholz, R. C. Bailey, J. T. Hupp, R. P. H. Chang, and H. Cao, “Detection of chemical species using ultraviolet microdisk lasers,” Appl. Phys. Lett. 85(17), 3666–3668 (2004). [CrossRef]
  5. H. Rong, S. Xu, Y.-H. Kuo, V. Sih, O. Cohen, O. Raday, and M. Paniccia, “Low-threshold continuous-wave Raman silicon laser,” Nat. Photonics 1(4), 232–237 (2007). [CrossRef]
  6. P. Thilakan, G. Sasikala, and I. Suemune, “Fabrication and characterization of a high Q microdisc laser using InAs quantum dot active regions,” Nanotechnology 18(5), 055401 (2007). [CrossRef]
  7. T. Grossmann, M. Hauser, T. Beck, C. Gohn-Kreuz, M. Karl, H. Kalt, C. Vannahme, and T. Mappes, “High-q conical polymeric microcavities,” Appl. Phys. Lett. 96(1), 013303 (2010). [CrossRef]
  8. A. M. Armani, A. Srinivasan, and K. J. Vahala, “Soft lithographic fabrication of high Q polymer microcavity arrays,” Nano Lett. 7(6), 1823–1826 (2007). [CrossRef] [PubMed]
  9. S. Klinkhammer, T. Grossmann, K. Lull, M. Hauser, C. Vannahme, T. Mappes, H. Kalt, and U. Lemmer, “Diode-pumped organic semiconductor microcone laser,” IEEE Photon. Technol. Lett. 23(8), 489–491 (2011). [CrossRef]
  10. L. Yang, T. Carmon, B. Min, S. M. Spillane, and K. J. Vahala, “Erbium-doped and Raman microlasers on a silicon chip fabricated by the sol-gel process,” Appl. Phys. Lett. 86(9), 091114 (2005). [CrossRef]
  11. T. Grossmann, S. Schleede, M. Hauser, M. B. Christiansen, C. Vannahme, C. Eschenbaum, S. Klinkhammer, T. Beck, J. Fuchs, G. U. Nienhaus, U. Lemmer, A. Kristensen, T. Mappes, and H. Kalt, “Low-threshold conical microcavity dye lasers,” Appl. Phys. Lett. 97(6), 063304 (2010). [CrossRef]
  12. B. Min, S. Kim, K. Okamoto, L. Yang, A. Scherer, H. Atwater, and K. Vahala, “Ultralow threshold on-chip microcavity nanocrystal quantum dot lasers,” Appl. Phys. Lett. 89(19), 191124 (2006). [CrossRef]
  13. A. Tulek, D. Akbulut, and M. Bayindir, “Ultralow threshold laser action from toroidal polymer microcavity,” Appl. Phys. Lett. 94(20), 203302 (2009). [CrossRef]
  14. H. S. Choi, X. Zhang, and A. M. Armani, “Hybrid silica-polymer ultra-high-Q microresonators,” Opt. Lett. 35(4), 459–461 (2010). [CrossRef] [PubMed]
  15. G.-D. Kim, G.-S. Son, H.-S. Lee, K.-D. Kim, and S.-S. Lee, “Refractometric sensor utilizing a vertically coupled polymeric microdisk resonator incorporating a high refractive index overlay,” Opt. Lett. 34(7), 1048–1050 (2009). [CrossRef] [PubMed]
  16. Y. Takezawa, N. Taketani, S. Tanno, and S. Ohara, “Empirical estimation method of intrinsic loss spectra in transparent amorphous polymers for plastic optical fibers,” J. Appl. Polym. Sci. 46(10), 1835–1841 (1992). [CrossRef]
  17. J. Pomplun, S. Burger, L. Zschiedrich, and F. Schmidt, “Adaptive finite element method for simulation of optical nano structures,” Phys. Status Solidi B 244(10), 3419–3434 (2007). [CrossRef]
  18. M. Oxborrow, “Traceable 2-D finite-element simulation of the whispering-gallery modes of axisymmetric electromagnetic resonators,” IEEE Trans. Microw. Theory Tech. 55(6), 1209–1218 (2007). [CrossRef]
  19. T. A. Beierlein, B. Ruhstaller, D. J. Gundlach, H. Riel, S. Karg, C. Rost, and W. Rieß, “Investigation of internal processes in organic light-emitting devices using thin sensing layers,” Synth. Met. 138(1-2), 213–221 (2003). [CrossRef]
  20. M. M. Mazumder, G. Chen, R. K. Chang, and J. B. Gillespie, “Wavelength shifts of dye lasing in microdroplets: effect of absorption change,” Opt. Lett. 20(8), 878–880 (1995). [CrossRef] [PubMed]

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